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1 The nature of the plant community: a reductionist view 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. Chapter 4. Mechanisms of coexistence 6 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 1 2 3 4 5 6 7 8 9 10 11 12 13 Alpha-niche Differentiation .................................................................................................................. 5 1.1 Resources (type of resource and time of availability) ........................................................... 5 1.2 Heterotroph-imposed niches ................................................................................................. 7 1.3 The niche extended by reaction ............................................................................................. 7 Environmental Fluctuation (seasonal, annual and decadal change) ..................................................... 9 Pest Pressure (heterotroph challenges) ............................................................................................... 13 3.1 Pathogens ............................................................................................................................ 14 3.2 Herbivory, general ............................................................................................................... 15 3.3 Herbivory of disseminules and seedlings ............................................................................ 16 3.4 Vegetative herbivory ........................................................................................................... 17 3.5 Pest Pressure conclusions .................................................................................................... 17 Circular Interference Networks .......................................................................................................... 18 Allogenic Disturbance (disrupting growth, mainly mechanically) ..................................................... 24 Interference/dispersal Tradeoff ........................................................................................................... 26 Initial Patch Composition ................................................................................................................... 27 Cyclic Succession: movement of community phases ......................................................................... 27 Equal Chance: neutrality..................................................................................................................... 27 Inertia .................................................................................................................................................. 29 10.1 Temporal Inertia .................................................................................................................. 29 10.2 Spatial Inertia: aggregation ................................................................................................. 30 Coevolution of Similar Interference Ability ....................................................................................... 31 Spatial Mass Effect (vicinism) ............................................................................................................ 31 Conclusion .......................................................................................................................................... 32 Were there no plant species coexistence, there would be no need for this book. However, most 32 plant communities comprise persisting populations of several species (This is a good ‘generalist’ 33 statement. I think it should be emphasized more frequently throughout the text. If it wasn’t stated in an 34 earlier chapter, it should be reiterated.. . Populations may increase or decrease through neutral drift or 35 weather fluctuations and species can immigrate or disappear from the local community. However, long- 36 term studies such as the Park Grass experiment (Silvertown 1987) and Bibury (Dunnett et al. 1998) 37 show that the basic tendency is persistence, for example, outbreaks are often followed by a decrease 38 back to the original abundance. This coexistence is the fundamental statement to be made about plant 39 communities, and how it is achieved is the fundamental problem. (good inro to chapter…) 40 Resources (e.g. light and nutrients) are almost always limiting. Competition, and thus 41 interference between individuals and species, is demonstrable in all types of habitat, except immediately 42 after disturbance (chap. 6, sect. 9.3 below; Clements et al. 1929). Interference abilities can never be J.B. Wilson & Agnew, chapter 4, Species coexistence, page 2 of 34 43 exactly equal, so the result should be the exclusion by interference of all but one species (Gause 1934). 44 Therefore, the amazing thing is not that the species in plant communities show any particular patterns of 45 coexistence, but that they coexist at all. Hutchinson (1941; 1961) asked: "How [is it] possible for a 46 number of species to coexist in a relatively isotrophic or unstructured environment, all competing for the 47 same sorts of materials?". He called it the “Paradox of the Plankton”.” Plants are not plankton and don’t 48 experience an ‘unstructured environment’ so why shouldn’t we expect coexistence?. 49 Monospecific stands of vegetation do exist, i.e. with only one vascular plant species (Plate 4.1). 50 Table 4.1 lists those that we have seen ourselves. They are often at land/water ecotones, in wet places 51 and especially in open water or extreme saline environments. In arid countries, a monotonous vegetation 52 of one halophytic species can dominate the landscape (Zohary 1973). We could generalise that these are 53 habitats where only one species is capable of growth due to a harsh environment, or where the 54 exuberance of one species excludes others by interference, but in some cases it is hard to know whether 55 to credit the extreme habitat or the high interference, e.g. Phragmites communis reedswamps. 56 Table 4.1. Some examples of monospecific stands. We exclude monospecificity in a single stratum or 57 guild of vegetation, such as a tree species or understorey species. Habitat Arid Saline Sand dunes Marine submerged Freshwater submerged Freshwater floating Freshwater edge Tidal/brackish edge River edge 58 Climatic zone Exemplar taxa Reference Sub-tropical Halocnemum strobilaceum Zohary 1973 Temperate Zygophyllum dumosum Zohary 1973 Ammophila arenaria Mediterranean Posidonia oceanica (also Den Den Hartog 1970 other marine Helobeae) Temperate Zostera marina Den Den Hartog 1970 Temperate Sagittaria sagittifolia Pieterse and Murphy 1993 Tropical Podostemon spp. Meijer 1976 Temperate Lemna minor Scunthorpe 1967 Azolla filiculoides Scunthorpe 1967 Tropical Eichornia crassipes Pieterse and Murphy 1993 Temperate Typha spp Weisner 1993 Cladium mariscus Tansley 1939 Tropical Cyperus papyrus Lind and Morrison 1974 Temperate Salicornia spp Tansley 1939 Subtropical Avicenna marina Batanouny 1981 Tropical Rhizophora mangle Gilmore and Snedaker 1993 Tropical Pandanus spp van Steenis 1981 At the other extreme is vegetation with high species richness. Values depend on the size of the 59 sample units, the life form guild being considered (e.g. often trees alone are recorded in tropical forests) 60 and the recording convention used (rooted or shoot presence, perennially or seasonally visible). 61 Whatever the sampling regime, extraordinarily high diversities can exist. Tropical rain forest is always 62 quoted as an example. Valencia et al. (1994) found 473 species of tree (individuals >5 cm dbh) in 1 ha 63 of Ecuadorian tropical rain forest, while Richards (1996) tabulates other examples with over 100 species 64 in 1 ha in New World and Malesian (not African!) tropical forests, Naveh and Whittaker (1979) J.B. Wilson & Agnew, chapter 4, Species coexistence, page 3 of 34 65 recorded 179 vascular species in 0.1 ha of a dry shrub/grass community in Israel. Mean species richness 66 of 18.3 per 0.01 m2 has been found in bryophyte carpets in the per-humid West Cape in New Zealand 67 (Steel et al. 2004) and 12.2 species at that scale in limestone grassland on Oeland, Sweden (van der 68 Maarel and Sykes 1993). (for another example, look to the Pine Savannas of Coastal Carolina; high 69 richness on the square meter scale….) 70 Box 4.1: Mechanisms of coexistence. 71 72 73 74 Stabilising mechanisms Niche-differentiation 1. Alpha-niche Differentiation (type of resource and time of availability) 2. Environmental Fluctuation – season, decadal and gradual change 75 76 77 Balances 3. Heterotroph challenges: Pest Pressure 4. Circular competitive networks 78 79 80 81 82 Escape through movement 5. Allogenic Disturbance – disrupting growth mainly mechanically 6. Competition/dispersal Tradeoff 7. Initial Patch Composition 8. Cyclic Succession: movement of community phases: 83 84 85 86 87 88 89 90 Equalising mechanisms 9. Equal Chance (neutrality) 10. Inertia Temporal Inertia Spatial Inertia: aggregation 11. Coevolution of Similar Interference Ability 12. Spatial Mass Effect (vicinism) Questions about coexistence must be asked at a particular spatial scale. The rainforest tree 91 Swietenia mahogoni (mahogany) occurs in the tropics and Colobanthus quitensis occurs in Antarctica; 92 they cannot be said to coexist. They grow in quite different places and environmental conditions, i.e. in 93 different beta niches. Similarly, Salicornia spp. (glasswort) occur on low-altitude saltmarshes and 94 Androsace spp. are alpines, again they occupy different beta niches. The Paradox of the Plankton as 95 defined by Hutchinson refers to how coexistence can occur in a “relatively isotrophic or unstructured 96 environment”. This scale is difficult to define, because allogenic environmental heterogeneity occurs 97 down to the very finest scales, so all species in a mixture exist as a pattern of abundance. The species’ 98 patterns will create further patchiness in resources since species differ in their resource economies: their 99 reaction on the environment. Numerous studies have shown that each species’ individuals affect its soils 100 (section 1.3), which can give autogenic heterogeneity. Also, each individual, by extending over space, 101 must sample a spectrum of resource and environmental qualities. Therefore, rather than specify a 102 particular scale, we state here that we are concerned with mechanisms that allow species to coexist 103 locally, i.e. mechanisms that are not due to imposed habitat heterogeneity within the area considered. J.B. Wilson & Agnew, chapter 4, Species coexistence, page 4 of 34 104 J.B. Wilson (1990) identified 12 distinct mechanisms by which coexistence could be maintained: 105 (1) Alpha-niche Differentiation, (2) Environmental Fluctuation, (3) Pest Pressure, (4) Circular 106 Interference Networks, (5) Allogenic Disturbance, (6) Interference/dispersal Tradeoff, (7) Initial Patch 107 Composition, (8) Cyclic Succession, (9) Equal Chance, (10) Inertia (temporal and spatial), 108 (11) Coevolution of Similar Interference Ability and (12) Spatial Mass Effect. We believe them to be 109 distinct and we know of no new ones, though we have adopted a different arrangement(is the box your 110 arrangement or his?) that we hope brings some new insights. 111 There is a basic distinction between stabilising mechanisms, which contain an increase-when- 112 rare mechanism, and equalising mechanisms, which make the differences between species in 113 replacement rates smaller (Box 4.1; Chesson 2000). Stabilising mechanisms (1 to 8 in Box 4.1) are 114 driven by Alpha-niche Differentiation, Environmental Fluctuation, balances unrelated to niches or 115 escape through movement (Box 4.1). Species abundances are bound to fluctuate and stabilising 116 mechanisms must include negative abundance-dependence to counter this. [For animals, ‘density- 117 dependence’ is often used, but since the concepts of ‘individual’ and ‘density’ are difficult in most 118 plants (chap. 1, sect. 1.1) the more general ‘abundance-dependence’ should be used.] This means that 119 when a species is at lower biomass in the community its plants must have higher fitness in terms of 120 long-term RGR, and its biomass should increase (Chesson in press). Simply, the one necessary and 121 sufficient phenomenon for maintaining a species in a mixture is ‘increase-when-rare’1. The corollary of 122 this is population limitation, i.e. reduced fitness as biomass increases. Either way, a species’ fitness 123 should be inversely related to its abundance. In the short term and in species that reproduce only 124 vegetatively we should consider vegetative RGR (relative growth rate). In the longer term, population 125 growth is the critical question. The increase-when-rare feature is incorporated into the Lotka-Volterra 126 logistic function, as every schoolboy knows. A feature that interferes with this in the real world is the 127 Allee effect, whereby populations cannot recover from very low numbers due to low success in mating. 128 We believe that it is rare in plants because, as we outlined in chapter 1, each individual is effectively a 129 colony and perennials have an indefinite life span. Nevertheless there are examples of the effect in 130 Banksia goodii, a shrub of dry savannah (Lamont et al. 1993), and in the outpollinated annual Clarkia 131 concinna in California (Groom 1998). Species with obligate outcrossing and/or scattered distributions 132 and/or monocarpic reproduction and/or specialised pollen vectors would be more liable to it. 133 True coexistence must be through a stabilising mechanism. Equalising processes do not contain 134 an increase-when-rare mechanism, but are ways in which species may persist for a time in unstable local 135 coexistence, slowing exclusion by interference. Moreover, if the difference between the interference 136 abilities of species is large, even the presence of a stabilising mechanism may not prevent exclusion by 137 interference and in this situation an equalising mechanism might reduce the difference in interference 138 ability between two species so that the stabilising mechanism is able to cause coexistence (Chesson J.B. Wilson & Agnew, chapter 4, Species coexistence, page 5 of 34 139 2000). Ultimately every plant has established itself by a process that can be explained by its tolerances 140 (its niche) and the environmental conditions prevailing during its ecesis, but the reasons for a species' 141 presence in a particular spot are usually obscure. For example, a tree may persist for so long that the 142 local soil/geomorphological conditions that allowed it to establish have since changed. It is therefore 143 tempting to suggest that each individual's presence owes as much to chance as to ecological 144 differentiation and a theory of equal chance has been proposed to explain species mixtures, as we 145 discuss below. Again, species may persist temporarily through inertia of individuals or populations in 146 time or aggregation in space. Any process causing similarity in interference ability is also equalising. 147 Lastly, and rather hesitantly, we include the Spatial Mass Effect as an equalising mechanism. 148 1 Alpha-niche Differentiation 149 We discussed alpha niches in chapter 1 (sect. 4.1). It has been pointed out that coexistence by 150 Alpha-niche Differentiation is impossible to disprove. Each species must by definition occupy a 151 different niche (chap. 1, sect. 2.1). Moreover, by reaction it uniquely constructs part of its niche. The 152 other side of the coin is that if redundancy really occurs, i.e. there are coexisting species that do not 153 differ in alpha niche, they will not coexist by this mechanism and some of the 11 other mechanisms 154 mentioned below must account for their presence. The ‘increase when rare’ element occurs here because 155 when a species is rare the resource that it particularly takes up and requires will be present in greater 156 abundance, that is, the niche is not fully occupied, though interferenceinterferencei would use another 157 word here as this means something specific in your text with this process could come through luxury 158 uptake of nutrients (Lipson et al. 1996). The population limitation on the other hand is due to full niche 159 occupancy, i.e. full use of its resource. The degree of niche separation required will increase with the 160 difference in interference ability between the species; if one species is a very strong competitor another 161 species will be able to coexist only if it is occupying a completely different niche. However, in contrast 162 to some stabilising mechanisms of coexistence, if the niche differentiation between two species is strong 163 enough, they can always coexist. 164 1.1 Resources (type of resource and time of availability) 165 Tilman (Titman 1976) demonstrated that coexistence was possible between two algae limited by 166 different nutrients, P and Si, and concluded that the number of species able to coexist is equal to the 167 number of resources and then only if each species is limited by a different resource. He confirmed this 168 in modelling (Tilman 1977), and there appears to have been no contradiction. Vance (1984) claimed to 169 show that two species can coexist on one limiting resource, but only “if each species interferes less with 170 resource acquisition by the other than with resource acquisition by itself”, which with pure competition J.B. Wilson & Agnew, chapter 4, Species coexistence, page 6 of 34 171 must mean niche differentiation (e.g. the one resource is water, but it is taken up from different soil 172 strata). 173 The primary resource requirements of most embryophytes are similar (light, water, CO2, N, P, K, 174 minor elements, sometimes pollination and dispersal). The concept of a resource gradient as niche 175 differentiation is simple for seed sizes as a resource for birds, but it applies less readily to plants, for 176 which most of the resources are discrete requirements. For example one species cannot require a low- 177 concentration type of P and another a high-concentration type, and the two cannot occur simultaneously 178 anyway. However, in other cases such as soil resources at different depths and pollinator service during 179 the season (section 2), the separation and specialisation of species along gradients are important 180 mechanisms of coexistence (Fig. 4.1; MacArthur and Levins 1967). An important question is how much 181 separation is needed, but in spite of the calculations of MacArthur and Levins this remains unanswered 182 for the real world. Even the existence of such niche limitation has been controversial and difficult to 183 prove (chapter 5). 184 Fig. 4.1. The MacArthur and Levins (1967) concept of niche separation along a gradient. Fig. 4.1. The MacArthur and Levins (1967) concept of niche separation along a gradient. 185 The above-ground structure of a plant is a light-capture mechanism. Therefore gross 186 characteristics of plant form have great relevance. Consider tree size and shape in rain forest, where 187 light is the prime resource; Kohyama (1992) and Akashi et al. (2003) concluded from modelling that 188 short species, with high seedling recruitment but with height-limited growth, could coexist with taller 189 species with lower recruitment rates. This is basically niche differentiation based on canopy strata. 190 Kohyama (1993) used the model to show that stable coexistence resulted without requiring a stand 191 mosaic and Yokozawa et al. (1996) demonstrated that two canopy shapes, conical and spheroidal, could 192 interact with different speeds of recruitment to give situations that allowed a diverse canopy flora. These 193 are subtle and devious ways in which resource differentiation takes place in this famous biome. 194 Stratification below ground, i.e. in rooting depth, is another important niche gradient (chap. 1, sect. 4.3), 195 especially when there is both precipitation and an accessible water table. 196 Two types of temporal gradient can be seen: (1) If growth is triggered by the resource itself, 197 species can differ in their speed of reaction to resource availability, their opportunism. Opportunistic 198 species react fast to resource availability, for example production of surface roots of succulents, J.B. Wilson & Agnew, chapter 4, Species coexistence, page 7 of 34 199 ephemeral leaves like Grewia spp. (cross berry) that have a leaf flush after every rain in African summer 200 deciduous bushland, and seasonally produced leaves that must survive periods of resource starvation not 201 an example of opportunism. (2) More commonly seasonal separation of species' growth patterns is 202 controlled not by the resource itself, but by signals such as daylength and temperature. This causes 203 regular seasonal phenological separation of species’ activity. Here, the mechanism overlaps with the 204 Storage effect (section 2). 205 1.2 Heterotroph-imposed niches 206 Pollination and dispersal can be switch mediators (chap. 3, sect. 5.4.G), but also means of niche 207 differentiation. Pollinators come in many sizes and specialisations: insects, birds, mammals and even 208 reptiles. Among insect pollinators there is huge variation in characteristics and their interplay with 209 plants can be rich and complex. There are robbers, mimics, rewards, guides and warnings discuss 210 further or drop this sentence. The pollination niche is liable to the Allee effect, both for self- 211 incompatible plants when there is no mate in the neighbourhood and for those specialised to particular 212 pollinating insects when the plant population is not large enough to attract the pollinator true, but 213 relevant to spp. Coexistence?. Dispersal tends to be less specialised, without an equivalent to the close 214 relation between flower morphology and pollinator morphology seen with some insects and birds, but 215 differences in fruiting times could reduce competition for dispersers. An Allee effect is possible in 216 dispersal if the population is too small to attract more specialised dispersers. Allee effects can neutralise 217 an increase-when-rare mechanism. 218 Vascular plants could occupy different niches by associating with different mycorrhizal fungi. 219 However, specificity within the two main groups of fungi (VAM, ecto, ericoid/epacrid, etc.) is 220 quantitative, in terms of efficacy rather than in absolute ability to colonise the roots. Moreover, the 221 effect on the higher plant with all types is on availability of soil nutrients (especially P) and water, and 222 the loss is in carbon. We conclude that niche diversification through mycorrhizae is unlikelytrue or just 223 unstudied? There are no references presented either way, just stated as fact. 224 1.3 The niche extended by reaction 225 The alpha niche is not a pre-existing box into which a species has to fit. We have emphasised 226 that the individuals of a species react on their environment, changing it and to a lesser or greater extent 227 constructing their own niche. Ramets of a species always show some sort of density pattern (chap. 3, 228 sect. 1) and this pattern must cause patchiness in the micro-environment of the habitat. Litter production 229 is often the basis for nutrient heterogeneity, but plant morphology and root growth can also be the cause 230 (Vogt et al. 1995). Habitats, therefore, must always be patchy in resource availability and physical 231 environmental actors and the size of the patches depends on the pattern of densities of each species. J.B. Wilson & Agnew, chapter 4, Species coexistence, page 8 of 34 232 There is evidence of this, particularly from forests where the sheer size of the trees makes their patches 233 large and easy to sample. For example, Pelletier et al. (1999) examined a mixed-species forest in 234 Québec, Canada, and found using ordination that forest-floor soil was different beneath different 235 species. For example, soil [Ca] was low below Fagus grandifolia (American beech) and they concluded 236 that it reduced the soil [Ca]. In most such observational studies there is a chicken-and-egg problem, that 237 perhaps the soil differences are determining which species grows at a point, not the reverse. However, 238 Pelletier et al. went two steps further: (a) they used spatial statistics to remove spatial correlations, so 239 that as far as possible they were examining the effects of individual trees, and (b) they offered evidence 240 that F. grandifolia produces litter which, from its Ca, lignin, polyphenol and tannin contents, was likely 241 to reduce soil [Ca]. The study of Ehrenfeld et al. (2001) went that step further in another way. They 242 found higher pH below two exotic species in a deciduous forest in New Jersey, USA, than beneath the 243 native Vaccinium spp., but they also grew the species in the greenhouse on field soil and found pH 244 differences in the same direction. The main problem for interpretation here is that the two exotics 245 (planted together) may have raised the pH more because of their greater growth. For another example, 246 you might look at a natural plant community found in the Southern Appalachians, the acidic cove forest 247 (see Schafle and Weakley 1998 for more info.). 248 Presumably plants can have the same effects (your prev, examples were also plants), but there 249 are few published studies. This argument is that every plant community must show pattern associated 250 with each of its constituent species, but there are other biotic forces making habitats more 251 heterogeneous. Litter is not only a significant niche factor, but one especially liable to cause change by 252 reaction. 253 The necessary presence of patches in a community is important to every species’ resource 254 foraging strategy. Large resource patches are best exploited by stoloniferous herbs (Wijesinghe and 255 Hutchings 1997). Jackson and Caldwell (1996) modelled the effect of root plasticity on the uptake of 256 nutrients from heterogenous versus homogenous environments and found that plasticity was 257 theoretically highly advantageous in sagebrush steppe conditions. In the same habitat the effect of 258 patchy soil nutrients differentially affected Agropyron desertorum (wheatgrass) and Artemisia tridentata 259 (big sagebrush), but only under shade conditions where carbon gain was reduced (Cui & Caldwell, 260 1997). Here A. desertorum outcompeted A. tridentata because, although its root proliferation was 261 reduced, its efficiency of P uptake was not and it could exploit rich patches. A. tridentata on the other 262 hand lost efficiency of P uptake and could not make use of patches of higher P availability. 263 Of course, the above-ground environment is altered too. Canopy trees create a 264 shade/temperature/humidity niche for understorey plants, that might not be able to survive without those 265 modifications. Many manifestly cannot do so in mixture with other species, since they are restricted to 266 forest understories. Trees also create the niche for climbers and epiphytes via their support, and they J.B. Wilson & Agnew, chapter 4, Species coexistence, page 9 of 34 267 create a niche for many epiphytes by water and nutrient stemflow. Parasite plants obviously occupy a 268 niche that would not exist without other plants; the term ‘niche construction’ is very appropriate here. 269 2 Environmental Fluctuation (seasonal, annual and decadal change) 270 Species can separate along annual and other changes in the environment, predictable and 271 unpredictable. We cannot talk of coexistence caused by millenial-scale fluctuation unless the plants are 272 very long-lived (say 500+ yr), because exclusion by interference may happen before the environmental- 273 fluctuation coexistence mechanism can operate. We discuss flowering and fruiting niche gradients, 274 mediated by pollinators and dispersers, in chapter 5 (sect. 6.2) and species can also separate along niche 275 axes of vegetative phenology. An excellent example is the vernal ground flora of deciduous forest. 276 Fargione and Tilman (2005b) found evidence that vegetative phenological niche differentiation added to 277 rooting-depth differences in facilitating the coexistence of species at Cedar Creek with the dominant 278 grass Schizachyrium scoparium (bluestem). Separation in flowering times will reduce competition for 279 pollinators, giving coexistence based on niche differentiation. 280 Environmental fluctuation can cause coexistence if it be on scales shorter than this but long 281 enough for there to be feedback on resources. The fluctuation can be one that affects vegetative growth, 282 for example the vernal flora of forests and spring ephemerals of semi-arid areas, though there is often an 283 accompanying fluctuation in reproduction. 284 As will be clear below, there has to be an interaction between growth and resource supply for 285 environmental fluctuation to cause coexistence. There has been confusion about this. Many authors have 286 claimed that simple variation in the environment and therefore in demographic parameters would allow 287 long-term coexistence. For example, Gigon (1997) wrote: “The fluctuations and their interferences mean 288 that no species encounters optimal growth conditions for a prolonged period of time. Therefore no species 289 can outcompete the others. Fluctuations are thus decisive for the coexistence of species”. Coexistence 290 cannot happen this way. For coexistence, the long-term growth rate of each species has to be RGR = 0.0 291 (r = 0.0, λ = 1.0). The long-term growth rate for a species is the arithmetic average of RGR (the 292 geometric average of λ) in each period. Variation in growth rate will not make it more likely that long- 293 term r is 0.0, in fact a value of exactly 0.0 due to such averaging is infinitely unlikely. Whilst it is true 294 that environmental variation can cause coexistence it can also promote exclusion by interference or have 295 no effect at all on coexistence/exclusion, depending on the biological response of the species to the 296 environment and to competition (Chesson 1990). There are only two ways in which temporal variation 297 can lead to the coexistence of two species: Relative non-linearity and Sub-additivity storage effect. 298 These can cause coexistence only if the interference unbalance between the two species is not too great, 299 and equalising mechanisms can contribute to this. J.B. Wilson & Agnew, chapter 4, Species coexistence, page 10 of 34 300 Relative non-linearity means that two species respond differently to levels of a resource for 301 which they are competing, and moreover that they respond by differently-shaped relations (Chesson in 302 press). For example, the three scenarios in Fig. 4.2 count as different shapes you only discuss the 3rd 303 graph below. The way to test for Relative non-linearity of shapes is to plot the values of RGR of one 304 species at each level of resource R against the values of the other: if the result is anything but a straight 305 line, the species are relatively non-linear. C D RGR A F B Rmean Resource level [R] Rmean Resource level [R] Rmean Resource level [R] E Fig. 4.2: Pairs of two species showing relative non-linearity. 306 307 Take the third 308 graph. C D F B E 309 310 Take the third graph. If [R], the level of resource R, is constant at the mean value (Rmean), species E has 311 a higher RGR than F. However, if there be environmental fluctuation depends on the degree of 312 fluctuation around the mean, the mean growth rate of F would be higher. Thus, low fluctuation in [R] 313 advantages E, high fluctuation advantages F. The reason this matters is that at low [R] species E grows 314 faster than F and therefore depletes the resources, at high [R] it grows little more than at Rmean, and 315 leaves much of the R unutilised. Both ways, when species E is in the majority it exacerbates the 316 fluctuations in [R]. Conversely when it is in the minority, fluctuation in [R] is lower, which favours it: 317 increase when rare is achieved. 318 In contrast, species F grows little at low [R], and will hardly deplete R. At high [R] it grows 319 disproportionately fast, absorbing R and therefore reducing [R]. Both ways, when species F is in the 320 majority it damps down the fluctuations in [R]. Conversely when it is in the minority, fluctuation in [R] 321 is higher, which favours it: increase when rare is achieved. J.B. Wilson & Agnew, chapter 4, Species coexistence, page 11 of 34 322 323 The second way that environmental fluctuations can cause coexistence is the Storage effect (Chesson in press. There are four requirements for the Storage effect to operate: 324 1. The species must be competing for a resource. 325 2. They must be affected by an environmental (i.e. non-resource) factor, and respond differently 326 327 to it. 3. There must be covariance between the environmental factor and the intensity of competition. 328 We would expect this, because when the plants are denser and/or larger, competition will 329 be more intense. That is, in ‘favourable’ conditions competition will be greater. 330 4. There must be subadditivity (= buffering, = an interaction between environment and 331 competition). That is to say, when environmental conditions are favourable to growth the 332 effect of competition on RGR is greater. So, whilst ‘3’ refers to the intensity of 333 competition, ‘4’ refers to the effect of competition. 334 In years (or other periods) when the environment is favourable for a species, if it is in the majority and 335 therefore competing against itself it cannot take much advantage of the favourable conditions because it 336 is competing against itself at high biomass: X in Fig. 4.3. J.B. Wilson & Agnew, chapter 4, Species coexistence, page 12 of 34 Competitive intensity X Low RGR High RGR Environmental favourability Fig. 4.3: The effect of competitive intensity and environmental favourability on RGR. For ‘X’, see the text. 337 Competitive intensity X Low RGR High RGR Environmental favourability Fig. 4.3: The effect of competitive intensity and environmental favourability on RGR. For ‘X’, see the text. 338 339 Chesson’s (1994) calculations indicate that the Storage effect is a considerably stronger force than 340 Relative non-linearity. Your explanation of the storage effect and fig 4.3 is not clear (though I 341 understand what the storage effect is) 342 These mechanisms clarify that the timescale on which environmental fluctuation can cause 343 coexistence is set by the timescale on which resource depletion can occur. Light intensity can change 344 instantaneously, it cannot be stored from one second to another and its effects in producing 345 photosynthate are quite short-term, so within-day fluctuation could suffice. Water depletion could occur 346 over a few days, and nutrient depletion over a few months. Soil nutrients often become more available 347 in the spring due to mineralisation, but slow uptake over winter, and are depleted during the period of 348 active growth, so the Storage effect can operate on within-season or between year variation in nutrient 349 use (reword, runs on ). For neither Non-linearity or the Storage effect do the species need to differ in the 350 resources they use. However, they do use them at different times. Seasonal differences in resource use J.B. Wilson & Agnew, chapter 4, Species coexistence, page 13 of 34 351 can be seen either as Alpha-niche Differentiation or as the Storage effect. (this first mechanism is 352 wanting of a better conclusion paragraph) 353 3 Pest Pressure (heterotroph challenges) 354 Both pathogens and herbivores (from insects to large mammal herbivores) have the potential to 355 give an increase-when-rare process (we use ‘pest’ to cover both pathogens and herbivores). For this, 356 three conditions are required. 357 358 1. Impact: the pests involved must have a significant impact on the growth and/or survival of the plant species, i.e. their fitness must be reduced. 359 2. Specificity: The pests involved must be to some degree specific to the plant species. It could be 360 sufficient for the species with lowest interference ability to have no specific pests/diseases, but 361 to benefit when the others are suppressed by pests/diseases. 362 3. Abundance-dependence: The challenge from pests must be less on a sparse than on an abundant 363 species. This represents an abundance-dependent effect. The requirement is for a lower impact 364 on the growth and reproduction of sparse species, but this will presumably be through reduced 365 infection. 366 (3.1-3.3 mechansims don’t follow all of the above requiredIf these three conditions… are all three still 367 considered coexistence mechanisms?)If these three conditions metobtain, when any one of the plant 368 species in the mixture becomes more abundant, the host-specific pest (Condition 2) will move more 369 rapidly amongst its host population and the degree of infestation will increase (Condition 3). This will 370 reduce its fitness (Condition 1). This will not directly impact other species, or will do so only to a lesser 371 extent (Condition 2). Conversely, when a species becomes sparse, infestation by its specific pests will 372 decrease and its fitness increase relative to its fitness when more abundant, giving the increase-when- 373 rare effect. Again, the strength of conditions 1-3 necessary for coexistence depends on the degree of 374 difference in interference ability, and equalising mechanisms of coexistence can allow Pest Pressure to 375 result in coexistence when it otherwise would not. 376 Condition ‘1’ can often be met, since pests have various effects on plant production. Basically, 377 the pest organism must have a carbon requirement, which is almost bound to result in lower production 378 and fitness for the plant. Many pests are quite specific to a species or group of plant species, meeting 379 condition 2. Ecologists tend more to question condition ‘3’ because it is less obvious how abundance- 380 dependence could operate. Possible mechanisms (Boudreau and Mundt 1997) are via: (a) a decreased 381 abundance of palatable / susceptible plants, which inhibits the dispersal of herbivores, disease-spores or 382 disease-vectors, (b) the flypaper effect for disease spores and possibly for insect pests, virus vectors and 383 hence for the viruses they carry, in which the pest is caught by a passive surface, (c) alteration in the air J.B. Wilson & Agnew, chapter 4, Species coexistence, page 14 of 34 384 flow and microclimate, (d) chemicals from associated species that repel herbivorous insects, and (e) 385 promotion by an associated species of natural enemies of the herbivorous insects, i.e. their predators. 386 We shall discuss diseases and herbivory separately since they act in different ways with different 387 dynamics. However, there is sometimes evidence for abundance-dependent mortality or a reduction in 388 growth that is circumstantial evidence for the process and therefore for the Pest Pressure mechanism, 389 that cannot be attributed to a particular pest. For example, Packer and Clay (2000) examined the 390 distribution of seedlings of Prunus serotina (black cherry). The greatest number of seeds germinated 391 quite close to their parent tree, 5-10 m. However, 4 months later and thereafter up to 28 months, 392 seedling survival was higher the greater the distance from the parent tree, up to the furthest distance 393 monitored (30 m). This is not always the pattern. Dalling et al. (1998) in tropical rainforest on Barro 394 Colorado Island found that seedlings tended to be denser nearer to an adult of the same species. For 395 temperate forests Houle (1992) found the seedling mortality of Acer saccharum (sugar maple) in an 396 Eastern American forest was not abundance-dependent; there was no particular spatial relation between 397 trees and seedlings. Hyatt et al. (2003), in a thorough review of the literature, found no evidence for an 398 effect of distance from conspecifics on seed survival in either temperate or tropical communities, but 399 there was a tendency for seedlings to show higher survival at distance, with hints that this occurred 400 especially in the tropical forests. This matches the conclusions of Wright (2002) who, with a rather 401 different review approach, found considerable evidence of low growth performance of saplings near 402 conspecific adults. When effects like this are found, they are assumed to be because of Pest Pressure, 403 though Wright discusses other explanations. This kind of pattern recalls the ‘Janzen-Connell’ hypothesis 404 that whilst the greatest density of fruit will be dispersed to near the parent plant, pests will have the 405 greatest impact there, so the maximal regeneration will occur at an intermediate distance from the 406 parent. This would be expected to be an important abundance-dependent method of species' 407 maintenance in diverse, stable communities of trees, specifically tropical rain forest, but it seems that it 408 is far from universal. 409 3.1 Pathogens 410 Pathogens act in the soil, in the plant systemically and in the plant’s photosynthetic and 411 reproductive systems. The impact of fungal pathogens can be considerable. Mihail et al. (1998) found 412 that in a greenhouse experiment with the annual legume Kummerowia stipulacea (Korean clover) the 413 fungus Rhizoctonia solani caused mortality that reduced plant density by 40 % whilst the fungus 414 Pythium irregulare reduced density by 80 %. C.E. Mitchell (2003) found that in an oldfield grassland at 415 Cedar Creek 8.9 % of the leaf area was infected by fungal pathogens, which decreased root production 416 by 25 % by decreasing leaf life, while herbivores had no effect. J.B. Wilson & Agnew, chapter 4, Species coexistence, page 15 of 34 417 418 The species-specificity requirement is less easy to meet. Gilbert (2002) concludes that the evidence so far indicates that in natural communities that most fungi infect a large number of hosts. 419 However, the abundance-dependence criterion can often be met. Most ephemeral pathogens are 420 transmitted aerially including rusts and smuts which affect leaves, stems and floral parts. Abundance of 421 the host plant affects the pathogen's population and its persistence, and the transmission of specialist 422 pathogens can be highly sensitive to the identity of other host species in the community (Boudreau and 423 Mundt 1997). An example is the Ustilago violacea smut on Silene alba (≡ S. latifolia; white campion), 424 for which Thrall and Jarosz (1994) experimentally compared the behaviour of the host and pathogen 425 populations to theoretical models. The match was good and showed that both density dependence and 426 frequency dependence occurred. An excellent study by Burdon et al. (1992) described the mortality of 427 Pinus sylvestris (Scots pine) caused by the snow blight fungus Phacidium infestans as being mostly 428 abundance-dependent, with greater mortality in subsites where the host had been denser the previous 429 year. This abundance-dependence has sometimes been shown to lead to a lower pathogen load in 430 mixtures, which must imply some host specificity. C.E. Mitchell et al. (2002) examined 147 plots in an 431 experiment at Cedar Creek established by Tilman and co-workers, sown and weeded to species richness 432 from 1 to 24 species. The percentage of each leaf visibly infected was guessed, using calibrated cards as 433 a guide. Infection dropped as species richness increased, the 24-species plots having only 37 % the 434 foliar fungal pathogen load of the mean monoculture (though more than the least-infected monoculture). 435 Similarly, C.E. Mitchell et al. (2003) analysed another Cedar Creek experiment sown and weeded to 1 436 to 16 species, and the pathogen load in the 16-species plots was only 34 % of that in the mean of 437 monocultures. In chapter 3, sections 6 and 7.5 we described the ‘selection’ artefact in overyield and 438 invasion-resistance experiments. A similar artefact would be possible here if the species less susceptible 439 to disease had thereby an interference advantage and increased its proportion in the mixture, so that the 440 mixture had a lower mean pathogen susceptibility and thus a lower pathogen load. However, C.E. 441 Mitchell et al. (2002; 2003) present evidence that this is not the cause of the effect they found. C.E. 442 Mitchell and Power (2006) conclude that “the transmission of specialist pathogens can be highly 443 sensitive to the identity of other host species in the community”. 444 Similar effects could be caused by below-ground pathogens. Bever (2003) modelled this, but 445 concluded that there is no evidence for it yet. 446 3.2 Herbivory, general (This section does not convince me that herbivory can 447 clearly act as a mechanism of coexistence. How can a plant species be selected 448 based on this mechanism? 449 450 Herbivores come in all sizes, specialisations and guilds. On vegetative parts there are leaf eaters, stem borers and root eaters. On reproductive systems there are flower exploiters, frugivores and J.B. Wilson & Agnew, chapter 4, Species coexistence, page 16 of 34 451 granivores. Plant species are variously adapted to herbivory, with chemical and physical defences, life 452 histories and growth patterns that have evolved seemingly to deal with the challenges. The potential 453 mechanism for coexistence via herbivory is similar to that for pathogens (chap. 2, sects. 7.3, 7.4). 454 However, whilst pathogens often reduce the functional efficiency of plant parts, most herbivores simply 455 remove plant material so that the plant needs to regrow to replace tissue and thus its resource base. 456 Obviously a great variety of relationships can be expected between plant species and their herbivores, 457 including symbiotic ones, such as Tegeticula spp. and Parategeticula spp. (yucca moths; James et al. 458 1993).does this fit in with a sect about herbivory?. In many of these systems herbivores exploit plant 459 populations in an abundance-dependent way. Grover (1994) modelled this and used the keystone 460 concept (chap. 5, sect. 11 below) to suggest that a controlling herbivore is one that holds down the 461 abundance of a potentially-dominating plant species and thus allows subordinate species to survive. We 462 may distinguish between abundance-dependent culling of seeds and seedlings and wholesale removal of 463 plant material, i.e. vegetative herbivory. 464 3.3 Herbivory of disseminules and seedlings 465 Seeds and seedlings are a rich nutritional resource and are heavily predated. Maron and Gardner 466 (2000) showed by modelling that herbivores can control adult population abundance by limiting the 467 seed input to the seedbank. Such limitation seems to be widespread. It occurs also via vegetative 468 disseminules. Thus, the ‘impact’ requirement of the Pest Pressure mechanism can be met by 469 disseminule/seedling herbivory. 470 Disseminule herbivory can often be abundance-dependent. Cygnus bewickii (swans) eat the 471 turions (disseminules, fleshy buds) of Potamogeton pectinatus (pondweed) in the autumn. Jonzen et al. 472 (2002) demonstrated clear abundance-dependent control of the P. pectinatus in which the denser patches 473 of turions were exploited, reducing their density, while areas of low turion densities were unexploited 474 and here the plant density subsequently increased. Edwards and Crawley (1999) examined four species 475 of British meadows and found that granivory by rodents was abundance-dependent, but its effects on 476 adult densities differed. Densities of species with bigger seeds (Arrhenatherum elatius, oat grass; and 477 Centaurea nigra, knapweed) appeared to be reduced, but in the smaller seeded Rumex acetosa (sorrel) 478 and Festuca rubra (fecundity?) survival increased to compensate for seed predation, with no overall 479 effect on plant density. Again, Ehrlen (1996) found that in Lathyrus vernus (spring pea), although seed 480 predation by a Bruchidae beetle was correlated with seed density in small plots and with inflorescence 481 size, this had no consistent effect on plant population recruitment. Thus, even the occurrence of 482 abundance-dependent seed predation is no guarantee that it will control the population and hence 483 contribute to species coexistence. J.B. Wilson & Agnew, chapter 4, Species coexistence, page 17 of 34 484 Another limitation of disseminule/seedling herbivory as a mechanism of coexistence may be that 485 most mammal granivores are not specific to one species. There is huge literature assuming that bird 486 granivores are restricted by beak size to a particular range of fruit/seed sizes, but not to one species. 487 However, species of Bruchid beetle are generally restricted to the seeds of one or a few species of 488 Fabaceae (legumes). Seedlings are presumably eaten by invertebrates, but they are likely to be quite 489 generalist. 490 3.4 Vegetative herbivory 491 Herbivory on vegetative parts can be considerable. Vertebrate herbivores (Jones 1933) are 492 involved, and insects both above- and below-ground (Brown and Gange 1989). The selectivity of 493 herbivores varies widely. Many large non-ruminant animals such as Loxodonta africana (African 494 elephant), even though they have preferences, will readily eat a wide range of species. More 495 importantly, some such as Equus spp. (horses) often graze finely-patterned vegetation at a relatively 496 coarse scale, necessarily taking in species with a range of palatability. Other ungulates such as sheep 497 and cattle are more selective, some tiny flower gall wasps are confined to one or a few plant species, 498 and some lepidopterans feed on only one species such as Tyria jacobaeae (cinnabar moth) on Senecio 499 jacobaea (ragwort): Plate 4.2. 500 There are cases when coexistence can be attributed rather clearly to insect vegetative herbivory. 501 For example, Carson and Root (2000) found that periodic plagues of folivorous chrysomelid beetles 502 checked populations of the dominant Solidago altissima (goldenrod) in an oldfield in New York state, 503 USA, and were responsible for the diversity and successional rates. The effect of vertebrate grazing in 504 increasing species diversity is well known, as shown by the rabbit exclosures erected by Tansley and 505 Adamson (1925). However, the effect there is surely that when the sward is higher, light competition is 506 more important, and especially there is more opportunity for the feedback between the outcome of 507 competition and interference ability to occur (chap. 2, sect. 2.3), allowing exclusion by 508 interference.evidence? This has nothing to do with any mechanism of coexistence between plant 509 species why not, seems to be a stabilizing mechanism to me. 510 3.5 Pest Pressure conclusions 511 The Pest Pressure effect seems most likely to operate via diseases. However, there can be 512 complex interactions. An example is seen on Netherlands sand dunes, on which there are relatively 513 uniform soils and clear successional sequences. The growth of the pioneer Ammophila arenaria 514 (marram grass) is impacted by nematodes and pathogens (Van der Stoel et al. 2002). The nematodes 515 reduce growth only early in succession and early in the season, and are not significant in the marked 516 decline in vigour commonly seen in older stands of A. arenaria. However, there is apparently a J.B. Wilson & Agnew, chapter 4, Species coexistence, page 18 of 34 517 synergistic effect between fungi, and between fungi and nematodes, which reduces A. arenaria’s growth 518 late or early in succession?, and this is important in mixture with Festuca rubra, the next dominant in 519 the succession (de Rooij-van der Groes 1995; Van der Putten and Peters 1997). Still Later in the 520 succession pathway , in nutrient poor grasslands, there is often a mosaic of Festuca rubra and Carex 521 arenaria (sand sedge). Here Olff et al. (2000) discovered that each species had phases of increased and 522 decreased vigour, replacing each other, and that this process was associated with pest phases, 523 particularly the plant-feeding nematodes. Each species seems to be affected by different groups of pests, 524 leading to the changing mosaic aspect of the vegetation, which might look superficially like cyclic 525 succession (chap. 3, sect. 4). 526 4 Circular Interference Networks 527 Interference relations between a set of species are said to be transitive if the species can be species A species B species C Fig. 4.4. A circular competitive network between three species. species A species C species B Fig. 4.4. A circular competitive network between three species. 528 arranged in a pecking order, such that a species higher in the order can always competitively exclude 529 one lower down. An alternateThe opposite situation is the existence of circular interference networks 530 (Fig. 4.4). If such networks exist, they would contain an increase-when-rare mechanism: as species A 531 starts to displace species B, species C increases because it has high interference ability against A, but 532 then it in turn is replaced by B, completing the cycle. 533 Simple questions are not always neatly answerable. First, we note that the question can be asked 534 only in one environment, for competitive abilities will change with the environment (Keddy et al. 2000; 535 Fynn et al. 2005). Clearly they must; that is the main reason there is different vegetation in different 536 places. Second, the species that dominates the mixture will be the one with the higher relative growth J.B. Wilson & Agnew, chapter 4, Species coexistence, page 19 of 34 537 rate, but as interference proceeds the proportions of the species will change and as a result the relative 538 RGRs of the two species may change. Therefore, the eventual result must be judged in terms of 539 exclusion by interference (often loosely referred to as ‘competitive exclusion’). Yet we know that for a 540 variety of reasons have been presented in this chapter for why(Chapter 4) exclusion by interference does 541 not always occur. For these cases, the question of transitivity cannot be asked. Several studies have determined interference ability by comparing of species’ performances in 542 543 mixture with those in monoculture. Connolly (1997) pointed logical flaws in this. Correction can be 544 made for the “size-bias”, but the basic error has been comparison with a monoculture. We are tempted 545 to conclude that if species A grows more slowly in mixture than in its monoculture whilst species B 546 grows faster in mixture than in its monoculture, B has the higher interference ability. Yet Connolly’s 547 table (4.2), over the undefined period of his artificial data and assuming a starting biomass of 1, gives an 548 example where A does worse in mixture than in monoculture, and B does better in mixture than in 549 monoculture. Yet A has the faster growth rate in mixture (loge 2.77 – loge 1 = 1.02) than Species B (loge 550 2.71 – loge 1 = 1.00) and will come to exclude its competitor from the mixture (subject to the conditions 551 mentioned above). If B goes extinct it can hardly be said to have had the higher interference ability. 552 How does this demonstrate a circular interference network? Table 4.2. Which species has the higher interference ability? The starting biomass for both species was 1.00 Species Biomass in Biomass monoculture in mixture A 3.00 decrease 2.77 Winner in mixture B 2.64 increase 2.71 553 It turns out that what is essential in designing such an experiment is not the monocultures, as 554 many people had thought, but two harvests so RGR can be calculated. This invalidates almost all the 555 studies of transitivity done so far. All we have to do is to wait, perhaps for close to infinite time, and see 556 which species has the higher growth rate as the mixture approaches one of themawkward way to say 557 this. This is coming to be one of those community ecology questions that are impossible to answer. 558 At the moment, it is interesting to look at the imperfect evidence available. Buss and Jackson 559 (1979) claimed several competitive cycles for coral reef sedentary organisms, as seen in static evidence 560 for overtopping. Likewise, Russ (1982) claimed non-transitive relations between species in the 561 overgrowth of sedentary marine organisms observed colonising experimental plastic sheets in the sea in 562 Australia, though no cycle can be made out of his results. J.B. Wilson & Agnew, chapter 4, Species coexistence, page 20 of 34 563 Turning to pure plant work, Mouquet et al. (2004) grew eight meadow herbs species in 564 replacement mixture in all possible pairs. Using relative yield (RYi,j = biomass of species i when 565 growing with species j / biomass of i in monoculture), if the species form a transitive hierarchy it should 566 be possible to arrange them so if species i is further up the hierarchy than species j, and RYi,j-RYj,I is 567 always positive. In his experiment, at both low and high density, it almost is, and with a very similar 568 order (Table 4.3). 569 Table 4.3. Competitive hierarchy from Mouquet et al. (2004), strong competitors at the top High density Holcus lanatus Rumex acetosella Cerastium glomeratum Anthoxanthum odoratum Festuca rubra Arabidopsis thaliana Lamium pupureum Veronica arvensis Low density Holcus lanatus Rumex acetosella Cerastium glomeratum Anthoxanthum odoratum Festuca rubra Lamium pupureum Arabidopsis thaliana Veronica arvensis 570 571 At each density, there is one negative RY1,2-RY2,1 indicating a conflict with the hierarchy, it is between 572 species not contiguous in the hierarchy, but it is of size -0.05 andor -0.06 respectively which is clearly 573 within the experimental error. 574 A study that returned a clear answer to the question of transitivity is that of Roxburgh and 575 Wilson (2000a). It relates to a real community, since the seven species used in the interference 576 experiment were taken from that community, the University of Otago Botany Lawn, grown in lawn soil 577 in boxes placed near the lawn. The use of 10 replicates in careful experimental conditions allowed 578 significance tests. The seven species could be arranged in a hierarchy to which all significant 579 competitive relations conformed, i.e. if species X is higher in the hierarchy and species Y lower, then 580 the suppressive effect of X on Y is greater than that of Y on X (Fig. 4.5). In fact, relations between all 581 pairs of species, significant or not, were compatible with the hierarchy. J.B. Wilson & Agnew, chapter 4, Species coexistence, page 21 of 34 582 583 Figure from most competitive to least? Adding an arrow to indicate this would be nice. Explain this 584 figure more. Why no arrows from Holcus to trifolium, Holcus to ranunculus, etc? 585 Fig. 4.5: Competitive relations in seven species from the University of Otago Botany Lawn. From 586 Roxburgh and Wilson (2000a). 587 The experimental design of Keddy et al. (1998) comprised planting a number of ‘wetland’ 588 species into a number of swards of wetland species. They report results for 18 species planted into five 589 swards. The 18 species tended to respond similarly to different swards, e.g. Kendal’s coefficient of 590 concordance took a rank of 0.7 (1.0 = complete agreement as to which target suffered more/less), highly 591 significant. Some of the variation in invader/sward? Explain this study more combinations could be due 592 to experimental error (no replication was possible), but some results are impressive, e.g. the rank of 593 Carex crinita (sedge) varied only from 14 to 17 across the 5 swards (18=suppressed most), and Lythrum 594 salicaria (purple loosestrife) varied from 4 to 7 (1= suppressed least). 595 In a different approach, Silvertown et al. (1992) used data from an experiment where several 596 species had been planted in adjacent hexagons, and invasion between hexagons recorded. Examining the 597 difference between the invasion of Species A into Species B and that of Species B into Species A, 598 replacement rates could be calculated. A pecking order can be formed from these results (Fig. 4.6), with 599 no discrepancies (though L. perenne (ryegrass) and C. cristatus (dog’s tail) could equally well exchange 600 positions). There are qualitative discrepancies. Since H. lanatus (Yorkshire fog) can invade P. trivialis 601 (A) and P. trivialis can strongly invade L. perenne (B), the expectation would be that H. lanatus would 602 be able to invade L. perenne even more strongly, but in fact their invasion rates are exactly balanced J.B. Wilson & Agnew, chapter 4, Species coexistence, page 22 of 34 603 (C). Moreover, although the species A. stolonifera (creeping bent) at the top of the order can invade C. 604 cristatus at the bottom, the rate of replacement is less than for other pairs (D). Agrostis stolonifera Holcus lanatus A Poa trivialis D B C Lolium perenne Cynosurus cristatus 605 Agrostis stolonifera Holcus lanatus A Poa trivialis D B C Lolium perenne Cynosurus cristatus 606 607 Key: Strong (> 0.2) difference in invasion rates 608 Weak-moderate difference in invasion rates 609 Invasion rates equal (i.e. no net invasion) 610 Fig. 4.6. The competitive hierarchy from invasion rates in data of Silvertown et al. (1992). 611 In a similar experiment Silvertown et al. (1994) used only four species, so there was less 612 opportunity for intransitivity, but in any case there was none in any of the four grazing treatments (Table 613 4.4). 614 Table 4.4: Competitive hierarchy of four species in four treatments in Silvertown et al. (1994). 615 Summer sward grazing height Winter and spring Invasion ability: greater → lesser 3 cm Grazed Lolium perenne → Festuca rubra → Schedonorus phoenix → Poa pratensis 3 cm Ungrazed Festuca rubra → Lolium perenne → Poa pratensis → Schedonorus phoenix 9 cm Grazed Festuca rubra → Lolium perenne → Schedonorus phoenix → Poa pratensis 9 cm Ungrazed Lolium perenne → Festuca rubra → Poa pratensis → Schedonorus phoenix J.B. Wilson & Agnew, chapter 4, Species coexistence, page 23 of 34 (1) (2) A A is taller than B and shades B out C produces an allelopathic chemical, toxic to A C C is shadetolerant, and scavenges nitrogen C 616 A o Tree A is taller than shrub B and shades B out grass C lowers the temperature, and suppresses seedlings of A B B is taller than C (3) A B C shrub B shades out grass C, and is not affected by lower temperature (4) A is taller than B and shades B out B is taller than C, and fixes N A A is taller than B and shades B out C with A is taller than it, and shades it out B C B B with C is taller than it, and shades C out . (1) A is taller than B and shades B out C produces an allelopathic chemical, toxic to A C C is shadetolerant, and scavenges nitrogen A o A Tree A is taller than shrub B and shades B out grass C lowers the temperature, and suppresses seedlings of A B B is taller than C (3) C (2) A B C shrub B shades out grass C, and is not affected by lower temperature (4) A is taller than B and shades B out B is taller than C, and fixes N B A C with A is taller than it, and shades it out C A is taller than B and shades B out B B with C is taller than it, and shades C out 617 618 619 620 Fig. 4.7. Possible causes of intransitivity between three species: A, B and C. It’s interesting to wonder what ecological processes would give rise to intransitivity (Fig. 4.7). In scenario ‘1’, we use an allelopathic chemical produced only by C and toxic only to A. This works, but J.B. Wilson & Agnew, chapter 4, Species coexistence, page 24 of 34 621 species-specific allelopathy is rather like Getafix’s magic potions in the Asterix books??: it can 622 perform/explain any wonder. Scenario ‘2’ is similar, except that the third factor is lower temperature 623 (Ball et al. 2002) rather than a toxin. In ‘3’, we have to ask why C can suppress A; presumably the 624 shade-tolerance of C minimises the competition for light, so competition for N becomes important, and 625 C has the lower Tilman R* (chapt. 6, sect. 7.1). Why cannot C suppress B? Perhaps because it is shorter 626 and so cannot compete for light, and its low R* for N does not help because B can fix N. Does this 627 work? Probably. In all three cases, not all pairs are interfering using the same resource/factor. Could we 628 envisage a 3-species solution using competition for light (‘4’)? How can there be heights of A>B, B>C 629 and C>A? Differential plasticity allows such magic: in this case probably by red:far-red effects (chap. 2, 630 sect. 2.6). However, we are again introducing a second factor: light spectrum in addition to light 631 intensity. All this is rather convoluted, which suggests that intransitivity will not be the norm. Move 632 this theoretical discussion to before the examples 633 The evidence is that circular interference networks are uncommon. They have not been observed 634 in plants. In retrospect we should have expected that, because we had not thought what mechanisms 635 would cause them, and such mechanisms are difficult to envisage. This is almost certainly not an 636 important mechanism of coexistence. 637 5 Allogenic Disturbance (disrupting growth, mainly mechanically) 638 Disturbance can have the same effect as climate variation (Roxburgh et al. 2004), but the true 639 Intermediate-timescale disturbance mechanism is a patch mechanism: within an area there are patches 640 of different time since disturbance, with different suites of species (J.B. Wilson 1994a). This gives a 641 successional mosaic. Whether this comprises coexistence depends on the scale at which the system is 642 viewed. Coexistence is seen only when considering a scale that is larger than the size of a disturbance 643 patch, so that it includes patches of differing time since disturbance – newly-disturbed versus recovered. 644 The different patch types are different beta niches, but on a small scale. A, and a species specialising in 645 a particular patch type will increase when rare because it will have more of its specific resources 646 available. In a sense, Allogenic Disturbance should not be counted as a mechanism of coexistence; we 647 do so here because it is so frequently seen as one, because of the impossibility of defining the target 648 scale, and because disturbances occur on all scales so that however small a scale we examine there will 649 still be disturbances within it. The disturbance does need to be sufficiently frequent that each patch will 650 usually include patches at various stages of recovery explain, or the mechanism will not operate. 651 All types of allogenic disturbances happen and it is not always easy to separate disturbance from 652 climatic stress, i.e. the environmental fluctuation discussed above (section 2). The characteristics of 653 disturbance are: (1) Plants not only fail to reproduce but are killed, at least above ground. (2) Most 654 species are killed, not just those that cannot tolerate a particular stress. (3) The event is sudden. (4) The J.B. Wilson & Agnew, chapter 4, Species coexistence, page 25 of 34 655 environmental effect is temporary, i.e. it is a pulse perturbation, so the original species can re-establish 656 the composition of the patch. However, the real difference is that Allogenic Disturbance is a between- 657 patch mechanism. Disturbance is common, creating gaps over the landscape at a range of scales from 658 meteor hits (many km2) to worm casts (about 0.03 m). Fossorial rodents, ants and termites act at scales 659 which can be important for individual plants. A good example is that of McGinley et al.’s (1994) 660 description of enriched harvester ant mounds in western Texas. It is possible that much of the variation 661 seen in communities is due to old disturbances, where vegetation cover has been regained, and obvious 662 pioneers have been eliminated, but differences in species composition remain. Perhaps we do not realise 663 this because ecologists fail to recognise mid-succession species as being such (Veblen and Stewart 664 1982). 665 The Allogenic Disturbance mechanism assumes that there are distinct pioneer and climax 666 species, i.e. r and K, R and C-S. However, we are talking of secondary succession, and cannot assume 667 this. Peterson and Pickett (1995) found that after windthrow disturbance in a North American conifer / 668 deciduous forest some species regenerated by the germination of seed and some from already-present 669 seedlings, but pioneer shade-intolerant species were sparse, apparently due to a lack of propagule input. 670 Autosuccession, in which the climax species immediately re-establishes after a disturbance, is known 671 from mesic areas such as after windthrow in temperate Nothofagus rainforest in New Zealand 672 (Cockayne 1926), but it is specially found under environmental stress, as predicted by C-S-R theory 673 (chap. 6, sect. 6.7 below; J.B. Wilson and Lee 2000). We might expect that the greater species richness 674 in tropical rain forests would include a good number of gap specialists. Indeed, Hubbell (2005) 675 demonstrated for Barro Colorado Island tropical rainforest a close negative correlation among species 676 between survival rate in shade and growth rate in full light (in gaps), though admitting there were rather 677 few gap species and their abundance was low. Wright et al. (2003) found that there was a continuous 678 distribution of gap-colonising species and those that avoided gaps in Barro Colorado Island, Panama, 679 but that the majority were rather indiscriminate. Similarly, Lieberman et al. (1995) found that 87 % of 680 the tree species in Costa Rican tropical forest had no significant canopy-gap / matrix specialization. 681 Poorter et al. (2005) found that only one of 47 species in a Liberian tropical rainforest was a shade 682 species for its whole life, and only one a light species for its whole life. It is clear that most species are 683 intermediate in this respect. This suggests that Allogenic Disturbance may not be an important 684 mechanism of coexistence in the very biome where we tend to envisage it. Yet in temperate forests there 685 may be greater opportunities for it to increase species richness: Poulson and Platt (1996) demonstrated 686 in Michigan that the size of the gap affects the species re-establishing, such that single treefalls favoured 687 Fagus grandifolia (American beech) but multiple fall gaps favoured Acer saccarum (sugar maple). This 688 is not a question of gap versus non-gap, but also of differences between different sorts of gap. J.B. Wilson & Agnew, chapter 4, Species coexistence, page 26 of 34 689 Gaps are by no means restricted to forests. Grubb (1982) suggested that in roadside communities 690 around Cambridge, England, the climax dominant amongst the grasses was Arrhenatherum elatius (oat 691 grass), but Dactylis glomerata (cocksfoot) and Plantago lanceolata (ribwort plantain) retained their 692 place in the community by being the first to invade small gaps. 693 6 Interference/dispersal Tradeoff 694 This concept originated simultaneously with Skellam (1951) and Hutchinson (1951). It has been 695 known under a variety of names (J.B. Wilson 1990), including ‘Life History Differences’ and the 696 endearing if not entirely accurate ‘Musical Chairs’ (Crawley 1986). Consider a model in which two 697 annual species occupy single-plant safe sites. Species C is the better competitor, and eliminates the 698 weaker competitor D if it reaches a site, but it has less efficient reproduction/dispersal than species D 699 and therefore fails to reach some sites. Species D has better dispersal and is therefore available to 700 colonise most of the sites that C has not reached. If C becomes sparse, there are many empty sites for its 701 offspring to occupy and its population growth rate increases; similarly if D becomes sparse, there are 702 many sites left over by C for it to occupy. This is increase-when-rare. The mechanism can be 703 distinguished from (‘1’) Niche differentiation in that no differences between species in resource use are 704 required. It can be distinguished from (‘5’) Allogenic Disturbance in that: (a) the gaps are caused by 705 monocarpic or seasonal death, not necessarily by external disturbance, though that is possible, and (b) 706 species C is limited only by dispersal, not by its ability to tolerate the environment of the gap. It can be 707 distinguished from (‘9’) Equal Chance in that, though there is a random element, it acts via dispersal; 708 the interference abilities of the two species are very different. 709 There have been many mathematical models of the mechanism, e.g. Levins and Culver (1971), 710 Nee and May (1992) and Tilman (1994). There is an assumption of a negative correlation, due to a 711 trade-off, between interference ability and dispersal ability, but Ehrlén and van Groenendael (1998) 712 surveyed the literature and found that this was un?common. Turnbull et al. (1999) demonstrated the 713 mechanism experimentally by sowing seven species from a limestone grassland, ranging from a seed 714 mass of 0.013 to 0.16, back into that grassland. When the seeds were sown at a high density, 83 % of the 715 resulting plants were from the three species with the largest seeds, but when a low density was sown this 716 percentage was reduced to 49 %. This is entirely compatible with the Interference/dispersal Tradeoff 717 mechanism: when there were enough seeds to reach almost all microsites the three big-seeded, strong 718 competitors occupied them, but when fewer seeds were sown there were microsites not occupied by the 719 big three, which the light seeded, probably well-dispersed species could occupy. The unlikely 720 Interference/dispersal Tradeoff theory is proved my intro bio students would loose points for a statement 721 like this in their lab reports. J.B. Wilson & Agnew, chapter 4, Species coexistence, page 27 of 34 722 723 7 Initial Patch Composition The coexistence model of Levin (1974) is that two species occupy small, transient patches. Some 724 patches will by chance have more individuals of one species than the other. The species in the majority 725 will suppress the other in that patch if intra-specific interference is less than inter-specific interference. 726 The latter condition is beloved of ecological modellers, but it seems unlikely in the real world. It would 727 be possible with mutual species-specific allelopathy: the Getafix potions? of community ecology. We do 728 not believe this model can apply to plants (or at all). Examples? Evidence? 729 8 Cyclic Succession: movement of community phases 730 This topic was covered in chapter 3, section 4. The increase-when-rare mechanism is similar to 731 that of (‘4’) Circular Interference Networks. The latter are between individual species whereas cyclic 732 succession involves the whole community, though in many of Watt’s (1947) examples the community 733 comprises one species. Cyclic succession involves reaction, but then interference also involves 734 environmental modification, be it more temporary. There could be cyclic succession between just two 735 phases, whereas there cannot logically be a circular interference network with fewer than three species. 736 Autoallelopathy, for example, would satisfy our criterion for increase-when-rare because a 737 species that is sparse finds less of its self-toxin in the soil, but there must be some kind of reaction 738 involved, giving abundance-dependence. A mosaic arises because a phase of the cycle that is replaced at 739 one point appears elsewhere; we therefore count it as a mechanism that uses movement to escape 740 exclusion by interference. This also means that the mechanism is scale-dependent: the scale examined 741 has to be one that includes patches of the mosaic in different phases. 742 9 Equal Chance: neutrality 743 It is a longstanding idea that there is an element of chance in which species occurs at a spot 744 (Lippmaa 1939). This has been especially invoked for tropical rain forests (e.g. Schulz 1960; Hubbell 745 and Foster 1986). Sale (1977) described it as a ‘lottery’ and Connell (1978) formally put it forward as a 746 mechanism of coexistence, the ‘Equal Chance’ mechanism. In this section, when we speak of chance, 747 we refer to processes such as dispersal that are so complex as to be unpredictable in practice, combined 748 with equally unpredictable climatic and catastrophic disturbance events. It is impossible to prove the 749 operation of chance, but some have implicated it. 750 Equal Chance means that any one of a number of species is equally likely to occupy and 751 pre-empt by reaction a particular microsite. One cause would be that the probability of a disseminule 752 reaching a site is proportional to its abundance this is equal chance?. Then, dispersal would determine 753 which species occupies a particular site (Schulz 1960). In New Zealand, Veblen and Stewart (1980) 754 used this as an explanation for the colonisation of canopy gaps by either Dacrydium cupressinum J.B. Wilson & Agnew, chapter 4, Species coexistence, page 28 of 34 755 (rimu), Weinmannia racemosa (kamahi) or Metrosideros umbellata (southern rata) as a function of 756 seed/seedling availability, mast seeding and the ability of many New Zealand tree species to remain as 757 suppressed seedlings. Equal interference abilities are likely to be invoked: the ‘Equivalence of 758 Competitors’ concept of Goldberg and Werner (1983) that often interference is intense, but many 759 species are similar in their interference ability. Sometimes the outcome of interference can be 760 established at the seedling stage, since when competition is for light and therefore cumulative, the first 761 plant to establish will exclude others, a type of inertia (chap. 2, sect. 2.3). This again invokes random 762 dispersal. Alternatively, both the probabilities of ecesis and their interference abilities might be different 763 between species, but the two balance. 764 The Equal Chance concept would result in variation in the species composition of communities 765 which it was impossible to correlate with any environmental factor, present or past. Some have used this 766 kind of negative result as evidence of chance. McCune and Allen (1985) in forests in Montana, USA, 767 R.B. Allen and Peet (1990) in forests in Colorado, USA, and Kazmierczak et al. (1995) in kettle-holes in 768 Poland found only weak correlation between species composition and the environment and invoked 769 chance. In such work, the weak correlation could be because there were important environmental factors 770 that had not been measured, or because some factors measured gave a non-linear response that the 771 analysis could not cope with. The Equal Chance hypothesis, used as an excuse for failing to find 772 vegetation/environment correlations is the last resort of the scoundrel (ouch, that’s pretty harsh. Are 773 you sure you are presenting McCune and Allen (1985) and R.B. Allen and Peet (1990) fairly and 774 accurately?. Lavorel and Lebreton (1992) compared the composition of the vegetation with that of the 775 seed pool in fields from southern France, and took the similarity as evidence of a random draw from the 776 seed pool. This is also doubtful evidence; it could equally well be caused by determinism. 777 The most well-known invocation of chance is the Island Biogeography model of MacArthur and 778 Wilson (1963), based on probabilistic immigration and extinction. However, Kelly et al. (1989) and 779 Tangney et al. (1990) could find little evidence for its operation in Lake Manapouri islands. On islands, 780 a direct test of determinacy v. chance (assuming that incidence functions are not important) is available 781 in a test for nesting. On the other hand, J.B. Wilson (1988d) found plant species nesting among these 782 islands to be significant, but far from complete. His analyses pointed to habitat control rather than 783 chance, at least for native species. J.B. Wilson et al. (1992a) sampled the algal flora of intertidal rock 784 pools, selected for habitat uniformity within a limited area, and analysed as virtual islands. The 785 distribution of species agreed closely with that expected at random, whether examined by the 786 distribution of associations, by nesting, by chequerboarding or by incidence functions. The simplest 787 explanation is that differences in specific composition between the pools are caused by chance, but that 788 is no proof; it is a minimalist default. The best example of chance – no difference between species if one 789 can ever have an example of no difference – is from Munday (2004), who investigated two small J.B. Wilson & Agnew, chapter 4, Species coexistence, page 29 of 34 790 congeneric coral-reef fish species, where there was evidence for interference, in field removal 791 experiments, and in lab colonisation. However, in none of these experiments, nor in field distribution, 792 was there any evidence of niche differentiation. 793 Even Equal Chance’s strongest advocates have been equivocal. Hubbell (2005), having 794 emphasised differences in niche between Barro Colorado Island tropical rain forest species, eventually 795 attributed coexistence to dispersal and recruitment limitation. This is in effect a resort to Equal Chance. 796 However, he immediately discussed negative abundance-dependence, which is stabilising, not 797 equalising. The Equal Chance mechanism is the equalising mechanism par excellence, and should be 798 seen as no more than that. 799 800 801 What about neutral models ala Hubbel? 10 Inertia Inertia is another type of equalising mechanism, slowing exclusion by interference and possibly 802 allowing stabilising mechanisms to operate. 803 10.1 Temporal Inertia 804 Temporal Inertia (Cowles 1901) can be an individual or population effect, effectively the same 805 because the ‘individual’ concept is not meaningful for plants. Trees stand where they stand and cause 806 inertia. If, when a tree fell over, there were a tendency for the niche it had constructed to favour its own 807 juveniles in the canopy gap created, this would represent a small-scale dispersal switch. This could 808 constitute inertia, slowing the ingress of a superior competitor. We mentioned above that Dalling et al. 809 (1998) found seedlings on Barro Colorado Island to be denser near to a conspecific adult. Species 810 differed little in the correlation of their growth rate with light intensity and they declared that differential 811 responses to soil and topography were rare. This left them to speculate that there was dispersal 812 limitation, as supported by the correlation between parent and juvenile being weaker for species with 813 small disseminules. This is inertia due to dispersal limitation. Annuals are a conspicuous life form in 814 arid climates where the rainfall is highly erratic and form a long-lived seed bank. This seed bank gives 815 inertia as well as a storage effect. A population with a seed bank has similarities to a tree, except that 816 this is multi-generation inertia. The decade-long dominance phases, with smooth increases and 817 decreases, that Watt (1981) found in the Breckland may owe something to inertia. A more active 818 mechanism of inertia would be delay in vegetation change caused by a switch (chap. 3, sect. 5.3). Inertia 819 caused by the length of life of trees must be common. Abrams and Scott (1989) describe a situation 820 where, until disturbance occurs, early-successional trees dominate the canopy with young plants of later 821 successional stages beneath them, and their model diagram shows the high species richness resulting at 822 this stage. J.B. Wilson & Agnew, chapter 4, Species coexistence, page 30 of 34 823 These situations are obvious in dryland vegetation where the rainfall is erratic and temperatures 824 are high. For many succulents, sufficient rainfall for the development of surface root hairs is itself a rare 825 event, yet by means of massive storage, CAM photosynthesis and often a high albedo, they can 826 withstand years of unavailable moisture. The establishment of such succulents can be a very rare event, 827 as exemplified by Agave macroacantha in Mexico (Arizaga and Ezcurra 2002). Minor rain events may 828 allow annuals to grow, while deep-rooted shrubs and trees maintain contact with a deep water resource 829 giving the appearance of a plant community. Clarke (2002) described a similar situation with woody 830 dryland vegetation in southwestern Australia, where no natural recruitment of shrubs was observed over 831 five years. However, the rare event required to cause the state change could be a different grazing 832 regime, as Prins and van der Jeugd (1993) found in Tanzania. Two pandemics in the herbivores in 1880 833 (rinderpest) and 1961 (anthrax) temporarily reduced browsing and allowed even-aged stands of Acacia 834 tortilis (umbrella thorn) to establish. These are now a conspicuous and apparently integral part of the 835 vegetation of national parks in the area, yet are present through inertia, not as maintained populations. 836 Here, the state change was anthropogenic, but a similar situation could occur naturally. Inertia may not 837 apply to all the species in a community, since many contain species that differ markedly in survival and 838 establishment probabilities. Extremely long-lived individuals of slow growth exist alongside perennials 839 with lifespans shorter by at least one order of magnitude. The long-lived individuals can establish only 840 during a rare event, which could be a disturbance such as flood, a 1/100 yr wet season. The probability 841 of such an event occurring in any one year is very low and does not change from year to year. Thus their 842 occurrence is stochastic yet within the time scale of very long-lived plants. 843 We still ask what the original coexistence was due to: if there is no coexistence, inertia cannot 844 prolong it. 845 10.2 Spatial Inertia: aggregation 846 Spatial aggregation of the plants of a species also gives inertia, delaying exclusion by 847 interference since it occurs only at patch boundaries. Presumably the aggregation was established due to 848 dispersal processes, an ‘ecological founder effect’. Stoll and Prati (2001) demonstrated beautifully the 849 slowing of exclusion by interference by experimental aggregation. Amongst four annuals they found 850 that the species with least interference ability (Cardamine hirsuta, bitter-cress) decreased over the 851 experiment to 6 % of the monoculture in a random arrangement but only to 26 % in an aggregated 852 arrangement. The species with lowest interference ability (Stellaria media, chickweed) increased its 853 biomass to 324 % of the monoculture in the random but to only 239 % in the aggregated1. This would 854 be a most potent mechanism for delaying exclusion by interference of a subservient species. Rebele 855 (2000) found a similar, but very slight, effect in an outdoor mesocosm experiment using mixtures of 856 Calamagrostis epigejos (reed) and Solidago canadensis (goldenrod). J.B. Wilson & Agnew, chapter 4, Species coexistence, page 31 of 34 857 Thórhallsdóttir (1990) had planted outdoors a hexagonal grid of adjacent plots. Each plot 858 contained one of five meadow species: Agrostis stolonifera (creeping bent), Holcus lanatus (Yorkshire 859 fog), Cynosurus cristatus, Poa trivialis (meadow grass), Lolium perenne (ryegrass) and Trifolium 860 repens (white clover). Silvertown et al. (1992) ran simulations to see in retrospect what effect 861 aggregation would have, given the invasion rates that Thórhallsdóttir found between the pairs of grass 862 species. After 50 time periods when the species were intermixed in a random pattern, the weakest 863 competitor Lolium perenne had almost disappeared (reduced from 20 % to 1 %), but with the species 864 ‘planted’ in bands, depending on the order of the species in the bands, it decreased only to 9 %, stayed 865 at 20 % or even increased slightly to 21 %. 866 Aggregation might also delay exclusion by interference via effects on herbivory (Parmesan 867 2000), fire spread (Hochberg et al. 1994) and other environmental factors. 868 11 Coevolution of Similar Interference Ability 869 Aarssen (1983) suggested that in a mixture of two species stronger selection pressure on the one 870 with lower interference ability would cause it to become the stronger competitor of the two, “Superiority 871 in competition therefore alternates between … members of the two populations”. He later (1989) 872 produced some evidenced for this: over two generations the interference ability of Senecio vulgaris 873 (groundsel) increased relative to a standard genotype of Phleum pratense (Timothy grass) with which it 874 was growing. Selection can result in small-scale genetic change in populations, as apparently occurred 875 in Trifolium repens (white clover) associated with different ecotypes of Lolium perenne (ryegrass) in the 876 pastures that Lüscher et al. (1992) investigated. However, neither this study nor that of McNeilly and 877 Roose (1996) could find evidence of co-adaptation between neighbouring ecotypes of associated L. 878 perenne. Eventual ecotypic evolution in response to neighbours would be expected, and has 879 occasionally been demonstrated (Martin and Harding 1981). However, Aarssen’s proposal is 880 unbelievable because it involves continual increases in interference ability, as Aarssen (1985) has since 881 concluded. The plastic response to interference (chap. 2, sect. 2.2) can also give a buffering effect. 882 12 Spatial Mass Effect (vicinism) 883 The Spatial Mass Effect refers to the maintenance of a population of a species by constant 884 immigration into a patch where the species cannot otherwise maintain itself (Zonneveld 1995). It has 885 been called the sink effect. The immigration could be by seeds, or in theory by rhizomes or stolons. 886 Populus tremuloides and related species (aspen) produce root suckers (Barnes 1966) and these can 887 appear beyond the canopy of the tree where there is no chance that they will survive to be self- 888 supporting, let alone sexually reproductive, for example in a lawn. Seed immigration is the most 889 common but difficult to demonstrate. It is difficult enough to monitor occasional seeds blowing in, and J.B. Wilson & Agnew, chapter 4, Species coexistence, page 32 of 34 890 even more difficult to demonstrate that the population into which they are blowing would have RGR < 891 0.0 without that subsidy. Snyder and Chesson (2004) have applied the concepts of the ‘storage effect’ 892 and non-linear dynamics to coexistence between species that have different tradeoffs of interference 893 versus fecundity+dispersal. Their model has Spatial Mass Effect, though also elements of (‘6’) 894 Interference/dispersal Tradeoff. The effect clearly maintains populations that are not susceptible to 895 considerations of abundance-dependence or increase when rare, the stabilising mechanisms we require 896 here, yet it can maintain coexistence indefinitely. 897 The Spatial Mass Effect has rarely been quantified. Kunin (1998) examined boundaries between 898 plots with different fertiliser treatment in the 150-year old Park Grass Experiment. There was a very 899 sharp pH change, within 50 cm of the boundary. Although there were many exceptions, the majority of 900 plots examined (34 out of 51 non-zero, 2-tailed p = 0.024) showed higher species richness towards the 901 boundary. The effect was seen especially where the two adjacent plots differed more in species 902 composition. The Spatial Mass Effect can be seen clearly in extreme cases where the recipient (sink) 903 population does not reproduce at all, like the 13 species of angiosperm that grow in the Lost World 904 Cavern, northern North Island, NZ, without any of them ever setting seed (de Lange and Stockley 905 1987). Studying an Argentinian steppe with the (palatable) grass Bromus pictus amongst tussocks of 906 unpalatable grasses Stipa spp. and Poa ligularis, Oesterheld and Oyarzábal (2004) found more B. pictus 907 in the upwind part of a grazing exclosure, showing that a seed subsidy was arriving from the grazed 908 area. The tussocks outcompeted the B. pictus when ungrazed, reducing the local seed output in the 909 exclosure. This situation may be the commonest way in which the spatial mass effect operates to 910 maintain species metapopulations. 911 13 Conclusion 912 We believe our review covers all the mechanisms by which species can coexist in stable 913 mixtures. Chesson’s terminology of stabilising versus equalising mechanisms is useful and important. It 914 has focussed attention on the fact that some proposed mechanisms of ‘coexistence’ do not, in fact, cause 915 long-term coexistence. It has also highlighted what few had recognised, that even though the equalising 916 mechanisms cannot on their own cause stable coexistence between two species, they can reduce the 917 difference in interference ability between species to the extent that a stabilising mechanism can operate. 918 We must speculate on the importance of each mechanism in order to build up in our minds a 919 vision of the plant community. The overwhelming reason for species coexistence is Alpha-niche 920 Differentiation. Environmental Fluctuation is probably important. It can be seen as niche differentiation 921 in time, but with special restrictions on when it can operate. In seasonal climates, local environment can 922 vary enormously, both stochastically and predictably. Each of the ecosystem attributes enumerated by 923 Reichle et al. (1975; see also chap. 1, sect. 1 above) must change during the year: the energy base J.B. Wilson & Agnew, chapter 4, Species coexistence, page 33 of 34 924 (affected by irradiance), the reservoir of energy, nutrient cycling (through mineralisation rates) and rate 925 regulation (temperature, water availability, herbivory). It seems that the available states of these 926 variates, factorially combined, should allow for the coexistence of a very large number of plant species. 927 Pest Pressure may be important; Gillett (1962) suggested that it is the major mechanism, but that 928 remains to be proved. The Spatial Mass Effect must be very common. Disturbance is clearly common, 929 and has potential to allow co-existence; surely all communities are successional mosaics. We earlier 930 discussed autogenic disturbance. It could have been listed as a separate mechanism here, it could have 931 been merged with Allogenic Disturbance since many disturbances are partly allogenic and partly 932 autogenic, or it could have been included with Cyclic Succession since it will often be a component. 933 Other mechanisms are probably of more minor importance. For example, circular interference 934 networks are an attractive idea, but remain undiscovered. Cyclic succession is more believable and from 935 time to time fashionable, but seldom observed and even more rarely proven to take place. 936 Autoallelopathy may be widespread but the soil is an intractable and infinitely complex medium where 937 clear chemical pathways and effects are difficult to prove. 938 Based on the evidence derived from the present literature, we list the mechanisms below in 939 increasing order of importance: This is not a very effective way of ordering these mechanisms…how 940 about grouping into minimal, moderate and strong influencial force? Or, maybe present them in 941 increasing/decreasing order of importance in the text. 942 Initial Patch Composition (7) 943 Co-Evolution of Similar Interference Ability (11) 944 Equal Chance (9) 945 Circular Interference Networks (4) 946 Cyclic Succession (8) 947 Temporal and Spatial Inertia (10) 948 Interference/Dispersal Tradeoffs (6) 949 Allogenic Disturbance (5) 950 Spatial Mass Effect (12) 951 Pest Pressure (3) 952 Environmental Fluctuation (2) 953 Alpha-niche Differentiation (1) 954 Switch to order of decreasing importance. Those lower in the ranking- their order seems 955 arbitrary or would vary with context 956 In a changing, disturbed world it will be increasingly difficult to separate stabilising mechanisms from 957 equalising ones. The temporal turnover of species in communities depends on some species 958 disappearing, others invading, so that many species in a community may be present by courtesy of one J.B. Wilson & Agnew, chapter 4, Species coexistence, page 34 of 34 959 of the equalising mechanisms and will ultimately be doomed. The multiplicity of possibilities for 960 coexistence should allow the coexistence of a very large number of plant species. The question then 961 becomes: “Why are there so few species in most habitats?”. 962 A plant, however, is sedentary and extends over a spatial volume, necessarily exposed to wide 963 range of environmental conditions. It therefore cannot be confined to a precisely-defined niche. It is the 964 interplay between the potential for high plant diversity in restricted niches and the necessity for plants to 965 tolerate a wide range of environments that encourages us to look for patterns in plant communities. If 966 adaptations to available niches were most of the reason for every species’ occurrence, our enquiry in 967 this book would be less interesting. 968 Footnotes 969 1 970 ‘sparse’ would be a better term than ‘rare’, but ‘increase when rare’ is ensconced in the literature, and so we use it here. 971 TABLES, ILLUSTRATIONS and PLATES 972 Table 4.1: Some Examples of monospecific stands. We exclude monospecificity in a single stratum or 973 974 guild of vegetation, such as a tree species or understorey species. Table 4.2: Which species has the higher interference ability? The starting biomass for both species was 975 1.00 976 Table 4.3: Competitive hierarchy from Mouquet et al. (2004), strong competitors at the top 977 Table 4.4: Competitive hierarchy of four species in four treatments in Silvertown et al. (1994). 978 Fig. 4.1: The MacArthur and Levins (1967) concept of niche separation along a gradient. 979 Fig. 4.2: Pairs of two species showing relative non-linearity. 980 Fig. 4.3: The effect of interference intensity and environmental favourability on RGR. 981 Fig. 4.4: A circular interference network between three species. 982 Fig. 4.5: Competitive relations in seven species from the University of Otago Botany Lawn. From 983 Roxburgh and Wilson (2000a). 984 Fig. 4.6. The competitive hierarchy from invasion rates in data of Silvertown et al. (1992). 985 Fig. 4.7. Possible causes of intransitivity between three species: A, B and C. 986 Plate 4.1: A monospecific community: Cladium mariscus stand. 987 Plate 4.2: Tyria jacobaeae (cinnabar moth) on Senecio jacobaea (ragwort) 1 all this is in the high-density treatment