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The nature of the plant community: a reductionist view 1 2 3 J. Bastow Wilson Botany Department, University of Otago, P.O. Box 56, Dunedin, New Zealand. 4 5 Andrew D.Q. Agnew Institute of Biological Sciences, University of Wales Aberystwyth, SY23 3DA, U.K. 6 Chapter 3: Community-level processes 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 1 2 3 4 5 6 7 8 Introduction ......................................................................................................................................... 1 Allogenic change: including secular change ....................................................................................... 3 Directional autogenic change .............................................................................................................. 6 3.1 Environmental change................................................................................................................. 7 3.2 Pioneer species ............................................................................................................................ 8 3.3 Decrease in biomass .................................................................................................................... 9 Cyclic succession: in fact or fiction .................................................................................................. 10 Switches: the positive feedback processes ........................................................................................ 13 5.1 The concept of the switch ......................................................................................................... 13 5.2 Types of switch ......................................................................................................................... 14 5.3 Outcomes of switches ............................................................................................................... 14 5.4 Mediating agents ....................................................................................................................... 15 5.4.A Mechanisms involving regional climate ........................................................................... 16 5.4.B Mechanisms involving water ............................................................................................ 16 5.4.C Mechanisms involving other aspects of microclimate ...................................................... 18 5.4.D Mechanisms involving physical substrate......................................................................... 18 5.4.E Mechanisms involving soil chemistry............................................................................... 19 5.4.F Mechanisms involving disturbance ................................................................................... 22 5.4.G Mechanisms involving heterotrophs ................................................................................. 23 5.5 Switch evolution ....................................................................................................................... 25 5.6 Alternative stable states ............................................................................................................ 26 5.7 Negative feedback ..................................................................................................................... 32 Diversity → productivity .................................................................................................................. 32 Stability ............................................................................................................................................. 35 7.1 Concept ..................................................................................................................................... 35 7.2 Reliability (constancy) .............................................................................................................. 36 7.3 Stability sensu stricto (Liapunov stability) ............................................................................... 37 7.4 Resistance to abiotic perturbation ............................................................................................. 40 7.5 Resistance to invasion ............................................................................................................... 41 7.6 Resilience .................................................................................................................................. 44 Conclusion ........................................................................................................................................ 46 38 39 40 1 Introduction The last chapter described the ways in which individuals of differing species can interact within 41 the space of a plant community. Most are based on reaction, which Clements (1916) defined as “The 42 effect which a plant or a community exerts on its habitat”. Admitting that classification was difficult, he 43 listed about 20 types of reaction. All plants and animals must alter their immediate surroundings (chapt. 44 2 above). Plants especially do this because of their bulk (biomass) and their necessity to produce litter, Wilson & Agnew, chapter 3, Community processes, page 2 of 50 45 leading to modifications of local light availability, climatic regimes, environmental chemistry and 46 geomorphological processes. This also affects the populations of herbivores and thus the system as a 47 whole. These interactions are the currency of community processes that can have only two outcomes: 48 stability or change. Change should be the expectation, firstly because the environment is never without 49 temporal variation and secondly because the reaction of the plants alters their own environmental 50 conditions. However, stabilising forces do exist, and aspects of stability can certainly be seen, even if 51 strict stability has never been shown. This chapter examines the mechanisms of community change and 52 stability. 53 The positive and negative interactions that we discussed in chapter 2 have counterparts at the 54 level of the entire plant community (Box 3.1), and these are the topic of this chapter, we consider the 55 implications of these effects for the entire plant community. The basic contrast is between a 56 species/community changing the environment to its disbenefit (facilitation, the concept though not the 57 term of Clements 1916) or benefit (switch: Odum 1971; Wilson and Agnew 1992). We therefore link 58 most vegetation change to facilitation and switches in this chapter. In either case, eventually there can 59 be negative feedback, causing stability. We also consider the cyclic succession of Watt (1947). 60 None of these processes can take place on a uniform stage. Plant communities can never be 61 homogeneous both because of the unique reaction of each species and also because each has unique 62 demographic processes controlling the arrangement of individuals. Peter Greig-Smith (1957) first used 63 sampling and analysis to show that all species’ individual occurrences had a non-random pattern. One 64 cause for this is the demographic (dispersal) behaviour and morphology of each species. In one 65 example, Kershaw (1964) examined North Wales pastures where Agrostis capillaris (= A. tenuis, bent) 66 showed no clumping to the naked eye, but where the tillers proved on sampling to have clumping 67 because disparate rhizomes upturned when nearing each other, forming patches of denser culms 68 (Kershaw, 1964). Patterns resulting from vegetative demography are clearly seen at the level of 69 individual clones in Harberd (1962)(chapt. 1, sect. 1.1 above) and in his similar results in Festuca rubra, 70 Trifolium repens and Holcus mollis. Using Mark Hill’s (1973) algebraic clarification, Agnew & Gitay 71 (1990) found that species and soil nutrients were similarly patterned in visually uniform dune slacks, 72 and that this pattern became more pronounced as species number increased. This is a general quality of 73 all plant communities and has implications for community function. 74 Wilson & Agnew, chapter 3, Community processes, page 3 of 50 species/guild X gives an absolute or relative advantage to itself = switch species/guild X gives a relative disadvantage to itself: the effect disappears at low density of X (negative feedback) = stability the effect is density-independent: facilitation and/or autointerference = succession 75 76 Box 3.1: Types of community interaction. 77 The patterns of change we can see are (Fig. 3.1): 78 Allogenic change – where is this in the figures below? 79 Directional autogenic change 80 Watt’s cyclic succession 81 Switches 82 However, we note that intermediates and combinations occur. Community X1 Community Y1 Community X2 Community X Community Y2 Time Community Z Clements’ relay floristics (succession to climax) Community X Time Community Y1 Time Community Z Community Y Watt’s cyclic succession Community Z1 Community Y2 Community Z2 A switch, giving alternative stable states Fig. 3.1: Three types of autogenic vegetation change through time. 83 84 85 2 Allogenic change: including secular change Vegetation changes on long timescales can be seen via the pollen record of temperate and arctic 86 lakes and mires, especially over the past 15,000 yr. In the northern Atlantic and Pacific areas, the most 87 abrupt, non-anthropogenic alteration was a reversion from birch scrub (Betula spp) to tundra herbs in 88 the Younger Dryas period (13,000 – 11,600 yr BP). When historical vegetation change is sudden, it may 89 be that the vegetation is tracking a sudden climate change, or the vegetation change may be abrupt 90 because there is a geographic-scale switch (sect. 5.4.A below). In the case of the Younger Dryas, there 91 does seem to have been a closely-correlated sudden temperature depression (Hu et al. 2002; Ruddiman 92 2003). Such changes are most likely to be seen at the geographical boundary of a vegetation type, and Wilson & Agnew, chapter 3, Community processes, page 4 of 50 93 the pollen profile of Kremenetsky et al. (1999) indicated that the steppe-forest boundary in the Ukraine 94 has moved at least twice since 6000 yr BP. 95 Soil processes can occur on a similar timescale. Soil leaching especially needs thousands of 96 undisturbed years to take place. It results in plant communities which are slow growing, often evergreen 97 and high in phenolics, on soils that are low in all nutrients, particularly phosphate. Northup et al. (1998) 98 describe a chronosequence of such soils in North California formed over an estimated 500,000 yr. 99 Recent soil there has pH (in CaCl2) over 6, while in the oldest soil it is less than 4. This environmental 100 change has been tracked not only ecologically but perhaps evolutionarily, since there are endemic 101 species and varieties of conifer particularly adapted to the soil conditions and contributing to them. 102 Fluctuations in the species composition of plant communities from year to year were described 103 by Clements (1934 pp. 41–42) as ‘annuation’, by Penfould (1964) as the ‘pulse phenomenon’ and by 104 Collins et al. (1987 %89) as ‘pulse-phase’. Investigations on this timescale, and the decade-long 105 changes that often accompany it, have been hindered by the paucity of datasets with continuous and 106 long records of approximately equilibrium vegetation. Notable exceptions are the Park Grass 107 experiment in lowland England (since 1856, though detailed recording has been somewhat intermittent) 108 and plots on Barro Colorado Island (since 1985; Chave 2003). Usher (1987) re-analysed Watt's (1960) 109 data from 21 years of observation in Breckland, Eastern England. The transition probabilities between 110 the vegetation elements appear to show that change is taking place towards an Agrostis/Festuca type 111 from what, don’t assume that the reader will be familiar enough with the study to know that sort of 112 detail. This is indeed gradual change between states, and does not appear to conform to an overall 113 succession. It is intuitively credible as a community process in changing environments, though Usher 114 had no ecological explanation. 115 When the environment changes, it would not be surprising for the plant community to do 116 likewise, but often the correlations are hard to see. Allen et al. (1995) found correlation with the rainfall 117 at one of the dry grassland sites examined, but not at four other, nearby sites. Collins et al. (1987) 118 similarly failed to find correlations between individual species and the climate. what sort of time scale 119 were these studies conducted in? Need more information about them to be able to evaluate how 120 effective theses studies could have been at detecting change. 121 Dunnett et al. (1998) correlated vegetation changes over 38 years in the roadside vegetation at 122 Bibury, southern England, with climatic fluctuations. There were over 100 grasses and forbs. More 123 correlations with weather indices were found than expected by chance. Warm dry summers increased 124 species favoured by environmental stress or disturbance (S or R of C-S-R theory), whereas wet growing 125 seasons favoured more productive species (C). Dunnett and Grime (1999) found by experiment that the 126 differential response of species to temperature was based on their physiological response, but the 127 differences between the species were amplified in competition. Species that were notably more Wilson & Agnew, chapter 3, Community processes, page 5 of 50 128 abundant in unusually warm summers included Achillea millefolium (yarrow) (Grime et al. 1994 %271) 129 which is distributed almost throughout the British Isles, being absent only in a very few cold/wet parts 130 (Preston et al. 2002), but Ranunculus repens (buttercup) and Trifolium repens (white clover) with 131 almost identical geographical distributions having identical geographic distributions doesn’t mean that 132 they have the same habitat requirements to A. millefolium, were less abundant in those years. 133 Brachypodium pinnatum (tor grass) was more abundant in warm summers, and is a species very 134 frequent in chalk/limestone areas with warm summers (July mean > 15 ˚C), but another grass with a 135 very similar distribution, Bromopsis erecta, was less abundant. This is puzzling. If the Brachypodium 136 and Bromopsis have different habitat affinities (as is suggested by the fact that Bromopsis isn’t 137 mentioned as a calcifile, then it is not surprising that they react differently to environmental chances 138 One of the most fascinating sets of observations in ecology is Watt’s (1981 %509) 38-year 139 record from Breckland in eastern England. Calcareous ‘Grassland A’ exhibited a sequential dominance, 140 from 1936 to 1950 by the small-tussock grass Festuca ovina (sheep’s fescue), from 1950 to 1958 by the 141 creeping herb Hieracium pilosella (hawkweed), from 1959 to 1964 co-dominance of the latter two with 142 the procumbent shrub Thymus polytrichus (= T. drucei, thyme), and from 1965 to the end of recordings 143 in 1973 by T. polytrichus on its own. These three species all showed a steady increase to a peak and a 144 steady reduction from it, though when H. pilosella decreased it held on to some microsites rather 145 tenaciously. Watt explains these dramatic patterns of sequential dominance by very subtle changes in 146 the weather, but as at Bibury the correlations are not convincing. Like Dunnett and Grime (1999) he saw 147 competition as important, which is probably is. Basically, these changes have never been explained. 148 Disease may have played a rôle. Based on what. There are other potential explanations not mentioned 149 Shorter-term changes can be seen too. Peco (1989) was able to show that low rainfall produced a 150 single-year retreat in an ordination axis that was correlated with successional time. This represents a 151 whole-community shift in a single year. Some species have been observed to show outbreaks: sudden 152 increases in abundance followed by a sudden decrease, all happening within a few years. In the Park 153 Grass Experiment, Dodd et al. (1995 % 277) found that of 43 species recorded on non-acidified plots, 154 10 showed outbreaks, i.e rather sudden increases and subsequent decreases on timescales in the order of 155 a decade. These species tended to be ruderals: self-fertilised and suppressed by competition. Outbreaks 156 are seen at Bibury too (Dunnett and Willis 2004 %18): repeated increases and decreases in species such 157 as Urtica dioica (stinging nettle) and Chamerion angustifolium (rosebay willowherb), and on shorter 158 timescales in species such as Galium aparine (goosegrass) and Rumex obtusifolius (dock); Dunnett and 159 Willis say many in the later group have ruderal traits, confirming the conclusion of Dodd et al. The 160 decade-long outbreaks in Watt’s (1981) Breckland site may have been due to the inertia of the shrubs 161 (Thymus polytrichus), tussocks (Festuca ovina) and clones (Hieracium pilosella). Wilson & Agnew, chapter 3, Community processes, page 6 of 50 162 At a fine scale, the elusive Carousel theory (van der Maarel and Sykes 1997) appears to say that 163 even if a community remains in equilibrium on a large scale, species presence on a small scale changes 164 from year to year, such that within a community most species are capable of occupying most microsites, 165 but in any year each species will leave some potential sites unoccupied, at random. This begs the 166 question of why the species sometimes choose (I would take points of an introductory bio student’s 167 paper for implying that species are choosing in this context) to be absent. The logical implication is that 168 there is niche limitation to the number of species that can co-exist locally, and there is evidence for this 169 (Wilson et al. 1995 %729). Baba (2005), also working in calcareous grassland, emphasised differences 170 in mobility between species: a ‘complex carousel’ rather than a single carousel. Van der Maael and 171 Sykes’ observations of turnover were made on a fine scale, e.g. 3 × 3 cm, which is less than the size of 172 most of the plants, and certainly less than their sphere of influence. It may represent that one year a leaf 173 of a plant overhangs the quadrat, and next year the same plant is present in the same place, but its leaves 174 are in a different direction. The first priority in understanding communities at this scale is to solve the 175 Paradox of the Plankton (chapt. 4 below). Then, we have to find how these mechanisms of coexistence 176 and those of competitive exclusion processes interact to produce the very fine coexistence we see in 177 many communities. Self-allelopathy for Hypochaeris radicata (cat’s ear) is one example, still not fully 178 established for the field, of the thousands of interactions that must be causal. The Carousel Theory is a 179 distraction from progress towards understanding them. 180 3 Directional autogenic change 181 The basis of autogenic succession is that a species or community, by reaction, changes the environment 182 (especially the soil), in such a way as to facilitate the entry of others. The new entrants are usually more 183 diverse, larger and longer-lived than their predecessors and therefore the process involves increases in 184 biomass and biodiversity with time (Clements 1916). In chapter 1, section 1, we listed the necessary 185 parts of ecosystems: an energy input apparatus, the capital of energy, the cycling of elements and rate 186 regulation (Reichel et al. 1975). Capital of energy is in biomass, while nutrient cycling (the resource 187 economy) requires biodiversity of heterotrophs. Nutrient capital in the system is part of the rate 188 regulation, other rates being set by the physiology (autecology) of the species. Monotonic increases in 189 the values of these system parts are not inevitable; there can be decreases in all of them, autogenically or 190 allogenically triggered (sect. 3.3 below). 191 ‘Succession’ has often been taken to be the linear gradient that was the norm for Clements 192 (1916), from pioneer stage to climax. This was characterised by Egler (1954) as “relay floristics” and by 193 others as “facilitation succession” and “succession to climax”. As always, Clements saw much, and 194 synthesised his observations into a general pattern. Like all generalisations, there are exceptions. 195 Clements saw these as well as anyone, and reported them. However, the general pattern that he Wilson & Agnew, chapter 3, Community processes, page 7 of 50 196 emphasised is a true one. Communities change through time for three reasons: (1) the environment 197 changes allogenically making invasion possible from the species pool, (2) new species arrive in the 198 regional pool, (3) species modify the environment by reaction, allowing other species to establish. I 199 second the comment blurb and mention that the section which seems to most closely follow these (#1) 200 points is an allogenic factor. It is only the last that we can call succession, either directional or cyclic. 201 3.1 Environmental change 202 If the initiation of this process is geomorphological, i.e. the natural creation of a new land 203 surface, the succession is said to be primary succession; if environmental modifications remain from 204 previous succession it is said to be secondary. This paragraph is unnecessary if your target audience is 205 adv. undergrads or higher 206 A classic description of a primary succession is that of Crocker and Major (1955) on deglaciated 207 terrain in Glacier Bay, Alaska. We must notice that this is a space-for-time substitution; as always with 208 such comparisons that the physical location and therefore the physical environment are not identical 209 between stages, neither were the weather and the propagule flux at each stage of succession. A classic 210 secondary succession is that in Buell-Small oldfields in New Jersey, USA (Bartha et al. 2003), where a 211 genuine temporal record was possible. Earlier workers (Clements 1916) envisaged a single end point of 212 a "climax community" in successions, but it has become clear that there are many potential pathways 213 through vegetation types. Thus in Glacier Bay the initial vegetation was of Dryas drummondii and Alnus 214 sinuata and A. rubra (alders), but the course of soil development in glacial til resulted in flats and 215 hollows where drainage became impeded and mire vegetation developed (‘muskeg’ with Pinus 216 contorta) while slopes became Picea sitchensis (sitka spruce) forest. Landform is not the only 217 determinant of successional sequences: Allen (1988) suggested that the causes of differing trajectories 218 of secondary succession in Wyoming sagebrush rangeland included anthropogenic topsoil alteration, 219 mycorrhizae and alien invaders. There may also be alternative stable states (sect. 5.6 below). awkward 220 sentence 221 The principal modification to the environment in mesic successions is in the soil/root system. 222 Nutrient pools and soil structure change as roots exploit and die in the soil, while the soil fauna uses 223 these resources and incidentally forces nutrient cycling. There is a trend towards soil stratification 224 imposed by surface litter deposition and water movement. Soil systems, like aerial systems in their 225 different ways, consist of a continuum of biota, all of which are leaky, so all components can affect all 226 others through diffusion and by mass flow of solute. Organisms are typically more closely intermixed 227 than above-ground. Fungal hyphae provide the potential for intimate connections between roots, and a 228 large body of research has demonstrated their importance in vascular plant nutrition and performance. 229 An example is the sand dune binding grass Ammophila breviligulata (beachgrass) in which Koske and 230 Gemma (1997) found that vascular arbuscular mycorrhizae (VAM) were critical in its rate of spread into Wilson & Agnew, chapter 3, Community processes, page 8 of 50 231 bare sand. The reaction that every plant has on its environment operates above ground as well (chapt 2., 232 sect. 2), such as the mutual shelter offered by alpine shrubs (Choler et al. 2001). However, the process 233 of facilitation has rarely been documented and [Walker et al. (1986 %1243) concluded that, interpreting 234 the zonation away from an Alaskan river as a chronosequence, there was no evidence that facilitation 235 was involved] Not enough info given on this study . I cant\’t evaluate this reference. unclear why 236 zonation away from the river was seen as a chronosequence- is there some reason to believe that the 237 river used to by broader and has shrunk?. Nitrogen fixers are clear examples of species that might 238 facilitate. Walker et al. (2003 %277) gave experimental evidence in succession after a volcanic eruption 239 in North Island, New Zealand, the net effect of the N-fixing shrub Coriaria arborea (tutu) on the 240 succeeding dominant tree Griselinia littoralis (broadleaf) was positive: considerable soil enrichment 241 outweighing the effect of shade. 242 3.2 Pioneer species 243 Pioneers are said to be mainly short-lived ruderals with high seed output. They have low 244 competitive ability, and are replaced by long-lived species producing fewer but larger seeds and often 245 with vegetative reproduction. These are often termed ‘r’ and ‘K’ selected respectively, in an analogy 246 with the Lotka Volterra logistic equations (Krebs 1978). It is tempting to equate pioneers with the ‘R’ of 247 C-S-R theory (Grime 2001) and ‘K’ with the ‘C’ of C-S-R theory (chapt. 6 below). However, under 248 Grime’s theory, whilst the secondary succession trajectory starts in the R corner, and starts to move 249 towards the C corner, it veers off towards ‘S’ (stressed) habitat and species characteristics. Moreover, 250 primary succession starts in the S corner, and after a trip towards the C-S-R centre also ends up also 251 back in the S corner. Grubb (1987) agrees with this: pointing out that in primary successions with 252 limited water and nutrients the colonisers are often S species. In temperate mesic situations there seems 253 little doubt that the plant strategies are associated with differing fungal relationships. Francis and Read 254 (1994) found in a survey that 70% of species form VAM associations. Most were apparently 255 mutualistic, but some species showed a negative effect and these were mostly ruderals. This might be an 256 additional reason for the inability of pioneers to persist in later-successional, closed communities. 257 Mycorrhizae are a sinkshouldn’t this be source? for photosynthate and their relationship with their hosts 258 depends upon whether the additional mineral nutrients that they obtain via the fungi enhance 259 photosynthetic efficiency enough to amortise their carbon cost (Tuomi et al. 2001). It is in nutrient-poor 260 conditions that mycorrhiza can benefit their hosts, especially in phosphate, and P generally decreases 261 through succession, making mycorrhizal associations more significant. Of course the benefit that hosts 262 obtain from their mycorrhizae depends on the availability of the other resources, light and water. 263 Zangaro et al. (2000) found that pioneers in a Brazilian secondary succession had abundant VAM, while 264 primary forest had much less; the juveniles in these secondary successions are in full light so that 265 nutrient gain gives a growth advantage, while saplings in the later, closed forest are shaded so that light, Wilson & Agnew, chapter 3, Community processes, page 9 of 50 266 not nutrients, is limiting. We can re-evaluate Francis and Read's (1995) survey and propose that one 267 reason for lower mycorrhizal association by ‘r’ selected species is that they are found in disturbed places 268 where nutrients are not limiting. Succession towards forest should have a special dynamic since the 269 system is storing nutrients (and carbon as biomass), potentially depleting soil resources. Thus Simard 270 and Sachs (2004) found that in young stands taller individuals were most affected by neighbours, while 271 in older stands shorter individuals were more sensitive. This is consistent with the idea that the resource 272 under competitive demand changed from light to nutrients, the opposite of Tilman’s interpretation for 273 Cedar Creek (*).?? What is this? 274 3.3 Decrease in biomass 275 Biomass can be lost in successional change by a switch process such as paludification (sect . 276 5.4.E, below). Rather similarly, the litter produced by some species, especially gymnosperms, is low in 277 bases and acidifies the soil, resulting in the leaching of nutrients. This is podsolisation. This litter is 278 often slow to break down, and therefore retains nutrients. These processes could theoretically lead to 279 reduced biomass. Wardle et al. (2004) compare sequences from six ‘chronosequences’, in different parts 280 of the world and on different substrates, all showing a decrease in biomass. The timescale on which this 281 process would occur makes it difficult to support with evidence, but they say that “For all the sequences, 282 the decline in basal area became noticeable within thousands to tens of thousands of years after the 283 start”. The sequences vary in the trends of litter N and P, litter decomposition, etc., but most conform to 284 podsolisation. 285 Another biomass-decreasing process which could occur late in a succession is salinisation, 286 caused by rheatophytes drawing on a sub-saline groundwater. Climate change and soil salinisation were 287 probably the cause of loss of casuarinid forest and an accompanying increase in salt-tolerant chenopods 288 in south-western New South Wales, Australia after 4500 yrs BP (Cupper et al. 2000). The salinization 289 could have been caused by the transpiration of the forest, in which case the process seems to be 290 autogenic. 291 292 Biomass loss caused by autogenic succession seems to be rare. Wilson & Agnew, chapter 3, Community processes, page 10 of 50 293 294 4 Cyclic succession: in fact or fiction 295 Watt (1947) emphasised the concept of self-generating, repeated cycles of change through 296 communities with different species composition, and often different biomass and biodiversity, to explain 297 observed vegetation mosaics. He concluded they were widespread and the usual cause of vegetation 298 mosaics. The idea fired many field ecologists worldwide to look for examples. 299 Cyclic systems have been widely inferred from observational evidence of patchiness in 300 vegetation consisting of shrubby and herbaceous phases (e.g. Agnew 1985). The mechanism is the 301 opposite to that used in the formulation of the switch process, that an individual of a major species 302 (usually only one) becomes unable to sustain its own presence or abundance and dies ("moving" 303 laterally), its place being taken often by members of a different guild, e.g. shrubs replaced by bare 304 ground with lichens. The latter eventually form an environment suitable for recolonisation by the 305 former, and so the cycle continues. All cycles are driven by the breakdown of one species and all cycles 306 allow the coexistence of several species in a moving mosaic. The effect could be via litter allelopathy, 307 pH, etc. (chapt. 2 above). The result is similar to plant pathogen increase at high densities, and may be 308 difficult to distinguish from it in field studies. The replant problem occurs where a crop fails to grow 309 satisfactorily on land on which the crop has already grown, and is important in commercial agriculture 310 and forestry. Zhang and Zu (2001) attempted to discern between pathogen loadings and autoallelopathy 311 in depressing growth of replantings of Cunninghamia lanceolata fir in China. They concluded that the 312 likely cause was allelopathy, not pathogens. However, Westphal et al. (2002) concluded that in an old 313 Californian vineyard the replant problem was caused by pathogens. We think that this effect may be 314 widespread in natural stands, and it is certainly a mechanism whereby perennial plant densities may be 315 limited, while allowing scattered (rare) seedlings to flourish. 316 It has been shown many times that leachates from a species’ shoot or occasionally roots can 317 inhibit that same species’ growth or seed germination (Singh et al 1999). Newman and Rovira (1975) 318 tested a set of British meadow herbs and found that Hypochaeris radicata (cat’s ear) and Plantago 319 lanceolata (ribwort plantain) were significantly inhibited by their own leachate, while Lolium perenne 320 (ryegrass) was not. They argued that this could explain why the former two species never formed pure 321 stands and individuals of them were short-lived in the meadow community, since performance would 322 decline with plant clump size, and would be best in remote single juveniles. The mechanism would 323 satisfy our criterion for increase-when-rare because a species that is sparse finds less of its self-toxin 324 around. A 2-phase cycle would be possible. There have been a few other reports of two-species 325 replacement cycles, when the two species are dominant. Rychnovska and Jakrlova (1990) describe 326 almost yearly alternating dominance by Sanguisorba officinalis (burnet) and the grass Nardus stricta in Wilson & Agnew, chapter 3, Community processes, page 11 of 50 327 a meadow in Bohemian Sudeten. These clearly allow species pairs to maintain strong populations in 328 spite of local or temporal dominance, thus there must be a mechanism of abundance-dependence 329 operating. 330 However, solid evidence has been hard to find. Miles (1987) states that Calluna vulgaris 331 (heather) alternates with Betula pendula and B. pubescens (birch) in Scotland, but his evidence was 332 from current observation, not from historical records that the cycle actually happened. At a smaller 333 scale, Turkington and Harper (1979 %201) suggested that there was cyclic succession between patches 334 of grasses and clover in a pasture, but they provided only very sketchy evidence. Cain et al. (1995) 335 explained the patch structure that Trifolium repens (white clover) exhibited in a lawn through density- 336 dependent growth, although the fluctuations in these patches were surprising. Herben (1990) in a 337 mountain grassland in Czechoslovakia carefully tracked species movement at a small scale, but the 338 pattern of transitions revealed no networks. 339 Seven suggestions have been made for cyclic successions in New Zealand forests, but none has 340 more than the slightest supporting evidence (Wilson 1990). For example, Mirams (1957) and others 341 suggested that stands of the valuable timber tree, Agathis australis (Kauri, Araucariaceae) in North 342 Island, New Zealand, are one phase of a cycle with broad leaf trees because A. australis may be unable 343 to regenerate under its own canopy due to the amount and pH of its litter (Blakemore and Miller 1968; 344 Mirams 1957). However, there is also evidence that seedlings are outcompeted by broad leaf tree 345 canopy and roots (Bieleski 1959; Mirams 1957), although A. australis litter is indeed slow to decay 346 (Enright and Ogden 1987). Bieleski, (1975a) suggests that under the succeeding angiosperm trees the 347 soil changes are reversed, the A. australis can then reinvade overtopping the angiosperm trees, 348 suppressing their growth, but no evidence is offered, and Ogden (1985) has questioned the basic 349 assumption of these suggestions, a lack of direct regeneration of A. australis, reporting that many A. 350 australis stands contain abundant seedlings of different sizes. Cyclic succession has probably been 351 confused with autogenic disturbance by litter. 352 Basically, cyclic succession has never been reliably confirmed by observation over time. None 353 of Watt's (1947) examples had this sort of evidence, and most have been proved wrong on inspection 354 over long time scales. This is true of the hummock-hollow alternation in bogs (Wilson and Agnew 355 1992; Belyea and Clymo 1998). Such a cycle had been proposed long before Watt took it as an example 356 and it appeared in textbooks, but the weight of evidence is now for the opposite process: a switch. Stasis 357 rather than alternation. Watt (1947) described cycling driven by the growth phases of Pteridium 358 aquilinum (bracken) where a vigorous front of P. aquilinum can invade Calluna vulgaris (heather) but 359 the C. vulgaris can invade an older P. aquilinum stand (Watt 1955 %490). However, Marrs and Hicks 360 (1986) found that after several decades the changes that Watt predicted had not happened. It would be 361 possible to see cycles in Watt’s (1960) own record from the control plot of acidic grass-heath in Wilson & Agnew, chapter 3, Community processes, page 12 of 50 362 Breckland, eastern England, but only for Luzula campestris (woodrush). In this grass-heath, Watt (1947) 363 had proposed that patches of A. capillaris (= tenuis) and A. canina (bent) spread, die out in the middle 364 to form a ring, and the patch is eventually re-colonised making a cycle. In Watt’s (960 records, those 365 species do indeed appear and then disappear within 10 years in two places in the plot, but they persist 366 for 20 years elsewhere in the plot. 367 In arid lands environmental extremes lead easily to facilitation. Sadek and Eldarier (1995) 368 describe how the succulent Arthrocnemum macrostachyum in Mediterranean Egypt traps sediment and 369 builds mounds in saline flats. The mounds are colonised by Cynodon dactylon (Bermuda grass) when A. 370 macrostachyum starts to die back and finally the mound breaks down. In dry grassland in Texas the 371 dead centres of spreading Eragrostis curvula (lovegrass) tussocks persist for some years before 372 recolonisation by the grass and is a cycle imposed by the species on itself. The dead centre was thought 373 to be due to tiller suppression by litter, but Wan and Sosebee, (2000) experimentally showed that litter 374 can enhance tillering when the tussocks are defoliated by grazing. They suggest that nutrient exhaustion 375 is an explanation for this process. Therefore it is clear that vegetation cycles can be driven by one 376 species and that this constitutes a method of maintaining a population by means of growth movement. 377 This could be a result of disease, which would not constitute cyclic succession because there is only one 378 species involved, and because there is no environmental change, i.e. no reaction. Even in arid lands the 379 detection of cyclic succession is difficult within a nexus of complex interactions. We are not in a 380 position to declare how widespread cycles are in nature. 381 One of the best examples of cyclic succession was provided by Yeaton (1978) in a Texas desert 382 (maybe this is just because I’m a Texan, but I’d like you to name the desert here) (Fig. 3.2). He 383 examined plant densities, the remains of dead plants below others, the vigour of associated plants, the 384 incidence of rodent burrows and soil erosion. The evidence suggests that Larrea tridentata (creosote 385 bush), a prolific seeder, can establish in bare ground. Birds perch in its branches and drop seeds of that 386 species and of a cactus, Opuntia leptocaulis. The O. leptocaulis establishes and grows well under L. 387 tridentata, perhaps because of fine wind-blown soil that accumulates there. The L. tridentata bush later 388 dies partly because of competition for soil water by the cactus, with its mat of shallow roots, O. 389 leptocaulis dies partly because of rodents burrowing below it and also because is soil erosion is liable to 390 occur below its shallow root system, without the stabilisation that the deeper roots of L. tridentata had 391 provided. This returns the patch to open ground. The data are observations at one time, rather than a 392 temporally observed cycle, but they build a convincing story. The mechanisms are speculation, but 393 believable. This study was in ’78 and no one has followed up on it?! 394 Wilson & Agnew, chapter 3, Community processes, page 13 of 50 Fig. 3.2: The Larrea tridentata (creosote bush) / Opuntia leptocaulis cyclic succession proposed by Yeaton (1978). 395 396 Although there is little firm evidence of vegetation cycles driven by facilitation, as we discuss 397 above, monocarpic (semelparous) self-replacing dominants provide irrefutable examples of cycles. This 398 is where all individuals of a dominant species flower and subsequently die over an appreciable area and 399 over in a short timescale. Successional communities with shorter-term dynamics then take over the 400 newly opened soil, starting with ruderals and often allowing lianas to establish, until the monocarpic 401 species re-establishes its dominance. Examples are many Bamboos (Janzen 1976; Agnew 1985), and 402 some tropical forest understorey shrubs such as Mimulopsis solmsii (Tweedie 1965). Fire appears to be 403 part of the bamboo cycle in Asia (Keeley and Bond 1999) but not in Africa (in our experience) or 404 Australia (Franklin and Bowman 2003). These situations cannot be considered switches since no plant 405 changes the environment in its favour, and are not stable states because the ruderal phase is not stable, 406 they are cycles caused by endogenous processes. 407 5 Switches: the positive feedback processes 408 5.1 The concept of the switch 409 The term ‘switch’ was coined by Howard Odum (1971) for the positive-feedback process 410 between biota and environment. We see the vegetation-environment switch as the key to plant 411 community ecology. The great majority of the effects that plants have on each other (chapt. 2 above) are 412 via the environment, the ‘Reaction’ concept of Clements (1916), rediscovered by Jones et al. (1994) as Wilson & Agnew, chapter 3, Community processes, page 14 of 50 413 ‘Ecological engineering’ and by Olding-Smee et al. (1996) as ‘Niche construction’, cute but redundant 414 termssounds a bit snarky. However, these concepts seem too general because at the community level, 415 there are two opposite possibilities: (1) The community makes the conditions relatively less favourable 416 for itself. This is the basis of Clements' theory relay floristics, based on facilitation. (2) The community 417 or a key species makes the conditions more favourable for itself, relative to other species. The reaction 418 reinforces the species’ hold on a site, there is community reinforcement by positive environmental 419 feedback, a switch (Wilson and Agnew 1992). The swich process has been known by a variety of terms: 420 "Constructiveness" (Braun-Blanquet 1932), "positive feedback" (Roberts 1987; Jefferies 1988), "self- 421 intensifying effect" (Frenzel 1983), "self-reinforcing trend" (Perry et al. 1987) and "bootstrapping" 422 (Perry et al. 1989). Switches have been under-studied, probably because the opposite process, Clements' 423 theory of facilitation, succession, still has a strong hold in the ecological psyche. The two elements of a 424 switch are: 425 Element i. Community X changes the environment, 426 Element ii. This change is relatively more favourable for community X than for community Y. 427 It is very difficult to find hard evidence for both elements ‘i’ and ‘ii’ for any single situation (Wilson 428 and Agnew 1992), though convincing evidence is emerging for a few cases. 429 5.2 Types of switch 430 431 432 Wilson and Agnew (1992) identified four types of switch: 1. One-sided switch: Community X changes the environment (e.g. increases the nutrient status or lowers the soil pH) in patches where it is present. 433 Zero-sum switch: Community X changes the environment in its patches, and in the process changes 434 the same environmental factor, in the opposite direction, in the patches where it is not present 435 (community Y), for example by channelling water or wind into those patches. We call this ‘zero- 436 sum’ here because it is caused by there being a limited amount of water or air. In Wilson and 437 Angew (1992) we called these ‘reaction’ switches, but we eschew that term to avoid a clash with 438 Clements’ prior use of it. 439 440 441 442 443 444 Symmetric switch: Community X changes an environmental factor in its patches, and Community Y simultaneously changes the same environmental factor in its patches, but in the opposite direction. Two-factor switch: Community X changes an environmental factor in its patches, and Community Y changes a different environmental factor in its patches. 5.3 Outcomes of switches An indication that a switch is operating is often a sharp spatial boundary without an obvious 445 abiotic cause; types ‘2’, ‘3’ and ‘4’ can provide this. However, there are four possible ecological 446 outcomes of the operation of switches (Fig. 3.3): Wilson & Agnew, chapter 3, Community processes, page 15 of 50 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 1. 2. 3. 4. Stable mosaic outcome: In a previously uniform environment a switch of types 2-4 can create a stable mosaic of communities, separated by sharp boundaries. Community X establishes at some sites by chance (whatever chance is), and its modification of the environment ensures it holds the sites. Elsewhere, Community Y establishes, and by the mechanism implied in that switch type also holds its sites. These are alternative stable states. A one-sided switch cannot give a stable mosaic because community X can further invade the unmodified environment. This is the ASS concept here right? if so you should make that clear Sharpening situation: Where there was originally a gradual environmental boundary, a switch (of any of the four types) can produce a sharp vegetation boundary on a gradient or in a less well-defined mosaic. Acceleration situation: If a switch (of any of the four types) be operated by invading species/community, their invasion can be accelerated. The invading species/community Y changes the environment to make it more suitable for community Y, and less suitable for the previous community, X. Delay situation: Existing species could delay vegetation change via a switch (of any of the four types). For example, a switch could delay succession, prolong the effect of initial patch composition, or delay the response to climate change. This operates by community X changing the environment to keep it more suitable for community (e.g. seral stage) X, and less suitable for the succeeding community, Y. The delaying effect of switches has been called ‘inertia’ (Nicholson 1970, Van Holle et al. 2003). 464 If a switch results in alternative stable states, they are technically stable in the Liapunov sense, so that 465 the state will be restored after a small perturbation. However, a large perturbation may cause a change to (a) Stable mosaic situation (b) Sharpening situation Fig. 3.3: Four possible outcomes of vegetation switches. (c) Acceleration situation (d) Delay situation 466 theses figures are illegible. also, the 1st (stable mosaic), is different from the figure that you use a t the 467 beginning. here there are 2 different initial states?(illegible)- x and y. in your figure at the beginning of 468 this chapter, there is only one initial state/ another of the stable states. We refer to these as ‘state 469 changes’ (cf. Brooks et al. 2004). 470 5.4 Mediating agents 471 We may generalize that there are eight types of agent that can cause a switch. We outline 472 examples briefly here, especially those reported in more recent literature, to complement the fuller 473 review in Wilson and Agnew (1992). Wilson & Agnew, chapter 3, Community processes, page 16 of 50 474 475 5.4.A Mechanisms involving regional climate G. Miller et al. (2005) proposed that early holocene human firing of the vegetation in the centre 476 of Australia so modified the climatic regime there that the vegetation changed from one with 477 considerable forest/shrub cover to a more open desert-scrub, that was both more fire-promoting and 478 more fire-tolerant, keeping the fire frequency high. They demonstrated through a general circulation 479 model that the resultant increase in the surface albedo and decrease in its roughness would change the 480 climate, increasing the fire frequency and reduce the extent to which monsoonal precipitation penetrated 481 into the area. Both these processes would hold the vegetation cover in its changed state. 482 In general, the different light reflectance, wind resistance (roughness length), energy flow and 483 evapotranspiration of forest vegetation (O’Brien 1996) results, according to models, in a regionally 484 warmer climate. In northern areas at least, this will which will favour the growth of forest and give a 485 regional- or global-scale switch (Bonan et al. 1992; Otto-Bliesner et al. 1997; Bergengren, et al. 2001). 486 According to Gaia theory, this is the reason Earth is habitable at all unlike Venus or Mars (Lovelock 487 1979). We list global-level climate switches as a separate category since they would occur on a spatial 488 scale orders of magnitude greater than our other examples. They are necessarily more speculative, being 489 based on word or simulation models. 490 5.4.B Mechanisms involving water 491 Water is an obvious mediating agent in switches. Occult rainfall is trapped from mist much more 492 by tree branches than by tussock grasses, but stiff erect tussocks collect more than soft flag-forms and 493 short grassland. The vegetation types that increase their own precipitation are thereby able to grow 494 faster, are less affected by drought, and therefore expand (Figs. 3.4, 3.5). Naturally this is a feature of 495 arid climates. Wilson & Agnew, chapter 3, Community processes, page 17 of 50 Higher precipitation and reduced evapotranspiration higher up the hill allows taller vegetation The taller vegetation traps fog precipitation, and further increases water status higher up the hill Fig. 3.4: A vegetation pattern caused by a fog-interception switch. Precipitation Higher water status allows taller vegetation With occult precipitation switch Without switch Grassland Fog forest Altitude 496 497 498 Fig. 3.5: Change in precipitation caused by a fog-interception switch. Scott and Hansell (2002) describe Picea glauca (white spruce) islands in Manitoba (Canada) 499 tundra where (isolated) young trees trap snow, which ameliorates low moisture and temperature. They 500 grow well, and peripheral branches root to form woody islands. However, as they age more branches 501 droop, needles abrade and the canopy thins, allowing lichen heath to establish, and permafrost to enter 502 the island. So the young trees operate the switch, but old trees do not. It is unclear how the young trees 503 establish. This is a one-sided switch, but unusually for a one-sided switch it leads to a mosaic because 504 the processes driving the switch no longer operate as the trees age. Wilson & Agnew, chapter 3, Community processes, page 18 of 50 Positive feedback between vegetation and water infiltration in arid conditions can give ‘tiger 505 506 bush’ alternative stable states (sect. 5.6 below). In mesic and wet climates ombrogenous bogs can form 507 by paludification, a nutrient/pH/waterlogging switch (chapt. 2, sect. 4.8 above). Wilson and Agnew 508 (1992) discuss water switches in saltmarsh pans. Very awkward. 3 sentences, 3 papers, 3 different ideas, 509 np connections made, no discussion 510 5.4.C Mechanisms involving other aspects of microclimate 511 Montane closed canopy forest usually ends abruptly at the uppermost altitudes at which tree 512 growth is maintained. A tree cover has a lower albedo, which tends to give a lower mean temperature in 513 the plant cover and in the soil (Körner 2003) and whilst radiation is more readily absorbed (Grace 1983) 514 it is also more readily lost. However, the crucial factor is frost, not average temperatures. Most tree 515 species require lower levels of frost for regeneration than the adults tolerate, and frost is ameliorated 516 under the canopy meaning that they can regenerate under their canopy but only with difficulty beyond, 517 giving a switch (Wilson and Agnew 1992). The latter authors describe how in alpine and arctic areas 518 shrubs and trees can ameliorate the frost, in both cases benefiting the plants and hence constituting a 519 switch. 520 The review also gives various examples where light is the mediating agent, mainly where a 521 species, by its canopy, produces low light levels that allow that species to regenerate but prevent 522 regeneration by others. Against expectation, some tropical forests are monodominant and Torti et al. 523 (2001) suggest that for the Congolese Gilbertiodendron dewevrei this is due in part to low light at the 524 ground surface caused by the dense canopy and the deep litter layer, though they suggested low nitrogen 525 was involved too. Another example is seen in Pteridium aquilinum (bracken), which performs best on 526 free draining brown earths, but often forms very dense and impenetrable litter, suppressing other 527 species. It often occurs as stable monospecific stands. There are numerous other examples. 528 5.4.D Mechanisms involving physical substrate 529 Plants can affect soil structures, such as soil particle size and porosity (Angers and Caron 1998). 530 They can effect soil structure and stability in the long term. Hellstrom and Lubke (1993) describe a 531 moving dune system in South Africa which has been colonized by the Australian shrub / small tree 532 Acacia cyclops (‘rooikrans’ in South Africa). The soil has been so altered that the dune cannot be 533 returned to its original state by the removal of the invading A. cyclops, and will presumably develop a 534 different vegetation type, the switch having been caused by the A. cyclops. This also happens through 535 sediment entrapment in estuarine salt marshes (De Leeuw et al. 1993). Often the entrapment is continuous 536 and the vegetation, often consisting of one species (e.g. Spartina anglica, cordgrass, Puccinellia maritima, 537 one of the saltmarsh grasses) starts a successional sequence, but sometimes a stable vegetation type Wilson & Agnew, chapter 3, Community processes, page 19 of 50 538 results. For example Castellanos et al. (1994) report Spartina maritima in south western Spain creates 539 raised flats with Sarcocornia (= Arthrocnemum) perennis (glasswort). 540 Probably the best-documented case of a sediment switch is for diatom growth on estuarine 541 bottoms (Fig. 3.6). Diatoms (Baccilariophyceae) produce extracellular polymeric substances (EPS), 542 largely carbohydrates. These bind the sediment together, leading to lower erosion of the silt component. 543 This encourages diatom growth. Here, one would expect the switch to be a zero-sum one if there were a 544 limited amount of silt in the estuary. This would result in a permanent, if shifting, mosaic. That would 545 give bimodality in the frequency of two alternative states, and this has been observed (van de Koppel 546 2001 %3457). Higher EPS → sediment erosion ↓ (Paterson 1989) Lower sediment erosion → silt content ↑ (Daborn et al. 1993) diatoms -> EPS produced (de Brouwer & Stal 2002) Higher silt content → diatom growth ↑ (van der Koppel et al.2001) 547 Fig. 3.6. A sediment-mediated switch. EPS = extracellular polymeric substances. 548 Plants could also affect soil fauna which can again change soil textures. These processes may well 549 comprise switches and form patchy landscapes but we have found no evidence of this. The nearest we 550 have is the influence of termites in tropical savannahs. For example, West African termite mounds 551 support better vegetation than the surroundings, and have better soil qualities: more clay, better porosity 552 and available moisture (Konate et al. 1998). Yet this cannot be regarded as a switch unless the termites 553 harvest the better vegetation so produced. But most termites harvest grasses, not on the shrubby 554 vegetation of termite mounds. 555 5.4.E Mechanisms involving soil chemistry 556 A plant can enhance its nutrient regime through trapping wind- or water-borne material around 557 its base, encountering mycorrhizae during soil exploration by its roots or facilitating the presence of 558 other species (both plants and animals) that enhance nutrient cycles in its favour and even provide 559 nutrient rich sites for its seedlings. Wilson & Agnew, chapter 3, Community processes, page 20 of 50 560 Such processes have often been described for isolated trees and shrubs in dry grassland.. Belsky 561 has, in a series of papers, shown how the shade zone of Acacia tortilis trees in Kenya is an enhanced soil 562 environment with lower evapotranspiration, dampened temperature fluctuations and a more adequate 563 population of soil micro-organisms than the open inter-tree grassland. This in turn attracts large 564 mammals whose defecations add to the effect. When seedling establishment takes place around an 565 isolated tree to form a copse, a switch is occurring. These patches of more mesic conditions in a 566 nutritionally poor arid environment have been called “resource islands” (Reynolds et al. 1999), but they 567 also represent wind switches (Wilson and Agnew 1992). They are a classical cause of vegetation 568 patchiness in a landscape. At the other extreme of rainfall, Agnew et al. (1993 %273) described a switch 569 mechanism of gymnospermous primal rain forest in western New Zealand, where the nutritionally rich 570 seedling sites were on fallen tree trunks. Trees were prevented from falling into an adjacent heathy mire 571 by a fringe of tall myrtaceous shrubs. 572 The preceding examples act through nutrient enhancement. In other switches, nutrients are 573 depleted. In pygmy forest in northern California, Northup et al. (1998) found that the leaves and 574 therefore the litter are high in polyphenols and condensed tannins. The decomposition of this litter is 575 slow, and the soil is therefore low in mineral nutrients, especially N. These conditions are tolerated by 576 the species that grow there but not by possible invaders. In Picea sitchensis (sikta spruce) forests in 577 northern Sweden, the low shrub Empetrum hermaphroditum (crowberry), already present as 578 understorey, can switch nutrient regimes in its favour after cooler forest fires (Mallik 2003). Its litter has 579 a high phenolic content, lowering the pH, sequestering available nitrogen and leaching out metallic ions; 580 conditions which it can tolerate. The phenolics also inhibit the development of conifer mycorrhizae. 581 Lowering of the pH and toleration of that low pH seems a common switch. For example, in many 582 Ericaceae, many Pinus species.these 2 sentences are awkward The acidification can be effected by either 583 their living or dead parts. In Britain Calluna vulgaris (heather) litter is acid, peat-forming and maintains 584 the acid, oligotrophic nature of the C. vulgaris habitat. Many switches combine a lowering of the pH 585 with a lowering of nutrient availability, as in the Empetrum hermaphroditum example. Low pH, low 586 nutrients and waterlogging are part of a similar switch in paludification and bog growth (chapt. 2, sect. 587 3.15 above; Wilson and Agnew 1992). 588 Species that interfere with their neighbours by allelopathy, and thereby benefit and are able to 589 further their allelopathic activity, also operate a switch (Wilson and Agnew 1992; Olding-Smee et al. 590 2003 %book). So can heavy-metal hyperaccumulators, species that grow on metaliferous soils and are 591 resistant to the high levels of Zn, Cd, Cr, Ni, etc. that occur there, but accumulate those metals to very 592 high concentrations (Wilson and Agnew 1992; Boyd and Jaffré 2001). Concentrations in the shoot have 593 been recorded in the order of 10,000 times that in the soil solution for Zn and 600 times for Cd (Knight 594 et al. 1997). The litter of these plants therefore has the potential to raise the available concentrations of Wilson & Agnew, chapter 3, Community processes, page 21 of 50 595 such elements in the surface soil to levels that they can tolerate but other species cannot (Boyd and 596 Jaffré 2001), reinforcing their abundance in the site. An analogous salt-mediated switch operates for 597 species which absorb saline water from the lower layers of the soil, increases the salinity of the surface 598 soil by its litter, and is able to tolerate this better than its competitors (Wilson and Agnew 1992). 599 A much studied, though complicated, switch occurs in some shallow lakes that can have two 600 alternative equilibria: clear and turbid (Scheffer et al. 1993). In the clear state, submerged aquatic 601 vegetation is abundant. Conditions are oligotrophic, low especially in P, allowing little phytoplankton 602 growth and few fish. Macrophytes such as members of the Nymphaeaceae (water lilies) can shelter algal 603 herbivores such as Cladocera species (water fleas), which then keep the algal populations low and 604 hence the water clear (Stansfield et al. 1997). Allelochemicals produced by Chara spp. may also be 605 involved in keeping the algae down (van Donk and van de Bund 2002). In the turbid state fish 606 movement causes turbidity, which stirs up sediment giving eutrophy and therefore phytoplankton 607 growth, but the sediment reduces light penetration and therefore suppresses submerged macrophyte 608 growth. Food chains are critical. Jones and Sayer (2003) suggest that, in eastern England, fish control 609 the epiphytic grazers, which control the periphyton (algae attached to macrophytes) which controls the 610 macrophytes. That is to say, with too many fish, the periphyton kills the macrophytes, and with no fish 611 the macrophytes are dominant in these shallow lakes. The switch mediating agents are nutrients, light 612 and fish. Although there are variants, this is an exceptionally well-documented switch. 613 van Nes et al. (2002) modelled the proposed clear/turbid mechanisms, and were able to produce 614 alternative stable states (ASS; sect. 5.6 below). The most critical parameter in their model was the 615 degree to which plants lowered the local turbidity. ASS developed most readily in shallow lakes with 616 uniform depth, or where there was little mixing, or to a lesser extent where there was little mixing of 617 water between areas of different depth. Evidence that alternative stable states exist alongside each other 618 in the real world can be seen in similar lakes, some in one state and some in the other, in natural state 619 changes, and in state changes effected by experimental manipulations (e.g. Mitchell et al. 1989; Scheffer 620 et al. 1993). Lakes naturally accumulate nutrients from inwash, and trap them through sedimentation 621 and in macrophytic vegetation. The critical point at which a change from one ASS to another occurs 622 appears to depend on initial state and the sedimentation rate (Janse 1997) which must have to do with 623 nutrients. Reversal may or may not occur readily (Körner 2001). For example, Blindow et al. (1993 624 %159) described two moderately eutrophic, shallow lakes in southern Sweden that have during the past 625 few decades changed several times between the two states. In both lakes, water level fluctuations were 626 the most common factor causing shifts. Blindow et al. (2002) examined one of these lakes in detail, and 627 found that the chlorophyll : total-phosphorus (dissolved, suspended + plankton) ratio declined from 628 April to September, strongly suggesting that some other factor came into play, such as carbon limitation 629 and/or by allelopathic chemicals produced by the submerged charophytes, might be controlling the Wilson & Agnew, chapter 3, Community processes, page 22 of 50 630 phytoplankton. Hargeby et al. (2004) recorded a change to the turbid state after there had been 30 years 631 of the clear-water state. It occurred over two years with mild winters which increased fish populations, 632 thus increasing bioturbation and nutrient cycling, combined with cool springs which hindered the 633 establishment of macrophytes. 634 Experiments are the key to understanding the workings of communities. Schrage and Downing 635 (2004) removed 75% of the benthivorous fish from a lake. One immediate effect was a decrease in 636 suspended sediment (probably due to reduced fish movement), and reduced phosphate and ammonia 637 (probably due to both the reduced sediment load and reduced fish excretion). There was an increase in 638 zooplankton, which seemed to lead to a reduction in phytoplankton. These two led to an increase in 639 water clarity, which may have been the cause of in an increase in macrophyte frequency. Thus all the 640 elements of a turbidity switch are present; however the lake reverted after two months to its previous 641 state, so an alternative stable state was not induced. 642 Scheffer and van Nes (2004) suggested that comparable ASS might exist in ecosystems of the 643 open ocean. 644 5.4.F Mechanisms involving disturbance 645 There are many ways by which a switch can operate through disturbance. We dealt with 646 autogenic disturbance in the last chapter. If lianas can facilitate gap formation (chapt. 2, sect. 3.5 above) 647 they would favour regrowth of their own life form in forest gaps, possibly constituting a switch by 648 which lianas maintain themselves. Where tidally inundated marsh vegetation occurs, ecesis is unlikely 649 because of disturbance. but Bertness and Yeh (1994) describe how the erect stems of marsh elder (Iva 650 frutescens) can trap so much water-borne litter that the underlying marsh vegetation dies, allowing its 651 own seedlings to survive. These plant canopies also reduce evapotranspiration and thus surface 652 salinities. This switch maintains marsh elder thickets and is dependent on litter for initiation. 653 Disturbance through fire is a major factor in creating vegetation edges because certain vegetation 654 types are fire resistant and fire promoting, whilst other types are susceptible to fire but do not readily 655 carry it. The South African fynbos shrubland is fire-prone and depends on fire for its persistence, while 656 adjacent closed forest is fire resistant (Manders and Richardson 1992). Both vegetation types benefit 657 from increased water availability (Manders and Smith 1992) but only during wet years can tree 658 seedlings establish in the shrubland. They can then grow to become fruiting nuclei of forest. The fire 659 regime maintains the balance between the two types. 660 Disturbance by humans can also be noted here. Humans follow paths and behaviour patterns in 661 vegetation which are repeated and predictable. The effect on vegetation is to reduce plant height and 662 production which in turn invite foot passage, so that this is a switch between vegetation types (Wilson 663 and King 1995). Wilson & Agnew, chapter 3, Community processes, page 23 of 50 664 5.4.G Mechanisms involving heterotrophs 665 Mycorrhizae present a somewhat different case, since they enhance nutrient uptake and need no 666 external driver, although they can be dependent on litter for long term survival (Vogt et al. 1995). They 667 are essential to nutrient uptake in many nutrient stressed situations. Although we have no examples 668 showing that mycorrhizae are solely responsible for the spread of a species or vegetation type, the 669 dominance of a vegetation type in an oligotrophic habitat and its abrupt edge can be due to this 670 association. The natural history of mycorrhizae is so complex and important that it is still difficult to 671 assess their rôle, whether imperative or facilitative, in plant community dynamics. There are many 672 examples of these interactions: Meadows resulting from abandoned beaver dams in northern Minnesota 673 resist invasion by the surrounding Picea mariana demonstrably through a lack of ectotrophic 674 mycorrhizal fungi (Terwilliger and Pastor 1999). It is clear that there is still much complexity to be 675 unravelled, some of which may constitute switch mechanisms, maintaining boundaries or changing 676 community structure. 677 Animal invasion can markedly alter plant communities, but this cannot comprise a switch unless 678 the community type both attracts and benefits from the animal exploitation. Much of the material is 679 speculative. We shall consider two major mechanisms: the effect of browsing and grazing and the effect 680 of frugivores on seed dispersal. 681 Grazers can be switch mediating agents if they are encouraged by a particular type of vegetation 682 cover and promote it. For example, in woodland grass openings, large mammal grazers crowd the 683 woodland/grassland ecotone for predator escape (Lamprey 1964). They sharpen the ecotone by 684 removing or trampling any outlying tree seedlings and prevent the woodland from spreading. Much of 685 the speculation on this situation has concerned East Africa, in explanation of the apparent ASS of 686 grassland and Acacia spp. woodland (van de Koppel and Prins 1998). On abandoned agricultural land 687 and range in western Europe, Equus caballus and E. assinus (horses and asses) defecate in preferred 688 sites which they fastidiously avoid for grazing, creating within the meadow thorny patches of Prunus 689 spinosus (blackthorn) and Rubus spp. (brambles) that that protect regenerating trees, leading to a mosaic 690 of forest with grassland openings (Bakker et al. 2004; Kuiters and Slim 2003). This seems to be a zero- 691 sum switch, because the thorny patches divert the equines onto the grassy areas, but the area of those 692 limits the animal population numbers. 693 Again, Augustine et al. (1998) describe how ASS can arise when high numbers of white-tailed 694 deer (Odocoileus virginianus) trample Laportea canadensis (Urticaceae) populations. At high 695 L. canadensis plant densities, the plant population survives because O. virginianus avoid the area, while 696 at initially low densities, L. canadensis is trampled as O. virginianus pass and it disappears. This also 697 seems to be a zero-sum switch. From this point on seems to be unrelated to the beginning of the 698 paragraph- very confusing,make it a new paragraph? Futuyma and Wasseman (1980) described a Wilson & Agnew, chapter 3, Community processes, page 24 of 50 699 situation in which Alsophila pometaria (fall cankerworm) adults emerge from pupae in spring. They lay 700 their eggs, the first larvae emerge at about the time Quercus coccinea (scarlet oak) comes into leaf and 701 larval emergence continues as Q. alba (white oak) leafs out. The larvae need young leaves to survive. 702 Therefore, in stands where Q. coccinea is in the minority it suffers more herbivory, presumably 703 reducing its fitness and maintaining the dominance of Q. alba. SO do most of the larvae emerge before 704 the Q. alba leafs out? if so then I agree with your statement. If not, it seems that Q. alba would suer 705 more damage b/c of its commonness unless Q. coccinea is a preferred food source. The converse 706 process would, in stands dominated by Q. coccinea, maintain its dominance. This process could result in 707 ASS in the field, but this has not been demonstrated. 708 Dispersal can mediate switches. We described resource islands above with regards to nutrients, 709 but patches of seedlings around foci can also be produced by perching frugivores. If the plant species is 710 a preferred food of the frugivore, this benefits the frugivore population, constituting a switch. 711 Germinable seeds can be dropped during eating, defecated by the bird around the tree, or carried by/in 712 the bird to establish a new focus (Herrera et al. 1994). In this way, patches of forest can arise around 713 isolated trees (Archer et al. 1988). Dispersers/consumers of nuts could operate a switch when they are 714 dependent on the fruit for food, but benefit the plant by protecting the fruit from other predators, bury 715 them in sites that are probably favourable for establishment, and disperse them away from the tree and 716 perhaps into new areas (Vanderwall 2001). A suite of Fabaceae (legumes) in African savannahs have 717 indehiscent, carbohydrate-loaded pods which are eaten by browsers. The seeds within are refractory and 718 they germinate faster after scarification, which can occur during their passage through the gut. 719 Deposition in dung is clearly helpful in seedling establishment. Many browsers concentrate their dry 720 season feeding on such legumes. Many of these are woodland trees (Baikiaea spp., Cassia spp., 721 Pterocarpus spp. in Miombo woodland; Stokke 1999), and the process maintains the vegetation type. 722 Acacia tortilis and A. erioloba often grow as isolated trees in rangeland, and can develop into thickets. 723 M.F. Miller (1995) and Dudley (1999) have shown that the pod resource is indeed heavily utilized by 724 browsers and ungulates, and that there is an enhanced germination effect, but other suites of rodents, 725 birds and beetles are also involved in a complex system. Pollinators could mediate a switch in a similar 726 way. 727 A similar switch could operate in which increase in the population of a host-specific pollinator 728 (perhaps an orchid) increased the seed set of species A, leading to greater abundance or a greater spread 729 of species A, leading to there being more nectar/pollen for the pollinator, increasing the population of 730 the pollinator: positive feedback. 731 Wilson & Agnew, chapter 3, Community processes, page 25 of 50 732 733 5.5 Switch evolution Wilson and Agnew (1992) suggested that switches would be more common than facilitation, 734 because there would be selection for genotypes with reactions that favoured the species producing them 735 over those with reactions that disfavoured them. This is a Gaia-type idea (Lenton and van Oijen 2002), 736 be it a rather mild one. At first sight, the argument seems logical. All plants, like other organisms, effect 737 reaction (Clements 1916; Miles 1985, van Breemen and Finzi 1988; Odling-Smee et al. 1996). For 738 example a plant is bound to shade another plant below it. A switch could arise by chance: characters 739 which were the result of selection for other reasons could happen to give the plant an ability to change its 740 environment in a way that gave it benefits relative to its neighbours. If reaction arose at random, we would 741 expect 50% of cases to be negative feedback (i.e. facilitation), and 50% positive feedback (i.e. switches). 742 However, the genotype of each plant will determine the direction and extent of its reaction, and natural 743 selection must operate on that genetic variation. 744 Olding-Smee et al. (2003) also conclude that ‘niche construction’ [i.e. reaction] “is a fact of 745 life”. They do not seem so concerned with a population’s affecting its own fitness, but with the causal 746 change: current population → environment modification → next-generation population. Applying this 747 to switch evolution, they say: “We define ecological inheritance as any case in which organisms 748 encounter a modified feature-factor relationship between themselves and their environment where the 749 change in the selective pressures is a consequence of the prior niche construction by parents or other 750 ancestral organisms”. They use the term ‘positive niche construction’ for a switch, giving the examples 751 of allelopathy and fire, and conclude that selection will occur for adaptations that allow organisms to 752 carry out their [presumably positive] niche construction with greater efficiency. Laland et al. (1996; 753 1999) also analysed the effect of reaction on the selection process, and concluded there could be strong 754 selection for switches, perhaps leading to the fixation of otherwise deleterious genes. 755 We have to be careful here, almost to the point of disagreeing with ourselves (Wilson and 756 Agnew 1992). Darwinian selection operates at the level of the individual genotype. A mutant that 757 produces a reaction favourable to that individual plant (genet, technically) could lead to the evolution of 758 a switch. However, this supposes an unlikely degree of independence between coexisting genotypes in 759 their reaction and response – that the mutation benefits only that one individual. We must be careful if 760 we apply this logic to a mutant that produces a reaction that benefits a whole population of the species, 761 because that mutation will have no selective advantage over the ‘wild type’. Selection cannot operate if 762 everyone benefits. What if your offspring benefit the most b/c they are closest to you? Moreover, many 763 of the plant features causing a switch will have a cost. For example, fire, allelopathy and herbivory 764 defence switches can involve the production of secondary compounds, which are expensive 765 energetically. If a character has a cost and benefits a plant’s neighbours, it must be characterised as 766 altruistic, and can evolve only by a group selection mechanism (Wade 1978). The most likely Wilson & Agnew, chapter 3, Community processes, page 26 of 50 767 mechanism for plants is local kin selection, in which a plant benefits its neighbours who are likely to be 768 somewhat related to it. However, such selection operates only within a limited range of parameters, and 769 is a weak force (Wilson 1987 %493). 770 We conclude that Darwinian selection would be possible if the reaction were so local that the 771 plant benefits only itself (e.g. produces litter just under itself), or at least with little benefit to neighbours 772 of the same species. If the effect were a little more widespread, but local enough for most of the plants 773 benefiting to be related to the mutant, the trait could possibly evolve by kin selection. The situation is 774 not as clear-cut as Wilson and Agnew (1992) supposed. Perhaps most switches arise by accident, not by 775 adaptation. 776 5.6 Alternative stable states 777 You’ve been discussing ASS’s for a while. Move this forward to the intro wherer you are 778 defining/explaining switches. In regions of apparently uniform terrain and climatic regime, such as 779 grassland on alluvial plains, there may be vegetation states that are persistent and different, but 780 impossible to relate to variations in the substrate or ambient conditions. The different states could 781 coexist side-by-side in a mosaic, or they could exist in the same site at different times. Such 782 communities are known as alternative stable states (ASS). It is assumed that either state could exist 783 there, but that each is stable, so there is a vegetation-dynamics barrier to be crossed moving from one to 784 another. There are three criteria for ASS (Connell and Sousa 1983): 785 1. The states, differing in species composition, occur in the same environment. 786 The perturbation causing the original change in species composition in space or time appeared as a 787 788 pulse, or by “chance”. The states are stable. 789 These criteria are difficult. For ‘1’, the states must be held by interactions caused by the different 790 species compositions, but then almost all species interactions are via reaction: i.e. a change in the 791 environment. Thus, criterion ‘1’ is impossible to meet literally. Some types of reaction such as light are 792 instantaneous but some such as soil characters have persistence. A realistic criterion would be that the 793 differences in environment are small and of a type probably caused by the plants themselves. For ‘2’, 794 we can rarely see the origin of the states in nature. A perturbation is normally taken to be an 795 environmental or biotic condition outside the normal range, appearing stochastically in time. We have to 796 distinguish between: (a) ‘press’ perturbations, where the difference between treatments is continued, e.g. 797 where there is continued grazing versus none, versus (b) pulse perturbations, where the change in 798 environment is temporary. Some pulse perturbations (shocks, instantaneous perturbations) really are 799 instantaneous, e.g. the removal of plants. Other types of ‘pulse’ perturbation can be applied 800 instantaneously but the environmental change persists for some time, such as a single fertiliser 801 application. However, in the context of ASS the origin of the different states should be historical factors Wilson & Agnew, chapter 3, Community processes, page 27 of 50 802 such as management or transient environmental differences that disappeared long ago, random dispersal, 803 the season of establishment by the community founders, or environmental patchiness subtle enough to 804 have been overwhelmed without the establishment of ASS. This would seem to be dependent on where 805 you place the biotic interactions The critical item is that no continuing environmental pressure is 806 applied that is different between the states. For ‘3’ we theoretically have to apply an infinitely small 807 perturbation and show that the system regains its original state, at least after infinite time (sect 7.3 808 below). Persistence in the face of natural perturbations is often used as a substitute. We see three 809 processes for maintaining an ASS state, that is a mosaic in space and/or time: 810 1. Spatial resource accumulation and depletion: A switch might result in spatial patches of better- 811 resourced systems in a depleted hinterland. Since accumulation causes depletion this is a Zero- 812 sum (Type 2) switch (Fig. 3.7). Depleted matrix Higherresource system patch Fig. 3.7. Maintenance of alternative stable states in space by a Zero-sum switch, comprising spatial resource accumulation and depletion. [X 3, and with two fuzzy edges] 813 2. Boundary sharpening: Patchiness in the physical environment such as a difference in soil water 814 flow, or variation in a biotic effect such as mycorrhizae – any of the initial differences that we 815 discussed above – could be converted to a spatial mosaic of ASS by boundary sharpening 816 through a switch of any of the types, 1-4. 817 818 3. Reinforcement. Once a state is established in an area, it can be held against a state change by a one-sided switch (Type 1). 819 The maintenance of alternative stable states is almost inevitably due to a switch. As in the ball-and-cup 820 analogy, although there is stability a sufficiently large pulse perturbation can overcome the resistance 821 and cause a state change (sect. 7.4 below). The ASS concept is close to the theoretical concepts of Wilson & Agnew, chapter 3, Community processes, page 28 of 50 822 "Several stable points" (Lewontin 1969) or "Multiple stable points from one initial condition" 823 (Sutherland 1974), though in such papers the crucial involvement of reaction has not generally been 824 elaborated. We should probably examine which type of switch is operating and through which 825 mediating agent before we draw curves of hysteresis or do matrix algebra on “multiple domains of 826 attraction” (Gilpin and Case 1976). 827 Theory 828 Contrary to our wish to examine ecological mechanisms before generating theory, much of the 829 consideration of ASS has been by mathematics or simulation. Luh and Pimm (1993) and Samuels and 830 Drake (1997) have proposed the Humpty-Dumpty principle: that it might not be possible for 831 communities to re-assemble from their constituent species after a perturbation. This might be because 832 the original species composition was based on a switch, originated by the elusive process of ‘chance’. 833 Platt and Connell (2003) argued that secondary successions might be more liable to the initiation of 834 ASS, since where there is stochastic establishment by survivors, alternative paths of vegetation 835 development may exist. Another possible cause of a Humpty-Dumpty effect is that succession fails 836 because intermediate species in the assembly process are missing (cf. Samuels and Drake 1997), 837 [though this seems a very rigorous interpretation of succession] please elaborate on this. Law and 838 Morton (1996) modelled the formation of a mosaic of ASS by this process. Van Nes and Scheffer (2004) 839 used a relatively simple Lotka Volterra model and a community matrix to show that alternative stable 840 states can arise in rich mixtures where intra- is less than inter-specific competition, a process driven by 841 negative feedback in contrast to the positive feedback of switches but which seems unlikely in the real 842 world (chapt. 4, sect. 7 below). Chase (2003) suggested that alternative stable states were more likely to 843 exist when the regional species pool (if we can ever define that) is large. He argued that with more 844 species there will be more redundancy (i.e. species similar in alpha niche). The trouble with this is that 845 if the species are so similar as to be redundant, the states will not be alternative in reaction. If they are 846 somewhat different, one species will replace another on a path that approaches equilibrium 847 convergence, so the states will not be ASS. Similar reasoning can be applied to his suggestion that less 848 dispersal will increase the likelihood of alternative stable states: a state is not stable just because species 849 find it hard to disperse to the site. 850 Examples 851 It is much harder to find evidence that ASS actually exist. Most investigations cited in reviews 852 as having good evidence of ASS are of freshwater lab microcosms, a few of mesocosms (Schröder et al. 853 2005). Even with those studies, a major problem has been lack of time for it to be clear that the 854 communities are not converging. For example, an experiment by Robinson and Dickerson (1987) 855 introduced species in two orders and at two slightly different rates, and found differences in species 856 frequencies, but the results were analysed on weeks 13-23 of the experiment when first-introductions of Wilson & Agnew, chapter 3, Community processes, page 29 of 50 857 a species occurred up to week 8, and re-introductions occurred right up to week 13. Drake (1991 %1) 858 performed similar experiments, again finding that the order of introduction of the autotrophs 859 significantly affected their relative abundance, but the last introduction of an autotroph was on day 180 860 and the final census only 15 days later. Chlamydomonas reinhardtii was the species most likely to end 861 up as a minor component, but this happened most notably in the two treatments where it was the first 862 autotroph to be introduced, i.e. where there had been most time for it to have been excluded by 863 interference. This gives the impression of a competitive hierarchy more than alternative stable states. 864 Coming to the field, Vandermeer et al. (2004) tracked regeneration in Nicaraguan tropical forest 865 over nine years after destructiondisturbance- destruction is value laden by a hurricane, and found floristic 866 divergence, especially initially. This would be an illustration of "multiple basins of attraction": ASS. Yet it 867 is questionable whether their time span was long enough, because four out of the six stand comparisons 868 became more similar in the last two years of observation reported. 869 Wilson & Agnew, chapter 3, Community processes, page 30 of 50 870 In very dry situations vegetation types and their litter clearly affect rain impact on the soil 871 surface and infiltration. Litter reduces the impact of raindrops on the soil surface, and roots provide soil 872 channels for infiltration. This can give a horizontal pattern imposed by the community itself, for 873 example where the drought-imposed gaps between woody plants are bare or covered with grasses and 874 their associates (Tongway et al. 2001). Such patterns can be seen in savannahs, shrublands and 875 grassland in semi-arid areas, on gravelly soil and in deserts. The most notable example is ‘tiger bush’, 876 lines of dense woody plant growth at right angles to direction of gently sloping land under low and 877 variable rainfall regimes. The phenomenon is certainly caused by surface run-off in the inter-stripe zone, 878 onto the vegetation stripe, which has been shown to have much higher soil infiltration rates (Berg and 879 Dunkerley 2004, in Central Australia). Shnerb et al. (2003) modelled the process, showing that the 880 formation of tiger bush occurred in a specific range of precipitation. It is tempting to regard vegetation 881 of striped landscapes as the most efficient water collecting system under these climatic regimes, but 882 Hierniaux and Gerard (1999) have shown this is not necessarily the case. The inter-stripe zone may be a 883 water collection site, but also evaporate small precipitation events. Wilson and Agnew (1992) 884 interpreted this as a zero-sum switch, because the increase in water by one phase (by infiltration) 885 reduced the water available to the other phase (by reduced runoff). Van de Koppel et al. (2002 %209) 886 modelled the hydrology of the system, showing such an effect. 887 So many switches have to do with water stress that it is not surprising that most claims of ASS 888 have been reported from arid lands. The frequently patchy nature of vegetation in deserts suggests the 889 possibility of ASS. Rainfall on desert land surfaces is erratic with much coming in storm events and 890 infiltration is often impeded if the bare soil surface is static, with a stone armour or with a cryptogamic 891 crust (Verrecchia et al. (1995). Water infiltration and nutrient capital can be increased by rodent 892 burrows, soil caches and ant nests, and these areas eventually become shrub patches (Whitford 1993). 893 At critical levels of soil water availability, surface redistribution of rainfall can also lead to vegetation 894 patches which may be self sustaining, i.e. ASS. Many semi-arid landscapes consist of mosaics of forest 895 and grassland which may constitute ASS by water / fire / grazing switches (Wilson and Agnew 1992). 896 Callaway and Davis (1998) investigated the importance of shrub facilitation on recruitment of Quercus 897 agrifolia (live oak) in central California, finding that oak forest patches which included shrubs were 898 stable as forest, while grasslands had virtually no oak recruitment and so were stable as grasslands. 899 These are stable states depending on the facilitation of oak ecesis by shrubs. Rietkerk and van de Koppel 900 (1997) describe how differences in plant density, rainfall infiltration and nutrient cycling can give rise to 901 ASS, triggered by herbivore grazing behaviour. Naturally it is difficult to confirm that these are long- 902 lived ASS. 903 904 Schröder et al. (2005) could find only two examples of ASS that they believed were convincingly demonstrated in the field. One is the situation examined in several studies by R.L. Jefferies Wilson & Agnew, chapter 3, Community processes, page 31 of 50 905 and co-workers, and summarised by Wilson and Agnew (1992) as a switch in “Grazing and nitrogen 906 cycling”. Schröder et al. cite a more recent paper (Handa et al. 2002) where the bare patches at the top 907 of the marsh were hardly being re-colonised, and vegetation could not be re-established. The soil had 908 become hyper-saline due to greater evaporation rates there (Srivastava and Jefferies 1995). This is akin 909 to a switch of the saltmarsh pan type (Wilson and Agnew 1992), except that Srivastava and Jefferies 910 suggest the salt is coming from buried sediments. Walker et al. (2003 %637) showed by modelling that 911 nitrogen dynamics, with input from N-fixing bluegreen algae and N export in goose emigration could 912 also explain the alternative stable states in this system. Thus, the mosaic on the ground is a clear hint 913 that a switch is operating, though state changes between the two ASS do not seem to have been reported 914 and it is not clear through which factor the switch is primarily mediated. Schröder et al.’s other example 915 was an experiment by Schmiz (2004) removing a predator spider Pisaurina mira, therefore releasing 916 grasshopper Melanoplus femurrubrum herbivores, which hid in and ate Poa pratensis (meadow grass), 917 releasing Solidago rugosa (goldenrod) which suppressed other forbs. When predators were allowed 918 back after 2 or 3 yr, the cover of S. rugosa remained high for 3 or 2 yr respectively. Schmitz offers no 919 explanation beyond “loss of top predator control”. It seems that, unfortunately, the cover values were 920 guessed. 921 Possibly the situation where the switch mechanism behind ASS has been most clearly 922 demonstrated is in dune slacks on Texel, The Netherlands. Adema et al. (2002) found two seral 923 stagesOne Littorella and one Schoenus? The example is unclear side by side, with a boundary that had 924 been static through 62 years of observation. Having investigated and dismissed all possible 925 environmental reasons for this stasis they surmise that it must be due to some “positive feedback 926 mechanism” (i.e. a switch) involving nutrient cycling or sulphide toxicity. Two early successional 927 species Littorella uniflora (shoreweed) and Schoenus nigricans (bog-rush), release oxygen into the soil 928 allowing greater oxidation of ammonium to nitrate (cf. Adema and Grootjans 2003), and then 929 denitrification to gaseous nitrogen (Adema et al. 2005 %374) . This could hold up succession. To 930 complete the story we would need to know that early successional species are more tolerant of low N, 931 and there is some experimental evidence that both L. uniflora and S. nigricans grow better at low N 932 availability and grow more slowly at high N availability compared to Carex nigra and Calamagrostis 933 epigejos (E.B. Adema pers. comm.). 934 If ASS exist in an area we should be able to see what type of perturbation can cause a state 935 change between them. There are many state-and-change flow diagrams in the literature, for example 936 comprising a whole issue of Tropical Grasslands, indicating the conditions under which each state 937 change occurs, but unfortunately they are all speculation. Olivia et al. (1998) was experimentally unable 938 to confirm a state-and-change system proposed from open grasslands to dwarf shrublands in Patagonia 939 in a 10-year trial, and Valone et al. (2002), who were interested in the grassland/shrubland mosaic, Wilson & Agnew, chapter 3, Community processes, page 32 of 50 940 found that after 20 years without grazing there was little change in the shrubland. They suggested that 941 there may be considerable lag times before any changes could be seen. The experience of land managers 942 is that intermediate states are much shorter-lived and state changes can be predicted from management 943 changes. We are not doubting that state changes occur, we are just calling for attempts to document 944 them. 945 Chase (2003) used examples to conclude that “… evidence suggests that community assembly 946 can lead to multiple stable equilibria”, and then goes on to start with an assumption: “Sometimes history 947 matters, creating multiple stable equilibria …”. We believe switches are common and important in 948 vegetation, and switches should often lead to ASS, but there is very little evidence yet to support their 949 existence. It may be that ASS are the explanation for much of the patchiness that we see in vegetation, 950 due to alternative trajectories of change (divergence) in vegetation in response the same disturbance, but 951 at different times with different biotic and/or physical environment (Walker 1993; Whisenant and 952 Wagstaff 1995; Lockwood and Lockwood 1993), or different vegetation conditions at the time (Watson 953 et al. 1996). This possibility has rarely been investigated, Walker and Wilson (2002) did so in dry 954 grassland in southern NZ, and found no evidence for it. Nevertheless, so far as our present knowledge 955 goes, to adapt Frank Egler’s phrase, ASS are “one of the more nauseating but delightful idiocies that 956 stem from the heart, not the mind, of otherwise respectable scientists”. 957 5.7 Negative feedback 958 Positive feedback means that a process P produces a condition/product C and that change in C 959 increases the rate of process P as a switch. Negative feedback means that a process P produces a 960 condition/product C and that change in C decreases the rate of process P. Negative feedback at the 961 whole-community level occur at the end of a succession (the climax) or in one of several alternative 962 stable states. The result is stability and the speed at which the negative feedback occurs is resilience. We 963 discuss these concepts below. However, we note that the negative feedback occurs primarily at the level 964 of individual species, be they interacting ones. 965 6 Diversity → productivity 966 967 There are two major possible outcomes of niche differentiation between species: overyield and stability. Wilson & Agnew, chapter 3, Community processes, page 33 of 50 Biomass of B Biomass of B Biomass of B Biomass of A (c) Transgressive overyielding Biomass of A (b) RYT relative overyielding Biomass of A (a) RYM arithmetic overyielding 100% A 50:50 100%B 100% A 50:50 100%B 50:50 100%B mixture mixture mixture Fig. 3.8. In ‘a’ there is arithmetic overyielding: the absolute gain by A in mixture is greater than the 100% A loss by B so RTM > 1.0. In ‘b’ the relative gain by B is greater than the loss by A so RYT > 1.0, in ‘c’ some mixtures yield more than either monoculture. For definitions of RYM and RYT see Wilson (1988 %279). interaction, = theoretical biomass of each species with no = actual biomass of each species, = total biomass of the mixture. 968 the figure is difficult to understand, spend a little time explaining. Overyield is the situation in which 969 higher species richness leads to higher total biomass, perhaps as a result of niche differentiation (Fig. 970 3.8). Investigation of this topic had been fraught with problems, the most important being the ‘selection 971 effect’. If a mixture of species is sown/planted in an experiment, the more competitive species will come 972 to dominate. The more species are planted in the mixture, the greater the probability that one or more of 973 these highly-competitive species will be included in the mix and will take over. If species with high 974 competitive ability also have high production (and note this ‘if’), the result will be higher higher 975 production for the mixture: the selection effect. In terms of seeking the results of niche differentiation 976 this is an artefact. There is no easy way looking at the biomass results to know whether simple 977 overyielding (Fig. 2.8a) has been caused by niche differentiation or by the selection effect. However, if 978 the production of the mixture is greater than that of any of the monocultures (‘transgressive 979 overyielding’: Fig. 3.8 c) the selection effect can be ruled out. Niche differentiation is the obvious cause, 980 though explanations in terms of subvention or interactions with heterotrophs are possible. Loreau and 981 Hector (2001) introduced a mathematical separation of the selection and niche differentiation effects. 982 They found that there was significant evidence of a positive selection effect in only two BioDepth 983 experimental localities out of nine, and a negative effect in one (suggesting that the stronger competitor 984 was of lower yield). In contrast, there was significant evidence for niche complementarity in four 985 localities. However, we suggest that if transgressive overyielding is not seen, the results are not of great 986 ecological significance. 987 988 Experiments to detect overyielding have been fashionable of late, but with mixed results. Tilman et al. (2001 %843) planted field plots at Cedar Creek in a range of species richnesses up to 16, from a Wilson & Agnew, chapter 3, Community processes, page 34 of 50 989 pool of 18 species in 1994, and analysed in 1999 and 2000. There was evidence that some mixtures of 990 grassland species could outyield the highest-yielding monocultures, especially several years after 991 establishment. Lambers et al. (2004 %661) examined the same plots, analysed up to 2002. 992 Over/underyielding spp were determined by regression of yield on diversity. Compared to the -1 slope 993 on log-log basis expected under a null model, six species significantly overyielded and four 994 underyielded. There was no time trend for overyielders to replace underyielders, which is strange. 995 Whittington and O’Brien (1968) planted field plots with Lolium perenne (ryegrass), Festuca pratensis 996 (meadow fescue) and their triploid hybrid. Transgressive yielding was seen in the ‘grazing’ (i.e. low- 997 cut) treatment only sporadically in the first two years, but almost constantly in the third. In two of the 998 mixtures, the two components had diverged in their proportional contribution to the sward. The 999 overyield can be attributed to niche differentiation in the depth of nutrient uptake. L. perenne and F. 1000 pratensis grown in monoculture took up very similar percentages of their P from 10, 30 and 60 cm 1001 (35/40/25 versus 38/35/27) (O’Brien et al. 1967 %513). Yet when they were grown together L. perenne 1002 took up its P from deeper (33/22/45) and F. pratensis from nearer the surface (41/34/25). In contrast, 1003 when grown with the deeper-rooted hybrid (monoculture uptake 18/36/47) L. perenne utilised the 1004 surface P (56/26/18). 1005 There is a tendency in such experiments that if overyield is reached, then it is after several years’ 1006 growth. One explanation for this is that time is required for the species to sort themselves into their 1007 respective niches, e.g. in the canopy or in rooting depth. Another possibility is that overyield is attained 1008 only with a certain optimum proportion of each species (as can be seen in many of the results of de 1009 Wit’s group) and time is needed for the species to reach these proportions. Possibly, as in Whittaker’s 1010 (1965) Niche-preemption concept, the dominant species takes all the resources it can and subordinate 1011 species remain in minor amounts to pick up the crumbs that fall from the strong man’s table. 1012 Overyielding does not necessarily develop. Hooper and Dukes (2004 %95) planted field plots 1013 with local grassland species and followed them for an impressive eight years, giving the species time to 1014 reach their optimal niches and their optimal proportions (as discussed above), but no significant 1015 transgressive overyielding was seen. On the other hand, Roscher et al. (2005) ran a field experiment in 1016 Germany for only one year, sowing nine species characteristic of grasslands of the site. Two-thirds of 1017 the plots showed transgressive overyielding, though the effect was highest in 2-species mixtures. 1018 Analyses to separate the selection effect from niche complementarity using the method of Loreau and 1019 Hector (2001) showed that complementarity reached its peak at 4-species. Wilson & Agnew, chapter 3, Community processes, page 35 of 50 1020 7 Stability 1021 7.1 1022 Concept The question is raised several times in this book of whether any plant community we see is an 1023 arbitrary assemblages of species, such that another assemblage might well have existed at that time and 1024 place, or whether determinism is involved. There are three possible answers: 1025 1026 1027 1028 1. Global stability, with one state: There is only one possible state, at least after there has been sufficient time for succession to have done its work. This is deterministic community structure. ASS, with a few states: There is a limited number of possible states. This is the Alternative Stable States concept discussed above. 1029 A continuous range of states. There is a large number of possible species assemblages, in terms of 1030 species presence and abundance, with all intermediates. The present assemblage exists for 1031 historical or even stochastic reasons, and after a perturbation or even as a result of drift another 1032 state might take its place. 1033 The critical test here is recovery from pulse perturbation (sects. 5.6 above, 7.3 below). If the community 1034 always recovers to the same state, it is globally stable, situation ‘1’ above. If the system is stable to 1035 small perturbations, but not to large ones, situation ‘2’ obtains. If the community does not recover to its 1036 original state after a perturbation, we assume situation ‘3’. We can doubt ‘3’ from our common 1037 observation as ecologists: if it were true we would have no predictive view of landscape. In fact we 1038 generally know what species to expect in a set of environmental conditions. We may sometimes be 1039 surprised, which is a prima facie case for ‘2’, alternative stable states. 1040 Seeing long-term continuance of a range of plant communities – forests, heaths, bogs and arctic 1041 alpines – we intuitively envisage the continuance as stability. However, the difficulty of defining this 1042 kind of stability, plus a little mathematics envy, has led ecology into accepting Lyapunov stability as the 1043 true stability. We therefore have various features of the ‘stability’ (sensu lato) of a community: 1044 1. Reliability: Constancy, lack of change probably in spite of minor perturbation. 1045 2. Stability sensu stricto, i.e. Liapunov stability: whether the community ever recovers from a small 1046 1047 1048 1049 pulse perturbation. 3. Resistance: Lack of change upon a pulse (i.e. temporary) perturbation. We separate it here into resistance to abiotic perturbation and resistance to invasion. 4. Resilience: Rate of recovery from perturbation. 1050 All these concepts assume that the ecological system is at equilibrium, and probably in the real world 1051 none ever is, but the concepts are real. All this leaves open the questions: change in what?, recovery in 1052 what? Here at least we are free from the mathematicians to ask whatever ecologically meaningful 1053 question we feel pressing. The change/recovery can be: Wilson & Agnew, chapter 3, Community processes, page 36 of 50 1054 1. Functional: This can be production (best defined, as we discussed in chapter 1, as energy passed 1055 on to the next trophic levels), nutrient cycling, water output, or any other result of whole- 1056 community processes. 1057 2. Guilds: The proportions of certain guilds, i.e. the guild structure. 1058 3. Species composition: Change/recovery in species presence and/or abundance, i.e. floristic 1059 structure. 1060 Most general discussions of stability refer to species-composition change/recovery, which is easier to 1061 measure. The categories are asymmetrically related – e.g. there can be functional constancy without 1062 constancy in species composition if species change but the productivity (e.g.) does not, and without 1063 constancy in guild representation. Species composition stability encompasses the other two types of 1064 stability, for if the species in the vegetation do not change, qualitatively and quantitatively, then guild 1065 structure and function of the vegetation are also preserved. A constant question in considering these 1066 concepts will be the effect of species richness: are communities with more species more ‘stable’ (sensu 1067 lato). 1068 7.2 Reliability (constancy) 1069 Reliability is the reciprocal of variation in the ecosystem (e.g. Naeem and Li 1997; Rastetter et 1070 al. 1999). The concept was behind ideas on ‘stability’ up to about 1975. For example, Orians (1975) 1071 wrote of ‘constancy’ as a type of stability. Species richness could increase reliability through the 1072 portfolio effect or through the covariance effect. In the portfolio effect, named after a portfolio of shares 1073 that varies less than individual shares, the community response is dampened because it is an average of 1074 the different responses of different species (Ives and Roughgarden 2002 %388). It will always operate 1075 unless the species are identical in their environmental responses. Since averaging over minor species has 1076 little effect, the portfolio effect increases with species evenness. The covariance effect is based on 1077 competition: when the environment disfavours one species others will increase by taking up the unused 1078 resources. This seems to be Whittaker’s (1975) concept of population ‘buffering’: the absorption of 1079 environmental fluctuations through compensation between different species or genotypes. However, we 1080 cannot assume that these effects will be stronger when species richness is higher. Hughes and 1081 Roughgarden (2000) modelled a community with density-dependent Lotka-Volterra competition 1082 equations, and found that with diffuse competition the reliability of biomass decreased with richness, 1083 except in systems with few species and very low interaction strengths. With each species competing 1084 with only its two nearest neighbours, as in MacArthur and Levins (1967) model, the richness/reliability 1085 relation was the reverse. DeWoody et al. (2003) used community models based on variations of 1086 MacArthur’s ‘broken stick’ and found that whilst reliability increased with richness it was, surprisingly, 1087 higher with more difference in competitive ability between the species. Hughes and Roughgarden 1088 (1998) found in a 2-species community-matrix model based on Lotka-Volterra equations that with the Wilson & Agnew, chapter 3, Community processes, page 37 of 50 1089 two species equal in competitive ability, biomass reliability was not affected by the strength of 1090 competition. As the competitive abilities of the two species became more unequal, reliability decreased. 1091 We shall have to descend to reality. 1092 Boreal forest is reliable in species composition in the short term of centuries. In the longer term 1093 of millenia it is functionally reliable but variable in species composition: even now the trees of Europe 1094 have not re-occupied their potential range after the last glacial retreat (Svenning and Skov 2004). 1095 Tropical forests appear to be observationally reliable in the short term and this is impressive considering 1096 their fast growth rates (Manokaran and Swaine 1994 in Malaysia; Sheil et al. 2000 in Uganda). Real 1097 examples of long-term stasis are found amongst stressed communities where the available flora is 1098 limited. The dwarf pitch pine shrubland on Mt Everett, Massachusetts (Motzkin et al. 2000), the Carex 1099 curvula zone in the European Alps (Steinger et al. 1996) and the ombrotrophic mires of Quebec 1100 (Klinger 1996) fit this syndrome. In contrast, O’Connor et al. (2001) found that whilst less common 1101 species increased biomass reliability in African grassland in a fluctuating rainfall this effect was evident 1102 only in rangelands of poor condition. 1103 Experimentally, McGrady-Steed and Morin (2000) found in microcosms of bacteria, micro- 1104 algae, micro-herbivores and bacteriovores and micro-predators that there was a U-shaped relation 1105 between species richness and the reliability of algal density, measured as variation in 3-day steps over 1106 42 days. Tilman (1996), in 207 plots with various nutrient treatments in four fields at Cedar Creek, 1107 found that species-rich plots had greater reliability (lower CV in biomass), though the plots differed in 1108 fertiliser application presumably confounding species richness with species composition. The 1109 experiment of Tilman et al. (2006 %6239) overcomes this problem by using plots sown at a range of 1110 species richnesses and maintained thus by weeding. The same increase in reliability was seen. This is 1111 the nearest we have so far to an answer. 1112 7.3 Stability sensu stricto (Liapunov stability) 1113 Defining stability 1114 May (1972) initiated the modern approach to stability with his community matrix analysis, based 1115 on the mathematical procedure of local stability analysis (= ‘Liapunov stability analysis’ = 1116 ‘neighbourhood stability analysis’). Stability sensu stricto is defined as the ability to recover completely 1117 from an infinitely small perturbation after an infinitely long time. Though we have phrased this to make 1118 it sound a little ecologically unrealistic, some approach to the concept is essential if we are to discuss 1119 stability. Defined thus, stability is like pregnancy: either one is or one isn’t. The ‘infinitely small’ is 1120 necessary to make the definition one of local stability, perhaps among alternative stable states. Global 1121 stability is a much more difficult idea conceptually and operationally. The phrase ‘after an infinitely 1122 long time’ is necessary to avoid arbitrary cutoffs, as well as to distinguish it from resilience. Stability 1123 can be predicted mathematically from the community matrix – a summary of all possible pairwise Wilson & Agnew, chapter 3, Community processes, page 38 of 50 1124 species interactions in an equilibrium community. The community matrix introduces three important 1125 aspects of community structure. The number of species involved, S, is simple. Another aspect is what 1126 proportion of the S×(S-1) off-diagonal matrix elements are non-zero (i.e. the species in that column does 1127 affect the species in that row). This is the connectance. Thirdly, the interaction strength is the mean 1128 absolute value of these non-zero elements. 1129 Is stability likely to occur? (simulations) 1130 Since May (1972), the community matrix has been used extensively in theoretical studies to 1131 predict what type of community would be stable (Hall and Raffaelli 1993). It turns out that, given a 1132 matrix comprising random values, communities with many species and/or with many or strong 1133 interspecific interactions are less likely to be stable. In fact, it is very unlikely that a community of 1134 randomly-assembled species will be stable at all. This represents an obvious conflict with our 1135 observation that multi-species communities commonly persist in nature. One explanation is that the 1136 species in natural communities have co-evolved to be able to co-exist. However, it has been found that 1137 the introduction of some basic ecological processes, especially simulating the assembly of a community 1138 by the immigration and extinction of species, or simply with a realistic pattern of extinction, allows 1139 complex multispecies communities to develop relatively easily without any assumption of coevolution 1140 (Tregonning and Roberts 1979; Taylor 1988 %569). In the simplest assembly models of Tokita and 1141 Yasutomi (2003) the community stabilized at about five species. When the introduced species were 1142 formed as minor mutants of a species already in the community, the number of species reached about 1143 10. However when, in their ‘local’ model, the characters of new species arriving were based on one 1144 species already present, except for its interactions with one other (abundant) species, the size of the 1145 community approached 50 and was still rising after 10 000 periods. Moreover, such communities were 1146 resistant to invasion how does this sentence square with the previous one?. They interpreted this as the 1147 temporary persistence of species only marginally inferior in competitive ability, that were able to exist 1148 stably once mutualistic partners appeared. This is stability by very simple coevolution. 1149 Tests for stability (the real world) 1150 Despite all the theoretical work on the community matrix, tests of stability in natural systems 1151 have been surprisingly sparse. The only valid method for estimating the community matrix for a given 1152 community is by the experimental manipulation of the organisms, quantifying the effect that each 1153 species has upon the growth rate of each species. We are aware of only a few such studies (Thomas and 1154 Pomerantz 1981; Seifert 1976; Thomas 1981; and see below). 1155 Schmitz (1997) used data from field and laboratory experiments to quantify the interaction 1156 strengths between grasshoppers, four old-field plants, and nitrogen supply. He used these values to 1157 parameterize the community matrix and thence to predict the response of each species to nitrogen and 1158 herbivore press perturbation. The predictions carried a high degree of uncertainty, but the observed Wilson & Agnew, chapter 3, Community processes, page 39 of 50 1159 results of the field press perturbations was close to, or within, that range in six out of eight cases. So 1160 this then is a positive test for community stability? Explain why better 1161 The most direct test of stability in a community has been on the University of Otago Botany 1162 Lawn (Roxburgh and Wilson 2000 %395, 2000 %409). A pre-requisite for the concept of stability is 1163 that the community should be at equilibrium, and this seemed to be true. Roxburgh and Wilson (2000 1164 %395) grew seven species from a lawn, including the six most abundant species, experimentally in soil 1165 from the lawn kept in two strata and placed in deep boxes only metres from the lawn. The species were 1166 grown in monocultures and in 2-species and other mixtures. A community matrix calculated from 1167 competition coefficients predicted the mixture of species to be unstable, but very close to the 1168 stability/instability boundary. It was certainly closer to the stability/instability boundary than matrices 1169 constructed under a range of null models. To test for the actual stability of the community, Roxburgh 1170 and Wilson (2000 %409) applied three types of pulse perturbation. After shading for 6 weeks and after 1171 mechanical perturbation (removal of vegetation), species composition recovered towards the original 1172 equilibrium state, although full recovery was not seen. Recovery from a herbicide specific to grasses 1173 was still quite incomplete after 2.5 years, apparently because the compensatory growth of dicots made it 1174 difficult for the grasses to reinvade. The pattern of recovery of the lawn from the perturbations was 1175 consistent with the prediction of marginal stability/instability given by the community matrix analysis. 1176 Simulations (Roxburgh & Wilson 2000 %395) showed that the stability of the community 1177 matrix, marginal though it was, could be attributed to the transitive competitive hierarchy found 1178 between the species (chapt. 5 below). A simplistic extrapolation from competitive abilities would be that 1179 species with a greater competitive ability in 2-species experiments would be more abundant in the 1180 community. Several authors have found such a correlation (e.g. Mitchley and Grubb 1986;Missing 1181 and Werner 1987 %1201 ). However, the strict hierarchy in competitive ability found in the 1182 pairwise box experiments, from Holcus lanatus (Yorkshire fog) as the most competitive species, to the 1183 herb Hydrocotyle heteromeria as the least, had no clear relation to abundances in the Botany Lawn 1184 (Roxburgh and Wilson 2000 %409). Such a lack of correlation has been found elsewhere (Aarssen 1988 1185 %3). Roxburgh and Wilson (2000 %409) suggested that it was due apparent mutualism in a multi- 1186 species mixture: the “My enemy’s enemy is my friend” effect. This effect will be more prominent the 1187 more species there are in the community. Apparent mutualism is implicitly included in the community 1188 matrix model, therefore the model should be able to predict abundances in the actual community (given 1189 that only seven species were included in the model). It could. For example, the model predicted that 1190 Trifolium repens (white clover), Holcus lanatus and Agrostis capillaris (bent) would be the three 1191 dominants, and these three are indeed among the four most abundant species in the lawn. From the 1192 pairwise experiments, H. lanatus headed the competitive hierarchy. However, it was only the second Wilson & Agnew, chapter 3, Community processes, page 40 of 50 1193 most abundant species in a experimental 7-species experimental mixture and in the lawn (Roxburgh and 1194 Wilson 2000 %395), an outcome correctly predicted by the community matrix model. 1195 7.4 Resistance to abiotic perturbation 1196 Resistance is a difficult concept. It is the response of a system to a natural or anthropogenic 1197 pulse perturbation, or to be more exact the lack of response. But what is a perturbation? Suppose a light 1198 wind blows over grassland, with no effect on community composition. Obviously the change was too 1199 small to be called a perturbation. The trouble is that if a perturbation is applied and the community does 1200 not react, it is impossible to say whether that was because it was a very resistant community, or because 1201 the treatment applied was not a real perturbation in the first place. We cannot even compare 1202 communities. If salt is applied to most communities, it will be a perturbation and plants may die. 1203 However, with a saltmarsh community any obligate halophytes there will jump for joy; it was no 1204 perturbation at all. This is analogous to the problems that Körner (2003 %141) saw in defining ‘stress’. 1205 The concept of resistance can apply at the level of the whole community or of individual species, and 1206 again in terms of function, guilds or species. We could expect some theory at the community level, but 1207 there is little. In Loreau and Behera’s (1999 %29) model, with plants limited by nutrients, the biomass 1208 resistancebiomass is onlyt one measure of resistance. Perhaps a mention of why this one was chosen 1209 generally decreased with increased ecological difference between the species. 1210 For evidence, we take two examples: drought and trampling. Drought can be basically a pulse 1211 perturbation. Pfisterer and Schmid (2002), in mesocosms of grassland species, found that species-rich 1212 plots were less resistant to drought in biomass, i.e. function. The same trend can be seen for species 1213 composition in the Cantabrian (Spain) mountain meadows that Gómezal-Sal (1994) examined. Other 1214 field comparisons and those with experimental plots that differ in fertiliser application have yielded the 1215 opposite result, though such comparisons confound species diversity with composition. Vegetation is 1216 complicated, so is its relation to soil, and the behaviour of each species is predicated by its evolutionary 1217 history, so it is little wonder that it is difficult to find generalisations. 1218 There is an extensive literature on the effects of human trampling. For example, Page et al. 1219 (1985) studied the above-ground production resistance of many species in Welsh coastal dune 1220 vegetation over two years after press perturbations of quantified trampling, and compared the results to 1221 controls. They found that species could be assigned to three behavioural classes, partly depending on 1222 their morphology, i.e. their guild / functional-type. In the closed-canopy vegetation of the fixed dunes, 1223 graminoid production was the most resistant, and forb production the least. Forbs and bryophytes of 1224 unstable habitats, counter intuitively, recovered only slowly. However, they have specialised reactions 1225 to particular environmental events. In Bellis perennis (lawn daisy) trampling triggers the swollen corm- 1226 like base, filled with starch, to produce stolons; in fact the species needs open ground for these stolons 1227 to be produced (A.D.Q. Agnew, pers. obs.). The moss Tortula ruraliformis has fast erect growth under Wilson & Agnew, chapter 3, Community processes, page 41 of 50 1228 sand burial,I thought you were talking about trampling? probably because it is peculiarly efficient in its 1229 ability to dry and re-hydrate fast with minimal respiratory cost (Willis 1964). The golden moss 1230 Homalothecium lutescens forms a weft with erect shoots which grow up after sand burial. Other authors 1231 using greenhouse and field experiments (e.g. Andersen 1995; Cole 1995 %215) have found that 1232 resistance to human foot and wheel pressure differs between plant guilds in similar ways, the geophytes 1233 and tussocky graminoids being consistently more resistant, upright forbs less resistant. Resistance seems 1234 to be individualistic to the species. 1235 7.5 Resistance to invasion 1236 Another type of resistance is that to invasion. The process of invasion can comprise: (1) the 1237 occupancy of an empty niche, (2) splitting an existing niche, (3) ousting an existing species from a 1238 niche, or (4) the invader’s constructing a new niche by reaction. Often, invasion is taken to mean by 1239 species exotic to the region (usually exotic to the continent). This confuses the ability of incomers to 1240 establish in a closed community with whether communities contain empty niches and with whether 1241 exotic species are somehow different from native ones.confusing sentence At other times, the topic 1242 means the invasion of local species that were not present at that spot before, which involves questions of 1243 community structure. The issues here are: 1244 1. Whether disturbance is required before invasion can occur. 1245 2. Whether continual disturbance is required for the invader to persist. 1246 3. The biotic diversity of the target community, and whether it is species-saturated. Behind this is 1247 whether there are empty niches, leaving resources available to the invader. 1248 4. Missing guilds (plant functional types). 1249 5. Community structure, for example whether communities with low interaction strength and/or 1250 1251 low connectance are more invasible, or possibly less so. 6. The existence of an alternative stable state which could keep an invader out, though it could 1252 form a table state once admitted. 1253 7. Subvention available to the invader, e.g. protection from grazing offered by a thorny shrub. 1254 8. Whether exotic species differ in some way from native ones. 1255 Exotic species will be discussed in chapter 5, but all the aspects above apply to them. Empty niches (‘3’, 1256 ‘4’) are especially relevant for exotics. So is ‘1’ because it is possible that the absence of an exotic 1257 species in a community is due to inertia, and lack of disturbance is only delaying up the inevitable 1258 invasion. Aspect ‘2’ will be relevant if many of the exotics are ruderal species. 1259 Theory 1260 Case (1991) found that in randomly-constructed Lotka-Volterra based communities resistance to 1261 an invader was higher when species richness was higher and there were strong interspecific interactions. Wilson & Agnew, chapter 3, Community processes, page 42 of 50 1262 Shea and Chesson (2002) concluded that theory is indeed now clear that, all other factors being equal, 1263 invasion resistance increases with species diversity. Intuitively, the explanation is that with high 1264 richness there are no resources left for an invader, with appeals, at least implied, to MacArthur and 1265 Wilson’s species packing. But then why, except for inertia, do the residents get the resources, not the 1266 invader? In partial refutation of Shea and Chesson (2002), the careful plant-community simulations of 1267 Moore et al. (2001) indicated that there is no simple relation between resistance and richness: it depends 1268 on the model and what caused the richness to vary. 1269 Evidence 1270 There have been many studies correlating species diversity and invasion across sites, and also 1271 studies experimentally adding invaders to a range of natural systems. We can draw no conclusions from 1272 these because there are always environmental/historical differences between sites that confound 1273 differences in species diversity. We like the idea that real communities can tell us more, but in this case 1274 experiments are needed in artificial communities. 1275 Dukes (2002) established small field microcosms on a soil/sand mix using grass and forb species 1276 found in a particular grassland in California. When Centaurea solstitialis was added as an invading 1277 species in the form of seeds, mixtures of two or more species reduced its biomass more than the average 1278 monoculture, but no mixture of up to 16 species inhibited invasion more than the most resistant single 1279 species: the annual forb Hemizonia congesta (it is a member of the same guild as C. solstitialis: hints of 1280 niche filling). No increase in resistance was seen beyond four species, though Dukes (2001 %563) 1281 attributed this to a guild effect, an artefact of the experimental design. This raises a complication. We 1282 are seeking a niche complementarity effect, in which with more species there will be a higher 1283 probability of including a species to fill each niche, in this case perhaps the annual forb niche, leaving 1284 few resources for an invader (leaving aside the question we raised above of why the residents should get 1285 them). As with overyield there is the possible artefact of the ‘selection effect’ (Wardle 2001 %161): 1286 when more species are sown there is more chance that a species with high interference ability is 1287 included, and that species will take over. This mimics complementarity if the species with the highest 1288 interference ability in the resident mixture also has high interference ability against the invader, but that 1289 is quite likely, for example a species with high cover or LAI reducing light intensity below itself. A 1290 counter to this artefact is finding that a mixture is more resistant to invasion, not just compared to an 1291 average mixture with fewer species, but than any mixture with fewer species or any monoculture. The 1292 experiment of Dukes (2001 %563; 2002) did not achieve this. 1293 Evidence for the effects of diversity on invasion comes from a 4-year old experiment at Cedar 1294 Creek. Tilman (1997 %81) sowed seeds of up to 54 species in to patches of native grassland. The 1295 proportion of the 54 added species that became established was negatively correlated with initial species 1296 richness of plots, suggesting that species-rich sites were more resistant to invasion. A problem is that Wilson & Agnew, chapter 3, Community processes, page 43 of 50 1297 some of the species added as seeds were already present in some of the plots, when such a species 1298 established it would not count as an invader, and there would be more chance of this happening at 1299 higher richnesses. This does not seem to be the complete explanation because the richness/resistance 1300 relation remains qualitatively the same if only species not already present are counted among those 1301 sown (D. Tilman, pers. comm.). However, the variation between plots was natural, and species 1302 composition may have covaried with richness. These problems were overcome in another experiment at 1303 Cedar Creek, when Fargione and Tilman (2005 %604) planted and weeded 169 plots to species 1304 richnesses from 1, through 2 to 16 for 6 years, then opened them to colonisation for two years. In 1305 higher-richness plots there was significantly lower invader richness (R2 = 0.31) and biomass (R2 = 1306 0.18). As usual, most of this effect on invader richness occurred at low resident richness, between one 1307 and two species. The effect seemed to be largely due to suppression of invaders by C4 grasses, and the 1308 authors suggest their effect is via reduction in soil nitrate. They admit this is most easily seen as a 1309 selection effect. However, the invasion resistance of some mixtures was greater than that of the most 1310 resistant monoculture (Schizachyrium scoparium, bluestem, a C4 grass), and this effect increased up to 1311 an 8-species mixture. Specifically, the percentage of plots that had lower invader biomass than did the 1312 best resident monoculture increased across the species richness gradient. This is all quite impressive of 1313 the complementarity effect. 1314 Wilsey and Polley (2002) attempted to outwit the sampling effect by experimentally varying the 1315 evenness of four grassland species (three grasses and one forb), not the diversity. Reducing evenness led 1316 to a greater number of dicot invaders, but had less of an effect on monocot invaders. Opportunity for 1317 testing comes also from guild-removal experiments; we discuss these chapter 5, section 7.4, coming to 1318 the conclusion that they offer little support for guild-based assembly rules. 1319 Resources, disturbance and species 1320 It is very frequently suggested that invasion is greater after disturbance (Davis et al. 2000 % 1321 421). This is true, but misleading. Of course, there must be sufficient resources for an invader to 1322 establish. Autogenic and allogenic disturbances are more likely in habitats with more nutrient resources, 1323 due to greater growth rates and herbivore muchiness respectively, and disturbance often releases 1324 resources. For example Burke and Grime (1996 %776) and Thompson et al. (2001) report an 1325 experiment in a limestone grassland at Buxton, northern England, where they added seed of 54 species, 1326 all widespread native herbs that are commonly found in climates and soils like those at the experimental 1327 site but which were not originally present there. There was greater establishment of the 54 species when 1328 the soil was disturbed and fertiliser added, though the combination of disturbance and fertility levels in 1329 which invasion was maximal was unique for each invading species. 1330 1331 Davis et al. (2000) generalised from experiments such as these to suggest that invasion requires resource enrichment or release, including that caused by disturbance. There have been many similar Wilson & Agnew, chapter 3, Community processes, page 44 of 50 1332 statements, but we must be careful. If the invaders are native then either: (a) they are better suited to the 1333 conditions than the residents, probably because the conditions have changed, in which case disturbance 1334 is simply overcoming inertia, or (b) they are not better suited, in which case they are probably ruderals 1335 taking advantage of the temporary gaps. Neither result is remarkable. If the invaders are exotics, the 1336 same argument applies; the only difference is that the possibility of their being better suited is greater, 1337 because: (a ii) there is the additional possibility that they are a worldwide super-species in those 1338 conditions, and (b ii) there is a greater probability of their being ruderals too, because many exotics are. 1339 With higher nutrients the argument is much the same. There are some careful and ingenious experiments 1340 here, but the conclusions are fraudulently trivial. 1341 We should be careful of over-simplification. Species identity matters. For example, Van Ruijven 1342 et al. (2003 %910) sowed plots with 1 to 8 species. They found an effect of species richness on 1343 invasibility, but it could be attributed to just two species, and mainly Leucanthemum vulgare (ox-eye 1344 daisy). However this was not a selection effect: L. vulgare was a low yielder in monoculture. Van 1345 Ruijven et al. suggested it may have suppressed invaders by harbouring root-feeding nematodes, and 1346 perhaps also transmitting viruses through them. Again, Crawley et al. (1999 %140) found little overall 1347 effect of species richness in decreasing volunteer invasion, but there was consistently lower invader 1348 richness and biomass in plots containing Alopecurus pratensis (foxtail). 1349 7.6 Resilience 1350 Resilience is a simple concept: the speed of recovery after a pulse perturbation. Again, recovery 1351 can be measured in an ecosystem function such as production, in guild structure or in species 1352 composition. If structure is described by the guilds present, then the guild proportions (i.e. the 1353 physiognomy and phenology of the vegetation) must be regained, which will restore community 1354 function. 1355 Walker (1992) argues that maintaining function after a perturbation should be easier if there are 1356 several members of each guild, a situation he calls species/ecological/functional redundancy. If a 1357 perturbation affects the dominant species in a guild, a subsidiary species will then be available to 1358 assume its rôle. Richness could lead to resilience this way. Walker et al. (1999) suggested that an 1359 Australian rangeland community was rather precisely organised this way: the most abundant species 1360 differed from each other functionally, the most functionally-similar species differed in abundance, and 1361 when a species decreased under heavy grazing another species increased that was functionally similar 1362 but had previously been sparse. For example, using his functional characters, when a large perennial 1363 grass with soft, thin leaves is reduced by grazing, another species with those characters should increase. 1364 This is not easy to imagine. In so far as those characters affect palatability and grazing tolerance, all the 1365 functionally-similar species should decrease together. The relevant theory is that environmental changes 1366 (here, grazing) lead to dominance by species of different beta niches but with the same alpha niches still Wilson & Agnew, chapter 3, Community processes, page 45 of 50 1367 mostly represented. The characters that Walker et al. (1999) used – height, width, life history, SLW and 1368 leaf ‘coarseness’ – do not fit easily into this model. Similarly, Cowling et al. (1994) suggested that in 1369 Cape Fynbos, South Africa, the high species diversity leads to considerable redundancy within guilds. 1370 After the massive fires and droughts which are endemic to the region some members of each guild could 1371 survive, so that the original guild structure is recovered. The concept of redundancy as espoused by 1372 Walker falls all too easily into teleology: “Why do the majority of species occur in low abundance? 1373 They … confer resilience on the community with respect to ecosystem function” (Walker et al. 1374 1999)Not sure why this sentence is teological, please explain. The real answers to their question come 1375 from the mechanisms of coexistence that we shall discuss in chapter 4, but we note that an alternative 1376 answer is that insurance could come not from redundancy but from migration (Loreau et al.’s 2003 1377 %12765). 1378 There have been many studies of the relation between productivity (usually above-ground 1379 biomass) and resilience. Nearly all deal with grasslands, and we therefore expect that generalisations 1380 may emerge about the effects involved. Sadly this is not the case. Moore et al. (1993), in a short review, 1381 state that productivity and resilience are inextricably linked, but Stone et al. (1996), using a range of 1382 mathematical models, suggest it is far from simple, and depends on the model used. Rietkerk et al. 1383 (1997) indicated similar complication using a more mechanistic model, concluding that if plant growth 1384 is water-limited the vegetation on sandy soils is more resilient, but if plant growth is nutrient-limited the 1385 vegetation on clayey soils is more resilient. However, they find that this depends on the extent of the 1386 plant’s reaction (sensu Clements) on the soil. Sometimes, it is not easy to see a deep meaning in 1387 differences in resilience. For example, Herbert et al. (1999 % 908) examined resilience in aboveground 1388 net primary productivity in Hawai’ian tropical rainforest after a hurricane. Plots that had been fertilised 1389 with P were less resistant but more resilient. Both seem a rather simple result of lush growth and high 1390 growth rates with fertilisation. 1391 High species richness would tend to imply high redundancy. Pfisterer and Schmid (2002) sowed 1392 BioDepth plots with a range of grass species richness, perturbed them with simulated drought, and 1393 found that the low-richness plots were more resilient in aboveground net primary productivity. 1394 Mathematical theory on resilience is sparse, but Haydon (1994 %14) used community-matrix models, 1395 first excluding unfeasible and unstable communities, which indicated that resilience should increase 1396 with connectance and interaction strength, and decrease with species richness. The latter prediction 1397 matches Pfistrer and Schmid’s finding, be it that Haydon’s prediction was for species-composition 1398 resilience. In contrast, when Engelhardt and Kadlec (2001) set up aquatic mesocosms with 1-5 aquatic 1399 species and applied the perturbation of clipping off the above-substrate shoots, there was no relation 1400 between species richness on biomass and respiration resilience. The answer is probably that species’ 1401 types differ, and we can expect no simple conclusions. For example, in the trampling experiments of Wilson & Agnew, chapter 3, Community processes, page 46 of 50 1402 Cole and Trull (1992) and Cole (1995)’s experimental trampling, resilience in cover (which was 1403 measured in the latter work!) was lower in communities containing more shrubs and chamaephytes. 1404 Correlation of resistance and resilience 1405 An inverse relationship between resistance and resilience has been repeatedly demonstrated, 1406 both across communities and across individual species. We saw this with Herbert et al. (1999) above. 1407 Taking our case study of trampling, Cole and Spildie (1998) found that when montane understorey 1408 communities were subjected to hiker, horse and llama traffic at two intensities, forb-dominated 1409 vegetation was highly vulnerable to vegetation impact but recovered rapidly, whilst shrub-dominated 1410 vegetation was more resistant but lacked resilience. At the between-species level, again with trampling, 1411 Cole (1995 p 215) found a negative correlation in mountain vegetation; the correlation was seen 1412 especially within the chamaephyte and graminoid guilds. In contrast, Pfisterer and Schmid (2002) found 1413 that species-poor plots were both more resistant and more resilient to drought. Again, simple 1414 generalisations are premature and probably wrong. 1415 8 Conclusion 1416 Many have tried to make generalisations about plant community change. Unfortunately, none are 1417 available. Even general phrases such as “Succession as a gradient in time” (Pickett 1976) can be 1418 misleading when a switch results in stasis or a mosaic, or when there is cyclic succession (Fig. 3.10). 1419 There have been fashions awkward sentence. Before Egler (1954), and for some time afterwards since 1420 most ecologists dismissed Egler as a crank (which he was), most ecologists held to Clements’ (1916) 1421 concepts of relay floristics in a much more simplistic way than Clements himself had. The reaction (no 1422 pun intended) began in earnest with Connell and Slatyer (1977), and progressed until some ecologists 1423 declared that the very word ‘climax’ should be avoided. Many have declared that the views of Gleason 1424 were “strongly opposing” the successional concepts of Clements, but in fact his concept of the process 1425 of succession (Gleason 1917; 1927) was almost identical. 1426 The geological background is included in Fig. 3.9, with the processes of speciation, 1427 biogeographic distribution, dispersal and environmental filtering / ecesis (steps A-D). In chapter 2 we 1428 discussed the interactions between species that follow (E and F). It is clear that given an area without 1429 plants, certain species will predominate first. Whether they do this through dispersal, fecundity or stress 1430 tolerance will depend on the environment. The later species will come in partly because, though not 1431 having those characteristics, they have greater competitive ability. The extent to which they rely on 1432 facilitation by the pioneer species will again depend on the environment and on the species (L.R. 1433 Walker et al. 1986 %1243; 2003 %277). A single, stable endpoint is present in one of the three possible 1434 pathways (Fig. 3.1). The ‘climax’ concept should not be thrown out with the bathwater. On the contrary, 1435 Relay floristics should be the null model, not because Clements was almost always right (though he Wilson & Agnew, chapter 3, Community processes, page 47 of 50 1436 was), but because it happens often, and it is more profitable to concentrate on the exceptions. The 1437 situations that Clements termed sub-climaxes are one exception to relay floristics, but his use of them 1438 was much more restrained that that of those peddling a simplified version of his theory. He saw fire and 1439 grazing subclimaxes as man-induced, but these days we are more likely to see fire suppression as man- 1440 induced. The only environmental factor that Clements (1916) saw as retarding succession to give a 1441 subclimax was saline/alkaline soils. The replacement of communities (sensu lato) in a cycles is an 1442 obvious exception from the null model, but evidence for it is so hard to find that can be fairly sure it is 1443 less common than Watt supposed. 1444 Reaction is clearly important, and reaction in the facilitative direction is part of the relay 1445 floristics. However, there is every reason to expect it to be commonly in the opposite direction, giving a 1446 switch, and this appears to be the case. Although we have emphasised that most putative switches are 1447 unproven, this is quite as true for relay floristics. Moreover, switches are probably often part of relay- 1448 floristics succession, either speeding it up in which case they will hardly be noticed, or slowing it. It is 1449 when switches sharpen a gradient or magnify small initial differences to give a mosaic of alternative 1450 stable states in space or time that they produce patterns that are hard to miss. Clements did eschew the 1451 idea of alternative pathways of succession (Clements 1916, 167), but he allowed two types of reaction 1452 that retarded succession: what we have described as the Sphagnum bog pH switch and the Ericaceae 1453 (e.g. Calluna vulgaris) switch which Clements ascribed to “acids and other harmful substances” 1454 (Clements 1916, 107). 1455 Throughout relay floristics, cyclic succession, the operation of a switch or for that matter 1456 inducing a state change there may be an allogenic environmental change (this chapter, sect. 2) or 1457 allogenic pulse perturbation (i.e. disturbance), changing the rules of the game. Response to and recovery 1458 from pulse perturbation were the subjects of sections 7.4 and 7.6 of this chapter, but for both we 1459 concluded that it is hard to see generalisations, and probably always will be because of individual 1460 responses by species. Resistance to invasion involves questions of alternative stable states (sect. 5.6 of 1461 this chapter), which are still better known in theory than in reality, and also questions of the nature of 1462 exotic species, which we shall deal with in chapter 5, section 12. 1463 Wilson & Agnew, chapter 3, Community processes, page 48 of 50 1464 1465 1466 1467 1468 1469 1470 1471 A Speciation B Biogeography: the species pool C Dispersal D Environmental filtering / ecesis E Interference filtering Geological processes 1472 Resources Biota 1473 1474 Resource use by biota 1475 1476 Growth, production assembly rules 1477 1478 1479 1480 1481 Vegetation change Autogenic factors interference subvention litter autogenic disturbance via other trophic levels reaction Cyclic Climax phase Cyclic phase ASS 1482 1483 1484 1485 Cyclic phase Fig. 3.9: The background of vegetation change. Allogenic factors short timescale Annual climate fluctuations (local) Millenial climatic shifts (regional) Geomorphological changes (local & regional) Geological changes (transcendent) long timescale Wilson & Agnew, chapter 3, Community processes, page 49 of 50 1486 Bare, stressful environment Bare, benign environment (probably secondary succession) (probably primary succession) Dispersal, ecesis, Dispersal, ecesis, facilitationtype reaction perhaps facilitation-type reaction Switchtype reaction Delaying or accelerating switch Switchtype reaction Mosaic/ sharpening switch Climax Perturbation Cyclic autogenic facilitation Regime Alternative stable state change Cyclic phase Cataclasm Fig. 3.10: Pathways of vegetation change 1487 ILLUSTRATIONS 1488 Fig. 3.1: Vegetation change through time by (a) Clements’ facilitation succession, (b) Watt’s cyclic 1489 1490 1491 succession, (c) a switch. Fig. 3.2: The Larrea tridentata (creosote bush) / Opuntia leptocaulis cyclic succession proposed by Yeaton (1978). 1492 Fig. 3.3: Four possible outcomes of vegetation switches. 1493 Fig. 3.4: A vegetation pattern caused by a fog-interception switch. 1494 Fig. 3.5: Change in precipitation caused by a fog-interception switch. Wilson & Agnew, chapter 3, Community processes, page 50 of 50 1495 1496 Fig. 3.6. A sediment-mediated switch. EPS = extracellular polymeric substances. 1497 Fig. 3.7. Maintenance of alternative stable states in space by a Zero-sum switch, comprising spatial 1498 1499 resource accumulation and depletion. Fig. 3.8. In ‘a’ arithmetic overyielding the absolute gain by A in mixture is greater than the loss by B so 1500 RTM > 1.0; in ‘b’ the relative gain by B is greater than the loss by A so RYT > 1.0, in ‘c’ some 1501 mixtures yield more than either monoculture. For definitions of RYM and RYT see Wilson 1502 (1988 %Jae). 1503 1504 = theoretical biomass of each species with no interaction, = actual biomass of each species, 1505 Fig. 3.10: Succession. 1506 Fig. 3.10: Succession. = total biomass of the mixture.