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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.
Chapter 4. Mechanisms of coexistence
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Alpha-niche Differentiation .................................................................................................................. 5
1.1 Resources (type of resource and time of availability) ........................................................... 5
1.2 Heterotroph-imposed niches ................................................................................................. 6
1.3 The niche extended by reaction ............................................................................................. 7
Environmental Fluctuation (seasonal, annual and decadal change) ..................................................... 8
Pest Pressure (heterotroph challenges) ............................................................................................... 11
3.1 Pathogens ............................................................................................................................ 12
3.2 Herbivory, general ............................................................................................................... 13
3.3 Herbivory of disseminules and seedlings ............................................................................ 14
3.4 Vegetative herbivory ........................................................................................................... 15
3.5 Pest Pressure conclusions .................................................................................................... 15
Circular Interference Networks .......................................................................................................... 16
Allogenic Disturbance (disrupting growth, mainly mechanically) ..................................................... 21
Interference/dispersal Tradeoff ........................................................................................................... 22
Initial Patch Composition ................................................................................................................... 23
Cyclic Succession: movement of community phases ......................................................................... 23
Equal Chance: neutrality..................................................................................................................... 24
Inertia .................................................................................................................................................. 25
10.1 Temporal Inertia .................................................................................................................. 25
10.2 Spatial Inertia: aggregation ................................................................................................. 26
Coevolution of Similar Interference Ability ....................................................................................... 27
Spatial Mass Effect (vicinism) ............................................................................................................ 28
Conclusion .......................................................................................................................................... 28
Were there no plant species coexistence, there would be no need for this book. However, most
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plant communities comprise persisting populations of several species. Populations may increase or
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decrease through neutral drift or weather fluctuations and species can immigrate or disappear from the
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local community. However, long-term studies such as the Park Grass experiment (Silvertown 1987) and
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Bibury (Dunnett et al. 1998) show that the basic tendency is persistence, for example, outbreaks are
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often followed by a decrease back to the original abundance. This coexistence is the fundamental
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statement to be made about plant communities, and how it is achieved is the fundamental problem.
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Resources (e.g. light and nutrients) are almost always limiting. Competition, and thus
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interference between individuals and species, is demonstrable in all types of habitat, except immediately
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after disturbance (chap. 6, sect. 9.3 below; Clements et al. 1929). Interference abilities can never be
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exactly equal, so the result should be the exclusion by interference of all but one species (Gause 1934).
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Therefore, the amazing thing is not that the species in plant communities show any particular patterns of
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 2 of 30
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coexistence, but that they coexist at all. Hutchinson (1941; 1961) asked: "How [is it] possible for a
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number of species to coexist in a relatively isotrophic or unstructured environment, all competing for the
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same sorts of materials?". He called it the “Paradox of the Plankton”.
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Monospecific stands of vegetation do exist, i.e. with only one vascular plant species (Plate 4.1).
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Table 4.1 lists those that we have seen ourselves. They are often at land/water ecotones, in wet places
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and especially in open water or extreme saline environments. In arid countries, a monotonous vegetation
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of one halophytic species can dominate the landscape (Zohary 1973). We could generalise that these are
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habitats where only one species is capable of growth due to a harsh environment, or where the
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exuberance of one species excludes others by interference, but in some cases it is hard to know whether
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to credit the extreme habitat or the high interference, e.g. Phragmites communis reedswamps.
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Table 4.1. Some examples of monospecific stands. We exclude monospecificity in a single stratum or
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guild of vegetation, such as a tree species or understorey species.
Habitat
Arid Saline
Sand dunes
Marine
submerged
Freshwater
submerged
Freshwater
floating
Freshwater
edge
Tidal/brackish
edge
River edge
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Climatic zone Exemplar taxa
Reference
Sub-tropical
Halocnemum strobilaceum
Zohary 1973
Temperate
Zygophyllum dumosum
Zohary 1973
Ammophila arenaria
Mediterranean Posidonia oceanica (also Den Den Hartog 1970
other marine Helobeae)
Temperate
Zostera marina
Den Den Hartog 1970
Temperate
Sagittaria sagittifolia
Pieterse and Murphy 1993
Tropical
Podostemon spp.
Meijer 1976
Temperate
Lemna minor
Scunthorpe 1967
Azolla filiculoides
Scunthorpe 1967
Tropical
Eichornia crassipes
Pieterse and Murphy 1993
Temperate
Typha spp
Weisner 1993
Cladium mariscus
Tansley 1939
Tropical
Cyperus papyrus
Lind and Morrison 1974
Temperate
Salicornia spp
Tansley 1939
Subtropical
Avicenna marina
Batanouny 1981
Tropical
Rhizophora mangle
Gilmore and Snedaker 1993
Tropical
Pandanus spp
van Steenis 1981
At the other extreme is vegetation with high species richness. Values depend on the size of the
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sample units, the life form guild being considered (e.g. often trees alone are recorded in tropical forests)
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and the recording convention used (rooted or shoot presence, perennially or seasonally visible).
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Whatever the sampling regime, extraordinarily high diversities can exist. Tropical rain forest is always
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quoted as an example. Valencia et al. (1994) found 473 species of tree (individuals >5 cm dbh) in 1 ha
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of Ecuadorian tropical rain forest, while Richards (1996) tabulates other examples with over 100 species
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in 1 ha in New World and Malesian (not African!) tropical forests, Naveh and Whittaker (1979)
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recorded 179 vascular species in 0.1 ha of a dry shrub/grass community in Israel. Mean species richness
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of 18.3 per 0.01 m2 has been found in bryophyte carpets in the per-humid West Cape in New Zealand
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(Steel et al. 2004) and 12.2 species at that scale in limestone grassland on Oeland, Sweden (van der
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Maarel and Sykes 1993).
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Box 4.1: Mechanisms of coexistence.
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Stabilising mechanisms
Niche-differentiation
1. Alpha-niche Differentiation (type of resource and time of availability)
2. Environmental Fluctuation – season, decadal and gradual change
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Balances
3. Heterotroph challenges: Pest Pressure
4. Circular competitive networks
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Escape through movement
5. Allogenic Disturbance – disrupting growth mainly mechanically
6. Competition/dispersal Tradeoff
7. Initial Patch Composition
8. Cyclic Succession: movement of community phases:
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Equalising mechanisms
9. Equal Chance (neutrality)
10. Inertia
Temporal Inertia
Spatial Inertia: aggregation
12. Coevolution of Similar Interference Ability
13. Spatial Mass Effect (vicinism)
Questions about coexistence must be asked at a particular spatial scale. The rainforest tree
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Swietenia mahogoni (mahogany) occurs in the tropics and Colobanthus quitensis occurs in Antarctica;
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they cannot be said to coexist. They grow in quite different places and environmental conditions, i.e. in
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different beta niches. Similarly, Salicornia spp. (glasswort) occur on low-altitude saltmarshes and
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Androsace spp. are alpines, again they occupy different beta niches. The Paradox of the Plankton as
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defined by Hutchinson refers to how coexistence can occur in a “relatively isotrophic or unstructured
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environment”. This scale is difficult to define, because allogenic environmental heterogeneity occurs
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down to the very finest scales, so all species in a mixture exist as a pattern of abundance. The species’
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patterns will create further patchiness in resources since species differ in their resource economies: their
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reaction on the environment. Numerous studies have shown that each species’ individuals affect its soils
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(section 1.3), which can give autogenic heterogeneity. Also, each individual, by extending over space,
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must sample a spectrum of resource and environmental qualities. Therefore, rather than specify a
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particular scale, we state here that we are concerned with mechanisms that allow species to coexist
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locally, i.e. mechanisms that are not due to imposed habitat heterogeneity within the area considered.
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J.B. Wilson (1990) identified 12 distinct mechanisms by which coexistence could be maintained:
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(1) Alpha-niche Differentiation, (2) Environmental Fluctuation, (3) Pest Pressure, (4) Circular
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Interference Networks, (5) Allogenic Disturbance, (6) Interference/dispersal Tradeoff, (7) Initial Patch
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Composition, (8) Cyclic Succession, (9) Equal Chance, (10) Inertia (temporal and spatial),
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(11) Coevolution of Similar Interference Ability and (12) Spatial Mass Effect. We believe them to be
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distinct and we know of no new ones, though we have adopted a different arrangement that we hope
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brings some new insights.
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There is a basic distinction between stabilising mechanisms, which contain an increase-when-
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rare mechanism, and equalising mechanisms, which make the differences between species in
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replacement rates smaller (Box 4.1; Chesson 2000). Stabilising mechanisms (1 to 8 in Box 4.1) are
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driven by Alpha-niche Differentiation, Environmental Fluctuation, balances unrelated to niches or
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escape through movement (Box 4.1). Species abundances are bound to fluctuate and stabilising
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mechanisms must include negative abundance-dependence to counter this. [For animals, ‘density-
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dependence’ is often used, but since the concepts of ‘individual’ and ‘density’ are difficult in most
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plants (chap. 1, sect. 1.1) the more general ‘abundance-dependence’ should be used.] This means that
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when a species is at lower biomass in the community its plants must have higher fitness in terms of
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long-term RGR, and its biomass should increase (Chesson in press). Simply, the one necessary and
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sufficient phenomenon for maintaining a species in a mixture is ‘increase-when-rare’1. The corollary of
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this is population limitation, i.e. reduced fitness as biomass increases. Either way, a species’ fitness
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should be inversely related to its abundance. In the short term and in species that reproduce only
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vegetatively we should consider vegetative RGR (relative growth rate). In the longer term, population
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growth is the critical question. The increase-when-rare feature is incorporated into the Lotka-Volterra
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logistic function, as every schoolboy knows. A feature that interferes with this in the real world is the
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Allee effect, whereby populations cannot recover from very low numbers due to low success in mating.
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We believe that it is rare in plants because, as we outlined in chapter 1, each individual is effectively a
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colony and perennials have an indefinite life span. Nevertheless there are examples of the effect in
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Banksia goodii, a shrub of dry savannah (Lamont et al. 1993), and in the outpollinated annual Clarkia
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concinna in California (Groom 1998). Species with obligate outcrossing and/or scattered distributions
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and/or monocarpic reproduction and/or specialised pollen vectors would be more liable to it.
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True coexistence must be through a stabilising mechanism. Equalising processes do not contain
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an increase-when-rare mechanism, but are ways in which species may persist for a time in unstable local
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coexistence, slowing exclusion by interference. Moreover, if the difference between the interference
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abilities of species is large, even the presence of a stabilising mechanism may not prevent exclusion by
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interference and in this situation an equalising mechanism might reduce the difference in interference
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ability between two species so that the stabilising mechanism is able to cause coexistence (Chesson
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2000). Ultimately every plant has established itself by a process that can be explained by its tolerances
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(its niche) and the environmental conditions prevailing during its ecesis, but the reasons for a species'
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presence in a particular spot are usually obscure. For example, a tree may persist for so long that the
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local soil/geomorphological conditions that allowed it to establish have since changed. It is therefore
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 5 of 30
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tempting to suggest that each individual's presence owes as much to chance as to ecological
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differentiation and a theory of equal chance has been proposed to explain species mixtures, as we
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discuss below. Again, species may persist temporarily through inertia of individuals or populations in
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time or aggregation in space. Any process causing similarity in interference ability is also equalising.
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Lastly, and rather hesitantly, we include the Spatial Mass Effect as an equalising mechanism.
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1 Alpha-niche Differentiation
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We discussed alpha niches in chapter 1 (sect. 4.1). It has been pointed out that coexistence by
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Alpha-niche Differentiation is impossible to disprove. Each species must by definition occupy a
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different niche (chap. 1, sect. 2.1). Moreover, by reaction it uniquely constructs part of its niche. The
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other side of the coin is that if redundancy really occurs, i.e. there are coexisting species that do not
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differ in alpha niche, they will not coexist by this mechanism and some of the 11 other mechanisms
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mentioned below must account for their presence. The ‘increase when rare’ element occurs here because
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when a species is rare the resource that it particularly takes up and requires will be present in greater
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abundance, that is, the niche is not fully occupied, though interference with this process could come
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through luxury uptake of nutrients (Lipson et al. 1996). The population limitation on the other hand is
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due to full niche occupancy, i.e. full use of its resource. The degree of niche separation required will
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increase with the difference in interference ability between the species; if one species is a very strong
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competitor another species will be able to coexist only if it is occupying a completely different niche.
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However, in contrast to some stabilising mechanisms of coexistence, if the niche differentiation between
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two species is strong enough, they can always coexist.
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1.1 Resources (type of resource and time of availability)
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Tilman (Titman 1976) demonstrated that coexistence was possible between two algae limited by
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different nutrients, P and Si, and concluded that the number of species able to coexist is equal to the
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number of resources and then only if each species is limited by a different resource. He confirmed this
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in modelling (Tilman 1977), and there appears to have been no contradiction. Vance (1984) claimed to
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show that two species can coexist on one limiting resource, but only “if each species interferes less with
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resource acquisition by the other than with resource acquisition by itself”, which with pure competition
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must mean niche differentiation (e.g. the one resource is water, but it is taken up from different soil
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strata).
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The primary resource requirements of most embryophytes are similar (light, water, CO2, N, P, K,
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minor elements, sometimes pollination and dispersal). The concept of a resource gradient as niche
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differentiation is simple for seed sizes as a resource for birds, but it applies less readily to plants, for
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which most of the resources are discrete requirements. For example one species cannot require a low-
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concentration type of P and another a high-concentration type, and the two cannot occur simultaneously
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 6 of 30
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anyway. However, in other cases such as soil resources at different depths and pollinator service during
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the season (section 2), the separation and specialisation of species along gradients are important
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mechanisms of coexistence (Fig. 4.1; MacArthur and Levins 1967). An important question is how much
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separation is needed, but in spite of the calculations of MacArthur and Levins this remains unanswered
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for the real world. Even the existence of such niche limitation has been controversial and difficult to
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prove (chapter 5).
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Fig. 4.1. The MacArthur and Levins (1967) concept of niche separation along a gradient.
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The above-ground structure of a plant is a light-capture mechanism. Therefore gross
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characteristics of plant form have great relevance. Consider tree size and shape in rain forest, where
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light is the prime resource; Kohyama (1992) and Akashi et al. (2003) concluded from modelling that
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short species, with high seedling recruitment but with height-limited growth, could coexist with taller
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species with lower recruitment rates. This is basically niche differentiation based on canopy strata.
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Kohyama (1993) used the model to show that stable coexistence resulted without requiring a stand
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mosaic and Yokozawa et al. (1996) demonstrated that two canopy shapes, conical and spheroidal, could
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interact with different speeds of recruitment to give situations that allowed a diverse canopy flora. These
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are subtle and devious ways in which resource differentiation takes place in this famous biome.
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Stratification below ground, i.e. in rooting depth, is another important niche gradient (chap. 1, sect. 4.3),
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especially when there is both precipitation and an accessible water table.
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Two types of temporal gradient can be seen: (1) If growth is triggered by the resource itself,
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species can differ in their speed of reaction to resource availability, their opportunism. Opportunistic
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species react fast to resource availability, for example production of surface roots of succulents,
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ephemeral leaves like Grewia spp. (cross berry) that have a leaf flush after every rain in African summer
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deciduous bushland, and seasonally produced leaves that must survive periods of resource starvation.
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(2) More commonly seasonal separation of species' growth patterns is controlled not by the resource
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itself, but by signals such as daylength and temperature. This causes regular seasonal phenological
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separation of species’ activity. Here, the mechanism overlaps with the Storage effect (section 2).
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1.2 Heterotroph-imposed niches
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Pollination and dispersal can be switch mediators (chap. 3, sect. 5.4.G), but also means of niche
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differentiation. Pollinators come in many sizes and specialisations: insects, birds, mammals and even
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reptiles. Among insect pollinators there is huge variation in characteristics and their interplay with
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plants can be rich and complex. There are robbers, mimics, rewards, guides and warnings. The
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pollination niche is liable to the Allee effect, both for self-incompatible plants when there is no mate in
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the neighbourhood and for those specialised to particular pollinating insects when the plant population is
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not large enough to attract the pollinator. Dispersal tends to be less specialised, without an equivalent to
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the close relation between flower morphology and pollinator morphology seen with some insects and
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birds, but differences in fruiting times could reduce competition for dispersers. An Allee effect is
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possible in dispersal if the population is too small to attract more specialised dispersers. Allee effects
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can neutralise an increase-when-rare mechanism.
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Vascular plants could occupy different niches by associating with different mycorrhizal fungi.
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However, specificity within the two main groups of fungi (VAM, ecto, ericoid/epacrid, etc.) is
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quantitative, in terms of efficacy rather than in absolute ability to colonise the roots. Moreover, the
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effect on the higher plant with all types is on availability of soil nutrients (especially P) and water, and
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the loss is in carbon. We conclude that niche diversification through mycorrhizae is unlikely.
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1.3 The niche extended by reaction
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The alpha niche is not a pre-existing box into which a species has to fit. We have emphasised
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that the individuals of a species react on their environment, changing it and to a lesser or greater extent
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constructing their own niche. Ramets of a species always show some sort of density pattern (chap. 3,
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sect. 1) and this pattern must cause patchiness in the micro-environment of the habitat. Litter production
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is often the basis for nutrient heterogeneity, but plant morphology and root growth can also be the cause
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(Vogt et al. 1995). Habitats, therefore, must always be patchy in resource availability and physical
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environmental actors and the size of the patches depends on the pattern of densities of each species.
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There is evidence of this, particularly from forests where the sheer size of the trees makes their patches
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large and easy to sample. For example, Pelletier et al. (1999) examined a mixed-species forest in
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Québec, Canada, and found using ordination that forest-floor soil was different beneath different
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species. For example, soil [Ca] was low below Fagus grandifolia (American beech) and they concluded
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that it reduced the soil [Ca]. In most such observational studies there is a chicken-and-egg problem, that
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perhaps the soil differences are determining which species grows at a point, not the reverse. However,
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Pelletier et al. went two steps further: (a) they used spatial statistics to remove spatial correlations, so
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that as far as possible they were examining the effects of individual trees, and (b) they offered evidence
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that F. grandifolia produces litter which, from its Ca, lignin, polyphenol and tannin contents, was likely
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to reduce soil [Ca]. The study of Ehrenfeld et al. (2001) went that step further in another way. They
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found higher pH below two exotic species in a deciduous forest in New Jersey, USA, than beneath the
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native Vaccinium spp., but they also grew the species in the greenhouse on field soil and found pH
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differences in the same direction. The main problem for interpretation here is that the two exotics
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(planted together) may have raised the pH more because of their greater growth.
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Presumably plants can have the same effects, but there are few published studies. This argument
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is that every plant community must show pattern associated with each of its constituent species, but
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there are other biotic forces making habitats more heterogeneous. Litter is not only a significant niche
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factor, but one especially liable to cause change by reaction.
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The necessary presence of patches in a community is important to every species’ resource
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foraging strategy. Large resource patches are best exploited by stoloniferous herbs (Wijesinghe and
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Hutchings 1997). Jackson and Caldwell (1996) modelled the effect of root plasticity on the uptake of
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nutrients from heterogenous versus homogenous environments and found that plasticity was
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theoretically highly advantageous in sagebrush steppe conditions. In the same habitat the effect of
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patchy soil nutrients differentially affected Agropyron desertorum (wheatgrass) and Artemisia tridentata
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(big sagebrush), but only under shade conditions where carbon gain was reduced (Cui & Caldwell,
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1997). Here A. desertorum outcompeted A. tridentata because, although its root proliferation was
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reduced, its efficiency of P uptake was not and it could exploit rich patches. A. tridentata on the other
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hand lost efficiency of P uptake and could not make use of patches of higher P availability.
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Of course, the above-ground environment is altered too. Canopy trees create a
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shade/temperature/humidity niche for understorey plants, that might not be able to survive without those
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modifications. Many manifestly cannot do so in mixture with other species, since they are restricted to
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forest understories. Trees also create the niche for climbers and epiphytes via their support, and they
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create a niche for many epiphytes by water and nutrient stemflow. Parasite plants obviously occupy a
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niche that would not exist without other plants; the term ‘niche construction’ is very appropriate here.
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2 Environmental Fluctuation (seasonal, annual and decadal change)
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Species can separate along annual and other changes in the environment, predictable and
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unpredictable. We cannot talk of coexistence caused by millenial-scale fluctuation unless the plants are
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very long-lived (say 500+ yr), because exclusion by interference may happen before the environmental-
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fluctuation coexistence mechanism can operate. We discuss flowering and fruiting niche gradients,
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mediated by pollinators and dispersers, in chapter 5 (sect. 6.2) and species can also separate along niche
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axes of vegetative phenology. An excellent example is the vernal ground flora of deciduous forest.
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Fargione and Tilman (2005b) found evidence that vegetative phenological niche differentiation added to
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rooting-depth differences in facilitating the coexistence of species at Cedar Creek with the dominant
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grass Schizachyrium scoparium (bluestem). Separation in flowering times will reduce competition for
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pollinators, giving coexistence based on niche differentiation.
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Environmental fluctuation can cause coexistence if it be on scales shorter than this but long
enough for there to be feedback on resources. The fluctuation can be one that affects vegetative growth,
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 9 of 30
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for example the vernal flora of forests and spring ephemerals of semi-arid areas, though there is often an
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accompanying fluctuation in reproduction.
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As will be clear below, there has to be an interaction between growth and resource supply for
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environmental fluctuation to cause coexistence. There has been confusion about this. Many authors have
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claimed that simple variation in the environment and therefore in demographic parameters would allow
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long-term coexistence. For example, Gigon (1997) wrote: “The fluctuations and their interferences mean
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that no species encounters optimal growth conditions for a prolonged period of time. Therefore no species
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can outcompete the others. Fluctuations are thus decisive for the coexistence of species”. Coexistence
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cannot happen this way. For coexistence, the long-term growth rate of each species has to be RGR = 0.0
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(r = 0.0, λ = 1.0). The long-term growth rate for a species is the arithmetic average of RGR (the
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geometric average of λ) in each period. Variation in growth rate will not make it more likely that long-
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term r is 0.0, in fact a value of exactly 0.0 due to such averaging is infinitely unlikely. Whilst it is true
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that environmental variation can cause coexistence it can also promote exclusion by interference or have
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no effect at all on coexistence/exclusion, depending on the biological response of the species to the
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environment and to competition (Chesson 1990). There are only two ways in which temporal variation
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can lead to the coexistence of two species: Relative non-linearity and Sub-additivity. These can cause
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coexistence only if the interference unbalance between the two species is not too great, and equalising
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mechanisms can contribute to this.
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Relative non-linearity means that two species respond differently to levels of a resource for
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which they are competing, and moreover that they respond by differently-shaped relations (Chesson in
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press). For example, the three scenarios in Fig. 4.2 count as different shapes. The way to test for
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Relative non-linearity of shapes is to plot the values of RGR of one species at each level of resource R
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against the values of the other: if the result is anything but a straight line, the species are relatively non-
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linear.
C
D
RGR
A
F
B
Rmean
Resource level [R]
Rmean
Resource level [R]
Rmean
Resource level [R]
E
Fig. 4.2: Pairs of two species showing relative non-linearity.
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Take the third graph. If [R], the level of resource R, is constant at the mean value (Rmean), species E has
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a higher RGR than F. However, if there be environmental fluctuation around the mean, the mean growth
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rate of F would be higher. Thus, low fluctuation in [R] advantages E, high fluctuation advantages F.
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The reason this matters is that at low [R] species E grows faster than F and therefore depletes the
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 10 of 30
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resources, at high [R] it grows little more than at Rmean, and leaves much of the R unutilised. Both ways,
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when species E is in the majority it exacerbates the fluctuations in [R]. Conversely when it is in the
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minority, fluctuation in [R] is lower, which favours it: increase when rare is achieved.
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In contrast, species F grows little at low [R], and will hardly deplete R. At high [R] it grows
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disproportionately fast, absorbing R and therefore reducing [R]. Both ways, when species F is in the
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majority it damps down the fluctuations in [R]. Conversely when it is in the minority, fluctuation in [R]
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is higher, which favours it: increase when rare is achieved.
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The second way that environmental fluctuations can cause coexistence is the Storage effect
(Chesson in press. There are four requirements for the Storage effect to operate:
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1. The species must be competing for a resource.
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2. They must be affected by an environmental (i.e. non-resource) factor, and respond differently
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to it.
3. There must be covariance between the environmental factor and the intensity of competition.
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We would expect this, because when the plants are denser and/or larger, competition will
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be more intense. That is, in ‘favourable’ conditions competition will be greater.
315
4. There must be subadditivity (= buffering, = an interaction between environment and
316
competition). That is to say, when environmental conditions are favourable to growth the
317
effect of competition on RGR is greater. So, whilst ‘3’ refers to the intensity of
318
competition, ‘4’ refers to the effect of competition.
319
In years (or other periods) when the environment is favourable for a species, if it is in the majority and
320
therefore competing against itself it cannot take much advantage of the favourable conditions because it
321
is competing against itself at high biomass: X in Fig. 4.3.
Competitive intensity
X
Low RGR
High RGR
Environmental favourability
Fig. 4.3: The effect of competitive intensity and environmental favourability on RGR. For ‘X’,
see the text.
322
323
Chesson’s (1994) calculations indicate that the Storage effect is a considerably stronger force than
324
Relative non-linearity.
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 11 of 30
325
These mechanisms clarify that the timescale on which environmental fluctuation can cause
326
coexistence is set by the timescale on which resource depletion can occur. Light intensity can change
327
instantaneously, it cannot be stored from one second to another and its effects in producing
328
photosynthate are quite short-term, so within-day fluctuation could suffice. Water depletion could occur
329
over a few days, and nutrient depletion over a few months. Soil nutrients often become more available
330
in the spring due to mineralisation, but slow uptake over winter, and are depleted during the period of
331
active growth, so the Storage effect can operate on within-season or between year variation in nutrient
332
use. For neither Non-linearity or the Storage effect do the species need to differ in the resources they
333
use. However, they do use them at different times. Seasonal differences in resource use can be seen
334
either as Alpha-niche Differentiation or as the Storage effect.
335
3 Pest Pressure (heterotroph challenges)
336
Both pathogens and herbivores (from insects to large mammal herbivores) have the potential to
337
give an increase-when-rare process (we use ‘pest’ to cover both pathogens and herbivores). For this,
338
three conditions are required.
339
1. Impact: the pests involved must have a significant impact on the growth and/or survival of the
340
plant species, i.e. their fitness must be reduced.
341
2. Specificity: The pests involved must be to some degree specific to the plant species. It could be
342
sufficient for the species with lowest interference ability to have no specific pests/diseases, but
343
to benefit when the others are suppressed by pests/diseases.
344
3. Abundance-dependence: The challenge from pests must be less on a sparse than on an abundant
345
species. This represents an abundance-dependent effect. The requirement is for a lower impact
346
on the growth and reproduction of sparse species, but this will presumably be through reduced
347
infection.
348
If these three conditions obtain, when any one of the plant species in the mixture becomes more
349
abundant, the host-specific pest (Condition 2) will move more rapidly amongst its host population and
350
the degree of infestation will increase (Condition 3). This will reduce its fitness (Condition 1). This will
351
not directly impact other species, or will do so only to a lesser extent (Condition 2). Conversely, when a
352
species becomes sparse, infestation by its specific pests will decrease and its fitness increase relative to
353
its fitness when more abundant, giving the increase-when-rare effect. Again, the strength of conditions
354
1-3 necessary for coexistence depends on the degree of difference in interference ability, and equalising
355
mechanisms of coexistence can allow Pest Pressure to result in coexistence when it otherwise would
356
not.
357
Condition ‘1’ can often be met, since pests have various effects on plant production. Basically,
358
the pest organism must have a carbon requirement, which is almost bound to result in lower production
359
and fitness for the plant. Many pests are quite specific to a species or group of plant species, meeting
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 12 of 30
360
condition 2. Ecologists tend more to question condition ‘3’ because it is less obvious how abundance-
361
dependence could operate. Possible mechanisms (Boudreau and Mundt 1997) are via: (a) a decreased
362
abundance of palatable / susceptible plants, which inhibits the dispersal of herbivores, disease-spores or
363
disease-vectors, (b) the flypaper effect for disease spores and possibly for insect pests, virus vectors and
364
hence for the viruses they carry, in which the pest is caught by a passive surface, (c) alteration in the air
365
flow and microclimate, (d) chemicals from associated species that repel herbivorous insects, and (e)
366
promotion by an associated species of natural enemies of the herbivorous insects, i.e. their predators.
367
We shall discuss diseases and herbivory separately since they act in different ways with different
368
dynamics. However, there is sometimes evidence for abundance-dependent mortality or a reduction in
369
growth that is circumstantial evidence for the process and therefore for the Pest Pressure mechanism,
370
that cannot be attributed to a particular pest. For example, Packer and Clay (2000) examined the
371
distribution of seedlings of Prunus serotina (black cherry). The greatest number of seeds germinated
372
quite close to their parent tree, 5-10 m. However, 4 months later and thereafter up to 28 months,
373
seedling survival was higher the greater the distance from the parent tree, up to the furthest distance
374
monitored (30 m). This is not always the pattern. Dalling et al. (1998) in tropical rainforest on Barro
375
Colorado Island found that seedlings tended to be denser nearer to an adult of the same species. For
376
temperate forests Houle (1992) found the seedling mortality of Acer saccharum (sugar maple) in an
377
Eastern American forest was not abundance-dependent; there was no particular spatial relation between
378
trees and seedlings. Hyatt et al. (2003), in a thorough review of the literature, found no evidence for an
379
effect of distance from conspecifics on seed survival in either temperate or tropical communities, but
380
there was a tendency for seedlings to show higher survival at distance, with hints that this occurred
381
especially in the tropical forests. This matches the conclusions of Wright (2002) who, with a rather
382
different review approach, found considerable evidence of low growth performance of saplings near
383
conspecific adults. When effects like this are found, they are assumed to be because of Pest Pressure,
384
though Wright discusses other explanations. This kind of pattern recalls the ‘Janzen-Connell’ hypothesis
385
that whilst the greatest density of fruit will be dispersed to near the parent plant, pests will have the
386
greatest impact there, so the maximal regeneration will occur at an intermediate distance from the
387
parent. This would be expected to be an important abundance-dependent method of species'
388
maintenance in diverse, stable communities of trees, specifically tropical rain forest, but it seems that it
389
is far from universal.
390
3.1 Pathogens
391
Pathogens act in the soil, in the plant systemically and in the plant’s photosynthetic and
392
reproductive systems. The impact of fungal pathogens can be considerable. Mihail et al. (1998) found
393
that in a greenhouse experiment with the annual legume Kummerowia stipulacea (Korean clover) the
394
fungus Rhizoctonia solani caused mortality that reduced plant density by 40 % whilst the fungus
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 13 of 30
395
Pythium irregulare reduced density by 80 %. C.E. Mitchell (2003) found that in an oldfield grassland at
396
Cedar Creek 8.9 % of the leaf area was infected by fungal pathogens, which decreased root production
397
by 25 % by decreasing leaf life, while herbivores had no effect.
398
399
The species-specificity requirement is less easy to meet. Gilbert (2002) concludes that the
evidence so far indicates that in natural communities that most fungi infect a large number of hosts.
400
However, the abundance-dependence criterion can often be met. Most ephemeral pathogens are
401
transmitted aerially including rusts and smuts which affect leaves, stems and floral parts. Abundance of
402
the host plant affects the pathogen's population and its persistence, and the transmission of specialist
403
pathogens can be highly sensitive to the identity of other host species in the community (Boudreau and
404
Mundt 1997). An example is the Ustilago violacea smut on Silene alba (≡ S. latifolia; white campion),
405
for which Thrall and Jarosz (1994) experimentally compared the behaviour of the host and pathogen
406
populations to theoretical models. The match was good and showed that both density dependence and
407
frequency dependence occurred. An excellent study by Burdon et al. (1992) described the mortality of
408
Pinus sylvestris (Scots pine) caused by the snow blight fungus Phacidium infestans as being mostly
409
abundance-dependent, with greater mortality in subsites where the host had been denser the previous
410
year. This abundance-dependence has sometimes been shown to lead to a lower pathogen load in
411
mixtures, which must imply some host specificity. C.E. Mitchell et al. (2002) examined 147 plots in an
412
experiment at Cedar Creek established by Tilman and co-workers, sown and weeded to species richness
413
from 1 to 24 species. The percentage of each leaf visibly infected was guessed, using calibrated cards as
414
a guide. Infection dropped as species richness increased, the 24-species plots having only 37 % the
415
foliar fungal pathogen load of the mean monoculture (though more than the least-infected monoculture).
416
Similarly, C.E. Mitchell et al. (2003) analysed another Cedar Creek experiment sown and weeded to 1
417
to 16 species, and the pathogen load in the 16-species plots was only 34 % of that in the mean of
418
monocultures. In chapter 3, sections 6 and 7.5 we described the ‘selection’ artefact in overyield and
419
invasion-resistance experiments. A similar artefact would be possible here if the species less susceptible
420
to disease had thereby an interference advantage and increased its proportion in the mixture, so that the
421
mixture had a lower mean pathogen susceptibility and thus a lower pathogen load. However, C.E.
422
Mitchell et al. (2002; 2003) present evidence that this is not the cause of the effect they found. C.E.
423
Mitchell and Power (2006) conclude that “the transmission of specialist pathogens can be highly
424
sensitive to the identity of other host species in the community”.
425
Similar effects could be caused by below-ground pathogens. Bever (2003) modelled this, but
426
concluded that there is no evidence for it yet.
427
3.2 Herbivory, general
428
429
Herbivores come in all sizes, specialisations and guilds. On vegetative parts there are leaf eaters,
stem borers and root eaters. On reproductive systems there are flower exploiters, frugivores and
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 14 of 30
430
granivores. Plant species are variously adapted to herbivory, with chemical and physical defences, life
431
histories and growth patterns that have evolved seemingly to deal with the challenges. The potential
432
mechanism for coexistence via herbivory is similar to that for pathogens (chap. 2, sects. 7.3, 7.4).
433
However, whilst pathogens often reduce the functional efficiency of plant parts, most herbivores simply
434
remove plant material so that the plant needs to regrow to replace tissue and thus its resource base.
435
Obviously a great variety of relationships can be expected between plant species and their herbivores,
436
including symbiotic ones, such as Tegeticula spp. and Parategeticula spp. (yucca moths; James et al.
437
1993). In many of these systems herbivores exploit plant populations in an abundance-dependent way.
438
Grover (1994) modelled this and used the keystone concept (chap. 5, sect. 11 below) to suggest that a
439
controlling herbivore is one that holds down the abundance of a potentially-dominating plant species
440
and thus allows subordinate species to survive. We may distinguish between abundance-dependent
441
culling of seeds and seedlings and wholesale removal of plant material, i.e. vegetative herbivory.
442
3.3 Herbivory of disseminules and seedlings
443
Seeds and seedlings are a rich nutritional resource and are heavily predated. Maron and Gardner
444
(2000) showed by modelling that herbivores can control adult population abundance by limiting the
445
seed input to the seedbank. Such limitation seems to be widespread. It occurs also via vegetative
446
disseminules. Thus, the ‘impact’ requirement of the Pest Pressure mechanism can be met by
447
disseminule/seedling herbivory.
448
Disseminule herbivory can often be abundance-dependent. Cygnus bewickii (swans) eat the
449
turions (disseminules, fleshy buds) of Potamogeton pectinatus (pondweed) in the autumn. Jonzen et al.
450
(2002) demonstrated clear abundance-dependent control of the P. pectinatus in which the denser patches
451
of turions were exploited, reducing their density, while areas of low turion densities were unexploited
452
and here the plant density subsequently increased. Edwards and Crawley (1999) examined four species
453
of British meadows and found that granivory by rodents was abundance-dependent, but its effects on
454
adult densities differed. Densities of species with bigger seeds (Arrhenatherum elatius, oat grass; and
455
Centaurea nigra, knapweed) appeared to be reduced, but in the smaller seeded Rumex acetosa (sorrel)
456
and Festuca rubra survival increased to compensate for seed predation, with no overall effect on plant
457
density. Again, Ehrlen (1996) found that in Lathyrus vernus (spring pea), although seed predation by a
458
Bruchidae beetle was correlated with seed density in small plots and with inflorescence size, this had no
459
consistent effect on plant population recruitment. Thus, even the occurrence of abundance-dependent
460
seed predation is no guarantee that it will control the population and hence contribute to species
461
coexistence.
462
Another limitation of disseminule/seedling herbivory as a mechanism of coexistence may be that
463
most mammal granivores are not specific to one species. There is huge literature assuming that bird
464
granivores are restricted by beak size to a particular range of fruit/seed sizes, but not to one species.
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 15 of 30
465
However, species of Bruchid beetle are generally restricted to the seeds of one or a few species of
466
Fabaceae (legumes). Seedlings are presumably eaten by invertebrates, but they are likely to be quite
467
generalist.
468
3.4 Vegetative herbivory
469
Herbivory on vegetative parts can be considerable. Vertebrate herbivores (Jones 1933) are
470
involved, and insects both above- and below-ground (Brown and Gange 1989). The selectivity of
471
herbivores varies widely. Many large non-ruminant animals such as Loxodonta africana (African
472
elephant), even though they have preferences, will readily eat a wide range of species. More
473
importantly, some such as Equus spp. (horses) often graze finely-patterned vegetation at a relatively
474
coarse scale, necessarily taking in species with a range of palatability. Other ungulates such as sheep
475
and cattle are more selective, some tiny flower gall wasps are confined to one or a few plant species,
476
and some lepidopterans feed on only one species such as Tyria jacobaeae (cinnabar moth) on Senecio
477
jacobaea (ragwort): Plate 4.2.
478
There are cases when coexistence can be attributed rather clearly to insect vegetative herbivory.
479
For example, Carson and Root (2000) found that periodic plagues of folivorous chrysomelid beetles
480
checked populations of the dominant Solidago altissima (goldenrod) in an oldfield in New York state,
481
USA, and were responsible for the diversity and successional rates. The effect of vertebrate grazing in
482
increasing species diversity is well known, as shown by the rabbit exclosures erected by Tansley and
483
Adamson (1925). However, the effect there is surely that when the sward is higher, light competition is
484
more important, and especially there is more opportunity for the feedback between the outcome of
485
competition and interference ability to occur (chap. 2, sect. 2.3), allowing exclusion by interference.
486
This has nothing to do with any mechanism of coexistence between plant species.
487
3.5 Pest Pressure conclusions
488
The Pest Pressure effect seems most likely to operate via diseases. However, there can be
489
complex interactions. An example is seen on Netherlands sand dunes, on which there are relatively
490
uniform soils and clear successional sequences. The growth of the pioneer Ammophila arenaria
491
(marram grass) is impacted by nematodes and pathogens (Van der Stoel et al. 2002). The nematodes
492
reduce growth only early in succession and early in the season, and are not significant in the marked
493
decline in vigour commonly seen in older stands of A. arenaria. However, there is apparently a
494
synergistic effect between fungi, and between fungi and nematodes, which reduces A. arenaria’s
495
growth, and this is important in mixture with Festuca rubra, the next dominant in the succession
496
(de Rooij-van der Groes 1995; Van der Putten and Peters 1997). Later in the succession, in nutrient poor
497
grassland, there is often a mosaic of Festuca rubra and Carex arenaria (sand sedge). Here Olff et al.
498
(2000) discovered that each species had phases of increased and decreased vigour, replacing each other,
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 16 of 30
499
and that this process was associated with pest phases, particularly the plant-feeding nematodes. Each
500
species seems to be affected by different groups of pests, leading to the changing mosaic aspect of the
501
vegetation, which might look superficially like cyclic succession (chap. 3, sect. 4).
502
4 Circular Interference Networks
503
Interference relations between a set of species are said to be transitive if the species can be
species A
species C
species B
Fig. 4.4. A circular competitive network between three species.
504
arranged in a pecking order, such that a species higher in the order can always competitively exclude
505
one lower down. The opposite situation is the existence of circular interference networks (Fig. 4.4). If
506
such networks exist, they would contain an increase-when-rare mechanism: as species A starts to
507
displace species B, species C increases because it has high interference ability against A, but then it in
508
turn is replaced by B, completing the cycle.
509
Simple questions are not always neatly answerable. First, we note that the question can be asked
510
only in one environment, for competitive abilities will change with the environment (Keddy et al. 2000;
511
Fynn et al. 2005). Clearly they must; that is the main reason there is different vegetation in different
512
places. Second, the species that dominates the mixture will be the one with the higher relative growth
513
rate, but as interference proceeds the proportions of the species will change and as a result the relative
514
RGRs of the two species may change. Therefore, the eventual result must be judged in terms of
515
exclusion by interference (often loosely referred to as ‘competitive exclusion’). Yet we know that for a
516
variety of reasons (Chapter 4) exclusion by interference does not always occur. For these cases, the
517
question of transitivity cannot be asked.
518
Several studies have determined interference ability by comparing of species’ performances in
519
mixture with those in monoculture. Connolly (1997) pointed logical flaws in this. Correction can be
520
made for the “size-bias”, but the basic error has been comparison with a monoculture. We are tempted
521
to conclude that if species A grows more slowly in mixture than in its monoculture whilst species B
522
grows faster in mixture than in its monoculture, B has the higher interference ability. Yet Connolly’s
523
table (4.2), over the undefined period of his artificial data and assuming a starting biomass of 1, gives an
524
example where A does worse in mixture than in monoculture, and B does better in mixture than in
525
monoculture. Yet A has the faster growth rate in mixture (loge 2.77 – loge 1 = 1.02) than Species B (loge
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 17 of 30
526
2.71 – loge 1 = 1.00) and will come to exclude its competitor from the mixture (subject to the conditions
527
mentioned above). If B goes extinct it can hardly be said to have had the higher interference ability.
Table 4.2. Which species has the higher interference ability? The
starting biomass for both species was 1.00
Species
Biomass in
Biomass
monoculture
in mixture
A
3.00
decrease
2.77 Winner in mixture
B
2.64
increase
2.71
528
It turns out that what is essential in designing such an experiment is not the monocultures, as
529
many people had thought, but two harvests so RGR can be calculated. This invalidates almost all the
530
studies of transitivity done so far. All we have to do is to wait, perhaps for close to infinite time, and see
531
which species has the higher growth rate as the mixture approaches one of them. This is coming to be
532
one of those community ecology questions that are impossible to answer.
533
At the moment, it is interesting to look at the imperfect evidence available. Buss and Jackson
534
(1979) claimed several competitive cycles for coral reef sedentary organisms, as seen in static evidence
535
for overtopping. Likewise, Russ (1982) claimed non-transitive relations between species in the
536
overgrowth of sedentary marine organisms observed colonising experimental plastic sheets in the sea in
537
Australia, though no cycle can be made out of his results.
538
Turning to pure plant work, Mouquet et al. (2004) grew eight meadow herbs species in
539
replacement mixture in all possible pairs. Using relative yield (RYi,j = biomass of species i when
540
growing with species j / biomass of i in monoculture), if the species form a transitive hierarchy it should
541
be possible to arrange them so if species i is further up the hierarchy than species j, and RYi,j-RYj,I is
542
always positive. In his experiment, at both low and high density, it almost is, and with a very similar
543
order (Table 4.3).
544
Table 4.3. Competitive hierarchy from Mouquet et al. (2004), strong competitors at the top
High density
Holcus lanatus
Rumex acetosella
Cerastium glomeratum
Anthoxanthum odoratum
Festuca rubra
Arabidopsis thaliana
Lamium pupureum
Veronica arvensis
545
Low density
Holcus lanatus
Rumex acetosella
Cerastium glomeratum
Anthoxanthum odoratum
Festuca rubra
Lamium pupureum
Arabidopsis thaliana
Veronica arvensis
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 18 of 30
546
At each density, there is one negative RY1,2-RY2,1 indicating a conflict with the hierarchy, it is between
547
species not contiguous in the hierarchy, but it is of size -0.05 or -0.06 which is clearly within the
548
experimental error.
549
A study that returned a clear answer to the question of transitivity is that of Roxburgh and
550
Wilson (2000a). It relates to a real community, since the seven species used in the interference
551
experiment were taken from that community, the University of Otago Botany Lawn, grown in lawn soil
552
in boxes placed near the lawn. The use of 10 replicates in careful experimental conditions allowed
553
significance tests. The seven species could be arranged in a hierarchy to which all significant
554
competitive relations conformed, i.e. if species X is higher in the hierarchy and species Y lower, then
555
the suppressive effect of X on Y is greater than that of Y on X (Fig. 4.5). In fact, relations between all
556
pairs of species, significant or not, were compatible with the hierarchy.
557
558
Fig. 4.5: Competitive relations in seven species from the University of Otago Botany Lawn. From
559
Roxburgh and Wilson (2000a).
560
The experimental design of Keddy et al. (1998) comprised planting a number of ‘wetland’
561
species into a number of swards of wetland species. They report results for 18 species planted into five
562
swards. The 18 species tended to respond similarly to different swards, e.g. Kendal’s coefficient of
563
concordance took a rank of 0.7 (1.0 = complete agreement as to which target suffered more/less), highly
564
significant. Some of the variation in invader/sward combinations could be due to experimental error (no
565
replication was possible), but some results are impressive, e.g. the rank of Carex crinita (sedge) varied
566
only from 14 to 17 across the 5 swards (18=suppressed most), and Lythrum salicaria (purple loosestrife)
567
varied from 4 to 7 (1= suppressed least).
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 19 of 30
568
In a different approach, Silvertown et al. (1992) used data from an experiment where several
569
species had been planted in adjacent hexagons, and invasion between hexagons recorded. Examining the
570
difference between the invasion of Species A into Species B and that of Species B into Species A,
571
replacement rates could be calculated. A pecking order can be formed from these results (Fig. 4.6), with
572
no discrepancies (though L. perenne (ryegrass) and C. cristatus (dog’s tail) could equally well exchange
573
positions). There are qualitative discrepancies. Since H. lanatus (Yorkshire fog) can invade P. trivialis
574
(A) and P. trivialis can strongly invade L. perenne (B), the expectation would be that H. lanatus would
575
be able to invade L. perenne even more strongly, but in fact their invasion rates are exactly balanced
576
(C). Moreover, although the species A. stolonifera (creeping bent) at the top of the order can invade C.
577
cristatus at the bottom, the rate of replacement is less than for other pairs (D).
Agrostis stolonifera
Holcus lanatus
A
Poa trivialis
D
B
C
Lolium perenne
Cynosurus cristatus
578
579
Key:
Strong (> 0.2) difference in invasion rates
580
Weak-moderate difference in invasion rates
581
Invasion rates equal (i.e. no net invasion)
582
Fig. 4.6. The competitive hierarchy from invasion rates in data of Silvertown et al. (1992).
583
In a similar experiment Silvertown et al. (1994) used only four species, so there was less
584
opportunity for intransitivity, but in any case there was none in any of the four grazing treatments (Table
585
4.4).
586
Table 4.4: Competitive hierarchy of four species in four treatments in Silvertown et al. (1994).
587
Summer sward
grazing height
Winter and
spring
Invasion ability: greater → lesser
3 cm
Grazed
Lolium perenne → Festuca rubra → Schedonorus phoenix → Poa pratensis
3 cm
Ungrazed
Festuca rubra → Lolium perenne → Poa pratensis → Schedonorus phoenix
9 cm
Grazed
Festuca rubra → Lolium perenne → Schedonorus phoenix → Poa pratensis
9 cm
Ungrazed
Lolium perenne → Festuca rubra → Poa pratensis → Schedonorus phoenix
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 20 of 30
(1)
A is taller
than B and
shades B
out
C produces an
allelopathic
chemical,
toxic to A
C
C is shadetolerant, and
scavenges
nitrogen
A
o
A
Tree A is taller
than shrub B
and
shades B
out
grass C lowers the
temperature, and
suppresses
seedlings of A
B
B is taller
than C
(3)
C
(2)
A
B
C
shrub B shades out grass
C, and is not affected by
lower temperature
(4)
A is taller
than B and
shades B
out
B is taller than C,
and fixes N
B
A
C with A is taller
than it, and
shades it out
C
B
B with C is taller than it,
and shades C out
588
.
589
Fig. 4.7. Possible causes of intransitivity between three species: A, B and C.
590
A is taller
than B and
shades B
out
It’s interesting to wonder what ecological processes would give rise to intransitivity (Fig. 4.7). In
591
scenario ‘1’, we use an allelopathic chemical produced only by C and toxic only to A. This works, but
592
species-specific allelopathy is rather like Getafix’s magic potions in the Asterix books: it can
593
perform/explain any wonder. Scenario ‘2’ is similar, except that the third factor is lower temperature
594
(Ball et al. 2002) rather than a toxin. In ‘3’, we have to ask why C can suppress A; presumably the
595
shade-tolerance of C minimises the competition for light, so competition for N becomes important, and
596
C has the lower Tilman R* (chapt. 6, sect. 7.1). Why cannot C suppress B? Perhaps because it is shorter
597
and so cannot compete for light, and its low R* for N does not help because B can fix N. Does this
598
work? Probably. In all three cases, not all pairs are interfering using the same resource/factor. Could we
599
envisage a 3-species solution using competition for light (‘4’)? How can there be heights of A>B, B>C
600
and C>A? Differential plasticity allows such magic: in this case probably by red:far-red effects (chap. 2,
601
sect. 2.6). However, we are again introducing a second factor: light spectrum in addition to light
602
intensity. All this is rather convoluted, which suggests that intransitivity will not be the norm.
603
The evidence is that circular interference networks are uncommon. They have not been observed
604
in plants. In retrospect we should have expected that, because we had not thought what mechanisms
605
would cause them, and such mechanisms are difficult to envisage. This is almost certainly not an
606
important mechanism of coexistence.
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 21 of 30
607
5 Allogenic Disturbance (disrupting growth, mainly mechanically)
608
Disturbance can have the same effect as climate variation (Roxburgh et al. 2004), but the true
609
Intermediate-timescale disturbance mechanism is a patch mechanism: within an area there are patches
610
of different time since disturbance, with different suites of species (J.B. Wilson 1994a). This gives a
611
successional mosaic. Whether this comprises coexistence depends on the scale at which the system is
612
viewed. Coexistence is seen only when considering a scale that is larger than the size of a disturbance
613
patch, so that it includes patches of differing time since disturbance – newly-disturbed versus recovered.
614
The different patch types are different beta niches, but on a small scale, and a species specialising in a
615
particular patch type will increase when rare because it will have more of its specific resources
616
available. In a sense, Allogenic Disturbance should not be counted as a mechanism of coexistence; we
617
do so here because it is so frequently seen as one, because of the impossibility of defining the target
618
scale, and because disturbances occur on all scales so that however small a scale we examine there will
619
still be disturbances within it. The disturbance does need to be sufficiently frequent that each patch will
620
usually include patches at various stages of recovery, or the mechanism will not operate.
621
All types of allogenic disturbances happen and it is not always easy to separate disturbance from
622
climatic stress, i.e. the environmental fluctuation discussed above (section 2). The characteristics of
623
disturbance are: (1) Plants not only fail to reproduce but are killed, at least above ground. (2) Most
624
species are killed, not just those that cannot tolerate a particular stress. (3) The event is sudden. (4) The
625
environmental effect is temporary, i.e. it is a pulse perturbation, so the original species can re-establish
626
the composition of the patch. However, the real difference is that Allogenic Disturbance is a between-
627
patch mechanism. Disturbance is common, creating gaps over the landscape at a range of scales from
628
meteor hits (many km2) to worm casts (about 0.03 m). Fossorial rodents, ants and termites act at scales
629
which can be important for individual plants. A good example is that of McGinley et al.’s (1994)
630
description of enriched harvester ant mounds in western Texas. It is possible that much of the variation
631
seen in communities is due to old disturbances, where vegetation cover has been regained, and obvious
632
pioneers have been eliminated, but differences in species composition remain. Perhaps we do not realise
633
this because ecologists fail to recognise mid-succession species as being such (Veblen and Stewart
634
1982).
635
The Allogenic Disturbance mechanism assumes that there are distinct pioneer and climax
636
species, i.e. r and K, R and C-S. However, we are talking of secondary succession, and cannot assume
637
this. Peterson and Pickett (1995) found that after windthrow disturbance in a North American conifer /
638
deciduous forest some species regenerated by the germination of seed and some from already-present
639
seedlings, but pioneer shade-intolerant species were sparse, apparently due to a lack of propagule input.
640
Autosuccession, in which the climax species immediately re-establishes after a disturbance, is known
641
from mesic areas such as after windthrow in temperate Nothofagus rainforest in New Zealand
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 22 of 30
642
(Cockayne 1926), but it is specially found under environmental stress, as predicted by C-S-R theory
643
(chap. 6, sect. 6.7 below; J.B. Wilson and Lee 2000). We might expect that the greater species richness
644
in tropical rain forests would include a good number of gap specialists. Indeed, Hubbell (2005)
645
demonstrated for Barro Colorado Island tropical rainforest a close negative correlation among species
646
between survival rate in shade and growth rate in full light (in gaps), though admitting there were rather
647
few gap species and their abundance was low. Wright et al. (2003) found that there was a continuous
648
distribution of gap-colonising species and those that avoided gaps in Barro Colorado Island, Panama,
649
but that the majority were rather indiscriminate. Similarly, Lieberman et al. (1995) found that 87 % of
650
the tree species in Costa Rican tropical forest had no significant canopy-gap / matrix specialization.
651
Poorter et al. (2005) found that only one of 47 species in a Liberian tropical rainforest was a shade
652
species for its whole life, and only one a light species for its whole life. It is clear that most species are
653
intermediate in this respect. This suggests that Allogenic Disturbance may not be an important
654
mechanism of coexistence in the very biome where we tend to envisage it. Yet in temperate forests there
655
may be greater opportunities for it to increase species richness: Poulson and Platt (1996) demonstrated
656
in Michigan that the size of the gap affects the species re-establishing, such that single treefalls favoured
657
Fagus grandifolia (American beech) but multiple fall gaps favoured Acer saccarum (sugar maple). This
658
is not a question of gap versus non-gap, but also of differences between different sorts of gap.
659
Gaps are by no means restricted to forests. Grubb (1982) suggested that in roadside communities
660
around Cambridge, England, the climax dominant amongst the grasses was Arrhenatherum elatius (oat
661
grass), but Dactylis glomerata (cocksfoot) and Plantago lanceolata (ribwort plantain) retained their
662
place in the community by being the first to invade small gaps.
663
6 Interference/dispersal Tradeoff
664
This concept originated simultaneously with Skellam (1951) and Hutchinson (1951). It has been
665
known under a variety of names (J.B. Wilson 1990), including ‘Life History Differences’ and the
666
endearing if not entirely accurate ‘Musical Chairs’ (Crawley 1986). Consider a model in which two
667
annual species occupy single-plant safe sites. Species C is the better competitor, and eliminates the
668
weaker competitor D if it reaches a site, but it has less efficient reproduction/dispersal than species D
669
and therefore fails to reach some sites. Species D has better dispersal and is therefore available to
670
colonise most of the sites that C has not reached. If C becomes sparse, there are many empty sites for its
671
offspring to occupy and its population growth rate increases; similarly if D becomes sparse, there are
672
many sites left over by C for it to occupy. This is increase-when-rare. The mechanism can be
673
distinguished from (‘1’) Niche differentiation in that no differences between species in resource use are
674
required. It can be distinguished from (‘5’) Allogenic Disturbance in that: (a) the gaps are caused by
675
monocarpic or seasonal death, not necessarily by external disturbance, though that is possible, and (b)
676
species C is limited only by dispersal, not by its ability to tolerate the environment of the gap. It can be
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 23 of 30
677
distinguished from (‘9’) Equal Chance in that, though there is a random element, it acts via dispersal;
678
the interference abilities of the two species are very different.
679
There have been many mathematical models of the mechanism, e.g. Levins and Culver (1971),
680
Nee and May (1992) and Tilman (1994). There is an assumption of a negative correlation, due to a
681
trade-off, between interference ability and dispersal ability, but Ehrlén and van Groenendael (1998)
682
surveyed the literature and found that this was common. Turnbull et al. (1999) demonstrated the
683
mechanism experimentally by sowing seven species from a limestone grassland, ranging from a seed
684
mass of 0.013 to 0.16, back into that grassland. When the seeds were sown at a high density, 83 % of the
685
resulting plants were from the three species with the largest seeds, but when a low density was sown this
686
percentage was reduced to 49 %. This is entirely compatible with the Interference/dispersal Tradeoff
687
mechanism: when there were enough seeds to reach almost all microsites the three big-seeded, strong
688
competitors occupied them, but when fewer seeds were sown there were microsites not occupied by the
689
big three, which the light seeded, probably well-dispersed species could occupy. The unlikely
690
Interference/dispersal Tradeoff theory is proved.
691
7 Initial Patch Composition
692
The coexistence model of Levin (1974) is that two species occupy small, transient patches. Some
693
patches will by chance have more individuals of one species than the other. The species in the majority
694
will suppress the other in that patch if intra-specific interference is less than inter-specific interference.
695
The latter condition is beloved of ecological modellers, but it seems unlikely in the real world. It would
696
be possible with mutual species-specific allelopathy: the Getafix potions of community ecology. We do
697
not believe this model can apply to plants (or at all).
698
8 Cyclic Succession: movement of community phases
699
This topic was covered in chapter 3, section 4. The increase-when-rare mechanism is similar to
700
that of (‘4’) Circular Interference Networks. The latter are between individual species whereas cyclic
701
succession involves the whole community, though in many of Watt’s (1947) examples the community
702
comprises one species. Cyclic succession involves reaction, but then interference also involves
703
environmental modification, be it more temporary. There could be cyclic succession between just two
704
phases, whereas there cannot logically be a circular interference network with fewer than three species.
705
Autoallelopathy, for example, would satisfy our criterion for increase-when-rare because a
706
species that is sparse finds less of its self-toxin in the soil, but there must be some kind of reaction
707
involved, giving abundance-dependence. A mosaic arises because a phase of the cycle that is replaced at
708
one point appears elsewhere; we therefore count it as a mechanism that uses movement to escape
709
exclusion by interference. This also means that the mechanism is scale-dependent: the scale examined
710
has to be one that includes patches of the mosaic in different phases.
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 24 of 30
711
712
9 Equal Chance: neutrality
It is a longstanding idea that there is an element of chance in which species occurs at a spot
713
(Lippmaa 1939). This has been especially invoked for tropical rain forests (e.g. Schulz 1960; Hubbell
714
and Foster 1986). Sale (1977) described it as a ‘lottery’ and Connell (1978) formally put it forward as a
715
mechanism of coexistence, the ‘Equal Chance’ mechanism. In this section, when we speak of chance,
716
we refer to processes such as dispersal that are so complex as to be unpredictable in practice, combined
717
with equally unpredictable climatic and catastrophic disturbance events. It is impossible to prove the
718
operation of chance, but some have implicated it.
719
Equal Chance means that any one of a number of species is equally likely to occupy and
720
pre-empt by reaction a particular microsite. One cause would be that the probability of a disseminule
721
reaching a site is proportional to its abundance. Then, dispersal would determine which species occupies
722
a particular site (Schulz 1960). In New Zealand, Veblen and Stewart (1980) used this as an explanation
723
for the colonisation of canopy gaps by either Dacrydium cupressinum (rimu), Weinmannia racemosa
724
(kamahi) or Metrosideros umbellata (southern rata) as a function of seed/seedling availability, mast
725
seeding and the ability of many New Zealand tree species to remain as suppressed seedlings. Equal
726
interference abilities are likely to be invoked: the ‘Equivalence of Competitors’ concept of Goldberg
727
and Werner (1983) that often interference is intense, but many species are similar in their interference
728
ability. Sometimes the outcome of interference can be established at the seedling stage, since when
729
competition is for light and therefore cumulative, the first plant to establish will exclude others, a type
730
of inertia (chap. 2, sect. 2.3). This again invokes random dispersal. Alternatively, both the probabilities
731
of ecesis and their interference abilities might be different between species, but the two balance.
732
The Equal Chance concept would result in variation in the species composition of communities
733
which it was impossible to correlate with any environmental factor, present or past. Some have used this
734
kind of negative result as evidence of chance. McCune and Allen (1985) in forests in Montana, USA,
735
R.B. Allen and Peet (1990) in forests in Colorado, USA, and Kazmierczak et al. (1995) in kettle-holes in
736
Poland found only weak correlation between species composition and the environment and invoked
737
chance. In such work, the weak correlation could be because there were important environmental factors
738
that had not been measured, or because some factors measured gave a non-linear response that the
739
analysis could not cope with. The Equal Chance hypothesis, used as an excuse for failing to find
740
vegetation/environment correlations is the last resort of the scoundrel. Lavorel and Lebreton (1992)
741
compared the composition of the vegetation with that of the seed pool in fields from southern France,
742
and took the similarity as evidence of a random draw from the seed pool. This is also doubtful evidence;
743
it could equally well be caused by determinism.
744
745
The most well-known invocation of chance is the Island Biogeography model of MacArthur and
Wilson (1963), based on probabilistic immigration and extinction. However, Kelly et al. (1989) and
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 25 of 30
746
Tangney et al. (1990) could find little evidence for its operation in Lake Manapouri islands. On islands,
747
a direct test of determinacy v. chance (assuming that incidence functions are not important) is available
748
in a test for nesting. On the other hand, J.B. Wilson (1988d) found plant species nesting among these
749
islands to be significant, but far from complete. His analyses pointed to habitat control rather than
750
chance, at least for native species. J.B. Wilson et al. (1992a) sampled the algal flora of intertidal rock
751
pools, selected for habitat uniformity within a limited area, and analysed as virtual islands. The
752
distribution of species agreed closely with that expected at random, whether examined by the
753
distribution of associations, by nesting, by chequerboarding or by incidence functions. The simplest
754
explanation is that differences in specific composition between the pools are caused by chance, but that
755
is no proof; it is a minimalist default. The best example of chance – no difference between species if one
756
can ever have an example of no difference – is from Munday (2004), who investigated two small
757
congeneric coral-reef fish species, where there was evidence for interference, in field removal
758
experiments, and in lab colonisation. However, in none of these experiments, nor in field distribution,
759
was there any evidence of niche differentiation.
760
Even Equal Chance’s strongest advocates have been equivocal. Hubbell (2005), having
761
emphasised differences in niche between Barro Colorado Island tropical rain forest species, eventually
762
attributed coexistence to dispersal and recruitment limitation. This is in effect a resort to Equal Chance.
763
However, he immediately discussed negative abundance-dependence, which is stabilising, not
764
equalising. The Equal Chance mechanism is the equalising mechanism par excellence, and should be
765
seen as no more than that.
766
10 Inertia
767
Inertia is another type of equalising mechanism, slowing exclusion by interference and possibly
768
allowing stabilising mechanisms to operate.
769
10.1 Temporal Inertia
770
Temporal Inertia (Cowles 1901) can be an individual or population effect, effectively the same
771
because the ‘individual’ concept is not meaningful for plants. Trees stand where they stand and cause
772
inertia. If, when a tree fell over, there were a tendency for the niche it had constructed to favour its own
773
juveniles in the canopy gap created, this would represent a small-scale dispersal switch. This could
774
constitute inertia, slowing the ingress of a superior competitor. We mentioned above that Dalling et al.
775
(1998) found seedlings on Barro Colorado Island to be denser near to a conspecific adult. Species
776
differed little in the correlation of their growth rate with light intensity and they declared that differential
777
responses to soil and topography were rare. This left them to speculate that there was dispersal
778
limitation, as supported by the correlation between parent and juvenile being weaker for species with
779
small disseminules. This is inertia due to dispersal limitation. Annuals are a conspicuous life form in
780
arid climates where the rainfall is highly erratic and form a long-lived seed bank. This seed bank gives
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 26 of 30
781
inertia as well as a storage effect. A population with a seed bank has similarities to a tree, except that
782
this is multi-generation inertia. The decade-long dominance phases, with smooth increases and
783
decreases, that Watt (1981) found in the Breckland may owe something to inertia. A more active
784
mechanism of inertia would be delay in vegetation change caused by a switch (chap. 3, sect. 5.3). Inertia
785
caused by the length of life of trees must be common. Abrams and Scott (1989) describe a situation
786
where, until disturbance occurs, early-successional trees dominate the canopy with young plants of later
787
successional stages beneath them, and their model diagram shows the high species richness resulting at
788
this stage.
789
These situations are obvious in dryland vegetation where the rainfall is erratic and temperatures
790
are high. For many succulents, sufficient rainfall for the development of surface root hairs is itself a rare
791
event, yet by means of massive storage, CAM photosynthesis and often a high albedo, they can
792
withstand years of unavailable moisture. The establishment of such succulents can be a very rare event,
793
as exemplified by Agave macroacantha in Mexico (Arizaga and Ezcurra 2002). Minor rain events may
794
allow annuals to grow, while deep-rooted shrubs and trees maintain contact with a deep water resource
795
giving the appearance of a plant community. Clarke (2002) described a similar situation with woody
796
dryland vegetation in southwestern Australia, where no natural recruitment of shrubs was observed over
797
five years. However, the rare event required to cause the state change could be a different grazing
798
regime, as Prins and van der Jeugd (1993) found in Tanzania. Two pandemics in the herbivores in 1880
799
(rinderpest) and 1961 (anthrax) temporarily reduced browsing and allowed even-aged stands of Acacia
800
tortilis (umbrella thorn) to establish. These are now a conspicuous and apparently integral part of the
801
vegetation of national parks in the area, yet are present through inertia, not as maintained populations.
802
Here, the state change was anthropogenic, but a similar situation could occur naturally. Inertia may not
803
apply to all the species in a community, since many contain species that differ markedly in survival and
804
establishment probabilities. Extremely long-lived individuals of slow growth exist alongside perennials
805
with lifespans shorter by at least one order of magnitude. The long-lived individuals can establish only
806
during a rare event, which could be a disturbance such as flood, a 1/100 yr wet season. The probability
807
of such an event occurring in any one year is very low and does not change from year to year. Thus their
808
occurrence is stochastic yet within the time scale of very long-lived plants.
809
We still ask what the original coexistence was due to: if there is no coexistence, inertia cannot
810
prolong it.
811
10.2 Spatial Inertia: aggregation
812
Spatial aggregation of the plants of a species also gives inertia, delaying exclusion by
813
interference since it occurs only at patch boundaries. Presumably the aggregation was established due to
814
dispersal processes, an ‘ecological founder effect’. Stoll and Prati (2001) demonstrated beautifully the
815
slowing of exclusion by interference by experimental aggregation. Amongst four annuals they found
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 27 of 30
816
that the species with least interference ability (Cardamine hirsuta, bitter-cress) decreased over the
817
experiment to 6 % of the monoculture in a random arrangement but only to 26 % in an aggregated
818
arrangement. The species with lowest interference ability (Stellaria media, chickweed) increased its
819
biomass to 324 % of the monoculture in the random but to only 239 % in the aggregated1. This would
820
be a most potent mechanism for delaying exclusion by interference of a subservient species. Rebele
821
(2000) found a similar, but very slight, effect in an outdoor mesocosm experiment using mixtures of
822
Calamagrostis epigejos (reed) and Solidago canadensis (goldenrod).
823
Thórhallsdóttir (1990) had planted outdoors a hexagonal grid of adjacent plots. Each plot
824
contained one of five meadow species: Agrostis stolonifera (creeping bent), Holcus lanatus (Yorkshire
825
fog), Cynosurus cristatus, Poa trivialis (meadow grass), Lolium perenne (ryegrass) and Trifolium
826
repens (white clover). Silvertown et al. (1992) ran simulations to see in retrospect what effect
827
aggregation would have, given the invasion rates that Thórhallsdóttir found between the pairs of grass
828
species. After 50 time periods when the species were intermixed in a random pattern, the weakest
829
competitor Lolium perenne had almost disappeared (reduced from 20 % to 1 %), but with the species
830
‘planted’ in bands, depending on the order of the species in the bands, it decreased only to 9 %, stayed
831
at 20 % or even increased slightly to 21 %.
832
Aggregation might also delay exclusion by interference via effects on herbivory (Parmesan
833
2000), fire spread (Hochberg et al. 1994) and other environmental factors.
834
11 Coevolution of Similar Interference Ability
835
Aarssen (1983) suggested that in a mixture of two species stronger selection pressure on the one
836
with lower interference ability would cause it to become the stronger competitor of the two, “Superiority
837
in competition therefore alternates between … members of the two populations”. He later (1989)
838
produced some evidenced for this: over two generations the interference ability of Senecio vulgaris
839
(groundsel) increased relative to a standard genotype of Phleum pratense (Timothy grass) with which it
840
was growing. Selection can result in small-scale genetic change in populations, as apparently occurred
841
in Trifolium repens (white clover) associated with different ecotypes of Lolium perenne (ryegrass) in the
842
pastures that Lüscher et al. (1992) investigated. However, neither this study nor that of McNeilly and
843
Roose (1996) could find evidence of co-adaptation between neighbouring ecotypes of associated L.
844
perenne. Eventual ecotypic evolution in response to neighbours would be expected, and has
845
occasionally been demonstrated (Martin and Harding 1981). However, Aarssen’s proposal is
846
unbelievable because it involves continual increases in interference ability, as Aarssen (1985) has since
847
concluded. The plastic response to interference (chap. 2, sect. 2.2) can also give a buffering effect.
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 28 of 30
848
849
12 Spatial Mass Effect (vicinism)
The Spatial Mass Effect refers to the maintenance of a population of a species by constant
850
immigration into a patch where the species cannot otherwise maintain itself (Zonneveld 1995). It has
851
been called the sink effect. The immigration could be by seeds, or in theory by rhizomes or stolons.
852
Populus tremuloides and related species (aspen) produce root suckers (Barnes 1966) and these can
853
appear beyond the canopy of the tree where there is no chance that they will survive to be self-
854
supporting, let alone sexually reproductive, for example in a lawn. Seed immigration is the most
855
common but difficult to demonstrate. It is difficult enough to monitor occasional seeds blowing in, and
856
even more difficult to demonstrate that the population into which they are blowing would have RGR <
857
0.0 without that subsidy. Snyder and Chesson (2004) have applied the concepts of the ‘storage effect’
858
and non-linear dynamics to coexistence between species that have different tradeoffs of interference
859
versus fecundity+dispersal. Their model has Spatial Mass Effect, though also elements of (‘6’)
860
Interference/dispersal Tradeoff. The effect clearly maintains populations that are not susceptible to
861
considerations of abundance-dependence or increase when rare, the stabilising mechanisms we require
862
here, yet it can maintain coexistence indefinitely.
863
The Spatial Mass Effect has rarely been quantified. Kunin (1998) examined boundaries between
864
plots with different fertiliser treatment in the 150-year old Park Grass Experiment. There was a very
865
sharp pH change, within 50 cm of the boundary. Although there were many exceptions, the majority of
866
plots examined (34 out of 51 non-zero, 2-tailed p = 0.024) showed higher species richness towards the
867
boundary. The effect was seen especially where the two adjacent plots differed more in species
868
composition. The Spatial Mass Effect can be seen clearly in extreme cases where the recipient (sink)
869
population does not reproduce at all, like the 13 species of angiosperm that grow in the Lost World
870
Cavern, northern North Island, NZ, without any of them ever setting seed (de Lange and Stockley
871
1987). Studying an Argentinian steppe with the (palatable) grass Bromus pictus amongst tussocks of
872
unpalatable grasses Stipa spp. and Poa ligularis, Oesterheld and Oyarzábal (2004) found more B. pictus
873
in the upwind part of a grazing exclosure, showing that a seed subsidy was arriving from the grazed
874
area. The tussocks outcompeted the B. pictus when ungrazed, reducing the local seed output in the
875
exclosure. This situation may be the commonest way in which the spatial mass effect operates to
876
maintain species metapopulations.
877
13 Conclusion
878
We believe our review covers all the mechanisms by which species can coexist in stable
879
mixtures. Chesson’s terminology of stabilising versus equalising mechanisms is useful and important. It
880
has focussed attention on the fact that some proposed mechanisms of ‘coexistence’ do not, in fact, cause
881
long-term coexistence. It has also highlighted what few had recognised, that even though the equalising
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 29 of 30
882
mechanisms cannot on their own cause stable coexistence between two species, they can reduce the
883
difference in interference ability between species to the extent that a stabilising mechanism can operate.
884
We must speculate on the importance of each mechanism in order to build up in our minds a
885
vision of the plant community. The overwhelming reason for species coexistence is Alpha-niche
886
Differentiation. Environmental Fluctuation is probably important. It can be seen as niche differentiation
887
in time, but with special restrictions on when it can operate. In seasonal climates, local environment can
888
vary enormously, both stochastically and predictably. Each of the ecosystem attributes enumerated by
889
Reichle et al. (1975; see also chap. 1, sect. 1 above) must change during the year: the energy base
890
(affected by irradiance), the reservoir of energy, nutrient cycling (through mineralisation rates) and rate
891
regulation (temperature, water availability, herbivory). It seems that the available states of these
892
variates, factorially combined, should allow for the coexistence of a very large number of plant species.
893
Pest Pressure may be important; Gillett (1962) suggested that it is the major mechanism, but that
894
remains to be proved. The Spatial Mass Effect must be very common. Disturbance is clearly common,
895
and has potential to allow co-existence; surely all communities are successional mosaics. We earlier
896
discussed autogenic disturbance. It could have been listed as a separate mechanism here, it could have
897
been merged with Allogenic Disturbance since many disturbances are partly allogenic and partly
898
autogenic, or it could have been included with Cyclic Succession since it will often be a component.
899
Other mechanisms are probably of more minor importance. For example, circular interference
900
networks are an attractive idea, but remain undiscovered. Cyclic succession is more believable and from
901
time to time fashionable, but seldom observed and even more rarely proven to take place.
902
Autoallelopathy may be widespread but the soil is an intractable and infinitely complex medium where
903
clear chemical pathways and effects are difficult to prove.
904
905
Based on the evidence derived from the present literature, we list the mechanisms below in
increasing order of importance:
906
Initial Patch Composition (7)
907
Co-Evolution of Similar Interference Ability (11)
908
Equal Chance (9)
909
Circular Interference Networks (4)
910
Cyclic Succession (8)
911
Temporal and Spatial Inertia (10)
912
Interference/Dispersal Tradeoffs (6)
913
Allogenic Disturbance (5)
914
Spatial Mass Effect (12)
915
Pest Pressure (3)
916
Environmental Fluctuation (2)
J.B. Wilson & Agnew, chapter 4, Species coexistence, page 30 of 30
917
Alpha-niche Differentiation (1)
918
In a changing, disturbed world it will be increasingly difficult to separate stabilising mechanisms from
919
equalising ones. The temporal turnover of species in communities depends on some species
920
disappearing, others invading, so that many species in a community may be present by courtesy of one
921
of the equalising mechanisms and will ultimately be doomed. The multiplicity of possibilities for
922
coexistence should allow the coexistence of a very large number of plant species. The question then
923
becomes: “Why are there so few species in most habitats?”.
924
A plant, however, is sedentary and extends over a spatial volume, necessarily exposed to wide
925
range of environmental conditions. It therefore cannot be confined to a precisely-defined niche. It is the
926
interplay between the potential for high plant diversity in restricted niches and the necessity for plants to
927
tolerate a wide range of environments that encourages us to look for patterns in plant communities. If
928
adaptations to available niches were most of the reason for every species’ occurrence, our enquiry in
929
this book would be less interesting.
930
Footnotes
931
1
932
‘sparse’ would be a better term than ‘rare’, but ‘increase when rare’ is ensconced in the literature, and
so we use it here.
933
TABLES, ILLUSTRATIONS and PLATES
934
Table 4.1: Some Examples of monospecific stands. We exclude monospecificity in a single stratum or
935
936
guild of vegetation, such as a tree species or understorey species.
Table 4.2: Which species has the higher interference ability? The starting biomass for both species was
937
1.00
938
Table 4.3: Competitive hierarchy from Mouquet et al. (2004), strong competitors at the top
939
Table 4.4: Competitive hierarchy of four species in four treatments in Silvertown et al. (1994).
940
Fig. 4.1: The MacArthur and Levins (1967) concept of niche separation along a gradient.
941
Fig. 4.2: Pairs of two species showing relative non-linearity.
942
Fig. 4.3: The effect of interference intensity and environmental favourability on RGR.
943
Fig. 4.4: A circular interference network between three species.
944
Fig. 4.5: Competitive relations in seven species from the University of Otago Botany Lawn. From
945
Roxburgh and Wilson (2000a).
946
Fig. 4.6. The competitive hierarchy from invasion rates in data of Silvertown et al. (1992).
947
Fig. 4.7. Possible causes of intransitivity between three species: A, B and C.
948
Plate 4.1: A monospecific community: Cladium mariscus stand.
949
Plate 4.2: Tyria jacobaeae (cinnabar moth) on Senecio jacobaea (ragwort)
1
all this is in the high-density treatment