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The nature of the plant community: a reductionist view
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J. Bastow Wilson
Botany Department, University of Otago, P.O. Box 56, Dunedin, New Zealand.
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Andrew D.Q. Agnew
Institute of Biological Sciences, University of Wales Aberystwyth, SY23 3DA, U.K.
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Chapter 3: Community-level processes
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Introduction ......................................................................................................................................... 1
Allogenic change: including secular change ....................................................................................... 3
Directional autogenic change .............................................................................................................. 6
3.1
Environmental change................................................................................................................. 7
3.2
Pioneer species ............................................................................................................................ 8
3.3
Decrease in biomass .................................................................................................................... 9
Cyclic succession: in fact or fiction .................................................................................................. 10
Switches: the positive feedback processes ........................................................................................ 13
5.1
The concept of the switch ......................................................................................................... 13
5.2
Types of switch ......................................................................................................................... 14
5.3
Outcomes of switches ............................................................................................................... 14
5.4
Mediating agents ....................................................................................................................... 15
5.4.A
Mechanisms involving regional climate ........................................................................... 16
5.4.B
Mechanisms involving water ............................................................................................ 16
5.4.C
Mechanisms involving other aspects of microclimate ...................................................... 18
5.4.D
Mechanisms involving physical substrate......................................................................... 18
5.4.E
Mechanisms involving soil chemistry............................................................................... 19
5.4.F
Mechanisms involving disturbance ................................................................................... 22
5.4.G
Mechanisms involving heterotrophs ................................................................................. 23
5.5
Switch evolution ....................................................................................................................... 25
5.6
Alternative stable states ............................................................................................................ 26
5.7
Negative feedback ..................................................................................................................... 32
Diversity → productivity .................................................................................................................. 32
Stability ............................................................................................................................................. 35
7.1
Concept ..................................................................................................................................... 35
7.2
Reliability (constancy) .............................................................................................................. 36
7.3
Stability sensu stricto (Liapunov stability) ............................................................................... 37
7.4
Resistance to abiotic perturbation ............................................................................................. 40
7.5
Resistance to invasion ............................................................................................................... 41
7.6
Resilience .................................................................................................................................. 44
Conclusion ........................................................................................................................................ 46
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1 Introduction
The last chapter described the ways in which individuals of differing species can interact within
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the space of a plant community. Most are based on reaction, which Clements (1916) defined as “The
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effect which a plant or a community exerts on its habitat”. Admitting that classification was difficult, he
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listed about 20 types of reaction. All plants and animals must alter their immediate surroundings (chapt.
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2 above). Plants especially do this because of their bulk (biomass) and their necessity to produce litter,
Wilson & Agnew, chapter 3, Community processes, page 2 of 50
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leading to modifications of local light availability, climatic regimes, environmental chemistry and
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geomorphological processes. This also affects the populations of herbivores and thus the system as a
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whole. These interactions are the currency of community processes that can have only two outcomes:
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stability or change. Change should be the expectation, firstly because the environment is never without
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temporal variation and secondly because the reaction of the plants alters their own environmental
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conditions. However, stabilising forces do exist, and aspects of stability can certainly be seen, even if
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strict stability has never been shown. This chapter examines the mechanisms of community change and
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stability.
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The positive and negative interactions that we discussed in chapter 2 have counterparts at the
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level of the entire plant community (Box 3.1), and these are the topic of this chapter, we consider the
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implications of these effects for the entire plant community. The basic contrast is between a
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species/community changing the environment to its disbenefit (facilitation, the concept though not the
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term of Clements 1916) or benefit (switch: Odum 1971; Wilson and Agnew 1992). We therefore link
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most vegetation change to facilitation and switches in this chapter. In either case, eventually there can
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be negative feedback, causing stability. We also consider the cyclic succession of Watt (1947).
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None of these processes can take place on a uniform stage. Plant communities can never be
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homogeneous both because of the unique reaction of each species and also because each has unique
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demographic processes controlling the arrangement of individuals. Peter Greig-Smith (1957) first used
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sampling and analysis to show that all species’ individual occurrences had a non-random pattern. One
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cause for this is the demographic (dispersal) behaviour and morphology of each species. In one
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example, Kershaw (1964) examined North Wales pastures where Agrostis capillaris (= A. tenuis, bent)
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showed no clumping to the naked eye, but where the tillers proved on sampling to have clumping
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because disparate rhizomes upturned when nearing each other, forming patches of denser culms
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(Kershaw, 1964). Patterns resulting from vegetative demography are clearly seen at the level of
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individual clones in Harberd (1962)(chapt. 1, sect. 1.1 above) and in his similar results in Festuca rubra,
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Trifolium repens and Holcus mollis. Using Mark Hill’s (1973) algebraic clarification, Agnew & Gitay
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(1990) found that species and soil nutrients were similarly patterned in visually uniform dune slacks,
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and that this pattern became more pronounced as species number increased. This is a general quality of
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all plant communities and has implications for community function.
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Wilson & Agnew, chapter 3, Community processes, page 3 of 50
species/guild X gives an absolute or relative advantage to itself = switch
species/guild X gives a relative disadvantage to itself:
the effect disappears at low density of X (negative feedback) = stability
the effect is density-independent: facilitation and/or autointerference = succession
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Box 3.1: Types of community interaction.
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The patterns of change we can see are (Fig. 3.1):
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Allogenic change – where is this in the figures below?
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Directional autogenic change
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Watt’s cyclic succession
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Switches
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However, we note that intermediates and combinations occur.
Community
X1
Community
Y1
Community
X2
Community
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Community
Y2
Time
Community
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Clements’ relay
floristics (succession to
climax)
Community
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Time
Community
Y1
Time
Community
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Community
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Watt’s cyclic
succession
Community
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Community
Y2
Community
Z2
A switch, giving
alternative stable states
Fig. 3.1: Three types of autogenic vegetation change through time.
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2 Allogenic change: including secular change
Vegetation changes on long timescales can be seen via the pollen record of temperate and arctic
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lakes and mires, especially over the past 15,000 yr. In the northern Atlantic and Pacific areas, the most
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abrupt, non-anthropogenic alteration was a reversion from birch scrub (Betula spp) to tundra herbs in
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the Younger Dryas period (13,000 – 11,600 yr BP). When historical vegetation change is sudden, it may
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be that the vegetation is tracking a sudden climate change, or the vegetation change may be abrupt
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because there is a geographic-scale switch (sect. 5.4.A below). In the case of the Younger Dryas, there
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does seem to have been a closely-correlated sudden temperature depression (Hu et al. 2002; Ruddiman
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2003). Such changes are most likely to be seen at the geographical boundary of a vegetation type, and
Wilson & Agnew, chapter 3, Community processes, page 4 of 50
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the pollen profile of Kremenetsky et al. (1999) indicated that the steppe-forest boundary in the Ukraine
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has moved at least twice since 6000 yr BP.
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Soil processes can occur on a similar timescale. Soil leaching especially needs thousands of
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undisturbed years to take place. It results in plant communities which are slow growing, often evergreen
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and high in phenolics, on soils that are low in all nutrients, particularly phosphate. Northup et al. (1998)
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describe a chronosequence of such soils in North California formed over an estimated 500,000 yr.
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Recent soil there has pH (in CaCl2) over 6, while in the oldest soil it is less than 4. This environmental
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change has been tracked not only ecologically but perhaps evolutionarily, since there are endemic
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species and varieties of conifer particularly adapted to the soil conditions and contributing to them.
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Fluctuations in the species composition of plant communities from year to year were described
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by Clements (1934 pp. 41–42) as ‘annuation’, by Penfould (1964) as the ‘pulse phenomenon’ and by
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Collins et al. (1987 %89) as ‘pulse-phase’. Investigations on this timescale, and the decade-long
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changes that often accompany it, have been hindered by the paucity of datasets with continuous and
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long records of approximately equilibrium vegetation. Notable exceptions are the Park Grass
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experiment in lowland England (since 1856, though detailed recording has been somewhat intermittent)
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and plots on Barro Colorado Island (since 1985; Chave 2003). Usher (1987) re-analysed Watt's (1960)
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data from 21 years of observation in Breckland, Eastern England. The transition probabilities between
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the vegetation elements appear to show that change is taking place towards an Agrostis/Festuca type
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from what, don’t assume that the reader will be familiar enough with the study to know that sort of
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detail. This is indeed gradual change between states, and does not appear to conform to an overall
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succession. It is intuitively credible as a community process in changing environments, though Usher
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had no ecological explanation.
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When the environment changes, it would not be surprising for the plant community to do
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likewise, but often the correlations are hard to see. Allen et al. (1995) found correlation with the rainfall
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at one of the dry grassland sites examined, but not at four other, nearby sites. Collins et al. (1987)
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similarly failed to find correlations between individual species and the climate. what sort of time scale
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were these studies conducted in? Need more information about them to be able to evaluate how
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effective theses studies could have been at detecting change.
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Dunnett et al. (1998) correlated vegetation changes over 38 years in the roadside vegetation at
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Bibury, southern England, with climatic fluctuations. There were over 100 grasses and forbs. More
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correlations with weather indices were found than expected by chance. Warm dry summers increased
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species favoured by environmental stress or disturbance (S or R of C-S-R theory), whereas wet growing
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seasons favoured more productive species (C). Dunnett and Grime (1999) found by experiment that the
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differential response of species to temperature was based on their physiological response, but the
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differences between the species were amplified in competition. Species that were notably more
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abundant in unusually warm summers included Achillea millefolium (yarrow) (Grime et al. 1994 %271)
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which is distributed almost throughout the British Isles, being absent only in a very few cold/wet parts
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(Preston et al. 2002), but Ranunculus repens (buttercup) and Trifolium repens (white clover) with
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almost identical geographical distributions having identical geographic distributions doesn’t mean that
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they have the same habitat requirements to A. millefolium, were less abundant in those years.
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Brachypodium pinnatum (tor grass) was more abundant in warm summers, and is a species very
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frequent in chalk/limestone areas with warm summers (July mean > 15 ˚C), but another grass with a
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very similar distribution, Bromopsis erecta, was less abundant. This is puzzling. If the Brachypodium
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and Bromopsis have different habitat affinities (as is suggested by the fact that Bromopsis isn’t
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mentioned as a calcifile, then it is not surprising that they react differently to environmental chances
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One of the most fascinating sets of observations in ecology is Watt’s (1981 %509) 38-year
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record from Breckland in eastern England. Calcareous ‘Grassland A’ exhibited a sequential dominance,
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from 1936 to 1950 by the small-tussock grass Festuca ovina (sheep’s fescue), from 1950 to 1958 by the
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creeping herb Hieracium pilosella (hawkweed), from 1959 to 1964 co-dominance of the latter two with
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the procumbent shrub Thymus polytrichus (= T. drucei, thyme), and from 1965 to the end of recordings
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in 1973 by T. polytrichus on its own. These three species all showed a steady increase to a peak and a
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steady reduction from it, though when H. pilosella decreased it held on to some microsites rather
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tenaciously. Watt explains these dramatic patterns of sequential dominance by very subtle changes in
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the weather, but as at Bibury the correlations are not convincing. Like Dunnett and Grime (1999) he saw
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competition as important, which is probably is. Basically, these changes have never been explained.
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Disease may have played a rôle. Based on what. There are other potential explanations not mentioned
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Shorter-term changes can be seen too. Peco (1989) was able to show that low rainfall produced a
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single-year retreat in an ordination axis that was correlated with successional time. This represents a
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whole-community shift in a single year. Some species have been observed to show outbreaks: sudden
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increases in abundance followed by a sudden decrease, all happening within a few years. In the Park
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Grass Experiment, Dodd et al. (1995 % 277) found that of 43 species recorded on non-acidified plots,
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10 showed outbreaks, i.e rather sudden increases and subsequent decreases on timescales in the order of
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a decade. These species tended to be ruderals: self-fertilised and suppressed by competition. Outbreaks
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are seen at Bibury too (Dunnett and Willis 2004 %18): repeated increases and decreases in species such
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as Urtica dioica (stinging nettle) and Chamerion angustifolium (rosebay willowherb), and on shorter
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timescales in species such as Galium aparine (goosegrass) and Rumex obtusifolius (dock); Dunnett and
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Willis say many in the later group have ruderal traits, confirming the conclusion of Dodd et al. The
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decade-long outbreaks in Watt’s (1981) Breckland site may have been due to the inertia of the shrubs
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(Thymus polytrichus), tussocks (Festuca ovina) and clones (Hieracium pilosella).
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At a fine scale, the elusive Carousel theory (van der Maarel and Sykes 1997) appears to say that
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even if a community remains in equilibrium on a large scale, species presence on a small scale changes
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from year to year, such that within a community most species are capable of occupying most microsites,
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but in any year each species will leave some potential sites unoccupied, at random. This begs the
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question of why the species sometimes choose (I would take points of an introductory bio student’s
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paper for implying that species are choosing in this context) to be absent. The logical implication is that
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there is niche limitation to the number of species that can co-exist locally, and there is evidence for this
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(Wilson et al. 1995 %729). Baba (2005), also working in calcareous grassland, emphasised differences
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in mobility between species: a ‘complex carousel’ rather than a single carousel. Van der Maael and
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Sykes’ observations of turnover were made on a fine scale, e.g. 3 × 3 cm, which is less than the size of
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most of the plants, and certainly less than their sphere of influence. It may represent that one year a leaf
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of a plant overhangs the quadrat, and next year the same plant is present in the same place, but its leaves
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are in a different direction. The first priority in understanding communities at this scale is to solve the
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Paradox of the Plankton (chapt. 4 below). Then, we have to find how these mechanisms of coexistence
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and those of competitive exclusion processes interact to produce the very fine coexistence we see in
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many communities. Self-allelopathy for Hypochaeris radicata (cat’s ear) is one example, still not fully
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established for the field, of the thousands of interactions that must be causal. The Carousel Theory is a
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distraction from progress towards understanding them.
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3 Directional autogenic change
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The basis of autogenic succession is that a species or community, by reaction, changes the environment
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(especially the soil), in such a way as to facilitate the entry of others. The new entrants are usually more
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diverse, larger and longer-lived than their predecessors and therefore the process involves increases in
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biomass and biodiversity with time (Clements 1916). In chapter 1, section 1, we listed the necessary
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parts of ecosystems: an energy input apparatus, the capital of energy, the cycling of elements and rate
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regulation (Reichel et al. 1975). Capital of energy is in biomass, while nutrient cycling (the resource
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economy) requires biodiversity of heterotrophs. Nutrient capital in the system is part of the rate
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regulation, other rates being set by the physiology (autecology) of the species. Monotonic increases in
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the values of these system parts are not inevitable; there can be decreases in all of them, autogenically or
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allogenically triggered (sect. 3.3 below).
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‘Succession’ has often been taken to be the linear gradient that was the norm for Clements
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(1916), from pioneer stage to climax. This was characterised by Egler (1954) as “relay floristics” and by
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others as “facilitation succession” and “succession to climax”. As always, Clements saw much, and
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synthesised his observations into a general pattern. Like all generalisations, there are exceptions.
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Clements saw these as well as anyone, and reported them. However, the general pattern that he
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emphasised is a true one. Communities change through time for three reasons: (1) the environment
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changes allogenically making invasion possible from the species pool, (2) new species arrive in the
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regional pool, (3) species modify the environment by reaction, allowing other species to establish. I
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second the comment blurb and mention that the section which seems to most closely follow these (#1)
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points is an allogenic factor. It is only the last that we can call succession, either directional or cyclic.
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3.1 Environmental change
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If the initiation of this process is geomorphological, i.e. the natural creation of a new land
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surface, the succession is said to be primary succession; if environmental modifications remain from
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previous succession it is said to be secondary. This paragraph is unnecessary if your target audience is
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adv. undergrads or higher
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A classic description of a primary succession is that of Crocker and Major (1955) on deglaciated
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terrain in Glacier Bay, Alaska. We must notice that this is a space-for-time substitution; as always with
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such comparisons that the physical location and therefore the physical environment are not identical
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between stages, neither were the weather and the propagule flux at each stage of succession. A classic
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secondary succession is that in Buell-Small oldfields in New Jersey, USA (Bartha et al. 2003), where a
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genuine temporal record was possible. Earlier workers (Clements 1916) envisaged a single end point of
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a "climax community" in successions, but it has become clear that there are many potential pathways
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through vegetation types. Thus in Glacier Bay the initial vegetation was of Dryas drummondii and Alnus
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sinuata and A. rubra (alders), but the course of soil development in glacial til resulted in flats and
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hollows where drainage became impeded and mire vegetation developed (‘muskeg’ with Pinus
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contorta) while slopes became Picea sitchensis (sitka spruce) forest. Landform is not the only
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determinant of successional sequences: Allen (1988) suggested that the causes of differing trajectories
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of secondary succession in Wyoming sagebrush rangeland included anthropogenic topsoil alteration,
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mycorrhizae and alien invaders. There may also be alternative stable states (sect. 5.6 below). awkward
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sentence
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The principal modification to the environment in mesic successions is in the soil/root system.
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Nutrient pools and soil structure change as roots exploit and die in the soil, while the soil fauna uses
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these resources and incidentally forces nutrient cycling. There is a trend towards soil stratification
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imposed by surface litter deposition and water movement. Soil systems, like aerial systems in their
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different ways, consist of a continuum of biota, all of which are leaky, so all components can affect all
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others through diffusion and by mass flow of solute. Organisms are typically more closely intermixed
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than above-ground. Fungal hyphae provide the potential for intimate connections between roots, and a
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large body of research has demonstrated their importance in vascular plant nutrition and performance.
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An example is the sand dune binding grass Ammophila breviligulata (beachgrass) in which Koske and
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Gemma (1997) found that vascular arbuscular mycorrhizae (VAM) were critical in its rate of spread into
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bare sand. The reaction that every plant has on its environment operates above ground as well (chapt 2.,
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sect. 2), such as the mutual shelter offered by alpine shrubs (Choler et al. 2001). However, the process
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of facilitation has rarely been documented and [Walker et al. (1986 %1243) concluded that, interpreting
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the zonation away from an Alaskan river as a chronosequence, there was no evidence that facilitation
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was involved] Not enough info given on this study . I cant\’t evaluate this reference. unclear why
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zonation away from the river was seen as a chronosequence- is there some reason to believe that the
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river used to by broader and has shrunk?. Nitrogen fixers are clear examples of species that might
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facilitate. Walker et al. (2003 %277) gave experimental evidence in succession after a volcanic eruption
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in North Island, New Zealand, the net effect of the N-fixing shrub Coriaria arborea (tutu) on the
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succeeding dominant tree Griselinia littoralis (broadleaf) was positive: considerable soil enrichment
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outweighing the effect of shade.
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3.2 Pioneer species
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Pioneers are said to be mainly short-lived ruderals with high seed output. They have low
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competitive ability, and are replaced by long-lived species producing fewer but larger seeds and often
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with vegetative reproduction. These are often termed ‘r’ and ‘K’ selected respectively, in an analogy
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with the Lotka Volterra logistic equations (Krebs 1978). It is tempting to equate pioneers with the ‘R’ of
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C-S-R theory (Grime 2001) and ‘K’ with the ‘C’ of C-S-R theory (chapt. 6 below). However, under
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Grime’s theory, whilst the secondary succession trajectory starts in the R corner, and starts to move
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towards the C corner, it veers off towards ‘S’ (stressed) habitat and species characteristics. Moreover,
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primary succession starts in the S corner, and after a trip towards the C-S-R centre also ends up also
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back in the S corner. Grubb (1987) agrees with this: pointing out that in primary successions with
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limited water and nutrients the colonisers are often S species. In temperate mesic situations there seems
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little doubt that the plant strategies are associated with differing fungal relationships. Francis and Read
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(1994) found in a survey that 70% of species form VAM associations. Most were apparently
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mutualistic, but some species showed a negative effect and these were mostly ruderals. This might be an
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additional reason for the inability of pioneers to persist in later-successional, closed communities.
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Mycorrhizae are a sinkshouldn’t this be source? for photosynthate and their relationship with their hosts
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depends upon whether the additional mineral nutrients that they obtain via the fungi enhance
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photosynthetic efficiency enough to amortise their carbon cost (Tuomi et al. 2001). It is in nutrient-poor
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conditions that mycorrhiza can benefit their hosts, especially in phosphate, and P generally decreases
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through succession, making mycorrhizal associations more significant. Of course the benefit that hosts
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obtain from their mycorrhizae depends on the availability of the other resources, light and water.
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Zangaro et al. (2000) found that pioneers in a Brazilian secondary succession had abundant VAM, while
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primary forest had much less; the juveniles in these secondary successions are in full light so that
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nutrient gain gives a growth advantage, while saplings in the later, closed forest are shaded so that light,
Wilson & Agnew, chapter 3, Community processes, page 9 of 50
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not nutrients, is limiting. We can re-evaluate Francis and Read's (1995) survey and propose that one
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reason for lower mycorrhizal association by ‘r’ selected species is that they are found in disturbed places
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where nutrients are not limiting. Succession towards forest should have a special dynamic since the
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system is storing nutrients (and carbon as biomass), potentially depleting soil resources. Thus Simard
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and Sachs (2004) found that in young stands taller individuals were most affected by neighbours, while
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in older stands shorter individuals were more sensitive. This is consistent with the idea that the resource
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under competitive demand changed from light to nutrients, the opposite of Tilman’s interpretation for
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Cedar Creek (*).?? What is this?
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3.3 Decrease in biomass
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Biomass can be lost in successional change by a switch process such as paludification (sect .
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5.4.E, below). Rather similarly, the litter produced by some species, especially gymnosperms, is low in
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bases and acidifies the soil, resulting in the leaching of nutrients. This is podsolisation. This litter is
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often slow to break down, and therefore retains nutrients. These processes could theoretically lead to
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reduced biomass. Wardle et al. (2004) compare sequences from six ‘chronosequences’, in different parts
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of the world and on different substrates, all showing a decrease in biomass. The timescale on which this
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process would occur makes it difficult to support with evidence, but they say that “For all the sequences,
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the decline in basal area became noticeable within thousands to tens of thousands of years after the
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start”. The sequences vary in the trends of litter N and P, litter decomposition, etc., but most conform to
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podsolisation.
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Another biomass-decreasing process which could occur late in a succession is salinisation,
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caused by rheatophytes drawing on a sub-saline groundwater. Climate change and soil salinisation were
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probably the cause of loss of casuarinid forest and an accompanying increase in salt-tolerant chenopods
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in south-western New South Wales, Australia after 4500 yrs BP (Cupper et al. 2000). The salinization
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could have been caused by the transpiration of the forest, in which case the process seems to be
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autogenic.
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Biomass loss caused by autogenic succession seems to be rare.
Wilson & Agnew, chapter 3, Community processes, page 10 of 50
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4 Cyclic succession: in fact or fiction
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Watt (1947) emphasised the concept of self-generating, repeated cycles of change through
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communities with different species composition, and often different biomass and biodiversity, to explain
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observed vegetation mosaics. He concluded they were widespread and the usual cause of vegetation
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mosaics. The idea fired many field ecologists worldwide to look for examples.
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Cyclic systems have been widely inferred from observational evidence of patchiness in
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vegetation consisting of shrubby and herbaceous phases (e.g. Agnew 1985). The mechanism is the
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opposite to that used in the formulation of the switch process, that an individual of a major species
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(usually only one) becomes unable to sustain its own presence or abundance and dies ("moving"
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laterally), its place being taken often by members of a different guild, e.g. shrubs replaced by bare
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ground with lichens. The latter eventually form an environment suitable for recolonisation by the
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former, and so the cycle continues. All cycles are driven by the breakdown of one species and all cycles
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allow the coexistence of several species in a moving mosaic. The effect could be via litter allelopathy,
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pH, etc. (chapt. 2 above). The result is similar to plant pathogen increase at high densities, and may be
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difficult to distinguish from it in field studies. The replant problem occurs where a crop fails to grow
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satisfactorily on land on which the crop has already grown, and is important in commercial agriculture
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and forestry. Zhang and Zu (2001) attempted to discern between pathogen loadings and autoallelopathy
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in depressing growth of replantings of Cunninghamia lanceolata fir in China. They concluded that the
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likely cause was allelopathy, not pathogens. However, Westphal et al. (2002) concluded that in an old
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Californian vineyard the replant problem was caused by pathogens. We think that this effect may be
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widespread in natural stands, and it is certainly a mechanism whereby perennial plant densities may be
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limited, while allowing scattered (rare) seedlings to flourish.
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It has been shown many times that leachates from a species’ shoot or occasionally roots can
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inhibit that same species’ growth or seed germination (Singh et al 1999). Newman and Rovira (1975)
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tested a set of British meadow herbs and found that Hypochaeris radicata (cat’s ear) and Plantago
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lanceolata (ribwort plantain) were significantly inhibited by their own leachate, while Lolium perenne
320
(ryegrass) was not. They argued that this could explain why the former two species never formed pure
321
stands and individuals of them were short-lived in the meadow community, since performance would
322
decline with plant clump size, and would be best in remote single juveniles. The mechanism would
323
satisfy our criterion for increase-when-rare because a species that is sparse finds less of its self-toxin
324
around. A 2-phase cycle would be possible. There have been a few other reports of two-species
325
replacement cycles, when the two species are dominant. Rychnovska and Jakrlova (1990) describe
326
almost yearly alternating dominance by Sanguisorba officinalis (burnet) and the grass Nardus stricta in
Wilson & Agnew, chapter 3, Community processes, page 11 of 50
327
a meadow in Bohemian Sudeten. These clearly allow species pairs to maintain strong populations in
328
spite of local or temporal dominance, thus there must be a mechanism of abundance-dependence
329
operating.
330
However, solid evidence has been hard to find. Miles (1987) states that Calluna vulgaris
331
(heather) alternates with Betula pendula and B. pubescens (birch) in Scotland, but his evidence was
332
from current observation, not from historical records that the cycle actually happened. At a smaller
333
scale, Turkington and Harper (1979 %201) suggested that there was cyclic succession between patches
334
of grasses and clover in a pasture, but they provided only very sketchy evidence. Cain et al. (1995)
335
explained the patch structure that Trifolium repens (white clover) exhibited in a lawn through density-
336
dependent growth, although the fluctuations in these patches were surprising. Herben (1990) in a
337
mountain grassland in Czechoslovakia carefully tracked species movement at a small scale, but the
338
pattern of transitions revealed no networks.
339
Seven suggestions have been made for cyclic successions in New Zealand forests, but none has
340
more than the slightest supporting evidence (Wilson 1990). For example, Mirams (1957) and others
341
suggested that stands of the valuable timber tree, Agathis australis (Kauri, Araucariaceae) in North
342
Island, New Zealand, are one phase of a cycle with broad leaf trees because A. australis may be unable
343
to regenerate under its own canopy due to the amount and pH of its litter (Blakemore and Miller 1968;
344
Mirams 1957). However, there is also evidence that seedlings are outcompeted by broad leaf tree
345
canopy and roots (Bieleski 1959; Mirams 1957), although A. australis litter is indeed slow to decay
346
(Enright and Ogden 1987). Bieleski, (1975a) suggests that under the succeeding angiosperm trees the
347
soil changes are reversed, the A. australis can then reinvade overtopping the angiosperm trees,
348
suppressing their growth, but no evidence is offered, and Ogden (1985) has questioned the basic
349
assumption of these suggestions, a lack of direct regeneration of A. australis, reporting that many A.
350
australis stands contain abundant seedlings of different sizes. Cyclic succession has probably been
351
confused with autogenic disturbance by litter.
352
Basically, cyclic succession has never been reliably confirmed by observation over time. None
353
of Watt's (1947) examples had this sort of evidence, and most have been proved wrong on inspection
354
over long time scales. This is true of the hummock-hollow alternation in bogs (Wilson and Agnew
355
1992; Belyea and Clymo 1998). Such a cycle had been proposed long before Watt took it as an example
356
and it appeared in textbooks, but the weight of evidence is now for the opposite process: a switch. Stasis
357
rather than alternation. Watt (1947) described cycling driven by the growth phases of Pteridium
358
aquilinum (bracken) where a vigorous front of P. aquilinum can invade Calluna vulgaris (heather) but
359
the C. vulgaris can invade an older P. aquilinum stand (Watt 1955 %490). However, Marrs and Hicks
360
(1986) found that after several decades the changes that Watt predicted had not happened. It would be
361
possible to see cycles in Watt’s (1960) own record from the control plot of acidic grass-heath in
Wilson & Agnew, chapter 3, Community processes, page 12 of 50
362
Breckland, eastern England, but only for Luzula campestris (woodrush). In this grass-heath, Watt (1947)
363
had proposed that patches of A. capillaris (= tenuis) and A. canina (bent) spread, die out in the middle
364
to form a ring, and the patch is eventually re-colonised making a cycle. In Watt’s (960 records, those
365
species do indeed appear and then disappear within 10 years in two places in the plot, but they persist
366
for 20 years elsewhere in the plot.
367
In arid lands environmental extremes lead easily to facilitation. Sadek and Eldarier (1995)
368
describe how the succulent Arthrocnemum macrostachyum in Mediterranean Egypt traps sediment and
369
builds mounds in saline flats. The mounds are colonised by Cynodon dactylon (Bermuda grass) when A.
370
macrostachyum starts to die back and finally the mound breaks down. In dry grassland in Texas the
371
dead centres of spreading Eragrostis curvula (lovegrass) tussocks persist for some years before
372
recolonisation by the grass and is a cycle imposed by the species on itself. The dead centre was thought
373
to be due to tiller suppression by litter, but Wan and Sosebee, (2000) experimentally showed that litter
374
can enhance tillering when the tussocks are defoliated by grazing. They suggest that nutrient exhaustion
375
is an explanation for this process. Therefore it is clear that vegetation cycles can be driven by one
376
species and that this constitutes a method of maintaining a population by means of growth movement.
377
This could be a result of disease, which would not constitute cyclic succession because there is only one
378
species involved, and because there is no environmental change, i.e. no reaction. Even in arid lands the
379
detection of cyclic succession is difficult within a nexus of complex interactions. We are not in a
380
position to declare how widespread cycles are in nature.
381
One of the best examples of cyclic succession was provided by Yeaton (1978) in a Texas desert
382
(maybe this is just because I’m a Texan, but I’d like you to name the desert here) (Fig. 3.2). He
383
examined plant densities, the remains of dead plants below others, the vigour of associated plants, the
384
incidence of rodent burrows and soil erosion. The evidence suggests that Larrea tridentata (creosote
385
bush), a prolific seeder, can establish in bare ground. Birds perch in its branches and drop seeds of that
386
species and of a cactus, Opuntia leptocaulis. The O. leptocaulis establishes and grows well under L.
387
tridentata, perhaps because of fine wind-blown soil that accumulates there. The L. tridentata bush later
388
dies partly because of competition for soil water by the cactus, with its mat of shallow roots, O.
389
leptocaulis dies partly because of rodents burrowing below it and also because is soil erosion is liable to
390
occur below its shallow root system, without the stabilisation that the deeper roots of L. tridentata had
391
provided. This returns the patch to open ground. The data are observations at one time, rather than a
392
temporally observed cycle, but they build a convincing story. The mechanisms are speculation, but
393
believable. This study was in ’78 and no one has followed up on it?!
394
Wilson & Agnew, chapter 3, Community processes, page 13 of 50
Fig. 3.2: The Larrea tridentata
(creosote bush) / Opuntia
leptocaulis cyclic
succession proposed by
Yeaton (1978).
395
396
Although there is little firm evidence of vegetation cycles driven by facilitation, as we discuss
397
above, monocarpic (semelparous) self-replacing dominants provide irrefutable examples of cycles. This
398
is where all individuals of a dominant species flower and subsequently die over an appreciable area and
399
over in a short timescale. Successional communities with shorter-term dynamics then take over the
400
newly opened soil, starting with ruderals and often allowing lianas to establish, until the monocarpic
401
species re-establishes its dominance. Examples are many Bamboos (Janzen 1976; Agnew 1985), and
402
some tropical forest understorey shrubs such as Mimulopsis solmsii (Tweedie 1965). Fire appears to be
403
part of the bamboo cycle in Asia (Keeley and Bond 1999) but not in Africa (in our experience) or
404
Australia (Franklin and Bowman 2003). These situations cannot be considered switches since no plant
405
changes the environment in its favour, and are not stable states because the ruderal phase is not stable,
406
they are cycles caused by endogenous processes.
407
5 Switches: the positive feedback processes
408
5.1 The concept of the switch
409
The term ‘switch’ was coined by Howard Odum (1971) for the positive-feedback process
410
between biota and environment. We see the vegetation-environment switch as the key to plant
411
community ecology. The great majority of the effects that plants have on each other (chapt. 2 above) are
412
via the environment, the ‘Reaction’ concept of Clements (1916), rediscovered by Jones et al. (1994) as
Wilson & Agnew, chapter 3, Community processes, page 14 of 50
413
‘Ecological engineering’ and by Olding-Smee et al. (1996) as ‘Niche construction’, cute but redundant
414
termssounds a bit snarky. However, these concepts seem too general because at the community level,
415
there are two opposite possibilities: (1) The community makes the conditions relatively less favourable
416
for itself. This is the basis of Clements' theory relay floristics, based on facilitation. (2) The community
417
or a key species makes the conditions more favourable for itself, relative to other species. The reaction
418
reinforces the species’ hold on a site, there is community reinforcement by positive environmental
419
feedback, a switch (Wilson and Agnew 1992). The swich process has been known by a variety of terms:
420
"Constructiveness" (Braun-Blanquet 1932), "positive feedback" (Roberts 1987; Jefferies 1988), "self-
421
intensifying effect" (Frenzel 1983), "self-reinforcing trend" (Perry et al. 1987) and "bootstrapping"
422
(Perry et al. 1989). Switches have been under-studied, probably because the opposite process, Clements'
423
theory of facilitation, succession, still has a strong hold in the ecological psyche. The two elements of a
424
switch are:
425
Element i. Community X changes the environment,
426
Element ii. This change is relatively more favourable for community X than for community Y.
427
It is very difficult to find hard evidence for both elements ‘i’ and ‘ii’ for any single situation (Wilson
428
and Agnew 1992), though convincing evidence is emerging for a few cases.
429
5.2 Types of switch
430
431
432
Wilson and Agnew (1992) identified four types of switch:
1. One-sided switch: Community X changes the environment (e.g. increases the nutrient status or
lowers the soil pH) in patches where it is present.
433
Zero-sum switch: Community X changes the environment in its patches, and in the process changes
434
the same environmental factor, in the opposite direction, in the patches where it is not present
435
(community Y), for example by channelling water or wind into those patches. We call this ‘zero-
436
sum’ here because it is caused by there being a limited amount of water or air. In Wilson and
437
Angew (1992) we called these ‘reaction’ switches, but we eschew that term to avoid a clash with
438
Clements’ prior use of it.
439
440
441
442
443
444
Symmetric switch: Community X changes an environmental factor in its patches, and Community Y
simultaneously changes the same environmental factor in its patches, but in the opposite direction.
Two-factor switch: Community X changes an environmental factor in its patches, and Community Y
changes a different environmental factor in its patches.
5.3 Outcomes of switches
An indication that a switch is operating is often a sharp spatial boundary without an obvious
445
abiotic cause; types ‘2’, ‘3’ and ‘4’ can provide this. However, there are four possible ecological
446
outcomes of the operation of switches (Fig. 3.3):
Wilson & Agnew, chapter 3, Community processes, page 15 of 50
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
1.
2.
3.
4.
Stable mosaic outcome: In a previously uniform environment a switch of types 2-4 can create a stable
mosaic of communities, separated by sharp boundaries. Community X establishes at some sites by chance
(whatever chance is), and its modification of the environment ensures it holds the sites. Elsewhere,
Community Y establishes, and by the mechanism implied in that switch type also holds its sites. These are
alternative stable states. A one-sided switch cannot give a stable mosaic because community X can further
invade the unmodified environment. This is the ASS concept here right? if so you should make that clear
Sharpening situation: Where there was originally a gradual environmental boundary, a switch (of any of
the four types) can produce a sharp vegetation boundary on a gradient or in a less well-defined mosaic.
Acceleration situation: If a switch (of any of the four types) be operated by invading species/community,
their invasion can be accelerated. The invading species/community Y changes the environment to make it
more suitable for community Y, and less suitable for the previous community, X.
Delay situation: Existing species could delay vegetation change via a switch (of any of the four types). For
example, a switch could delay succession, prolong the effect of initial patch composition, or delay the
response to climate change. This operates by community X changing the environment to keep it more
suitable for community (e.g. seral stage) X, and less suitable for the succeeding community, Y. The
delaying effect of switches has been called ‘inertia’ (Nicholson 1970, Van Holle et al. 2003).
464
If a switch results in alternative stable states, they are technically stable in the Liapunov sense, so that
465
the state will be restored after a small perturbation. However, a large perturbation may cause a change to
(a) Stable mosaic situation
(b) Sharpening situation
Fig. 3.3: Four
possible
outcomes of
vegetation
switches.
(c) Acceleration situation
(d) Delay situation
466
theses figures are illegible. also, the 1st (stable mosaic), is different from the figure that you use a t the
467
beginning. here there are 2 different initial states?(illegible)- x and y. in your figure at the beginning of
468
this chapter, there is only one initial state/ another of the stable states. We refer to these as ‘state
469
changes’ (cf. Brooks et al. 2004).
470
5.4 Mediating agents
471
We may generalize that there are eight types of agent that can cause a switch. We outline
472
examples briefly here, especially those reported in more recent literature, to complement the fuller
473
review in Wilson and Agnew (1992).
Wilson & Agnew, chapter 3, Community processes, page 16 of 50
474
475
5.4.A Mechanisms involving regional climate
G. Miller et al. (2005) proposed that early holocene human firing of the vegetation in the centre
476
of Australia so modified the climatic regime there that the vegetation changed from one with
477
considerable forest/shrub cover to a more open desert-scrub, that was both more fire-promoting and
478
more fire-tolerant, keeping the fire frequency high. They demonstrated through a general circulation
479
model that the resultant increase in the surface albedo and decrease in its roughness would change the
480
climate, increasing the fire frequency and reduce the extent to which monsoonal precipitation penetrated
481
into the area. Both these processes would hold the vegetation cover in its changed state.
482
In general, the different light reflectance, wind resistance (roughness length), energy flow and
483
evapotranspiration of forest vegetation (O’Brien 1996) results, according to models, in a regionally
484
warmer climate. In northern areas at least, this will which will favour the growth of forest and give a
485
regional- or global-scale switch (Bonan et al. 1992; Otto-Bliesner et al. 1997; Bergengren, et al. 2001).
486
According to Gaia theory, this is the reason Earth is habitable at all unlike Venus or Mars (Lovelock
487
1979). We list global-level climate switches as a separate category since they would occur on a spatial
488
scale orders of magnitude greater than our other examples. They are necessarily more speculative, being
489
based on word or simulation models.
490
5.4.B Mechanisms involving water
491
Water is an obvious mediating agent in switches. Occult rainfall is trapped from mist much more
492
by tree branches than by tussock grasses, but stiff erect tussocks collect more than soft flag-forms and
493
short grassland. The vegetation types that increase their own precipitation are thereby able to grow
494
faster, are less affected by drought, and therefore expand (Figs. 3.4, 3.5). Naturally this is a feature of
495
arid climates.
Wilson & Agnew, chapter 3, Community processes, page 17 of 50
Higher precipitation
and reduced
evapotranspiration
higher up the hill allows
taller vegetation
The taller vegetation
traps fog precipitation,
and further increases
water status higher up
the hill
Fig. 3.4: A vegetation pattern
caused by a
fog-interception switch.
Precipitation
Higher water
status allows taller
vegetation
With occult
precipitation
switch
Without
switch
Grassland
Fog forest
Altitude
496
497
498
Fig. 3.5: Change in precipitation caused by a fog-interception switch.
Scott and Hansell (2002) describe Picea glauca (white spruce) islands in Manitoba (Canada)
499
tundra where (isolated) young trees trap snow, which ameliorates low moisture and temperature. They
500
grow well, and peripheral branches root to form woody islands. However, as they age more branches
501
droop, needles abrade and the canopy thins, allowing lichen heath to establish, and permafrost to enter
502
the island. So the young trees operate the switch, but old trees do not. It is unclear how the young trees
503
establish. This is a one-sided switch, but unusually for a one-sided switch it leads to a mosaic because
504
the processes driving the switch no longer operate as the trees age.
Wilson & Agnew, chapter 3, Community processes, page 18 of 50
Positive feedback between vegetation and water infiltration in arid conditions can give ‘tiger
505
506
bush’ alternative stable states (sect. 5.6 below). In mesic and wet climates ombrogenous bogs can form
507
by paludification, a nutrient/pH/waterlogging switch (chapt. 2, sect. 4.8 above). Wilson and Agnew
508
(1992) discuss water switches in saltmarsh pans. Very awkward. 3 sentences, 3 papers, 3 different ideas,
509
np connections made, no discussion
510
5.4.C Mechanisms involving other aspects of microclimate
511
Montane closed canopy forest usually ends abruptly at the uppermost altitudes at which tree
512
growth is maintained. A tree cover has a lower albedo, which tends to give a lower mean temperature in
513
the plant cover and in the soil (Körner 2003) and whilst radiation is more readily absorbed (Grace 1983)
514
it is also more readily lost. However, the crucial factor is frost, not average temperatures. Most tree
515
species require lower levels of frost for regeneration than the adults tolerate, and frost is ameliorated
516
under the canopy meaning that they can regenerate under their canopy but only with difficulty beyond,
517
giving a switch (Wilson and Agnew 1992). The latter authors describe how in alpine and arctic areas
518
shrubs and trees can ameliorate the frost, in both cases benefiting the plants and hence constituting a
519
switch.
520
The review also gives various examples where light is the mediating agent, mainly where a
521
species, by its canopy, produces low light levels that allow that species to regenerate but prevent
522
regeneration by others. Against expectation, some tropical forests are monodominant and Torti et al.
523
(2001) suggest that for the Congolese Gilbertiodendron dewevrei this is due in part to low light at the
524
ground surface caused by the dense canopy and the deep litter layer, though they suggested low nitrogen
525
was involved too. Another example is seen in Pteridium aquilinum (bracken), which performs best on
526
free draining brown earths, but often forms very dense and impenetrable litter, suppressing other
527
species. It often occurs as stable monospecific stands. There are numerous other examples.
528
5.4.D Mechanisms involving physical substrate
529
Plants can affect soil structures, such as soil particle size and porosity (Angers and Caron 1998).
530
They can effect soil structure and stability in the long term. Hellstrom and Lubke (1993) describe a
531
moving dune system in South Africa which has been colonized by the Australian shrub / small tree
532
Acacia cyclops (‘rooikrans’ in South Africa). The soil has been so altered that the dune cannot be
533
returned to its original state by the removal of the invading A. cyclops, and will presumably develop a
534
different vegetation type, the switch having been caused by the A. cyclops. This also happens through
535
sediment entrapment in estuarine salt marshes (De Leeuw et al. 1993). Often the entrapment is continuous
536
and the vegetation, often consisting of one species (e.g. Spartina anglica, cordgrass, Puccinellia maritima,
537
one of the saltmarsh grasses) starts a successional sequence, but sometimes a stable vegetation type
Wilson & Agnew, chapter 3, Community processes, page 19 of 50
538
results. For example Castellanos et al. (1994) report Spartina maritima in south western Spain creates
539
raised flats with Sarcocornia (= Arthrocnemum) perennis (glasswort).
540
Probably the best-documented case of a sediment switch is for diatom growth on estuarine
541
bottoms (Fig. 3.6). Diatoms (Baccilariophyceae) produce extracellular polymeric substances (EPS),
542
largely carbohydrates. These bind the sediment together, leading to lower erosion of the silt component.
543
This encourages diatom growth. Here, one would expect the switch to be a zero-sum one if there were a
544
limited amount of silt in the estuary. This would result in a permanent, if shifting, mosaic. That would
545
give bimodality in the frequency of two alternative states, and this has been observed (van de Koppel
546
2001 %3457).
Higher EPS →
sediment erosion ↓
(Paterson 1989)
Lower sediment erosion
→ silt content ↑
(Daborn et al. 1993)
diatoms -> EPS produced
(de Brouwer & Stal 2002)
Higher silt content →
diatom growth ↑
(van der Koppel et al.2001)
547
Fig. 3.6. A sediment-mediated switch. EPS = extracellular polymeric substances.
548
Plants could also affect soil fauna which can again change soil textures. These processes may well
549
comprise switches and form patchy landscapes but we have found no evidence of this. The nearest we
550
have is the influence of termites in tropical savannahs. For example, West African termite mounds
551
support better vegetation than the surroundings, and have better soil qualities: more clay, better porosity
552
and available moisture (Konate et al. 1998). Yet this cannot be regarded as a switch unless the termites
553
harvest the better vegetation so produced. But most termites harvest grasses, not on the shrubby
554
vegetation of termite mounds.
555
5.4.E Mechanisms involving soil chemistry
556
A plant can enhance its nutrient regime through trapping wind- or water-borne material around
557
its base, encountering mycorrhizae during soil exploration by its roots or facilitating the presence of
558
other species (both plants and animals) that enhance nutrient cycles in its favour and even provide
559
nutrient rich sites for its seedlings.
Wilson & Agnew, chapter 3, Community processes, page 20 of 50
560
Such processes have often been described for isolated trees and shrubs in dry grassland.. Belsky
561
has, in a series of papers, shown how the shade zone of Acacia tortilis trees in Kenya is an enhanced soil
562
environment with lower evapotranspiration, dampened temperature fluctuations and a more adequate
563
population of soil micro-organisms than the open inter-tree grassland. This in turn attracts large
564
mammals whose defecations add to the effect. When seedling establishment takes place around an
565
isolated tree to form a copse, a switch is occurring. These patches of more mesic conditions in a
566
nutritionally poor arid environment have been called “resource islands” (Reynolds et al. 1999), but they
567
also represent wind switches (Wilson and Agnew 1992). They are a classical cause of vegetation
568
patchiness in a landscape. At the other extreme of rainfall, Agnew et al. (1993 %273) described a switch
569
mechanism of gymnospermous primal rain forest in western New Zealand, where the nutritionally rich
570
seedling sites were on fallen tree trunks. Trees were prevented from falling into an adjacent heathy mire
571
by a fringe of tall myrtaceous shrubs.
572
The preceding examples act through nutrient enhancement. In other switches, nutrients are
573
depleted. In pygmy forest in northern California, Northup et al. (1998) found that the leaves and
574
therefore the litter are high in polyphenols and condensed tannins. The decomposition of this litter is
575
slow, and the soil is therefore low in mineral nutrients, especially N. These conditions are tolerated by
576
the species that grow there but not by possible invaders. In Picea sitchensis (sikta spruce) forests in
577
northern Sweden, the low shrub Empetrum hermaphroditum (crowberry), already present as
578
understorey, can switch nutrient regimes in its favour after cooler forest fires (Mallik 2003). Its litter has
579
a high phenolic content, lowering the pH, sequestering available nitrogen and leaching out metallic ions;
580
conditions which it can tolerate. The phenolics also inhibit the development of conifer mycorrhizae.
581
Lowering of the pH and toleration of that low pH seems a common switch. For example, in many
582
Ericaceae, many Pinus species.these 2 sentences are awkward The acidification can be effected by either
583
their living or dead parts. In Britain Calluna vulgaris (heather) litter is acid, peat-forming and maintains
584
the acid, oligotrophic nature of the C. vulgaris habitat. Many switches combine a lowering of the pH
585
with a lowering of nutrient availability, as in the Empetrum hermaphroditum example. Low pH, low
586
nutrients and waterlogging are part of a similar switch in paludification and bog growth (chapt. 2, sect.
587
3.15 above; Wilson and Agnew 1992).
588
Species that interfere with their neighbours by allelopathy, and thereby benefit and are able to
589
further their allelopathic activity, also operate a switch (Wilson and Agnew 1992; Olding-Smee et al.
590
2003 %book). So can heavy-metal hyperaccumulators, species that grow on metaliferous soils and are
591
resistant to the high levels of Zn, Cd, Cr, Ni, etc. that occur there, but accumulate those metals to very
592
high concentrations (Wilson and Agnew 1992; Boyd and Jaffré 2001). Concentrations in the shoot have
593
been recorded in the order of 10,000 times that in the soil solution for Zn and 600 times for Cd (Knight
594
et al. 1997). The litter of these plants therefore has the potential to raise the available concentrations of
Wilson & Agnew, chapter 3, Community processes, page 21 of 50
595
such elements in the surface soil to levels that they can tolerate but other species cannot (Boyd and
596
Jaffré 2001), reinforcing their abundance in the site. An analogous salt-mediated switch operates for
597
species which absorb saline water from the lower layers of the soil, increases the salinity of the surface
598
soil by its litter, and is able to tolerate this better than its competitors (Wilson and Agnew 1992).
599
A much studied, though complicated, switch occurs in some shallow lakes that can have two
600
alternative equilibria: clear and turbid (Scheffer et al. 1993). In the clear state, submerged aquatic
601
vegetation is abundant. Conditions are oligotrophic, low especially in P, allowing little phytoplankton
602
growth and few fish. Macrophytes such as members of the Nymphaeaceae (water lilies) can shelter algal
603
herbivores such as Cladocera species (water fleas), which then keep the algal populations low and
604
hence the water clear (Stansfield et al. 1997). Allelochemicals produced by Chara spp. may also be
605
involved in keeping the algae down (van Donk and van de Bund 2002). In the turbid state fish
606
movement causes turbidity, which stirs up sediment giving eutrophy and therefore phytoplankton
607
growth, but the sediment reduces light penetration and therefore suppresses submerged macrophyte
608
growth. Food chains are critical. Jones and Sayer (2003) suggest that, in eastern England, fish control
609
the epiphytic grazers, which control the periphyton (algae attached to macrophytes) which controls the
610
macrophytes. That is to say, with too many fish, the periphyton kills the macrophytes, and with no fish
611
the macrophytes are dominant in these shallow lakes. The switch mediating agents are nutrients, light
612
and fish. Although there are variants, this is an exceptionally well-documented switch.
613
van Nes et al. (2002) modelled the proposed clear/turbid mechanisms, and were able to produce
614
alternative stable states (ASS; sect. 5.6 below). The most critical parameter in their model was the
615
degree to which plants lowered the local turbidity. ASS developed most readily in shallow lakes with
616
uniform depth, or where there was little mixing, or to a lesser extent where there was little mixing of
617
water between areas of different depth. Evidence that alternative stable states exist alongside each other
618
in the real world can be seen in similar lakes, some in one state and some in the other, in natural state
619
changes, and in state changes effected by experimental manipulations (e.g. Mitchell et al. 1989; Scheffer
620
et al. 1993). Lakes naturally accumulate nutrients from inwash, and trap them through sedimentation
621
and in macrophytic vegetation. The critical point at which a change from one ASS to another occurs
622
appears to depend on initial state and the sedimentation rate (Janse 1997) which must have to do with
623
nutrients. Reversal may or may not occur readily (Körner 2001). For example, Blindow et al. (1993
624
%159) described two moderately eutrophic, shallow lakes in southern Sweden that have during the past
625
few decades changed several times between the two states. In both lakes, water level fluctuations were
626
the most common factor causing shifts. Blindow et al. (2002) examined one of these lakes in detail, and
627
found that the chlorophyll : total-phosphorus (dissolved, suspended + plankton) ratio declined from
628
April to September, strongly suggesting that some other factor came into play, such as carbon limitation
629
and/or by allelopathic chemicals produced by the submerged charophytes, might be controlling the
Wilson & Agnew, chapter 3, Community processes, page 22 of 50
630
phytoplankton. Hargeby et al. (2004) recorded a change to the turbid state after there had been 30 years
631
of the clear-water state. It occurred over two years with mild winters which increased fish populations,
632
thus increasing bioturbation and nutrient cycling, combined with cool springs which hindered the
633
establishment of macrophytes.
634
Experiments are the key to understanding the workings of communities. Schrage and Downing
635
(2004) removed 75% of the benthivorous fish from a lake. One immediate effect was a decrease in
636
suspended sediment (probably due to reduced fish movement), and reduced phosphate and ammonia
637
(probably due to both the reduced sediment load and reduced fish excretion). There was an increase in
638
zooplankton, which seemed to lead to a reduction in phytoplankton. These two led to an increase in
639
water clarity, which may have been the cause of in an increase in macrophyte frequency. Thus all the
640
elements of a turbidity switch are present; however the lake reverted after two months to its previous
641
state, so an alternative stable state was not induced.
642
Scheffer and van Nes (2004) suggested that comparable ASS might exist in ecosystems of the
643
open ocean.
644
5.4.F Mechanisms involving disturbance
645
There are many ways by which a switch can operate through disturbance. We dealt with
646
autogenic disturbance in the last chapter. If lianas can facilitate gap formation (chapt. 2, sect. 3.5 above)
647
they would favour regrowth of their own life form in forest gaps, possibly constituting a switch by
648
which lianas maintain themselves. Where tidally inundated marsh vegetation occurs, ecesis is unlikely
649
because of disturbance. but Bertness and Yeh (1994) describe how the erect stems of marsh elder (Iva
650
frutescens) can trap so much water-borne litter that the underlying marsh vegetation dies, allowing its
651
own seedlings to survive. These plant canopies also reduce evapotranspiration and thus surface
652
salinities. This switch maintains marsh elder thickets and is dependent on litter for initiation.
653
Disturbance through fire is a major factor in creating vegetation edges because certain vegetation
654
types are fire resistant and fire promoting, whilst other types are susceptible to fire but do not readily
655
carry it. The South African fynbos shrubland is fire-prone and depends on fire for its persistence, while
656
adjacent closed forest is fire resistant (Manders and Richardson 1992). Both vegetation types benefit
657
from increased water availability (Manders and Smith 1992) but only during wet years can tree
658
seedlings establish in the shrubland. They can then grow to become fruiting nuclei of forest. The fire
659
regime maintains the balance between the two types.
660
Disturbance by humans can also be noted here. Humans follow paths and behaviour patterns in
661
vegetation which are repeated and predictable. The effect on vegetation is to reduce plant height and
662
production which in turn invite foot passage, so that this is a switch between vegetation types (Wilson
663
and King 1995).
Wilson & Agnew, chapter 3, Community processes, page 23 of 50
664
5.4.G Mechanisms involving heterotrophs
665
Mycorrhizae present a somewhat different case, since they enhance nutrient uptake and need no
666
external driver, although they can be dependent on litter for long term survival (Vogt et al. 1995). They
667
are essential to nutrient uptake in many nutrient stressed situations. Although we have no examples
668
showing that mycorrhizae are solely responsible for the spread of a species or vegetation type, the
669
dominance of a vegetation type in an oligotrophic habitat and its abrupt edge can be due to this
670
association. The natural history of mycorrhizae is so complex and important that it is still difficult to
671
assess their rôle, whether imperative or facilitative, in plant community dynamics. There are many
672
examples of these interactions: Meadows resulting from abandoned beaver dams in northern Minnesota
673
resist invasion by the surrounding Picea mariana demonstrably through a lack of ectotrophic
674
mycorrhizal fungi (Terwilliger and Pastor 1999). It is clear that there is still much complexity to be
675
unravelled, some of which may constitute switch mechanisms, maintaining boundaries or changing
676
community structure.
677
Animal invasion can markedly alter plant communities, but this cannot comprise a switch unless
678
the community type both attracts and benefits from the animal exploitation. Much of the material is
679
speculative. We shall consider two major mechanisms: the effect of browsing and grazing and the effect
680
of frugivores on seed dispersal.
681
Grazers can be switch mediating agents if they are encouraged by a particular type of vegetation
682
cover and promote it. For example, in woodland grass openings, large mammal grazers crowd the
683
woodland/grassland ecotone for predator escape (Lamprey 1964). They sharpen the ecotone by
684
removing or trampling any outlying tree seedlings and prevent the woodland from spreading. Much of
685
the speculation on this situation has concerned East Africa, in explanation of the apparent ASS of
686
grassland and Acacia spp. woodland (van de Koppel and Prins 1998). On abandoned agricultural land
687
and range in western Europe, Equus caballus and E. assinus (horses and asses) defecate in preferred
688
sites which they fastidiously avoid for grazing, creating within the meadow thorny patches of Prunus
689
spinosus (blackthorn) and Rubus spp. (brambles) that that protect regenerating trees, leading to a mosaic
690
of forest with grassland openings (Bakker et al. 2004; Kuiters and Slim 2003). This seems to be a zero-
691
sum switch, because the thorny patches divert the equines onto the grassy areas, but the area of those
692
limits the animal population numbers.
693
Again, Augustine et al. (1998) describe how ASS can arise when high numbers of white-tailed
694
deer (Odocoileus virginianus) trample Laportea canadensis (Urticaceae) populations. At high
695
L. canadensis plant densities, the plant population survives because O. virginianus avoid the area, while
696
at initially low densities, L. canadensis is trampled as O. virginianus pass and it disappears. This also
697
seems to be a zero-sum switch. From this point on seems to be unrelated to the beginning of the
698
paragraph- very confusing,make it a new paragraph? Futuyma and Wasseman (1980) described a
Wilson & Agnew, chapter 3, Community processes, page 24 of 50
699
situation in which Alsophila pometaria (fall cankerworm) adults emerge from pupae in spring. They lay
700
their eggs, the first larvae emerge at about the time Quercus coccinea (scarlet oak) comes into leaf and
701
larval emergence continues as Q. alba (white oak) leafs out. The larvae need young leaves to survive.
702
Therefore, in stands where Q. coccinea is in the minority it suffers more herbivory, presumably
703
reducing its fitness and maintaining the dominance of Q. alba. SO do most of the larvae emerge before
704
the Q. alba leafs out? if so then I agree with your statement. If not, it seems that Q. alba would suer
705
more damage b/c of its commonness unless Q. coccinea is a preferred food source. The converse
706
process would, in stands dominated by Q. coccinea, maintain its dominance. This process could result in
707
ASS in the field, but this has not been demonstrated.
708
Dispersal can mediate switches. We described resource islands above with regards to nutrients,
709
but patches of seedlings around foci can also be produced by perching frugivores. If the plant species is
710
a preferred food of the frugivore, this benefits the frugivore population, constituting a switch.
711
Germinable seeds can be dropped during eating, defecated by the bird around the tree, or carried by/in
712
the bird to establish a new focus (Herrera et al. 1994). In this way, patches of forest can arise around
713
isolated trees (Archer et al. 1988). Dispersers/consumers of nuts could operate a switch when they are
714
dependent on the fruit for food, but benefit the plant by protecting the fruit from other predators, bury
715
them in sites that are probably favourable for establishment, and disperse them away from the tree and
716
perhaps into new areas (Vanderwall 2001). A suite of Fabaceae (legumes) in African savannahs have
717
indehiscent, carbohydrate-loaded pods which are eaten by browsers. The seeds within are refractory and
718
they germinate faster after scarification, which can occur during their passage through the gut.
719
Deposition in dung is clearly helpful in seedling establishment. Many browsers concentrate their dry
720
season feeding on such legumes. Many of these are woodland trees (Baikiaea spp., Cassia spp.,
721
Pterocarpus spp. in Miombo woodland; Stokke 1999), and the process maintains the vegetation type.
722
Acacia tortilis and A. erioloba often grow as isolated trees in rangeland, and can develop into thickets.
723
M.F. Miller (1995) and Dudley (1999) have shown that the pod resource is indeed heavily utilized by
724
browsers and ungulates, and that there is an enhanced germination effect, but other suites of rodents,
725
birds and beetles are also involved in a complex system. Pollinators could mediate a switch in a similar
726
way.
727
A similar switch could operate in which increase in the population of a host-specific pollinator
728
(perhaps an orchid) increased the seed set of species A, leading to greater abundance or a greater spread
729
of species A, leading to there being more nectar/pollen for the pollinator, increasing the population of
730
the pollinator: positive feedback.
731
Wilson & Agnew, chapter 3, Community processes, page 25 of 50
732
733
5.5 Switch evolution
Wilson and Agnew (1992) suggested that switches would be more common than facilitation,
734
because there would be selection for genotypes with reactions that favoured the species producing them
735
over those with reactions that disfavoured them. This is a Gaia-type idea (Lenton and van Oijen 2002),
736
be it a rather mild one. At first sight, the argument seems logical. All plants, like other organisms, effect
737
reaction (Clements 1916; Miles 1985, van Breemen and Finzi 1988; Odling-Smee et al. 1996). For
738
example a plant is bound to shade another plant below it. A switch could arise by chance: characters
739
which were the result of selection for other reasons could happen to give the plant an ability to change its
740
environment in a way that gave it benefits relative to its neighbours. If reaction arose at random, we would
741
expect 50% of cases to be negative feedback (i.e. facilitation), and 50% positive feedback (i.e. switches).
742
However, the genotype of each plant will determine the direction and extent of its reaction, and natural
743
selection must operate on that genetic variation.
744
Olding-Smee et al. (2003) also conclude that ‘niche construction’ [i.e. reaction] “is a fact of
745
life”. They do not seem so concerned with a population’s affecting its own fitness, but with the causal
746
change: current population → environment modification → next-generation population. Applying this
747
to switch evolution, they say: “We define ecological inheritance as any case in which organisms
748
encounter a modified feature-factor relationship between themselves and their environment where the
749
change in the selective pressures is a consequence of the prior niche construction by parents or other
750
ancestral organisms”. They use the term ‘positive niche construction’ for a switch, giving the examples
751
of allelopathy and fire, and conclude that selection will occur for adaptations that allow organisms to
752
carry out their [presumably positive] niche construction with greater efficiency. Laland et al. (1996;
753
1999) also analysed the effect of reaction on the selection process, and concluded there could be strong
754
selection for switches, perhaps leading to the fixation of otherwise deleterious genes.
755
We have to be careful here, almost to the point of disagreeing with ourselves (Wilson and
756
Agnew 1992). Darwinian selection operates at the level of the individual genotype. A mutant that
757
produces a reaction favourable to that individual plant (genet, technically) could lead to the evolution of
758
a switch. However, this supposes an unlikely degree of independence between coexisting genotypes in
759
their reaction and response – that the mutation benefits only that one individual. We must be careful if
760
we apply this logic to a mutant that produces a reaction that benefits a whole population of the species,
761
because that mutation will have no selective advantage over the ‘wild type’. Selection cannot operate if
762
everyone benefits. What if your offspring benefit the most b/c they are closest to you? Moreover, many
763
of the plant features causing a switch will have a cost. For example, fire, allelopathy and herbivory
764
defence switches can involve the production of secondary compounds, which are expensive
765
energetically. If a character has a cost and benefits a plant’s neighbours, it must be characterised as
766
altruistic, and can evolve only by a group selection mechanism (Wade 1978). The most likely
Wilson & Agnew, chapter 3, Community processes, page 26 of 50
767
mechanism for plants is local kin selection, in which a plant benefits its neighbours who are likely to be
768
somewhat related to it. However, such selection operates only within a limited range of parameters, and
769
is a weak force (Wilson 1987 %493).
770
We conclude that Darwinian selection would be possible if the reaction were so local that the
771
plant benefits only itself (e.g. produces litter just under itself), or at least with little benefit to neighbours
772
of the same species. If the effect were a little more widespread, but local enough for most of the plants
773
benefiting to be related to the mutant, the trait could possibly evolve by kin selection. The situation is
774
not as clear-cut as Wilson and Agnew (1992) supposed. Perhaps most switches arise by accident, not by
775
adaptation.
776
5.6 Alternative stable states
777
You’ve been discussing ASS’s for a while. Move this forward to the intro wherer you are
778
defining/explaining switches. In regions of apparently uniform terrain and climatic regime, such as
779
grassland on alluvial plains, there may be vegetation states that are persistent and different, but
780
impossible to relate to variations in the substrate or ambient conditions. The different states could
781
coexist side-by-side in a mosaic, or they could exist in the same site at different times. Such
782
communities are known as alternative stable states (ASS). It is assumed that either state could exist
783
there, but that each is stable, so there is a vegetation-dynamics barrier to be crossed moving from one to
784
another. There are three criteria for ASS (Connell and Sousa 1983):
785
1. The states, differing in species composition, occur in the same environment.
786
The perturbation causing the original change in species composition in space or time appeared as a
787
788
pulse, or by “chance”.
The states are stable.
789
These criteria are difficult. For ‘1’, the states must be held by interactions caused by the different
790
species compositions, but then almost all species interactions are via reaction: i.e. a change in the
791
environment. Thus, criterion ‘1’ is impossible to meet literally. Some types of reaction such as light are
792
instantaneous but some such as soil characters have persistence. A realistic criterion would be that the
793
differences in environment are small and of a type probably caused by the plants themselves. For ‘2’,
794
we can rarely see the origin of the states in nature. A perturbation is normally taken to be an
795
environmental or biotic condition outside the normal range, appearing stochastically in time. We have to
796
distinguish between: (a) ‘press’ perturbations, where the difference between treatments is continued, e.g.
797
where there is continued grazing versus none, versus (b) pulse perturbations, where the change in
798
environment is temporary. Some pulse perturbations (shocks, instantaneous perturbations) really are
799
instantaneous, e.g. the removal of plants. Other types of ‘pulse’ perturbation can be applied
800
instantaneously but the environmental change persists for some time, such as a single fertiliser
801
application. However, in the context of ASS the origin of the different states should be historical factors
Wilson & Agnew, chapter 3, Community processes, page 27 of 50
802
such as management or transient environmental differences that disappeared long ago, random dispersal,
803
the season of establishment by the community founders, or environmental patchiness subtle enough to
804
have been overwhelmed without the establishment of ASS. This would seem to be dependent on where
805
you place the biotic interactions The critical item is that no continuing environmental pressure is
806
applied that is different between the states. For ‘3’ we theoretically have to apply an infinitely small
807
perturbation and show that the system regains its original state, at least after infinite time (sect 7.3
808
below). Persistence in the face of natural perturbations is often used as a substitute. We see three
809
processes for maintaining an ASS state, that is a mosaic in space and/or time:
810
1. Spatial resource accumulation and depletion: A switch might result in spatial patches of better-
811
resourced systems in a depleted hinterland. Since accumulation causes depletion this is a Zero-
812
sum (Type 2) switch (Fig. 3.7).
Depleted
matrix
Higherresource
system
patch
Fig. 3.7. Maintenance of alternative stable states in space by a Zero-sum switch, comprising
spatial resource accumulation and depletion. [X 3, and with two fuzzy edges]
813
2. Boundary sharpening: Patchiness in the physical environment such as a difference in soil water
814
flow, or variation in a biotic effect such as mycorrhizae – any of the initial differences that we
815
discussed above – could be converted to a spatial mosaic of ASS by boundary sharpening
816
through a switch of any of the types, 1-4.
817
818
3. Reinforcement. Once a state is established in an area, it can be held against a state change by a
one-sided switch (Type 1).
819
The maintenance of alternative stable states is almost inevitably due to a switch. As in the ball-and-cup
820
analogy, although there is stability a sufficiently large pulse perturbation can overcome the resistance
821
and cause a state change (sect. 7.4 below). The ASS concept is close to the theoretical concepts of
Wilson & Agnew, chapter 3, Community processes, page 28 of 50
822
"Several stable points" (Lewontin 1969) or "Multiple stable points from one initial condition"
823
(Sutherland 1974), though in such papers the crucial involvement of reaction has not generally been
824
elaborated. We should probably examine which type of switch is operating and through which
825
mediating agent before we draw curves of hysteresis or do matrix algebra on “multiple domains of
826
attraction” (Gilpin and Case 1976).
827
Theory
828
Contrary to our wish to examine ecological mechanisms before generating theory, much of the
829
consideration of ASS has been by mathematics or simulation. Luh and Pimm (1993) and Samuels and
830
Drake (1997) have proposed the Humpty-Dumpty principle: that it might not be possible for
831
communities to re-assemble from their constituent species after a perturbation. This might be because
832
the original species composition was based on a switch, originated by the elusive process of ‘chance’.
833
Platt and Connell (2003) argued that secondary successions might be more liable to the initiation of
834
ASS, since where there is stochastic establishment by survivors, alternative paths of vegetation
835
development may exist. Another possible cause of a Humpty-Dumpty effect is that succession fails
836
because intermediate species in the assembly process are missing (cf. Samuels and Drake 1997),
837
[though this seems a very rigorous interpretation of succession] please elaborate on this. Law and
838
Morton (1996) modelled the formation of a mosaic of ASS by this process. Van Nes and Scheffer (2004)
839
used a relatively simple Lotka Volterra model and a community matrix to show that alternative stable
840
states can arise in rich mixtures where intra- is less than inter-specific competition, a process driven by
841
negative feedback in contrast to the positive feedback of switches but which seems unlikely in the real
842
world (chapt. 4, sect. 7 below). Chase (2003) suggested that alternative stable states were more likely to
843
exist when the regional species pool (if we can ever define that) is large. He argued that with more
844
species there will be more redundancy (i.e. species similar in alpha niche). The trouble with this is that
845
if the species are so similar as to be redundant, the states will not be alternative in reaction. If they are
846
somewhat different, one species will replace another on a path that approaches equilibrium
847
convergence, so the states will not be ASS. Similar reasoning can be applied to his suggestion that less
848
dispersal will increase the likelihood of alternative stable states: a state is not stable just because species
849
find it hard to disperse to the site.
850
Examples
851
It is much harder to find evidence that ASS actually exist. Most investigations cited in reviews
852
as having good evidence of ASS are of freshwater lab microcosms, a few of mesocosms (Schröder et al.
853
2005). Even with those studies, a major problem has been lack of time for it to be clear that the
854
communities are not converging. For example, an experiment by Robinson and Dickerson (1987)
855
introduced species in two orders and at two slightly different rates, and found differences in species
856
frequencies, but the results were analysed on weeks 13-23 of the experiment when first-introductions of
Wilson & Agnew, chapter 3, Community processes, page 29 of 50
857
a species occurred up to week 8, and re-introductions occurred right up to week 13. Drake (1991 %1)
858
performed similar experiments, again finding that the order of introduction of the autotrophs
859
significantly affected their relative abundance, but the last introduction of an autotroph was on day 180
860
and the final census only 15 days later. Chlamydomonas reinhardtii was the species most likely to end
861
up as a minor component, but this happened most notably in the two treatments where it was the first
862
autotroph to be introduced, i.e. where there had been most time for it to have been excluded by
863
interference. This gives the impression of a competitive hierarchy more than alternative stable states.
864
Coming to the field, Vandermeer et al. (2004) tracked regeneration in Nicaraguan tropical forest
865
over nine years after destructiondisturbance- destruction is value laden by a hurricane, and found floristic
866
divergence, especially initially. This would be an illustration of "multiple basins of attraction": ASS. Yet it
867
is questionable whether their time span was long enough, because four out of the six stand comparisons
868
became more similar in the last two years of observation reported.
869
Wilson & Agnew, chapter 3, Community processes, page 30 of 50
870
In very dry situations vegetation types and their litter clearly affect rain impact on the soil
871
surface and infiltration. Litter reduces the impact of raindrops on the soil surface, and roots provide soil
872
channels for infiltration. This can give a horizontal pattern imposed by the community itself, for
873
example where the drought-imposed gaps between woody plants are bare or covered with grasses and
874
their associates (Tongway et al. 2001). Such patterns can be seen in savannahs, shrublands and
875
grassland in semi-arid areas, on gravelly soil and in deserts. The most notable example is ‘tiger bush’,
876
lines of dense woody plant growth at right angles to direction of gently sloping land under low and
877
variable rainfall regimes. The phenomenon is certainly caused by surface run-off in the inter-stripe zone,
878
onto the vegetation stripe, which has been shown to have much higher soil infiltration rates (Berg and
879
Dunkerley 2004, in Central Australia). Shnerb et al. (2003) modelled the process, showing that the
880
formation of tiger bush occurred in a specific range of precipitation. It is tempting to regard vegetation
881
of striped landscapes as the most efficient water collecting system under these climatic regimes, but
882
Hierniaux and Gerard (1999) have shown this is not necessarily the case. The inter-stripe zone may be a
883
water collection site, but also evaporate small precipitation events. Wilson and Agnew (1992)
884
interpreted this as a zero-sum switch, because the increase in water by one phase (by infiltration)
885
reduced the water available to the other phase (by reduced runoff). Van de Koppel et al. (2002 %209)
886
modelled the hydrology of the system, showing such an effect.
887
So many switches have to do with water stress that it is not surprising that most claims of ASS
888
have been reported from arid lands. The frequently patchy nature of vegetation in deserts suggests the
889
possibility of ASS. Rainfall on desert land surfaces is erratic with much coming in storm events and
890
infiltration is often impeded if the bare soil surface is static, with a stone armour or with a cryptogamic
891
crust (Verrecchia et al. (1995). Water infiltration and nutrient capital can be increased by rodent
892
burrows, soil caches and ant nests, and these areas eventually become shrub patches (Whitford 1993).
893
At critical levels of soil water availability, surface redistribution of rainfall can also lead to vegetation
894
patches which may be self sustaining, i.e. ASS. Many semi-arid landscapes consist of mosaics of forest
895
and grassland which may constitute ASS by water / fire / grazing switches (Wilson and Agnew 1992).
896
Callaway and Davis (1998) investigated the importance of shrub facilitation on recruitment of Quercus
897
agrifolia (live oak) in central California, finding that oak forest patches which included shrubs were
898
stable as forest, while grasslands had virtually no oak recruitment and so were stable as grasslands.
899
These are stable states depending on the facilitation of oak ecesis by shrubs. Rietkerk and van de Koppel
900
(1997) describe how differences in plant density, rainfall infiltration and nutrient cycling can give rise to
901
ASS, triggered by herbivore grazing behaviour. Naturally it is difficult to confirm that these are long-
902
lived ASS.
903
904
Schröder et al. (2005) could find only two examples of ASS that they believed were
convincingly demonstrated in the field. One is the situation examined in several studies by R.L. Jefferies
Wilson & Agnew, chapter 3, Community processes, page 31 of 50
905
and co-workers, and summarised by Wilson and Agnew (1992) as a switch in “Grazing and nitrogen
906
cycling”. Schröder et al. cite a more recent paper (Handa et al. 2002) where the bare patches at the top
907
of the marsh were hardly being re-colonised, and vegetation could not be re-established. The soil had
908
become hyper-saline due to greater evaporation rates there (Srivastava and Jefferies 1995). This is akin
909
to a switch of the saltmarsh pan type (Wilson and Agnew 1992), except that Srivastava and Jefferies
910
suggest the salt is coming from buried sediments. Walker et al. (2003 %637) showed by modelling that
911
nitrogen dynamics, with input from N-fixing bluegreen algae and N export in goose emigration could
912
also explain the alternative stable states in this system. Thus, the mosaic on the ground is a clear hint
913
that a switch is operating, though state changes between the two ASS do not seem to have been reported
914
and it is not clear through which factor the switch is primarily mediated. Schröder et al.’s other example
915
was an experiment by Schmiz (2004) removing a predator spider Pisaurina mira, therefore releasing
916
grasshopper Melanoplus femurrubrum herbivores, which hid in and ate Poa pratensis (meadow grass),
917
releasing Solidago rugosa (goldenrod) which suppressed other forbs. When predators were allowed
918
back after 2 or 3 yr, the cover of S. rugosa remained high for 3 or 2 yr respectively. Schmitz offers no
919
explanation beyond “loss of top predator control”. It seems that, unfortunately, the cover values were
920
guessed.
921
Possibly the situation where the switch mechanism behind ASS has been most clearly
922
demonstrated is in dune slacks on Texel, The Netherlands. Adema et al. (2002) found two seral
923
stagesOne Littorella and one Schoenus? The example is unclear side by side, with a boundary that had
924
been static through 62 years of observation. Having investigated and dismissed all possible
925
environmental reasons for this stasis they surmise that it must be due to some “positive feedback
926
mechanism” (i.e. a switch) involving nutrient cycling or sulphide toxicity. Two early successional
927
species Littorella uniflora (shoreweed) and Schoenus nigricans (bog-rush), release oxygen into the soil
928
allowing greater oxidation of ammonium to nitrate (cf. Adema and Grootjans 2003), and then
929
denitrification to gaseous nitrogen (Adema et al. 2005 %374) . This could hold up succession. To
930
complete the story we would need to know that early successional species are more tolerant of low N,
931
and there is some experimental evidence that both L. uniflora and S. nigricans grow better at low N
932
availability and grow more slowly at high N availability compared to Carex nigra and Calamagrostis
933
epigejos (E.B. Adema pers. comm.).
934
If ASS exist in an area we should be able to see what type of perturbation can cause a state
935
change between them. There are many state-and-change flow diagrams in the literature, for example
936
comprising a whole issue of Tropical Grasslands, indicating the conditions under which each state
937
change occurs, but unfortunately they are all speculation. Olivia et al. (1998) was experimentally unable
938
to confirm a state-and-change system proposed from open grasslands to dwarf shrublands in Patagonia
939
in a 10-year trial, and Valone et al. (2002), who were interested in the grassland/shrubland mosaic,
Wilson & Agnew, chapter 3, Community processes, page 32 of 50
940
found that after 20 years without grazing there was little change in the shrubland. They suggested that
941
there may be considerable lag times before any changes could be seen. The experience of land managers
942
is that intermediate states are much shorter-lived and state changes can be predicted from management
943
changes. We are not doubting that state changes occur, we are just calling for attempts to document
944
them.
945
Chase (2003) used examples to conclude that “… evidence suggests that community assembly
946
can lead to multiple stable equilibria”, and then goes on to start with an assumption: “Sometimes history
947
matters, creating multiple stable equilibria …”. We believe switches are common and important in
948
vegetation, and switches should often lead to ASS, but there is very little evidence yet to support their
949
existence. It may be that ASS are the explanation for much of the patchiness that we see in vegetation,
950
due to alternative trajectories of change (divergence) in vegetation in response the same disturbance, but
951
at different times with different biotic and/or physical environment (Walker 1993; Whisenant and
952
Wagstaff 1995; Lockwood and Lockwood 1993), or different vegetation conditions at the time (Watson
953
et al. 1996). This possibility has rarely been investigated, Walker and Wilson (2002) did so in dry
954
grassland in southern NZ, and found no evidence for it. Nevertheless, so far as our present knowledge
955
goes, to adapt Frank Egler’s phrase, ASS are “one of the more nauseating but delightful idiocies that
956
stem from the heart, not the mind, of otherwise respectable scientists”.
957
5.7 Negative feedback
958
Positive feedback means that a process P produces a condition/product C and that change in C
959
increases the rate of process P as a switch. Negative feedback means that a process P produces a
960
condition/product C and that change in C decreases the rate of process P. Negative feedback at the
961
whole-community level occur at the end of a succession (the climax) or in one of several alternative
962
stable states. The result is stability and the speed at which the negative feedback occurs is resilience. We
963
discuss these concepts below. However, we note that the negative feedback occurs primarily at the level
964
of individual species, be they interacting ones.
965
6 Diversity → productivity
966
967
There are two major possible outcomes of niche differentiation between species: overyield and
stability.
Wilson & Agnew, chapter 3, Community processes, page 33 of 50
Biomass of B
Biomass of B
Biomass of B
Biomass of A
(c) Transgressive
overyielding
Biomass of A
(b) RYT relative
overyielding
Biomass of A
(a) RYM arithmetic
overyielding
100% A 50:50 100%B
100% A 50:50 100%B
50:50 100%B
mixture
mixture
mixture
Fig. 3.8. In ‘a’ there is arithmetic overyielding: the absolute gain by A in mixture is greater than the
100% A
loss by B so RTM > 1.0. In ‘b’ the relative gain by B is greater than the loss by A so RYT >
1.0, in ‘c’ some mixtures yield more than either monoculture. For definitions of RYM and
RYT see Wilson (1988 %279).
interaction,
= theoretical biomass of each species with no
= actual biomass of each species,
= total biomass of the mixture.
968
the figure is difficult to understand, spend a little time explaining. Overyield is the situation in which
969
higher species richness leads to higher total biomass, perhaps as a result of niche differentiation (Fig.
970
3.8). Investigation of this topic had been fraught with problems, the most important being the ‘selection
971
effect’. If a mixture of species is sown/planted in an experiment, the more competitive species will come
972
to dominate. The more species are planted in the mixture, the greater the probability that one or more of
973
these highly-competitive species will be included in the mix and will take over. If species with high
974
competitive ability also have high production (and note this ‘if’), the result will be higher higher
975
production for the mixture: the selection effect. In terms of seeking the results of niche differentiation
976
this is an artefact. There is no easy way looking at the biomass results to know whether simple
977
overyielding (Fig. 2.8a) has been caused by niche differentiation or by the selection effect. However, if
978
the production of the mixture is greater than that of any of the monocultures (‘transgressive
979
overyielding’: Fig. 3.8 c) the selection effect can be ruled out. Niche differentiation is the obvious cause,
980
though explanations in terms of subvention or interactions with heterotrophs are possible. Loreau and
981
Hector (2001) introduced a mathematical separation of the selection and niche differentiation effects.
982
They found that there was significant evidence of a positive selection effect in only two BioDepth
983
experimental localities out of nine, and a negative effect in one (suggesting that the stronger competitor
984
was of lower yield). In contrast, there was significant evidence for niche complementarity in four
985
localities. However, we suggest that if transgressive overyielding is not seen, the results are not of great
986
ecological significance.
987
988
Experiments to detect overyielding have been fashionable of late, but with mixed results. Tilman
et al. (2001 %843) planted field plots at Cedar Creek in a range of species richnesses up to 16, from a
Wilson & Agnew, chapter 3, Community processes, page 34 of 50
989
pool of 18 species in 1994, and analysed in 1999 and 2000. There was evidence that some mixtures of
990
grassland species could outyield the highest-yielding monocultures, especially several years after
991
establishment. Lambers et al. (2004 %661) examined the same plots, analysed up to 2002.
992
Over/underyielding spp were determined by regression of yield on diversity. Compared to the -1 slope
993
on log-log basis expected under a null model, six species significantly overyielded and four
994
underyielded. There was no time trend for overyielders to replace underyielders, which is strange.
995
Whittington and O’Brien (1968) planted field plots with Lolium perenne (ryegrass), Festuca pratensis
996
(meadow fescue) and their triploid hybrid. Transgressive yielding was seen in the ‘grazing’ (i.e. low-
997
cut) treatment only sporadically in the first two years, but almost constantly in the third. In two of the
998
mixtures, the two components had diverged in their proportional contribution to the sward. The
999
overyield can be attributed to niche differentiation in the depth of nutrient uptake. L. perenne and F.
1000
pratensis grown in monoculture took up very similar percentages of their P from 10, 30 and 60 cm
1001
(35/40/25 versus 38/35/27) (O’Brien et al. 1967 %513). Yet when they were grown together L. perenne
1002
took up its P from deeper (33/22/45) and F. pratensis from nearer the surface (41/34/25). In contrast,
1003
when grown with the deeper-rooted hybrid (monoculture uptake 18/36/47) L. perenne utilised the
1004
surface P (56/26/18).
1005
There is a tendency in such experiments that if overyield is reached, then it is after several years’
1006
growth. One explanation for this is that time is required for the species to sort themselves into their
1007
respective niches, e.g. in the canopy or in rooting depth. Another possibility is that overyield is attained
1008
only with a certain optimum proportion of each species (as can be seen in many of the results of de
1009
Wit’s group) and time is needed for the species to reach these proportions. Possibly, as in Whittaker’s
1010
(1965) Niche-preemption concept, the dominant species takes all the resources it can and subordinate
1011
species remain in minor amounts to pick up the crumbs that fall from the strong man’s table.
1012
Overyielding does not necessarily develop. Hooper and Dukes (2004 %95) planted field plots
1013
with local grassland species and followed them for an impressive eight years, giving the species time to
1014
reach their optimal niches and their optimal proportions (as discussed above), but no significant
1015
transgressive overyielding was seen. On the other hand, Roscher et al. (2005) ran a field experiment in
1016
Germany for only one year, sowing nine species characteristic of grasslands of the site. Two-thirds of
1017
the plots showed transgressive overyielding, though the effect was highest in 2-species mixtures.
1018
Analyses to separate the selection effect from niche complementarity using the method of Loreau and
1019
Hector (2001) showed that complementarity reached its peak at 4-species.
Wilson & Agnew, chapter 3, Community processes, page 35 of 50
1020
7 Stability
1021
7.1
1022
Concept
The question is raised several times in this book of whether any plant community we see is an
1023
arbitrary assemblages of species, such that another assemblage might well have existed at that time and
1024
place, or whether determinism is involved. There are three possible answers:
1025
1026
1027
1028
1. Global stability, with one state: There is only one possible state, at least after there has been
sufficient time for succession to have done its work. This is deterministic community structure.
ASS, with a few states: There is a limited number of possible states. This is the Alternative Stable
States concept discussed above.
1029
A continuous range of states. There is a large number of possible species assemblages, in terms of
1030
species presence and abundance, with all intermediates. The present assemblage exists for
1031
historical or even stochastic reasons, and after a perturbation or even as a result of drift another
1032
state might take its place.
1033
The critical test here is recovery from pulse perturbation (sects. 5.6 above, 7.3 below). If the community
1034
always recovers to the same state, it is globally stable, situation ‘1’ above. If the system is stable to
1035
small perturbations, but not to large ones, situation ‘2’ obtains. If the community does not recover to its
1036
original state after a perturbation, we assume situation ‘3’. We can doubt ‘3’ from our common
1037
observation as ecologists: if it were true we would have no predictive view of landscape. In fact we
1038
generally know what species to expect in a set of environmental conditions. We may sometimes be
1039
surprised, which is a prima facie case for ‘2’, alternative stable states.
1040
Seeing long-term continuance of a range of plant communities – forests, heaths, bogs and arctic
1041
alpines – we intuitively envisage the continuance as stability. However, the difficulty of defining this
1042
kind of stability, plus a little mathematics envy, has led ecology into accepting Lyapunov stability as the
1043
true stability. We therefore have various features of the ‘stability’ (sensu lato) of a community:
1044
1. Reliability: Constancy, lack of change probably in spite of minor perturbation.
1045
2. Stability sensu stricto, i.e. Liapunov stability: whether the community ever recovers from a small
1046
1047
1048
1049
pulse perturbation.
3. Resistance: Lack of change upon a pulse (i.e. temporary) perturbation. We separate it here into
resistance to abiotic perturbation and resistance to invasion.
4. Resilience: Rate of recovery from perturbation.
1050
All these concepts assume that the ecological system is at equilibrium, and probably in the real world
1051
none ever is, but the concepts are real. All this leaves open the questions: change in what?, recovery in
1052
what? Here at least we are free from the mathematicians to ask whatever ecologically meaningful
1053
question we feel pressing. The change/recovery can be:
Wilson & Agnew, chapter 3, Community processes, page 36 of 50
1054
1. Functional: This can be production (best defined, as we discussed in chapter 1, as energy passed
1055
on to the next trophic levels), nutrient cycling, water output, or any other result of whole-
1056
community processes.
1057
2. Guilds: The proportions of certain guilds, i.e. the guild structure.
1058
3. Species composition: Change/recovery in species presence and/or abundance, i.e. floristic
1059
structure.
1060
Most general discussions of stability refer to species-composition change/recovery, which is easier to
1061
measure. The categories are asymmetrically related – e.g. there can be functional constancy without
1062
constancy in species composition if species change but the productivity (e.g.) does not, and without
1063
constancy in guild representation. Species composition stability encompasses the other two types of
1064
stability, for if the species in the vegetation do not change, qualitatively and quantitatively, then guild
1065
structure and function of the vegetation are also preserved. A constant question in considering these
1066
concepts will be the effect of species richness: are communities with more species more ‘stable’ (sensu
1067
lato).
1068
7.2 Reliability (constancy)
1069
Reliability is the reciprocal of variation in the ecosystem (e.g. Naeem and Li 1997; Rastetter et
1070
al. 1999). The concept was behind ideas on ‘stability’ up to about 1975. For example, Orians (1975)
1071
wrote of ‘constancy’ as a type of stability. Species richness could increase reliability through the
1072
portfolio effect or through the covariance effect. In the portfolio effect, named after a portfolio of shares
1073
that varies less than individual shares, the community response is dampened because it is an average of
1074
the different responses of different species (Ives and Roughgarden 2002 %388). It will always operate
1075
unless the species are identical in their environmental responses. Since averaging over minor species has
1076
little effect, the portfolio effect increases with species evenness. The covariance effect is based on
1077
competition: when the environment disfavours one species others will increase by taking up the unused
1078
resources. This seems to be Whittaker’s (1975) concept of population ‘buffering’: the absorption of
1079
environmental fluctuations through compensation between different species or genotypes. However, we
1080
cannot assume that these effects will be stronger when species richness is higher. Hughes and
1081
Roughgarden (2000) modelled a community with density-dependent Lotka-Volterra competition
1082
equations, and found that with diffuse competition the reliability of biomass decreased with richness,
1083
except in systems with few species and very low interaction strengths. With each species competing
1084
with only its two nearest neighbours, as in MacArthur and Levins (1967) model, the richness/reliability
1085
relation was the reverse. DeWoody et al. (2003) used community models based on variations of
1086
MacArthur’s ‘broken stick’ and found that whilst reliability increased with richness it was, surprisingly,
1087
higher with more difference in competitive ability between the species. Hughes and Roughgarden
1088
(1998) found in a 2-species community-matrix model based on Lotka-Volterra equations that with the
Wilson & Agnew, chapter 3, Community processes, page 37 of 50
1089
two species equal in competitive ability, biomass reliability was not affected by the strength of
1090
competition. As the competitive abilities of the two species became more unequal, reliability decreased.
1091
We shall have to descend to reality.
1092
Boreal forest is reliable in species composition in the short term of centuries. In the longer term
1093
of millenia it is functionally reliable but variable in species composition: even now the trees of Europe
1094
have not re-occupied their potential range after the last glacial retreat (Svenning and Skov 2004).
1095
Tropical forests appear to be observationally reliable in the short term and this is impressive considering
1096
their fast growth rates (Manokaran and Swaine 1994 in Malaysia; Sheil et al. 2000 in Uganda). Real
1097
examples of long-term stasis are found amongst stressed communities where the available flora is
1098
limited. The dwarf pitch pine shrubland on Mt Everett, Massachusetts (Motzkin et al. 2000), the Carex
1099
curvula zone in the European Alps (Steinger et al. 1996) and the ombrotrophic mires of Quebec
1100
(Klinger 1996) fit this syndrome. In contrast, O’Connor et al. (2001) found that whilst less common
1101
species increased biomass reliability in African grassland in a fluctuating rainfall this effect was evident
1102
only in rangelands of poor condition.
1103
Experimentally, McGrady-Steed and Morin (2000) found in microcosms of bacteria, micro-
1104
algae, micro-herbivores and bacteriovores and micro-predators that there was a U-shaped relation
1105
between species richness and the reliability of algal density, measured as variation in 3-day steps over
1106
42 days. Tilman (1996), in 207 plots with various nutrient treatments in four fields at Cedar Creek,
1107
found that species-rich plots had greater reliability (lower CV in biomass), though the plots differed in
1108
fertiliser application presumably confounding species richness with species composition. The
1109
experiment of Tilman et al. (2006 %6239) overcomes this problem by using plots sown at a range of
1110
species richnesses and maintained thus by weeding. The same increase in reliability was seen. This is
1111
the nearest we have so far to an answer.
1112
7.3 Stability sensu stricto (Liapunov stability)
1113
Defining stability
1114
May (1972) initiated the modern approach to stability with his community matrix analysis, based
1115
on the mathematical procedure of local stability analysis (= ‘Liapunov stability analysis’ =
1116
‘neighbourhood stability analysis’). Stability sensu stricto is defined as the ability to recover completely
1117
from an infinitely small perturbation after an infinitely long time. Though we have phrased this to make
1118
it sound a little ecologically unrealistic, some approach to the concept is essential if we are to discuss
1119
stability. Defined thus, stability is like pregnancy: either one is or one isn’t. The ‘infinitely small’ is
1120
necessary to make the definition one of local stability, perhaps among alternative stable states. Global
1121
stability is a much more difficult idea conceptually and operationally. The phrase ‘after an infinitely
1122
long time’ is necessary to avoid arbitrary cutoffs, as well as to distinguish it from resilience. Stability
1123
can be predicted mathematically from the community matrix – a summary of all possible pairwise
Wilson & Agnew, chapter 3, Community processes, page 38 of 50
1124
species interactions in an equilibrium community. The community matrix introduces three important
1125
aspects of community structure. The number of species involved, S, is simple. Another aspect is what
1126
proportion of the S×(S-1) off-diagonal matrix elements are non-zero (i.e. the species in that column does
1127
affect the species in that row). This is the connectance. Thirdly, the interaction strength is the mean
1128
absolute value of these non-zero elements.
1129
Is stability likely to occur? (simulations)
1130
Since May (1972), the community matrix has been used extensively in theoretical studies to
1131
predict what type of community would be stable (Hall and Raffaelli 1993). It turns out that, given a
1132
matrix comprising random values, communities with many species and/or with many or strong
1133
interspecific interactions are less likely to be stable. In fact, it is very unlikely that a community of
1134
randomly-assembled species will be stable at all. This represents an obvious conflict with our
1135
observation that multi-species communities commonly persist in nature. One explanation is that the
1136
species in natural communities have co-evolved to be able to co-exist. However, it has been found that
1137
the introduction of some basic ecological processes, especially simulating the assembly of a community
1138
by the immigration and extinction of species, or simply with a realistic pattern of extinction, allows
1139
complex multispecies communities to develop relatively easily without any assumption of coevolution
1140
(Tregonning and Roberts 1979; Taylor 1988 %569). In the simplest assembly models of Tokita and
1141
Yasutomi (2003) the community stabilized at about five species. When the introduced species were
1142
formed as minor mutants of a species already in the community, the number of species reached about
1143
10. However when, in their ‘local’ model, the characters of new species arriving were based on one
1144
species already present, except for its interactions with one other (abundant) species, the size of the
1145
community approached 50 and was still rising after 10 000 periods. Moreover, such communities were
1146
resistant to invasion how does this sentence square with the previous one?. They interpreted this as the
1147
temporary persistence of species only marginally inferior in competitive ability, that were able to exist
1148
stably once mutualistic partners appeared. This is stability by very simple coevolution.
1149
Tests for stability (the real world)
1150
Despite all the theoretical work on the community matrix, tests of stability in natural systems
1151
have been surprisingly sparse. The only valid method for estimating the community matrix for a given
1152
community is by the experimental manipulation of the organisms, quantifying the effect that each
1153
species has upon the growth rate of each species. We are aware of only a few such studies (Thomas and
1154
Pomerantz 1981; Seifert 1976; Thomas 1981; and see below).
1155
Schmitz (1997) used data from field and laboratory experiments to quantify the interaction
1156
strengths between grasshoppers, four old-field plants, and nitrogen supply. He used these values to
1157
parameterize the community matrix and thence to predict the response of each species to nitrogen and
1158
herbivore press perturbation. The predictions carried a high degree of uncertainty, but the observed
Wilson & Agnew, chapter 3, Community processes, page 39 of 50
1159
results of the field press perturbations was close to, or within, that range in six out of eight cases. So
1160
this then is a positive test for community stability? Explain why better
1161
The most direct test of stability in a community has been on the University of Otago Botany
1162
Lawn (Roxburgh and Wilson 2000 %395, 2000 %409). A pre-requisite for the concept of stability is
1163
that the community should be at equilibrium, and this seemed to be true. Roxburgh and Wilson (2000
1164
%395) grew seven species from a lawn, including the six most abundant species, experimentally in soil
1165
from the lawn kept in two strata and placed in deep boxes only metres from the lawn. The species were
1166
grown in monocultures and in 2-species and other mixtures. A community matrix calculated from
1167
competition coefficients predicted the mixture of species to be unstable, but very close to the
1168
stability/instability boundary. It was certainly closer to the stability/instability boundary than matrices
1169
constructed under a range of null models. To test for the actual stability of the community, Roxburgh
1170
and Wilson (2000 %409) applied three types of pulse perturbation. After shading for 6 weeks and after
1171
mechanical perturbation (removal of vegetation), species composition recovered towards the original
1172
equilibrium state, although full recovery was not seen. Recovery from a herbicide specific to grasses
1173
was still quite incomplete after 2.5 years, apparently because the compensatory growth of dicots made it
1174
difficult for the grasses to reinvade. The pattern of recovery of the lawn from the perturbations was
1175
consistent with the prediction of marginal stability/instability given by the community matrix analysis.
1176
Simulations (Roxburgh & Wilson 2000 %395) showed that the stability of the community
1177
matrix, marginal though it was, could be attributed to the transitive competitive hierarchy found
1178
between the species (chapt. 5 below). A simplistic extrapolation from competitive abilities would be that
1179
species with a greater competitive ability in 2-species experiments would be more abundant in the
1180
community. Several authors have found such a correlation (e.g. Mitchley and Grubb 1986;Missing
1181
and Werner 1987 %1201 ). However, the strict hierarchy in competitive ability found in the
1182
pairwise box experiments, from Holcus lanatus (Yorkshire fog) as the most competitive species, to the
1183
herb Hydrocotyle heteromeria as the least, had no clear relation to abundances in the Botany Lawn
1184
(Roxburgh and Wilson 2000 %409). Such a lack of correlation has been found elsewhere (Aarssen 1988
1185
%3). Roxburgh and Wilson (2000 %409) suggested that it was due apparent mutualism in a multi-
1186
species mixture: the “My enemy’s enemy is my friend” effect. This effect will be more prominent the
1187
more species there are in the community. Apparent mutualism is implicitly included in the community
1188
matrix model, therefore the model should be able to predict abundances in the actual community (given
1189
that only seven species were included in the model). It could. For example, the model predicted that
1190
Trifolium repens (white clover), Holcus lanatus and Agrostis capillaris (bent) would be the three
1191
dominants, and these three are indeed among the four most abundant species in the lawn. From the
1192
pairwise experiments, H. lanatus headed the competitive hierarchy. However, it was only the second
Wilson & Agnew, chapter 3, Community processes, page 40 of 50
1193
most abundant species in a experimental 7-species experimental mixture and in the lawn (Roxburgh and
1194
Wilson 2000 %395), an outcome correctly predicted by the community matrix model.
1195
7.4 Resistance to abiotic perturbation
1196
Resistance is a difficult concept. It is the response of a system to a natural or anthropogenic
1197
pulse perturbation, or to be more exact the lack of response. But what is a perturbation? Suppose a light
1198
wind blows over grassland, with no effect on community composition. Obviously the change was too
1199
small to be called a perturbation. The trouble is that if a perturbation is applied and the community does
1200
not react, it is impossible to say whether that was because it was a very resistant community, or because
1201
the treatment applied was not a real perturbation in the first place. We cannot even compare
1202
communities. If salt is applied to most communities, it will be a perturbation and plants may die.
1203
However, with a saltmarsh community any obligate halophytes there will jump for joy; it was no
1204
perturbation at all. This is analogous to the problems that Körner (2003 %141) saw in defining ‘stress’.
1205
The concept of resistance can apply at the level of the whole community or of individual species, and
1206
again in terms of function, guilds or species. We could expect some theory at the community level, but
1207
there is little. In Loreau and Behera’s (1999 %29) model, with plants limited by nutrients, the biomass
1208
resistancebiomass is onlyt one measure of resistance. Perhaps a mention of why this one was chosen
1209
generally decreased with increased ecological difference between the species.
1210
For evidence, we take two examples: drought and trampling. Drought can be basically a pulse
1211
perturbation. Pfisterer and Schmid (2002), in mesocosms of grassland species, found that species-rich
1212
plots were less resistant to drought in biomass, i.e. function. The same trend can be seen for species
1213
composition in the Cantabrian (Spain) mountain meadows that Gómezal-Sal (1994) examined. Other
1214
field comparisons and those with experimental plots that differ in fertiliser application have yielded the
1215
opposite result, though such comparisons confound species diversity with composition. Vegetation is
1216
complicated, so is its relation to soil, and the behaviour of each species is predicated by its evolutionary
1217
history, so it is little wonder that it is difficult to find generalisations.
1218
There is an extensive literature on the effects of human trampling. For example, Page et al.
1219
(1985) studied the above-ground production resistance of many species in Welsh coastal dune
1220
vegetation over two years after press perturbations of quantified trampling, and compared the results to
1221
controls. They found that species could be assigned to three behavioural classes, partly depending on
1222
their morphology, i.e. their guild / functional-type. In the closed-canopy vegetation of the fixed dunes,
1223
graminoid production was the most resistant, and forb production the least. Forbs and bryophytes of
1224
unstable habitats, counter intuitively, recovered only slowly. However, they have specialised reactions
1225
to particular environmental events. In Bellis perennis (lawn daisy) trampling triggers the swollen corm-
1226
like base, filled with starch, to produce stolons; in fact the species needs open ground for these stolons
1227
to be produced (A.D.Q. Agnew, pers. obs.). The moss Tortula ruraliformis has fast erect growth under
Wilson & Agnew, chapter 3, Community processes, page 41 of 50
1228
sand burial,I thought you were talking about trampling? probably because it is peculiarly efficient in its
1229
ability to dry and re-hydrate fast with minimal respiratory cost (Willis 1964). The golden moss
1230
Homalothecium lutescens forms a weft with erect shoots which grow up after sand burial. Other authors
1231
using greenhouse and field experiments (e.g. Andersen 1995; Cole 1995 %215) have found that
1232
resistance to human foot and wheel pressure differs between plant guilds in similar ways, the geophytes
1233
and tussocky graminoids being consistently more resistant, upright forbs less resistant. Resistance seems
1234
to be individualistic to the species.
1235
7.5 Resistance to invasion
1236
Another type of resistance is that to invasion. The process of invasion can comprise: (1) the
1237
occupancy of an empty niche, (2) splitting an existing niche, (3) ousting an existing species from a
1238
niche, or (4) the invader’s constructing a new niche by reaction. Often, invasion is taken to mean by
1239
species exotic to the region (usually exotic to the continent). This confuses the ability of incomers to
1240
establish in a closed community with whether communities contain empty niches and with whether
1241
exotic species are somehow different from native ones.confusing sentence At other times, the topic
1242
means the invasion of local species that were not present at that spot before, which involves questions of
1243
community structure. The issues here are:
1244
1. Whether disturbance is required before invasion can occur.
1245
2. Whether continual disturbance is required for the invader to persist.
1246
3. The biotic diversity of the target community, and whether it is species-saturated. Behind this is
1247
whether there are empty niches, leaving resources available to the invader.
1248
4. Missing guilds (plant functional types).
1249
5. Community structure, for example whether communities with low interaction strength and/or
1250
1251
low connectance are more invasible, or possibly less so.
6. The existence of an alternative stable state which could keep an invader out, though it could
1252
form a table state once admitted.
1253
7. Subvention available to the invader, e.g. protection from grazing offered by a thorny shrub.
1254
8. Whether exotic species differ in some way from native ones.
1255
Exotic species will be discussed in chapter 5, but all the aspects above apply to them. Empty niches (‘3’,
1256
‘4’) are especially relevant for exotics. So is ‘1’ because it is possible that the absence of an exotic
1257
species in a community is due to inertia, and lack of disturbance is only delaying up the inevitable
1258
invasion. Aspect ‘2’ will be relevant if many of the exotics are ruderal species.
1259
Theory
1260
Case (1991) found that in randomly-constructed Lotka-Volterra based communities resistance to
1261
an invader was higher when species richness was higher and there were strong interspecific interactions.
Wilson & Agnew, chapter 3, Community processes, page 42 of 50
1262
Shea and Chesson (2002) concluded that theory is indeed now clear that, all other factors being equal,
1263
invasion resistance increases with species diversity. Intuitively, the explanation is that with high
1264
richness there are no resources left for an invader, with appeals, at least implied, to MacArthur and
1265
Wilson’s species packing. But then why, except for inertia, do the residents get the resources, not the
1266
invader? In partial refutation of Shea and Chesson (2002), the careful plant-community simulations of
1267
Moore et al. (2001) indicated that there is no simple relation between resistance and richness: it depends
1268
on the model and what caused the richness to vary.
1269
Evidence
1270
There have been many studies correlating species diversity and invasion across sites, and also
1271
studies experimentally adding invaders to a range of natural systems. We can draw no conclusions from
1272
these because there are always environmental/historical differences between sites that confound
1273
differences in species diversity. We like the idea that real communities can tell us more, but in this case
1274
experiments are needed in artificial communities.
1275
Dukes (2002) established small field microcosms on a soil/sand mix using grass and forb species
1276
found in a particular grassland in California. When Centaurea solstitialis was added as an invading
1277
species in the form of seeds, mixtures of two or more species reduced its biomass more than the average
1278
monoculture, but no mixture of up to 16 species inhibited invasion more than the most resistant single
1279
species: the annual forb Hemizonia congesta (it is a member of the same guild as C. solstitialis: hints of
1280
niche filling). No increase in resistance was seen beyond four species, though Dukes (2001 %563)
1281
attributed this to a guild effect, an artefact of the experimental design. This raises a complication. We
1282
are seeking a niche complementarity effect, in which with more species there will be a higher
1283
probability of including a species to fill each niche, in this case perhaps the annual forb niche, leaving
1284
few resources for an invader (leaving aside the question we raised above of why the residents should get
1285
them). As with overyield there is the possible artefact of the ‘selection effect’ (Wardle 2001 %161):
1286
when more species are sown there is more chance that a species with high interference ability is
1287
included, and that species will take over. This mimics complementarity if the species with the highest
1288
interference ability in the resident mixture also has high interference ability against the invader, but that
1289
is quite likely, for example a species with high cover or LAI reducing light intensity below itself. A
1290
counter to this artefact is finding that a mixture is more resistant to invasion, not just compared to an
1291
average mixture with fewer species, but than any mixture with fewer species or any monoculture. The
1292
experiment of Dukes (2001 %563; 2002) did not achieve this.
1293
Evidence for the effects of diversity on invasion comes from a 4-year old experiment at Cedar
1294
Creek. Tilman (1997 %81) sowed seeds of up to 54 species in to patches of native grassland. The
1295
proportion of the 54 added species that became established was negatively correlated with initial species
1296
richness of plots, suggesting that species-rich sites were more resistant to invasion. A problem is that
Wilson & Agnew, chapter 3, Community processes, page 43 of 50
1297
some of the species added as seeds were already present in some of the plots, when such a species
1298
established it would not count as an invader, and there would be more chance of this happening at
1299
higher richnesses. This does not seem to be the complete explanation because the richness/resistance
1300
relation remains qualitatively the same if only species not already present are counted among those
1301
sown (D. Tilman, pers. comm.). However, the variation between plots was natural, and species
1302
composition may have covaried with richness. These problems were overcome in another experiment at
1303
Cedar Creek, when Fargione and Tilman (2005 %604) planted and weeded 169 plots to species
1304
richnesses from 1, through 2 to 16 for 6 years, then opened them to colonisation for two years. In
1305
higher-richness plots there was significantly lower invader richness (R2 = 0.31) and biomass (R2 =
1306
0.18). As usual, most of this effect on invader richness occurred at low resident richness, between one
1307
and two species. The effect seemed to be largely due to suppression of invaders by C4 grasses, and the
1308
authors suggest their effect is via reduction in soil nitrate. They admit this is most easily seen as a
1309
selection effect. However, the invasion resistance of some mixtures was greater than that of the most
1310
resistant monoculture (Schizachyrium scoparium, bluestem, a C4 grass), and this effect increased up to
1311
an 8-species mixture. Specifically, the percentage of plots that had lower invader biomass than did the
1312
best resident monoculture increased across the species richness gradient. This is all quite impressive of
1313
the complementarity effect.
1314
Wilsey and Polley (2002) attempted to outwit the sampling effect by experimentally varying the
1315
evenness of four grassland species (three grasses and one forb), not the diversity. Reducing evenness led
1316
to a greater number of dicot invaders, but had less of an effect on monocot invaders. Opportunity for
1317
testing comes also from guild-removal experiments; we discuss these chapter 5, section 7.4, coming to
1318
the conclusion that they offer little support for guild-based assembly rules.
1319
Resources, disturbance and species
1320
It is very frequently suggested that invasion is greater after disturbance (Davis et al. 2000 %
1321
421). This is true, but misleading. Of course, there must be sufficient resources for an invader to
1322
establish. Autogenic and allogenic disturbances are more likely in habitats with more nutrient resources,
1323
due to greater growth rates and herbivore muchiness respectively, and disturbance often releases
1324
resources. For example Burke and Grime (1996 %776) and Thompson et al. (2001) report an
1325
experiment in a limestone grassland at Buxton, northern England, where they added seed of 54 species,
1326
all widespread native herbs that are commonly found in climates and soils like those at the experimental
1327
site but which were not originally present there. There was greater establishment of the 54 species when
1328
the soil was disturbed and fertiliser added, though the combination of disturbance and fertility levels in
1329
which invasion was maximal was unique for each invading species.
1330
1331
Davis et al. (2000) generalised from experiments such as these to suggest that invasion requires
resource enrichment or release, including that caused by disturbance. There have been many similar
Wilson & Agnew, chapter 3, Community processes, page 44 of 50
1332
statements, but we must be careful. If the invaders are native then either: (a) they are better suited to the
1333
conditions than the residents, probably because the conditions have changed, in which case disturbance
1334
is simply overcoming inertia, or (b) they are not better suited, in which case they are probably ruderals
1335
taking advantage of the temporary gaps. Neither result is remarkable. If the invaders are exotics, the
1336
same argument applies; the only difference is that the possibility of their being better suited is greater,
1337
because: (a ii) there is the additional possibility that they are a worldwide super-species in those
1338
conditions, and (b ii) there is a greater probability of their being ruderals too, because many exotics are.
1339
With higher nutrients the argument is much the same. There are some careful and ingenious experiments
1340
here, but the conclusions are fraudulently trivial.
1341
We should be careful of over-simplification. Species identity matters. For example, Van Ruijven
1342
et al. (2003 %910) sowed plots with 1 to 8 species. They found an effect of species richness on
1343
invasibility, but it could be attributed to just two species, and mainly Leucanthemum vulgare (ox-eye
1344
daisy). However this was not a selection effect: L. vulgare was a low yielder in monoculture. Van
1345
Ruijven et al. suggested it may have suppressed invaders by harbouring root-feeding nematodes, and
1346
perhaps also transmitting viruses through them. Again, Crawley et al. (1999 %140) found little overall
1347
effect of species richness in decreasing volunteer invasion, but there was consistently lower invader
1348
richness and biomass in plots containing Alopecurus pratensis (foxtail).
1349
7.6 Resilience
1350
Resilience is a simple concept: the speed of recovery after a pulse perturbation. Again, recovery
1351
can be measured in an ecosystem function such as production, in guild structure or in species
1352
composition. If structure is described by the guilds present, then the guild proportions (i.e. the
1353
physiognomy and phenology of the vegetation) must be regained, which will restore community
1354
function.
1355
Walker (1992) argues that maintaining function after a perturbation should be easier if there are
1356
several members of each guild, a situation he calls species/ecological/functional redundancy. If a
1357
perturbation affects the dominant species in a guild, a subsidiary species will then be available to
1358
assume its rôle. Richness could lead to resilience this way. Walker et al. (1999) suggested that an
1359
Australian rangeland community was rather precisely organised this way: the most abundant species
1360
differed from each other functionally, the most functionally-similar species differed in abundance, and
1361
when a species decreased under heavy grazing another species increased that was functionally similar
1362
but had previously been sparse. For example, using his functional characters, when a large perennial
1363
grass with soft, thin leaves is reduced by grazing, another species with those characters should increase.
1364
This is not easy to imagine. In so far as those characters affect palatability and grazing tolerance, all the
1365
functionally-similar species should decrease together. The relevant theory is that environmental changes
1366
(here, grazing) lead to dominance by species of different beta niches but with the same alpha niches still
Wilson & Agnew, chapter 3, Community processes, page 45 of 50
1367
mostly represented. The characters that Walker et al. (1999) used – height, width, life history, SLW and
1368
leaf ‘coarseness’ – do not fit easily into this model. Similarly, Cowling et al. (1994) suggested that in
1369
Cape Fynbos, South Africa, the high species diversity leads to considerable redundancy within guilds.
1370
After the massive fires and droughts which are endemic to the region some members of each guild could
1371
survive, so that the original guild structure is recovered. The concept of redundancy as espoused by
1372
Walker falls all too easily into teleology: “Why do the majority of species occur in low abundance?
1373
They … confer resilience on the community with respect to ecosystem function” (Walker et al.
1374
1999)Not sure why this sentence is teological, please explain. The real answers to their question come
1375
from the mechanisms of coexistence that we shall discuss in chapter 4, but we note that an alternative
1376
answer is that insurance could come not from redundancy but from migration (Loreau et al.’s 2003
1377
%12765).
1378
There have been many studies of the relation between productivity (usually above-ground
1379
biomass) and resilience. Nearly all deal with grasslands, and we therefore expect that generalisations
1380
may emerge about the effects involved. Sadly this is not the case. Moore et al. (1993), in a short review,
1381
state that productivity and resilience are inextricably linked, but Stone et al. (1996), using a range of
1382
mathematical models, suggest it is far from simple, and depends on the model used. Rietkerk et al.
1383
(1997) indicated similar complication using a more mechanistic model, concluding that if plant growth
1384
is water-limited the vegetation on sandy soils is more resilient, but if plant growth is nutrient-limited the
1385
vegetation on clayey soils is more resilient. However, they find that this depends on the extent of the
1386
plant’s reaction (sensu Clements) on the soil. Sometimes, it is not easy to see a deep meaning in
1387
differences in resilience. For example, Herbert et al. (1999 % 908) examined resilience in aboveground
1388
net primary productivity in Hawai’ian tropical rainforest after a hurricane. Plots that had been fertilised
1389
with P were less resistant but more resilient. Both seem a rather simple result of lush growth and high
1390
growth rates with fertilisation.
1391
High species richness would tend to imply high redundancy. Pfisterer and Schmid (2002) sowed
1392
BioDepth plots with a range of grass species richness, perturbed them with simulated drought, and
1393
found that the low-richness plots were more resilient in aboveground net primary productivity.
1394
Mathematical theory on resilience is sparse, but Haydon (1994 %14) used community-matrix models,
1395
first excluding unfeasible and unstable communities, which indicated that resilience should increase
1396
with connectance and interaction strength, and decrease with species richness. The latter prediction
1397
matches Pfistrer and Schmid’s finding, be it that Haydon’s prediction was for species-composition
1398
resilience. In contrast, when Engelhardt and Kadlec (2001) set up aquatic mesocosms with 1-5 aquatic
1399
species and applied the perturbation of clipping off the above-substrate shoots, there was no relation
1400
between species richness on biomass and respiration resilience. The answer is probably that species’
1401
types differ, and we can expect no simple conclusions. For example, in the trampling experiments of
Wilson & Agnew, chapter 3, Community processes, page 46 of 50
1402
Cole and Trull (1992) and Cole (1995)’s experimental trampling, resilience in cover (which was
1403
measured in the latter work!) was lower in communities containing more shrubs and chamaephytes.
1404
Correlation of resistance and resilience
1405
An inverse relationship between resistance and resilience has been repeatedly demonstrated,
1406
both across communities and across individual species. We saw this with Herbert et al. (1999) above.
1407
Taking our case study of trampling, Cole and Spildie (1998) found that when montane understorey
1408
communities were subjected to hiker, horse and llama traffic at two intensities, forb-dominated
1409
vegetation was highly vulnerable to vegetation impact but recovered rapidly, whilst shrub-dominated
1410
vegetation was more resistant but lacked resilience. At the between-species level, again with trampling,
1411
Cole (1995 p 215) found a negative correlation in mountain vegetation; the correlation was seen
1412
especially within the chamaephyte and graminoid guilds. In contrast, Pfisterer and Schmid (2002) found
1413
that species-poor plots were both more resistant and more resilient to drought. Again, simple
1414
generalisations are premature and probably wrong.
1415
8 Conclusion
1416
Many have tried to make generalisations about plant community change. Unfortunately, none are
1417
available. Even general phrases such as “Succession as a gradient in time” (Pickett 1976) can be
1418
misleading when a switch results in stasis or a mosaic, or when there is cyclic succession (Fig. 3.10).
1419
There have been fashions awkward sentence. Before Egler (1954), and for some time afterwards since
1420
most ecologists dismissed Egler as a crank (which he was), most ecologists held to Clements’ (1916)
1421
concepts of relay floristics in a much more simplistic way than Clements himself had. The reaction (no
1422
pun intended) began in earnest with Connell and Slatyer (1977), and progressed until some ecologists
1423
declared that the very word ‘climax’ should be avoided. Many have declared that the views of Gleason
1424
were “strongly opposing” the successional concepts of Clements, but in fact his concept of the process
1425
of succession (Gleason 1917; 1927) was almost identical.
1426
The geological background is included in Fig. 3.9, with the processes of speciation,
1427
biogeographic distribution, dispersal and environmental filtering / ecesis (steps A-D). In chapter 2 we
1428
discussed the interactions between species that follow (E and F). It is clear that given an area without
1429
plants, certain species will predominate first. Whether they do this through dispersal, fecundity or stress
1430
tolerance will depend on the environment. The later species will come in partly because, though not
1431
having those characteristics, they have greater competitive ability. The extent to which they rely on
1432
facilitation by the pioneer species will again depend on the environment and on the species (L.R.
1433
Walker et al. 1986 %1243; 2003 %277). A single, stable endpoint is present in one of the three possible
1434
pathways (Fig. 3.1). The ‘climax’ concept should not be thrown out with the bathwater. On the contrary,
1435
Relay floristics should be the null model, not because Clements was almost always right (though he
Wilson & Agnew, chapter 3, Community processes, page 47 of 50
1436
was), but because it happens often, and it is more profitable to concentrate on the exceptions. The
1437
situations that Clements termed sub-climaxes are one exception to relay floristics, but his use of them
1438
was much more restrained that that of those peddling a simplified version of his theory. He saw fire and
1439
grazing subclimaxes as man-induced, but these days we are more likely to see fire suppression as man-
1440
induced. The only environmental factor that Clements (1916) saw as retarding succession to give a
1441
subclimax was saline/alkaline soils. The replacement of communities (sensu lato) in a cycles is an
1442
obvious exception from the null model, but evidence for it is so hard to find that can be fairly sure it is
1443
less common than Watt supposed.
1444
Reaction is clearly important, and reaction in the facilitative direction is part of the relay
1445
floristics. However, there is every reason to expect it to be commonly in the opposite direction, giving a
1446
switch, and this appears to be the case. Although we have emphasised that most putative switches are
1447
unproven, this is quite as true for relay floristics. Moreover, switches are probably often part of relay-
1448
floristics succession, either speeding it up in which case they will hardly be noticed, or slowing it. It is
1449
when switches sharpen a gradient or magnify small initial differences to give a mosaic of alternative
1450
stable states in space or time that they produce patterns that are hard to miss. Clements did eschew the
1451
idea of alternative pathways of succession (Clements 1916, 167), but he allowed two types of reaction
1452
that retarded succession: what we have described as the Sphagnum bog pH switch and the Ericaceae
1453
(e.g. Calluna vulgaris) switch which Clements ascribed to “acids and other harmful substances”
1454
(Clements 1916, 107).
1455
Throughout relay floristics, cyclic succession, the operation of a switch or for that matter
1456
inducing a state change there may be an allogenic environmental change (this chapter, sect. 2) or
1457
allogenic pulse perturbation (i.e. disturbance), changing the rules of the game. Response to and recovery
1458
from pulse perturbation were the subjects of sections 7.4 and 7.6 of this chapter, but for both we
1459
concluded that it is hard to see generalisations, and probably always will be because of individual
1460
responses by species. Resistance to invasion involves questions of alternative stable states (sect. 5.6 of
1461
this chapter), which are still better known in theory than in reality, and also questions of the nature of
1462
exotic species, which we shall deal with in chapter 5, section 12.
1463
Wilson & Agnew, chapter 3, Community processes, page 48 of 50
1464
1465
1466
1467
1468
1469
1470
1471
A Speciation
B Biogeography: the species pool
C Dispersal
D Environmental filtering / ecesis
E Interference filtering
Geological
processes
1472
Resources
Biota
1473
1474
Resource use by biota
1475
1476
Growth, production
assembly rules
1477
1478
1479
1480
1481
Vegetation change
Autogenic factors
interference
subvention
litter
autogenic disturbance
via other trophic levels
reaction
Cyclic
Climax
phase
Cyclic
phase
ASS
1482
1483
1484
1485
Cyclic
phase
Fig. 3.9: The background of vegetation change.
Allogenic factors
short
timescale
Annual climate fluctuations
(local)
Millenial climatic shifts
(regional)
Geomorphological changes
(local & regional)
Geological changes
(transcendent)
long
timescale
Wilson & Agnew, chapter 3, Community processes, page 49 of 50
1486
Bare,
stressful
environment
Bare, benign
environment
(probably
secondary
succession)
(probably
primary
succession)
Dispersal,
ecesis,
Dispersal,
ecesis,
facilitationtype
reaction
perhaps
facilitation-type
reaction
Switchtype
reaction
Delaying or
accelerating
switch
Switchtype
reaction
Mosaic/
sharpening
switch
Climax
Perturbation
Cyclic
autogenic
facilitation
Regime
Alternative
stable state
change
Cyclic
phase
Cataclasm
Fig. 3.10: Pathways of vegetation change
1487
ILLUSTRATIONS
1488
Fig. 3.1: Vegetation change through time by (a) Clements’ facilitation succession, (b) Watt’s cyclic
1489
1490
1491
succession, (c) a switch.
Fig. 3.2: The Larrea tridentata (creosote bush) / Opuntia leptocaulis cyclic succession proposed by
Yeaton (1978).
1492
Fig. 3.3: Four possible outcomes of vegetation switches.
1493
Fig. 3.4: A vegetation pattern caused by a fog-interception switch.
1494
Fig. 3.5: Change in precipitation caused by a fog-interception switch.
Wilson & Agnew, chapter 3, Community processes, page 50 of 50
1495
1496
Fig. 3.6. A sediment-mediated switch. EPS = extracellular polymeric substances.
1497
Fig. 3.7. Maintenance of alternative stable states in space by a Zero-sum switch, comprising spatial
1498
1499
resource accumulation and depletion.
Fig. 3.8. In ‘a’ arithmetic overyielding the absolute gain by A in mixture is greater than the loss by B so
1500
RTM > 1.0; in ‘b’ the relative gain by B is greater than the loss by A so RYT > 1.0, in ‘c’ some
1501
mixtures yield more than either monoculture. For definitions of RYM and RYT see Wilson
1502
(1988 %Jae).
1503
1504
= theoretical biomass of each species with no interaction,
= actual biomass of each species,
1505
Fig. 3.10: Succession.
1506
Fig. 3.10: Succession.
= total biomass of the mixture.