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1 The nature of the plant community: a reductionist view 2 J. Bastow Wilson 3 Botany Department, University of Otago, P.O. Box 56, Dunedin, New Zealand. 4 Andrew D.Q. Agnew 5 Institute of Biological Sciences, University of Wales Aberystwyth, SY23 3DA, U.K. 6 Chapter 1: Plants are strange and wondrous things 7 1 From plants to communities ................................................................................................... 2 8 1.1 Features of all land plants that predetermine their natural history .................................. 5 9 1.2 What is a plant community? .......................................................................................... 11 10 2 The accession of species into mixtures ................................................................................. 13 11 2.1 Step A, Speciation: What is a species? .......................................................................... 13 12 2.2 Step B, Biogeography: The species pool ....................................................................... 15 13 2.3 Step C, Dispersal ........................................................................................................... 16 14 2.4 Step D, Environmental filtering / ecesis ........................................................................ 18 15 2.5 Step E, Interference filtering (mainly competition) ....................................................... 21 16 2.6 Step F: Assembly rules .................................................................................................. 24 17 3 Geographical boundaries ...................................................................................................... 25 18 4 Concepts of the space occupied by one species .................................................................... 26 19 4.1 The niche ....................................................................................................................... 26 20 4.2 Guilds ............................................................................................................................. 30 21 4.3 Stratification .................................................................................................................. 32 22 5 Conclusion ............................................................................................................................ 34 23 24 25 A problem I had repeatedly as I read the chapter was that I could not figure out who the authors 26 expected to be in the target audience. Many cases exist where jargon enters into the text without 27 prior definition and importance ideas from the field are assumed without explanation. This 28 suggests that the book must be intended for no one earlier in their career than roughly a third-year 29 graduate student (which will limit sales). At other times simple ideas are discussed in a way that 30 suggests advanced undergraduates. Either level, or one in between, would work, but parts of the Wilson & Agnew, chapter 1, Plants, page 2 of 34 31 book need to be adjusted to so that the target isat least consistent. The Preface states that “the 32 book is deliberately unbalanced,” but this is not a facet I think should adhere to this underlying 33 philosophy. 34 35 In general, I like the approach. It seems the authors are attempting to demonstrate the high 36 amount of uncertainty in the field of plant community ecology. However, I am hopeful that after 37 the deconstruction, a framework remains from which we can build upon. In general I would 38 recommend providing some definition for all major terms. For example, the authors did not 39 provide definitions for individual or species pool although there was a great deal of discussion 40 regarding different interpretations of these terms. 41 42 General comment: In general, I am enjoying this new take on community ecology. Since many of 43 the ideas are slightly different from traditional textbooks and do not quite fit in with intro ecology 44 concepts (e.g. throwing out the concept of “individual”, saying that all previous attempts have 45 “failed”, etc.), I see this book as more of a companion book to these more traditional texts. 46 47 48 1 From plants to communities 49 Our aim in this book is to explore the workings of plant communities and especially the 50 forces that limit the coexistence of some species and promote the coexistence of others. We are 51 searching for generalisations that can be applied to plant assemblages, working from the bottom 52 up. We shall rarely discuss animals: this book is about plants. First we explain our view of 53 vegetation and of the plants that comprise it. 54 The landforms of the earth result from an underlying geological diversity, moulded by 55 geomorphological forces and mostly clothed with vegetation. Even in arid climates, any 56 scattering of plants intrudes and holds the human eye. Like the architectural heritage of the built 57 environment, landscape has the power to be emotionally and spiritually uplifting, or depressing. 58 Our reaction depends on our cultural history, our background experience and often current 59 fashion. We, the authors, have been able to study vegetation during our full working lives, and it 60 has been enormously rewarding and emotionally satisfying. Such studies are in some way a 61 homage to nature and to God.[personal taste perhaps, but I think God has no place in a book 62 about science; nature is, in contrast, the process we are trying to study.] Or, the authors could use Wilson & Agnew, chapter 1, Plants, page 3 of 34 63 a statement such as “We consider such studies a homage to nature and to God.” I see nothing 64 wrong with admitting the emotional, aesthetic and even spiritual motivations of scientific 65 research, and the authors have the prerogative of conveying their personal tastes on such matters. 66 However, we also enjoy the application of science to the natural world, behoving us to seek the 67 processes behind the vegetation that we see, to search for general patterns, and to attempt the 68 formulation of community-level theories. 69 From the beginnings of plant ecology, some scientists have concentrated on describing 70 the myriad of combinations in which species occur (e.g. Lawesson 2004). Others have used a 71 reductionist approach, examining a process by which species A affects species B, but have sought 72 no deeper generalisations. Yet others have developed theories into which they hope the world 73 will fit (see Bio 2000 –not accessible; use different reference). Such is the complexity of plant 74 communities that, whether the theories have been primarily deductive (e.g. MacArthur 1969) or 75 empirical (e.g. Grime 1979), all have basically failed This is a big statement here with no backing 76 provided? Are the authors suggesting that we have learned nothing and can make no generalities 77 about plant ecology?. This book is an attempt to move reality and theories closer. The authors 78 state that deductive and empirical attempts to describe plant communities have basically failed. 79 However, there is no further explanation or qualification of that statement. Perhaps they failed at 80 proving that communities exist without exception? Maybe they failed at developing a “law” of 81 community ecology? 82 There are plenty of theories to test, some more trivial than others, but it seems none have 83 reliable truth. Suppose we take a group of students into the field, tell them that there is a ‘theory’ 84 that species richness is higher in ecotones (boundaries) and have them sample. Will they find 85 that? Probably not. Suppose we tell them of the opposite ‘theory’ that can be found in the 86 literature – species richness is lower in ecotones – will they take community ecology seriously as 87 a science? Suppose we draw out of the hat a theory on where species evenness will be high, or 88 where the relative abundance distribution will be a particular shape; will the students find it? 89 Probably not.[This book needs to address why not] The only reason that students put up with this 90 ‘science’ is that they, like us, find being in the field more pleasant than being in the lab. 91 Nevertheless, it is our duty as scientists to start solving these problems. Is a theory disproved if 92 it can’t be demonstrated within a class period? 93 94 We shall emphasise terrestrial vascular plants, because more is known about them, and most of the processes to be found are found in them. However, it is likely that many of the same Wilson & Agnew, chapter 1, Plants, page 4 of 34 95 principles apply to lower plants, down to macro-algae and plankton (Tilman 1981; Wilson et al. 96 1995 %689; Steel et al. 2004), and we shall take examples from any group of plants when we 97 fancy. Very rarely do we see a plant species persisting on its own even when we try to make it do 98 so in a garden or farm, so this book is about plant communities. However, in keeping with our 99 reductionist approach we start by examining the importance and nature of plants. 100 The importance of plants 101 Plants, as the dominant carbon fixers in the biosphere, control all ecosystems. The 102 terrestrial part of the biosphere is overwhelmingly vascular plant cover. Plant communities have a 103 global entropic effect. Visible light from the sun is intercepted by our planet, and is dissipated at 104 longer wavelengths into space (D.H. Miller 1981). This represents a gain in entropy, i.e. a trend 105 towards homogenization of the universe. Yet life captures energy and by its maintenance and 106 replication creates order/decreases entropy. The accumulation of biomass and organic matter 107 suggest that at the level of planet entropy is decreased. The plant covering of the Earth increases, 108 be it almost immeasurably, this entropy gain. It does this by fixing a tiny part of solar energy into 109 organic matter and, through evolutionary processes, maximising the efficiency of its utilisation 110 (Ulanowicz and Hannon 1987) so that even more energy is re-radiated as long wave radiation. 111 [Chemical energy storage is small compared to absorption and reradiation, which accomplishes 112 the same thing, and that storage might well be construed to be the opposite of increased entropy, 113 bought at the price of some of the much larger entropy increase associated with absorption and 114 reradiation.] This is quite temporary for an individual living plant, but forests hold a long-term 115 store of energy as reduced carbon and terrestrial plant products can remain for longer in soil, peat 116 and eventually in subfossil and fossil deposits. The result is the maintenance of the oxygenated 117 atmospheric state, of no small importance to us all [and itself a stronger example of neg-entropy 118 storage ]. In fact, the vegetation cover has multifarious feedbacks on the climate (Hayden 1998). 119 Plant communities affect the rocks and soil too, exercising major geomorphological 120 controls on the earth’s land surface and landforms. They intercept precipitation and wind, damp 121 down environmental fluctuations, reduce erosional rates, affect soil formation and dominate 122 geochemical cycles (Trudgill 1977). Local and regional hydrology are profoundly affected by 123 vegetation through evapotranspiration, which reduces the amount of water available in soil and 124 the catchment outflow. Plant cover may accrue wind- and water-borne deposits and thus build 125 landscapes. Every plant affects the local environment in ways that are again multifarious (Eviner Wilson & Agnew, chapter 1, Plants, page 5 of 34 126 and Chapin 2003). This is the ‘reaction’ of Clements (1904; 1916) and Gleason (1927). 127 [Reference will not be clear to the reader ] 128 Plants are also almost the sole basis for the food chain. Reichle et al. (1975) itemise the 129 four essential parts of ecosystem function as: (1) energy input in photosynthesis (‘energy base’), 130 (2) the capital of energy [not ecosystem ‘function’, though perhaps an ecosystem ‘service’, 131 whatever that is]in photosynthetic biomass (‘reservoir of energy’) – I would rework #2 as energy 132 flow through the trophic structure, (3) cycling, especially of elements, and (4) the control of the 133 rates of these and other processes by factors such as temperature and the availability of 134 heterotrophs (‘rate regulation’) – #4 also seems not a function, but a background condition. On 135 land, green vascular plants comprise almost the whole of the energy base and the resevoir of 136 energy, and they make major contributions to cycling and rate regulation. 137 1.1 Features of all land plants that predetermine their natural history 138 Terrestrial green plants are so familiar to us that we often lose our sense of wonder at 139 them, even as their features become more extraordinary as our knowledge of biology deepens. 140 We argue that: 141 1. Land plants root in the soil to obtain mineral nutrients, water and anchorage. Therefore, 142 they are sedentary, so defence from herbivores can be only by structure and chemistry, 143 not by escape. – Two separate things going on here, and it confuses things to fully merge 144 them. One is obtaining nutrients and water from the environment and the other is 145 anchorage. They need not be linked. Plants anchor in other places, like as epiphytes. 146 This provides, among other things, a stable place from which to array aboveground parts 147 for light foraging. Only when we invoke a direct linkage like mycorrhize does fixed 148 location become important for resource extraction 149 2. This puts a selective premium on cell walls that are low in food value to herbivores, 150 basically cellulose, which is also strong enough to support cell turgor. However, cellulose 151 cannot be efficiently recycled. Therefore plants almost always have to grow by the 152 replacement of modules, such as leaves. Discarded modules are a necessary byproduct, 153 comprising litter. -- Would be productive to contrast the modular growth of plants with 154 the generally non-modular growth of most higher animals? 155 3. Because of modular growth, the number of cell divisions between generations (i.e. 156 gamete-to-gamete) is indeterminate and large [perhaps note nondeterminate animals like 157 fish and turtles in contrast with determinate animals like mammals, birds, and most Wilson & Agnew, chapter 1, Plants, page 6 of 34 158 dinosaurs]. In the process of module production somatic mutations can occur, so all 159 ‘individuals’ are potentially genetic mosaics. Genetic mosaic is not a readily-available 160 term in the average student’s mind. It would be good to briefly define this term here, or 161 use another, more obvious, group of words . The germ cells are defined only just before 162 the meiotic process, so they include these somatic mutations. This contrasts with animals, 163 where the germ cells are defined at an early stage and migrate to the gonads (Gilbert 164 1997) with few cell divisions between one generation and the next and hence little 165 opportunity for somatic mutations to accumulate and be passed on. [I hope you will return 166 to why these three things are important as we move through the book ] 167 Another result of modular growth is movement. Motile animals move around but, having grown, 168 usually stay within approximately the same adult body, replacing organs cell-by-cell or molecule- 169 by-molecule until death. Plants are sedentary [or certainly do not ‘actively’ move] , but their 170 organs and elements of their living transport system have a limited length of useful life and must 171 be replaced by new ones (Larcher 1980).[this varies – some plants – especially annuals – are 172 determinant in growth – nonetheless, they do grow ] The photosynthetic rate of a leaf is maximal 173 early in its life and declines thereafter, so leaves and their supporting organs are generally 174 replaced several times during the lifespan of a plant.[in perennial woody plants at least] These 175 replacement leaves are formed distally on the stem, or on side branches. This means that plants 176 can never persist in an unchanged physical space; they must grow and in the process explore and 177 expand into new space. [again, a subset of plants] Even cacti must increase in size during their 178 life (de Kroon and van Groenendael 1997). This remorseless renewal of all modules of growth, 179 the discard of old dead plants as litter and exploration of new space results in disturbance to 180 neighbours. In other words: plants move, animals don’t.[sounds cute and is thought provoking, 181 but does not hold up to careful scrutiny] [Discussion of symmetric vs asymmetric competition 182 seems to be relevant here.] I think this should be changed to something like “plants move in a 183 different way than animals do”. The authors have a pretty good argument that plants do move, 184 but the converse that animals do not move is not really true, or at least the text provides no 185 background for this statement. 186 This seems to be a point Wilson has been dying to make, and is therefore blinded by his 187 own logic. There are many ways to dissect the functions, assemblages, and relationships of living 188 organisms, and I feel that making this statement leaves the reader not only confused, but also 189 short of the whole truth. Anyone can manipulate the definition of movement to make a point Wilson & Agnew, chapter 1, Plants, page 7 of 34 190 Some colonial, sedentaryi animals are similar to plants in that they must grow to stay 191 192 alive: some Urochordata (tunicates), corals and Porifera (sponges). As a result they have several 193 similarities to plants. They have similar genetic characteristics. They filter water for carbon just 194 as plants can be said to be filtering water and air. The sedentary tunicates and corals have 195 exoskeletons somewhat resistant to decay and predation (tunicin and calcium carbonate 196 respectively), comparable to the epidermis of plants. However, there are differences. The 197 modules causing the mandatory growth of corals are not discarded in the way leaves are, though 198 the xylem in the heartwood of trees is retained too.[but the filtering parts are replaced! And don’t 199 forget the symbiotic algae in the corals are at least as important as the filtering of stuff from the 200 water. ] Although animals that accumulate calcium carbonate have profound effects on marine 201 geomorphology and the biosphere, no animals on land have byproducts similar to the litter of 202 dead waste parts produced by living plants (chapt. 2, sect. 2 below). In the arthropods there is a 203 periodically-shed exoskeleton that includes cellulose-like material, but there is sufficient protein 204 in the exoskeleton to make it readily decomposable and anyway the biomass of herbivores can 205 never approach that of the primary producers and therefore cannot modify the environment of 206 entire systems as can plants. Is it worth thinking about aquatic and marine systems where the 207 biomass of photosynthetic organisms can be lower than that of the next trophic level and perhaps 208 that higher trphic level and modify the environment more because of the amount nutrient that gets 209 stored/removed by its actions? Lots of stuff ends up on the sea floor, and it is not decomposed 210 and recycled, but for very different reasons than on land.. I feel that this minor section which 211 speaks of the biomass left behind by plants is also a little misleading. It may be true that 212 historical plant biomass surpasses animal biomass. However, there is no recognition of the mass 213 limestone across the earth, which was contributed by animals in the past. How these amounts 214 compare isn’t the point for me—it’s an issue of Wilson giving inaccurate examples to make his 215 point 216 The problem of the individual 217 The problem of recognising individuals in plant populations is longstanding. It is reflected 218 in discussions of the terms biotype, genet and ramet [here we again have a jargon issue that needs 219 to be rsolved in terms of the expected background of the target audience. I don’t think we can 220 expect all the readers to know these terms (Harper 1977) as well as more philosophical 221 discussions of the nature of the plant individual (Firn 2004).[A rather oblique reference to a Wilson & Agnew, chapter 1, Plants, page 8 of 34 222 rather little know but interesting paper – the appropriateness will vary with the target audience. 223 You seem to be aiming at professionals and advanced graduate students, whereas I think you 224 should aim at new graduate students as they are the ones who can be shaped and who have the 225 time to read books.] In an annual with no vegetative reproduction it is clear what an individual is. 226 In vegetatively-reproducing plants with ramets gradually becoming independent (Marshall 1996), 227 perhaps with the clone then splitting into several discrete patches (Harberd 1962), ‘individual’ 228 has no demographic meaning. The same issue arises with the apomictic offspring in genera such 229 as Crepis (hawksbeard), Poa and Taraxacum (dandelion) that are potentially identical in 230 genotype.[Do identical twins in humans function as individuals? Usually! I don’t think we want 231 to link the definition of individual too tightly to genetic identity.] Another problem with applying 232 the animal ‘individual’ concept to plants is that whilst most animals [only because most animals 233 are insects and we conveniently ignore their larval stages because they are harder to find and less 234 pretty.] are relatively constant in size at any particular age, individuals of one plant genotype can 235 differ in biomass by several orders of magnitude (Harper 1977). There is some evidence that the 236 root system of an individual genet or even ramet can differentiate between roots from its own 237 parent and other individuals of its own or other species. The experimental evidence of Gersani et 238 al. (2001) using Glycine max (soybean) plants, of Gruntman and Novoplansky (2004) in Buchloe 239 dactyloides (buffalo grass) and the neurotransmission speculations of Baluska et al. (2004) are 240 fascinating in this respect and need confirmation. [Even in an advanced textbook these probably 241 need to be defined rather than assuming the papers have been read by the reader. ] 242 The section on genetic change seems only tangentially related here. I would have preferred a 243 summary paragraph on the definitions of the individual and the consequences of using these 244 definitions for interpretation in plant community ecology. 245 It is unclear what the relevance of this discussion about the individual is. The concept of 246 individual seems to have been thrown out because it may not always mean the same thing. The 247 fact that you would define an individual differently for a clonal plant than you would for an 248 animal doesn’t mean that the concept isn’t valid or that different uses of the concept might not be 249 important to address particular questions, it just means that you need to be clear as to what you 250 mean by it in a given use. 251 252 253 Somatic mutations complicate the issue further. There can be mutations as Taraxacum plants reproduce (King and Schaal 1990) so the apomictic offspring need not be genetically Wilson & Agnew, chapter 1, Plants, page 9 of 34 254 identical. Somatic mutations can occur in vegetatively-reproducing plants and during growth 255 (Gill et al. 1995). Using the plant cell sizes in the classic Strasburger's textbook of botany 256 (Harder et al. 1965) with a conservative estimate of mean cambial cell length of 0.1 mm it is clear 257 that there could be of the order of 220 cell divisions between separate sectors of growth in a tree, 258 with a consequent probability of mitotic errors. Therefore, even an apparently ‘individual’ plant 259 cannot reliably be taken as a single genotype, and has to be regarded as a colony of apical 260 meristems, even a colony of apical meristem segments (Fig. 1.1). Could you not say the same 261 thing of every organism? Are we all not “colonies” of cells with slightly different genetic codes? 262 Every apex and therefore each flower can be genetically unique, or perhaps every sector within 263 an apex (Newbury et al. 2000). [In this day of molecular studies it is not sufficient to simply 264 assert this diversity, but rather papers documenting the degree of variation should be cited/]The 265 modules of a physiological individual such as a tree also differ in their environment (e.g. light 266 intensity) and often the cause of that variation (shade, in our example) can be the individual itself 267 (self-shading). [For this to be ecologically interesting we need some evidence of selection within 268 the plant, rather than simple genetic diversity – that is, show us that it matters for something.] 269 However, physiological interdependence between the modules overcomes this to some extent. Litter Fig. 1.1: A stylised dicotyledonous plant as a colony of active and inactive apices 270 271 We conclude that because of vegetative reproduction and apomixis, variation in size, somatic 272 mutation and plasticity, the animal concept of ‘individual’ is not appropriate or useful in plants. 273 [more helpful to clearly articulate the differences and then indicate what we mean by an 274 individual. ] This doesn’t mean that there isn’t a plant version that would be. The statement that 275 the concept of ‘individual’ is not appropriate for plants is taking things a step too far. Certainly Wilson & Agnew, chapter 1, Plants, page 10 of 34 276 this concept is useful to ecologists, but the definition should depend on the question being asked. 277 If a genetic analysis is being performed, then an individual may be any genetically-unique shoot. 278 For a survey of regeneration following disturbance, perhaps each shoot should be counted as an 279 individual. 280 281 The features we have been discussing make genetic change difficult.[explain why they 282 make genetic change difficult – seems like you are referring to all that within-plant genetic 283 diversity, but perhaps referring in a confusing way back earlier to plastic and indeterminant 284 growth – unclear at any rate] We must ask why plants need genetic change when they can change 285 plastically. One answer to this paradox has been the controversial theory of ‘genetic assimilation’ 286 (Pigliucci and Murren 2003): that plastic changes can become incorporated into the genotype. 287 Bradshaw’s (1973) answer was that plants are genetically ‘sown into their winter underwear’, 288 because their plastic response to an adverse environmental shock would be too slow. [a bit too 289 obscure to follow without more explanation; wit is nice but clarity of expression is perhaps better 290 ]A third answer is that they do not actually become adapted genetically: Rapson and Wilson 291 (1988; 1992) found that though significant genetic differences had developed in Agrostis 292 capillaris (bent) in southern New Zealand since it was introduced in 1853, there was no sign that 293 populations were differentially adapted to the habitat they were growing in [can this be tested in a 294 two-suided way, or can we only find or fail to find specific evidence of adaptation – easy to not 295 test the right thing when there is somuch to test.]. Perhaps genetic conservatism is a result of 296 duplication of alleles on chromosomes and of duplication of genomes (polyploidy). Ought to 297 expand discussion of polypoidy and its importance in plant evolution. Of course, populations and 298 eventually species do change sometimes, giving in some cases dramatic ecotypic adaptation and 299 eventually leading to the 400,000 flowering plant species that we see today and the millions that 300 rest in peace. Citation? Is this just conjecture? 301 Interaction with other trophic levels 302 Plants are mostly autotrophic, but they interact with all other trophic levels. Since our 303 thrust is plant communities, we shall generally discuss this only so far as it mediates plant-plant 304 interactions. Wardle (2002) has discussed interactions with decomposers. [seems insufficient to 305 simply allude to work without any mention of what was found] Plants meet and usually withstand 306 challenges from herbivores and diseases, usually in two totally differing environments: the 307 relatively humid soil below ground and the comparative aridity of sunlight above ground (chapts. Wilson & Agnew, chapter 1, Plants, page 11 of 34 308 2 and 4). Many plants rely on animals for pollination and dispersal (chapts. 2 and 4).Top 309 carnivores will have indirect effects. Mycorrhizae are crucial for many species, and will be 310 discussed especially in chapter 2. In addition to their rôle in nutrition and water acquisition, it 311 seems that vesicular-arbuscular mycorrhizae (VAM) can restrict the development of pathogen 312 loads in their host (Larsen and Bødker 2001). Endophytic fungi and bacteria are also widespread 313 and have a multiplicity of effects on plant growth. There is usually an extensive microflora in the 314 phyllosphere and in the rhizosphere. Some plants form special relationships with ants, to which 315 we shall refer. In some plants mites inhabit small pits in leaves (domatia), and apparently protect 316 the plant against other herbivores or against pathogenic fungi (Grostal and O’Dowd 1994). This 317 brief list of interactions is surely far from exhaustive. [seems to tease the reader –we perhaps 318 need more at the level of impact on the community] 319 1.2 What is a plant community? 320 To test community theories we need communities. Unfortunately it is not possible to 321 provide a definition of ‘community’ that includes areal extent, uniformity of environment, 322 closeness to equilibrium, etc. All sorts of species mixtures exist, in all sorts of environments, and 323 there are no discontinuities in the hierarchy of this variation I find this statement awkward. 324 Furthermore, species mixtures are constantly changing. We believe the plethora of terms that 325 have been applied to species mixtures (phytocoenose, association, nodum, etc.) represents 326 attempts to persuade vegetation ecologists that the study of this aspect of the natural world can 327 yield general statements and predictive rules, but it cannot. [They do provide contingent 328 generalizations in the sense of May. The smaller the distances in time and space, the greater our 329 ability to predict. It is at the level global level that predication . I agree that there are several 330 seemingly ambiguous or unnecessary terms for communities in ecology, and that it can 331 sometimes be difficult to discern the boundaries between community types on a landscape. 332 However, I do not think these terms necessarily exist to persuade us that community ecology can 333 “yield general statements and predictive rules”. I think that most of these concepts exist so that 334 we can talk about plant community ecology concepts more easily. 335 336 How close can we get to defining ‘plant community’? A degree of repeatability between 337 samples (i.e. quadrats) would be a useful restriction, but this again is difficult to prescribe (chapt. 338 6, sect. 2 below). We need to specify scale at some point in our argument; Gleason (1936) 339 suggested it should be one plant of a largest species but we do not feel able to insist on this.. Wilson & Agnew, chapter 1, Plants, page 12 of 34 340 Mueller-Dombois and Ellenberg (1974) give a historical summary and agree that no rigid 341 definition is possible. However, they distinguish between conceptual communities which are the 342 abstract units of plant community classification and ‘concrete’ communities that are the actual 343 plant species mixtures encountered in the field. We hope that all our discussions can be related to 344 real, actual examples of plant communities, the concrete ones, for we are not persuaded of the 345 relevance of conceptual communities.[But, are you not seeking generality????] We could use the 346 splendidly neutral and practical statement of Tansley and Chip (1926) that “A plant community 347 may be defined as any naturally growing collection of plants which, for the purposes of the study 348 of vegetation, can be usefully treated as an entity.” 349 To include environmental relations, stability and change in the community, and spatial contiguity 350 we here see the plant community as: Naturally generated plant stands where the environment of 351 the individuals of one species potentially, predictably and persistently includes individuals of its 352 own and usually a restricted number of other species.[ wow – he used the bad word ] 353 The definition of plant community is confusing. The wording “potentially, predictably 354 and persistently” is the most confusing part of the definition, as it is difficult to understand how 355 and when something can “potentially” occur, as well as “predictably and persistently” occur. 356 357 358 359 [It seems rather odd to make a definition of community using the term “individual” after just spending an entire section throwing that term out as inappropriate for plants!] I like the Tansley and Chip definition much better than the Wilson and Agnew one. The Wilson and Agnew one seems to lead the reader even more confused with vagueness. 360 361 This excludes mixtures deliberately planted, such as a mixed shrubbery, but planted 362 gardens and agricultural fields can contain a rich weed flora and are valid objects of study. Of 363 course, indirect human intervention such as fertilisation and the release of grazers is quite 364 acceptable: they often mimic perturbations in natural communities, and in any case it is 365 fascinating to see how a mixture of species responds (e.g. Fuhlendorf and Smeins 1997; 366 Silvertown 2006). I didn’t have the impression that the authors agreed with their definition of 367 plant community. It is unclear to me still what the authors take to be a “community”. They say 368 earlier that they “are not persuaded of the relevance of conceptual communities”. but this seems 369 like a very conceptual def. to me. 370 371 We are trying to make sense of nature, starting with a vision about plants and plant communities, and looking for underlying predictability and repeatability so we can claim Wilson & Agnew, chapter 1, Plants, page 13 of 34 372 community ecology as a science. As the great Robert MacArthur (1972) said: “To do science is to 373 search for repeated patterns”. [But in that same book MacArthur makes the point that the intrinsic 374 complexity of ecosystems forces ecologists to search for generalizations that are contingent on 375 numerous and often quite specific initial conditions.] The major difficulty for us is that we do not 376 know what sort of pattern to look for (chapt. 5 below). One issue in dealing with samples of plant 377 mixtures is the concept of phantom species. These are species present in the general area (“in the 378 community”), potentially available in samples but not actually recorded.[the species pool?] This 379 may be a valid concern for animal communities where species at low density can be around and 380 sometimes walk/swim/fly though the sample area/volume, but happened not to be there at the 381 recording time. This is less relevant for plant communities and we follow Pielou’s (1990) 382 suggestion: “a biological collection … should be treated as a universe in its own right”, rejecting 383 the concept of phantom species as a figment of the theoretical ecologist’s imagination. 384 2 The accession of species into mixtures 385 386 We discuss the processes that initiate plant communities in six steps, in some developmental order: 387 A. Speciation: Life has originated, and the species must have evolved. 388 B. Biogeography: The species must be in the regional species pool. 389 C. Dispersal: The species in the regional species pool must reach the particular site. 390 D. Environmental filtering / ecesis: The species must be able to germinate/develop from its 391 propagule and then grow to reproduction under the physical environmental conditions 392 prevailing. 393 E. Productivity and biotic filtering: The species must be able to ecise and reproduce under 394 the general interference pressure from the other species present: competition etc. (chapt. 2 395 below). 396 397 398 F. Assembly rules: The species must withstand restrictions from the particular species or types of species present (chapt. 5 below). 2.1 Step A, Speciation: What is a species? Wilson & Agnew, chapter 1, Plants, page 14 of 34 399 We shall deal only peripherally with sub-specific evolution and not at all with the 400 evolution of species, but they are the first required taxonomic category above the plant. The 401 recognition, description and diagnosis of species allow us to predict much of a plant’s It would be Species pool (metacommunity) Dispersal Challenge Niche constructed Niche available Niche unavailable No entry! Population Establishment Fig. 1.2: Pathways from the species pool to community entry. 402 This figure is completely reliant on the niche theory. There’s no attempt to “oust” the neutral 403 theory, which still gets a fair amount of research attention. Am I out of the loop here? It would 404 be nice it the figure and the steps A-F followed each other more directly. morphology and 405 behaviour after the identification of a scrap (the use of ‘morphospecies’ does not allow this). This 406 predictability was the basis for the development of the science of Botany in the eighteenth 407 century, and our ability to describe the vegetation around us. Unlike with animals, plant 408 taxonomists (Stace 1989) are happy to allow species with only incomplete restrictions to gene 409 exchange. However, each species is required to have a distinct phenotype. It must therefore have 410 a unique environmental tolerance and a unique reaction on the environment, even if the 411 difference from other species is sometimes small. Wilson & Agnew, chapter 1, Plants, page 15 of 34 412 413 2.2 Step B, Biogeography: The species pool The plant community can, in the short term, comprise only species present in the region, 414 which is the species pool (Fig. 1.2). The pool is difficult to define and quite as difficult to 415 determine, because we never know the distances over which species have the ability to disperse 416 or the frequency of dispersal events. However, different processes do occur on different spatial 417 scales. The regional species distribution for many species comprises a metapopulation: a series of 418 populations that are partly independent but connected by occasional migration events. In practice 419 the metapopulations of many species will show similar distributions due to similar habitat 420 requirements, giving a metacommunity (Holyoak et al. 2005). It is a nice distinction as to 421 whether a disseminule arrives via long-distance dispersal or from the metacommunity hinterland, 422 and in any case the resulting processes of establishment must be similar. 423 Questions about the species pool are dependent on the time frame: how long are we 424 prepared to wait for the species to arrive? Were time the only limitation to dispersal, disseminules 425 from far and wide would arrive anywhere, 400,000 species, and clearly this does not happen. 426 Continents have very different floras. Many European tree species have failed to occupy their 427 potential ranges in spite of several thousand years in which to spread across the continent 428 (Svenning and Skov 2004). The school of panbiogeography sees many present-day restrictions in 429 distribution between and within land areas as a reflection of the geography millions of years ago 430 (Fig. 1.3), and its analyses of species distributions that have repeatedly been borne out by 431 subsequent geological discoveries (Heads 2005). Clements and Shelford (1939) agree that 432 whereas migration of propagules is common, establishment of them is “altogether exceptional”. 433 [some discussion of waifs and mass effect seems to fit here] There are also restrictions on the 434 scale of hundreds of years: Matlack (2005) modelled the distribution of species in eastern USA 435 and concluded that the frequencies of species in the modern landscape was controlled by the time 436 available for spread in the last 300 years, with vertebrate-dispersed species occupying 437 considerably more of their potential geographical range than other species. It is often unclear on 438 which timescale the distribution limitation has occurred; for example a gap in the distribution of 439 Nothofagus spp. in the South Island of New Zealand (Fig. 1.3) has variously been correlated with 440 geological movements (Heads 1989), the last glaciation (Wardle 1980) and the current 441 environment (Haase 1990). Therefore, the closest we can come to definition is to say that over 442 realistic time spans most members of the area’s species pool could arrive, and we have to explain 443 the restricted subset of species found in each plant community. Wilson & Agnew, chapter 1, Plants, page 16 of 34 444 The concept of the species pool has sometimes included only species suited to the 445 environment of the habitat in question. In Europe, species have sometimes been excluded from 446 the pool using their Ellenberg ecological-tolerance rating (Ellenberg 1974). These values, 447 originally crude, have been progressively refined. Outside Europe, little information exists on 448 species tolerances for whole floras. A confounding question is whether the species pool is 449 defined before or after interference. If the species pool comprises all those species 450 physiologically able to tolerate the physical conditions at the site, it would include many never 451 found there, because of interference (Steps E and F). Many reports concerning filtering and Fig. 1.3a: Disjunct distribution (●) of the subshrub Kelleria laxa in South Island, New Zealand, interpreted as an originally contiguous distribution torn apart by tectonic movement ( ) along the Alpine Fault 2-10 million years ago and the ‘beech gap’ ( ). From Heads (1989). 452 interference assume that the species pool includes species that can tolerate the environment, but 453 cannot stand interference. However, the species lists are often taken from post-interference 454 communities and so the argument is circular. An example of this syndrome is when climate 455 change models of future vegetation are based on physiological parameters derived from 456 distributions (i.e. from the realised niches; sect. 4.1 below), and used in models as physiological 457 parameters (e.g. Sykes and Prentice 1996). [approaches to species pools seems to assume prior 458 knowledge] What are the authors going to use as their definition of the species pool? 459 2.3 Step C, Dispersal 460 Propagules 461 462 Propagule types are various. Within the angiosperms, seeds can be produced sexually (after meiosis and fertilisation), apomictically (with no meiosis and no involvement by pollen) or Wilson & Agnew, chapter 1, Plants, page 17 of 34 463 by pseudogamy (pollen is needed for seed development, and fertilises the endosperm, but the 464 embryo itself is produced apomictically). Vegetative reproduction can occur via bulbils, stolons, 465 rhizomes, layering of branches (e.g. Salix cinerea, willow tree), root suckers, etc. There is no 466 basic distinction between the apomictic seeds of Taraxacum spp. (dandelion), ‘vegetative 467 reproduction’ such as the production of Kalanchoe daigremontiana plantlets from the leaf 468 margin, the growth of an Elytrigia repens (couch grass) clone by rhizomes, the growth of a 469 Populus tremuloides (aspen) clone by root suckers and the growth of an axillary bud on a tree 470 branch to give new leaf modules. All replicate an original genotype but after many mitotic 471 divisions which can accumulate errors. [common theme without relevance noted] 472 The immediate fate of these propagules is various. Bulbils and viviparous seeds both 473 develop as plantlets on the parent. Ramets produced by stolon or rhizome are initially dependent 474 on the parent, then for a period are physiologically independent unless a change occurs, such as 475 defoliation or shading, when ramets subsidise each other (Marshall 1996), and then become fully 476 independent as the connecting stolons/rhizomes wither. Seeds are usually dispersed by wind, 477 water or animals, although a few plants produce hypogeal seeds (i.e. belowground). Tree and 478 herb sectors behave similarly to clonal tillers with limited integration, except that there is a 479 greater tendency for branches to overtop one another competitively (Novoplansky 1996). 480 Migration 481 Dispersal is the means by which species move around the landscape. The two critical 482 considerations are the distance and frequency with which disseminules move outside their source 483 habitat, which is negatively related to disseminule size, and their potential for establishment as a 484 seedling in a new site amongst existing plants which is positively related to disseminule size 485 (Salisbury 1942). 486 487 488 Plant dispersal usually has a long tail (Fig. 1.4), i.e. it is leptokurtic, and is often best fitted by a negative exponential function.[tail even longer than that??] Indeed, there is work by James Clark (1999, 1998 among them) that explores a diversity 489 of functions and found that other distributions often fit better. That is, most dispersal of 490 disseminules of every type is surprisingly short-distance, with rare long-distance events, as Carey 491 and Watkinson (1993) found for mechanical scatter of the seeds of an annual festucoid grass and 492 Matlack (2005) for dispersal of ingested seeds. The reason is probably that most species are 493 dispersed by two or more mechanisms: for example Agnew and Flux (1970) found in the Rift 494 Valley, Kenya, that though many grass disseminules had a large wing apparently adapted for Wilson & Agnew, chapter 1, Plants, page 18 of 34 495 wind dispersal, the longer distance dispersal seemed to occur when the fruit became entangled in 496 the coats of Lepus capensis (hares). Occasionally, the direction can be towards suitable habitat, 497 for example ants dispersing seeds along their runways (Huxley and Cutler 1991). In general, the 498 number of disseminules arriving (the ‘propagule pressure’) will not matter: a smaller number will 499 delay an invasion but will not prevent it.[propagule pressure and disturbance can together cause 500 considerable influx] The exception is when an Allee effect is operating. 70 Number of seeds 60 50 40 30 20 10 0 -80 501 -60 -40 -20 0 20 40 60 80 100 120 Distance (m) Fig. 1.3. Fig. 1.4. The leptokurtic curve of dispersal: Juncus effusus seed rain around a single plant. 502 The population of dormant seeds in the soil or in aerial fruits – the ‘seed pool’ or ‘seed 503 bank’ – is a buffer against elimination of a species. This is important for an annual species in an 504 adverse year, and for species surviving through disasters such as fire. The seed pool is not only 505 local, it can have a metapopulation structure comparable to that of adults when seeds are moved 506 around by floods or large herbivores such as elephants. This can be seen as spatial mass effect 507 [not defined](chapt. 4, sect. 12 below). 508 2.4 Step D, Environmental filtering / ecesis 509 Propagule germination and establishment 510 The germination of a propagule starts phase of an invasion when it is the challenged by 511 the conditions in a new site (Fig. 1.2). Awkward sentence Sometimes seeds germinate only under 512 conditions more mesic than those tolerated by their parents, and these events may be rare in 513 stressed habitats. For example, many halophytes need unusually low salinity on a saltmarsh 514 before they germinate, when their more glycophytic seedlings can become established 515 (Alexander and Dunton 2002). Arid land species show very precise adaptations for effective Wilson & Agnew, chapter 1, Plants, page 19 of 34 516 dispersal and germination in the highly variable rainfall patterns found in these habitats 517 (Gutterman 2002). For example, Pake and Venable (1996) found that different species of winter 518 annual in the Sonoran Desert tended to germinate in different years, and species tended to 519 germinate more in years that turned out to give them higher reproductive success. 520 Plants of all species need to pass through a juvenile phase before becoming reproductive. 521 This part of the challenge in occupying a new site is called ecesis (Clements 1904). As Clements 522 (1916) wrote: “Ecesis is the adjustment of the plant to a new home. It consists of three essential 523 processes, germination, growth, and reproduction. ... Ecesis comprises all the processes exhibited 524 by an invading germule from the time it enters a new area until it is thoroughly established there. 525 Hence it really includes competition and other types of interference, except in the case of 526 pioneers in bare areas.”. This definition is of course far too broad, because it seems to include the 527 whole life cycle. We need to separate out the first stages for our reductionist view, so we restrict 528 ‘ecesis’ to post-germination survival, growth and establishment: the part of the species filtering 529 process that determines which species survive the initial dispersal and germination phases of 530 plant community establishment. [reference to Grubb’s regeneration niche?] 531 Move to start of STEP D and combine with the environmental filtering section. 532 533 534 Invasion patterns Move invasion patterns prior to Step D Invasion can be seen on all scales, from movement to and fro across 2 m within a decade 535 in links (Olff et al. 2000) to movement over thousands of years. The patterns of invasion seen are 536 much the same at any scale, and fall into three types. Phalanx invasion is dense and over a broad, 537 solid front. Guerrilla invasion comprises invasion by isolated individuals, which gradually fills in 538 the space available (Hutchings 1986). It is difficult to measure invasion processes because 539 a-priori assessment of habitat suitability is problematic. However, an example of an invasion 540 where at least part of the flora used guerrilla invasion is the advance of the herb flora of ancient 541 forest into adjacent secondary growth in Sweden, where Brunet et al. (2000) concluded that 542 distance (equivalent to time for dispersal) and the soil environment almost equally controlled 543 species composition. However, by far the most common seems to be a third type of invasion – a 544 combination of the former two – infiltration invasion in which there is occasional long-distance 545 dispersal (as in guerrilla invasion), followed by local short-range dispersal from these foci (as in 546 phalanx) (Fig. 1.5; Wilson and Lee 1989). This matches the leptokurtic / two-mechanism 547 dispersal typical of plant disseminules.[nucleation well documented in primary succession 548 studies, such as Mt St. Helens] It can be seen at a variety of scales, e.g. over kilometres (Lee et al. Wilson & Agnew, chapter 1, Plants, page 20 of 34 549 1991) to centimetres. A small-scale example occurs when most of a tussock grass’ tillers are 550 produced within the leaf sheath, but a few are pushed greater distances by animal hooves 551 (Harberd 1962), an example of the leptokurtic / two-mechanism dispersal to which we referred in 552 section 2.3 above. Egler (1977) describes all these patterns, with details, though using different 553 terms. Expanding foci Guerrilla individuals g clumps 554 555 556 557 Fig. 1.5: Infiltration invasion by Olearia lyallii in the Auckland Islands. Environmental filtering No habitat holds all the available species from its hinterland pool. There are physical 558 environmental conditions, such as soil type, hydrology, climatic regimes and altitude, that 559 prevent the immigrants’ growing (Honnay et al. 2001). This has been called environmental 560 filtering or abiotic filtering (Weiher and Keddy 1995). It occurs largely during ecesis. The 561 existence of this filter is obvious. Sophisticated methods can be used to record the response 562 surface (e.g. Bio et al. 1998), but it remains what Warming (1909) described as “this easy task”. 563 In this book, we generally take the physical restrictions as given, and concentrate on those 564 community processes that control species composition. However, we need to return to this topic 565 in considering the niche. 566 Reaction 567 The physical environment is not strictly abiotic, for the receptor community can alter the 568 environment to create or close invasible sites, a process for which Clements (1904) coined the 569 term ‘reaction’: “By the term reaction is understood the effect which a plant or a community 570 exerts upon its habitat. … Direct reactions of importance are confined almost wholly to physical 571 factors” (Clements 1916) This term was first used much earlier in the text but not defined there, I Wilson & Agnew, chapter 1, Plants, page 21 of 34 572 suggest moving this def to the first use of the work. Any organism must cause reaction, though 573 the environmental modification varies from slight to major, and the causal species' autoresponse 574 from negative (facilitation: Clements 1916) to positive (switch: Wilson & Agnew 1992). 575 Reaction is the basis of almost all plant-plant interactions (Clements 1904). Since Clements, 576 other terms have been used for the same effect, such as ‘ecosystem engineer’ (Jones et al. 1994), 577 and ‘niche construction’ (Laland et al. 1996). They seem to be later synonyms, but we shall 578 sometimes use the latter when discussing the niche. 579 Sometimes, the favourable microsites for establishment are gaps, but Ryser (1993) found 580 in a temperate calcareous grassland that the favourable microsites were those where the reaction 581 of established plants in the community provided shelter from frost; unvegetated gaps were not 582 colonised. Litter production is a common mode of reaction, forming the seedbed of an invader, 583 and enabling or inhibiting its germination (chapt. 2 below). needs rewording to emphasize that 584 these are examples of reaction. 585 2.5 Step E, Productivity and biotic filtering 586 Communities develop a regime of carbon cycling, of which the autotrophic production is 587 our concern. We devote space to productivity here because it is easy to think rather loosely about 588 it. Productivity is “The potential rate of incorporation or generation of energy or organic 589 matter … per unit area …” (Lincoln et al. 1982), but there is a lot of complication behind this 590 definition: 591 592 593 594 1. Carbon is fixed from CO2 by the C3, C4 or CAM mechanisms [define, discuss?]. We suppose this is gross productivity. 2. Immediately, some of the fixed C is lost by photorespiration, though in C4 plants it is retained in the leaf, and can be re-assimilated. 595 3. Later, at night, some of the fixed C is lost by dark respiration. 596 4. The remaining C is transported to sinks at sites of cell division (secondary cambium, root, 597 shoot apex, inflorescence, etc.), where it is incorporated into cell wall tissue, storage 598 carbohydrates or cytoplasm, with shared potential fates. In this process, it can be: 599 A. Lost by respiration whilst still incorporated in soluble C compounds, etc. 600 B. Lost to aphids on the way: Heizmann et al. (2001) wrote: “during midday depression 601 of photosynthesis, a high percentage of the total C delivery was provided to the 602 leaves by the transpiration stream (83 to 91%). Apparently, attack by phloem-feeding 603 aphids lowered the assimilate transport from roots to shoots; as a consequence the Wilson & Agnew, chapter 1, Plants, page 22 of 34 604 portion of C available to the leaves from xylem transport amounted to only 12 to 605 16%.” 606 C. Leached from leaves as soluble C compounds (Czech and Kappen 1997). 607 D. Lost by roots as exudate of soluble C compounds (Kuzyakov and Siniakina 2001) and 608 609 610 611 as mycorrhizal growth and respiration (Johnson et al. 2002) E. Converted into plant material mainly as cell wall. This material, with the cell contents, can be: i. Eaten by pests (vertebrates, invertebrates and pathogens). 612 ii. Removed by allogenic or autogenic damage or abscised. The amount lost varies 613 from parts of leaves to tree branches: cell walls plus modified cell contents. 614 iii. Lost at the death of tissues (wood, bark), organs (roots, leaves flowers and fruit), 615 616 or the whole plant. F. In living tissues, C in carbohydrate storage and cytoplasm can be lost by respiration as 617 the root becomes old or the leaf becomes shaded and/or old, or it can be translocated 618 with attendant respiratory costs. However, cell wall C cannot be lost this way, 619 because no autolysis of cellulose or lignin occurs within living plants. (This contrasts 620 with animal tissue, where all C is part of the labile pool except for some dermal 621 structures.) 622 In the face of this complexity there is no consensus as to what productivity is or how it should be 623 measured. Logic and simplicity would suggest that the real definition of productivity [ecological 624 efficiency]is the amount of C that reaches the next trophic level, herbivores or decomposers. This 625 top down definition would comprise only E1-3 in the above schema. ‘Gross productivity’ can be 626 measured in the field through gas analysis (though this omits photorespiration: Step 2), and ‘net 627 productivity’ as this less all the later losses. Productivity is most often estimated by sequential 628 sampling. Suppose a habitat holding mature stable vegetation in an approximately steady state. If 629 we sample at one time and then resample the same area 12 months later, in many areas there 630 would be no change in the absence of climate change and apart from sampling error – neither 631 accumulation nor loss of biomass – so we would arrive at an estimate of zero productivity. This is 632 either accurate or misleadingly trivial, depending on your definition of productivity. However, all 633 systems show some seasonal development and change, and most estimates of plant productivity 634 rest on the successive sampling of harvest biomass (standing crop) during the season of maximal 635 growth (Perkins et al. 1978). This working approximation is a wild under-estimate of actual Wilson & Agnew, chapter 1, Plants, page 23 of 34 636 productivity, yet has physical presence and ecological meaning. Actually, in much discussion of 637 productivity, e.g. in testing for a humped-back curve (Grime 1979) [not defined??], standing crop 638 is used as a substitute, and it is a very poor one. 639 The productivity potential of a site controls what plant community develops in three ways. 640 Firstly, the readiness of the soil surface to provide sites for invasion and thus augment the 641 community: often with greater productivity more litter will be available affecting the ecesis of 642 invaders (sect. 2.4 above; chapt. 2 below). It should definitely be briefly revised here HOW the 643 ecesis of invaders is affected, e.g. positively or negatively? 644 Secondly, disturbance of the community through herbivory and often fire: high 645 productivity attracts herbivory, while pronounced dry seasons between productive growing 646 seasons favour fire. However, in general these first two factors affect the rate of invasion more 647 than the eventual fate of an invasion.[really? ]A statement like that should have references to 648 back it up. The third aspect is the competitive status of the community: greater productivity 649 means more competition and more difficulty for additional species to establish-what effect do 650 you hypothesize that this will have on the outcome and rate of invasion?. For example, Cantero et 651 al. (1999) concluded that the diversity of short grasslands in Argentina was affected by 652 surrounding species pools, while that of tall grasslands with more competition was not. This is 653 the interference filter, in which a species is able to tolerate the physical environment of a site, but 654 cannot grow well enough there to withstand the general level of interference present there: 655 competition, allelopathy, etc. It is an effect not specially dependent on the identity of the 656 associates. This is the classic distinction between the fundamental and realised niche (sect. 4.1 657 below). It is clearly a major factor, as can be seen by the ready cultivation of many species in 658 botanic gardens outside their natural edaphic and/or climatic range. 659 Organisms of other trophic levels affect ecesis and reproduction (sect. 1.1 above, 660 “Interaction with other trophic levels”). A species might be able to maintain a positive population 661 growth rate without heterotrophs, but be pushed into negative growth when pathogens or 662 herbivores take their toll. This could be environmentally dependent: a potentially fatal herbivore 663 might be absent because the environment is beyond its tolerance. On the other hand, a plant may 664 be unable to reproduce because a normally subventing pollinator is beyond its environmental 665 range, and hence absent. This would be a type of assembly rule (Step F), though in this book we 666 consider only plant-plant assembly rules as such. Wilson & Agnew, chapter 1, Plants, page 24 of 34 667 2.6 Step F: Assembly rules and micro-evolution 668 Did not provide enough background information for me. It could be written more generally reserving the specific 669 examples for chapter 5. 670 Assembly rules are "restrictions on the observed patterns of species presence or 671 abundance that are based on the presence or abundance of one or other species or groups of 672 species …" (Wilson 1999 %chapter). We discuss them in chapter 5. It is clearly a simplification 673 to take species as fixed units and we do so only to limit the scope of this book. To glimpse into 674 the world of ecotypes and micro-evolution as they affect community structure we examine the 675 work of Turkington and Harper (1979), taking plants of Trifolium repens (white clover) from 676 patches of a field dominated by four different grass species. When they planted them into boxes 677 of a standardised soil sown with the four grass species, each T. repens genotype was the best 678 performer against the species from whose neighbourhood it had been taken in the field – an 679 amazingly neat result. They interpreted this as genetic coadaptation within the field. A 680 mechanism such as the quality of transmitted light is possible (Thompson and Harper 1988). 681 Awkward paragraph 682 Aarssen and Turkington (1985 %605) performed a similar experiment in a pasture in 683 British Columbia, Canada, but using different patches/genotypes of Lolium perenne (ryegrass). 684 They obtained similar results for T. repens. However, they also examined variation within the 685 associated L. perenne and the results for it were the opposite – three of the four L. perenne 686 genotypes had their lowest competitive ability against the T. repens genotype with which they 687 had been growing in the field. This would tend to keep the grass/clover competitive abilities 688 balanced. There remains a fear that the effects were due to carry-over, i.e. maternal effects in the 689 vegetative material. Aarssen (1988) found that collecting seed rather than using vegetative 690 material (ramets) gave quite different results, which he attributed to screening of the gene pool 691 between seed and adult populations. Such screening has certainly been seen in heavy-metal 692 ecotypes, though under conditions where gene flow was high and the selective differentials 693 extreme. However, Turkington and Harper (1979) had used preconditioning periods of only 3 694 months. Evans and Turkington (1988) in Canada, collecting plants in a similar way to Turkington 695 and Harper (1979) – from below four different grass species – found morphological differences 696 between T. repens of the four origins after 4 months growth in a common garden which 697 disappeared after 27 months growth. Chanway et al. (1989) suggested that the difference between 698 T. repens material might be in the specific Rhizobium strains carried with it, not in the T. repens 699 itself. This could still be a force in structuring communities. Wilson & Agnew, chapter 1, Plants, page 25 of 34 700 The Turkington and Harper (1979) result would have been small-scale character 701 displacement. It is almost impossible to prove that character displacement has occurred because 702 the evidence must involve comparisons between areas, and those areas might differ in other ways 703 (Strong 1983). However, some cases are suggestive. If found, character displacement would be 704 evidence that species interactions were a strong force in genetic selection, and therefore also in 705 ecological selection, implying deterministic community structure. 706 3 Geographical boundaries 707 The behaviour of a species at its distributional limit can be fascinating. Often the habitat 708 range of a species becomes more restricted towards its boundary. Pigott (1970) reported that near 709 the limit of Ilex aquifolium (holly) in Britain it becomes increasingly restricted to forest, and 710 Cirsium acaule (stemless thistle) at its northern limit becomes confined to southern (warm) 711 aspects. On the other hand, Diekmann and Lawesson (1999) found four potential examples where 712 species had wider ecological amplitudes towards their range margin in northern Sweden, and 713 suggested that there is such climatic stress in that region that a smaller flora is present and 714 important competitors are absent. Usually the plants are smaller and less fecund towards the 715 limit, leading to populations becoming smaller and absent from apparently suitable habitat 716 patches (Carey et al. 1995; Nantel and Gagnon 1999; Jump and Woodward 2003). Lower 717 fecundity also makes populations more sensitive to disturbance. For example, fire restricts 718 Canadian Abies balsamea (balsam fir; Sirois 1997) and Pinus resinosa (red pine; Flannigan and 719 Bergeron 1998) to islands and isolated populations at the northern edge of their ranges. In some 720 cases there is a sudden cut-off point in a species with no reduction in vigour near the limit 721 (Lactuca serriola, prickly lettuce; Carter and Prince 1985). This may not be exceptional. Griggs 722 (1914) made an early and careful observational study of Sugar Creek, in a “tension zone” in Ohio 723 where over 120 species have geographical boundaries, asking whether populations became sparse 724 or less fecund in this region. He found no consistency of behaviour, but most edge-of-range 725 species were abundant and flowered and fruited successfully up to the geographical limit, as in L. 726 serriola. Griggs could only hypothesise that competition sharpened boundaries to make them 727 abrupt. There can be many different reasons for a species’ failing to expand its distribution, 728 sometimes surprising ones. Pigott and Huntley (1981) found that the environmental filter for Tilia 729 cordata (linden) at the northern limit of its range in England was that the pollen tube could not Wilson & Agnew, chapter 1, Plants, page 26 of 34 730 grow fast enough in the low temperature to reach the ovule, leaving a relictual population now 731 unable to reproduce by seed. At present it seems that the behaviour of species at the margins of their ranges is complex 732 733 and unpredictable. For example, if the range limit were due to individuals being selectively 734 eliminated we might expect lower variances of morphological measurements at geographic 735 edges, but Wilson et al. (1991 %780) could find no such effect [I find it interesting Bastow did 736 not hit on the classic bell-curve of abundance of Whittaker – since some species tend to be either 737 abundant or absent. One sacred cow was forgotten in the pasture]. 738 4 Concepts of the space occupied by one species 739 Our purpose in this book is to examine the way species fit together in a mixture. Two powerful 740 conceptual tools, developed for this purpose, are the niche and the guild. A table or diagram 741 would be useful for comparing niche and guild and also showing relationships between 742 alpha/beta niche/guild More discussion of the niche makes one thirst for more on neutral and why only niche 743 744 works…perhaps this is another (much smaller) book? 745 4.1 746 747 748 The niche The discussion of niches is interesting, and the concepts of alpha and beta niche were ones that seem useful, and original. The end point of a species' pilgrimage from the pool into a community is occupancy of a 749 niche. The niche is an old concept. Grinnell (1904) and Elton (1927) introduced the term, and 750 both used it to describe an area available within habitat space, broadly defined by physical and 751 trophic parameters. Hutchinson (1944) formalised this to “a region in n-dimensional hyperspace” 752 where the dimensions are all the environmental, resource or behavioural (e.g. phenology, 753 foraging) parameters that permit an organism to live. 754 Since Hutchinson’s overarching statement it has been tempting to regard species presence 755 as the only definition of the niche. Thus, Levins and Lewontin (1985) advocated that “ecological 756 niches are defined only by the organisms in them”. Olding-Smee et al. (2003 %book) believed 757 that for Hutchinson “a niche cannot exist without an occupant”. We see no reason to understand 758 Hutchinson thus. The crunch comes with the empty niche. Under the “the species is the niche” 759 concept “the idea of an ecological niche without an organism filling it loses all meaning” (Levins 760 and Lewontin 1985)-Surely this is a strawman argument. Would someone make the same claim Wilson & Agnew, chapter 1, Plants, page 27 of 34 761 today?. However, the empty niche is a necessary concept in theory, especially in relation to 762 invasions. The absurdity of the “the species is the niche” is seen by observing innovative 763 invaders. Did no niche for a cactus exist in central Australia until Opuntia stricta (prickly pear 764 cactus) was introduced (Hosking et al. 1994)? Was there no niche for a cactus-eating insect 765 before the moth Cactoblastis cactorum was introduced for biological control of O. stricta? Was 766 there no niche below the saltmeadow in British estuaries until Spartina ×townsendii / anglica 767 (cord grass) created itself by hybridisation in 1887? Was there no niche for an emergent tree in 768 Bonin Island shrublands until Pinus lutchuensis (a pine from elsewhere in Japan) was introduced 769 (Shimizu and Tabata 1985)? It seems better to regard all these as empty niches that were later 770 filled. To be sure, the identification of empty niches is very hard. There must be areas of 771 hyperspace that it is impossible for plants to fill: floating in the air, growing on the ice at the 772 South Pole or growing at 100 °C in hydrothermal steam vents? 773 Tilman (1997 %81) claimed to find evidence of empty niches. In 1991 he sowed seeds of up to 774 54 species into native grassland at Cedar Creek. Many became established [but for how long?]. 775 However, this did not cause extinctions among the species originally present in 1991: the 776 proportion of those lost was not correlated with the number of species added (r = +0.16, R2 = 777 2.6 %, not significant). Even more interestingly, the total cover of those species present in 1991 778 did not decrease (r = 0.04, R2 = 0.16%, not significant). The R2 values are impressively low so 779 we could conclude, as Tilman did, that the added species occupied empty niches. There is a 780 problem that the species composition probably co-varied with the species richness. The use of 781 ‘total cover’ is odd. If two leaves of different species are vertically aligned, both count towards 782 cover, and if two leaves of the same species are horizontally aligned both count, but if two leaves 783 of the same species are vertically aligned only one counts ?? I’m not certain what the the authors 784 are trying to describe here. I agree that total cover is generally not a very standardized method 785 when collecting field data. However, I think there are more methods in vegetation sampling that 786 are probably equally as flawed. 787 ‘Total cover’ is not a sensible concept. In this case, the ‘cover’ of each species was, 788 unfortunately, guessed. Cardboard cutouts were used to guide the guessing but we do not take 789 aids to guessing as removing the fact that cover was guessed. Philip Grime’s group at Sheffield 790 always uses objective measurements: presence/absence, local frequency, point quadrats or sorted 791 biomass as appropriate. Why can’t everyone? [seems self evident] We mention this issue because 792 it will repeatedly mar results that we report. We shall not shirk from pointing out when the data Wilson & Agnew, chapter 1, Plants, page 28 of 34 793 of this type are used, nor shall we use euphemisms like “estimated by eye”, because we believe 794 this practice is a blot on our science (even if we have occasionally been guilty ourselves in evil 795 places far away and naughty times forever gone) [“cover guessing” measures can be valid and 796 helpful in many contexts. As with many methods (most statistical methods, for example), you 797 need to be aware to the possible problems, the accuracy, the biases, and so forth. I am worried 798 that the book tends to be repeatedly dogmatic and negative without any attempt to provide a 799 balanced perspective – it is living up to its goal in the Preface of being unbalanced.] 800 How does the use of presence/absence data allow us to differentiate between dominant 801 and trace species within a plot? Not to mention that this throws out indicator species analysis, 802 ordination, community classification(a given), and a whole host of other measures for how 803 vegetation in one spot is different from vegetation in another.. Very soap-box. Some valid points 804 but I still think that there can be valid/appropriate uses of “estimated” cover. While it may be fine 805 (and quite Feasible!) to actually use repeatable, objective measures in herb/grass communitys, it 806 is not always a reasonable option. 807 The second reason for rejecting the “the species is the niche” concept is that it takes 808 species presence in the field as the de facto description of its niche, but this is affected by 809 interaction with other biota (competition, herbivory etc), introducing an imponderable set of 810 variables over space and time. It is more useful to separate biotic variables as restricting the 811 occurrence of a species to its realised niche, whereas its environmental tolerance defines its 812 fundamental niche or physiological tolerance (Hutchinson 1957). This distinction was known to 813 Tansley (1917) and Gleason (1917), though dismissed by Clements (1907) as “merely 814 migration”. The niche width of a group of species is usually considerably narrower, and hence 815 their niche overlap is less, when they are grown in an experimental mixed community than when 816 they are grown alone in the same conditions. That is, their realised niche is smaller than their 817 fundamental niche. Silvertown et al. (1999 %61) found this re-analysing data of Ellenberg’s with 818 six grass species and a water table gradient: the mean niche overlap was considerably higher in 819 the 6-species mixture than comparing monocultures, and the niche modes spread out to a range of 820 5-100 cm depth-to-water-table in the mixture compared to a range of 20-35 cm among the 821 monocultures. However, definitions become difficult because of reaction: any organism must 822 alter its own environment and this may cause niche construction, potentially leading to realised 823 niches that are larger than the fundamental ones (Odling-Smee, et al. 2003)[real or definitional 824 problem??] Wilson & Agnew, chapter 1, Plants, page 29 of 34 825 The niche includes a species' developmental requirements (temperature etc.), its material 826 requirements (resources) and its relations with neighbouring species. A complete circumscription 827 of these is almost impossible, requiring knowledge of every aspect of the species' physiology and 828 life history, but two types of niche can be distinguished. The beta (β) niche is the range of 829 physical environmental conditions under which the fitness of a species is maintained (Alley 830 1985), e.g. its temperature tolerance, and therefore its potential geographical limits. It is related to 831 Chesson’s (in press) concept of ‘environment’ as a factor that does not form a feedback loop, i.e. 832 is not appreciably affected by the organisms themselves. The alpha (α) niche represents the 833 resources used within a community/site, the “‘profession’ or functional role” (Alley 1985), e.g. 834 different rooting depths. Many methods of analysis, e.g. the calculation of niche width and 835 overlap, can be used for both alpha and beta niches, and there are areas of character overlap. 836 However, when we use the niche concept we generally need either one or the other. Much 837 ecological discussion has been confused by failing to take the distinction into account. I have to 838 say that this (the alpha and beta niches and guilds) is the part of the book that I have so far 839 enjoyed the most. Here is a presentation of a new/different way of looking at things rather than a 840 tearing down of what has been 841 842 843 The axes of the alpha niche are controlled by the morphology and physiology of the plant, its growth and its chemistry: 1. Morphology and its plasticity influence resource foraging and capture (light, nutrients, 844 water source), persistence (storage organs, wood), autogenic disturbance (through litter 845 and physical environmental effects), heat budget (convective, transpirative, radiative), 846 physical defence against herbivores (glands, hairs, thorns), pollination and dispersal 847 biology. An example of these factors is synusiae in forest, such as epiphytes and lianas, 848 and indeed stratification in almost all communities. Another example is the parasitic 849 habit. 850 2. Growth phenology as the plant's response to environmental signals comprises the 851 seasonality of growth and reproduction (pollination and dispersal). Examples are the 852 progression of flowering in temperate vegetation and leaf flushes in tropical forests. 853 3. The chemical functioning of a plant ultimately controls everything, but we may list as 854 examples phototype (C3, C4 or CAM), light requirement, mycotrophy, P sources via root 855 phosphatase exudate, N source (N2, NH4 or NO3) and chemical defence against herbivore 856 and pathogen challenge. Wilson & Agnew, chapter 1, Plants, page 30 of 34 857 4. Any of the above niche axes can influence plant/plant interactions, through interference or 858 subvention by neighbours (chapt. 2 below), for example in the morphological pre-emption 859 of soil resources, though the overgrowth of competitors and through the toxic chemical 860 countering of competition known as allelopathy. 861 862 5. Additional resources are gathered by the community. This is reaction causing niche construction. 863 Beta niche axes are the environmental features of the locality and its biota, that is to say the 864 habitat. Complexities between factors are more apparent than with alpha axes. Aspects are: 865 1. Climate delivers solar insolation, water availability, CO2 (though the latter is rarely local), 866 nutrients (when ombrotrophic), some pollination vectors and rate regulators such as 867 temperature. These interact with the chemistry below. Climate also delivers exposure to 868 atmospheric humidity, wind, aeration and snow. This affects morphology, for example as 869 a partial determinant of the Raunkiaer life form (the life form can also differ between 870 alpha niches, for example in forest stratification). 871 2. Chemical features of soils (calcareous versus non-calcareous, pH, salinity, etc.) affect 872 system function (nutrient availability, cycling); these overlap with geomorphology below. 873 In the short term, mineral nutrients are often dominant. 874 3. Geomorphology delivers allogenic disturbance and soil substrate. 875 4. Biota deliver allogenic disturbance, generalised herbivore pressure and animal pollinators. 876 There can sometimes be overlap between the concepts of alpha-niche and beta-niche in terms of 877 characters: e.g. low growth is a feature of the ground alpha-niche in a forest but also of arctic 878 plants. However, the effects are opposite: species of the same beta niche will tend to co-occur 879 because they have the same environmental tolerances; species of the same alpha niche will have 880 no such tendency to co-occur, and if competitive exclusion is operating they will tend not to co- 881 occur (Wilson, submitted). 882 4.2 Guilds 883 The ecological term ‘guild’ was coined by Drude (1885) as the German 884 ‘Artengenossenschaften’ to refer to a group of species moving from one region to another, such 885 as exotic species. It was used thus by Clements (1904; 1905) and Wilson (1989 %223). Perhaps 886 independently, Schimper (1898; 1903) used the term ‘Artengenossenschaften’ / ‘guild’ to mean a 887 synusia (e.g. stratum) in a forest. Tansley (1920) used it in the same way, writing of “guilds of 888 the same dependent life-form, such for instance as lianes”. Root (1967) ignored these established Wilson & Agnew, chapter 1, Plants, page 31 of 34 889 usages and with animal assemblages in mind re-defined the guild as a “group of species using 890 similar resources in a similar way”. This is not directly useful for plants, since almost all use the 891 same resources (the sun’s energy, water, CO2, N, P, K and minor elements). The guild is a 892 category that is intended to be ecological rather than taxonomic, and Wilson (1999) defined it as: 893 “a group of species that are similar in some way that is ecologically relevant, or might be”. It is 894 unusual to find “Or might be” in a scientific definition; it is necessary here because we hardly 895 ever known at the beginning of an investigation whether the guilds we are using are the real ones, 896 and often not at the end (but see the discussion of intrinsic guilds: chapt. 5, sect. 7.6 below). The 897 phrase “or might be” in the definition of guild is troubling. I understand the point that some plant 898 species are put into guilds a priori so that the guild may or may not be actually relevant. 899 However, the idea is that the guild is relevant, and if it turns out not to be relevant, then we 900 should change the way we have grouped the species, not the definition of guild. In spite of the 901 lack of precedence of Root’s usage, and the impossibility of applying it strictly to plants, a 902 similar usage can have value: a guild as a group of species that occupy similar niches. 903 Wilson (1999) pointed out that there are two basic types of guilds, corresponding to the 904 distinction between alpha-niches and beta-niches. Again, the outcomes are opposite: species that 905 are in the same beta-guild and therefore have similar environmental tolerances will generally co- 906 occur; species that are in the same alpha-guild and therefore use similar resources will tend 907 exclude each other. The species within one alpha-guild are similar in their resource use. For 908 example, within northern European forests, species that are within the same alpha-guild might be 909 the trees Tilia cordata (linden), Quercus petraea (sessile oak) and Fagus sylvatica (beech). They 910 are using similar resources: the light at the top of the canopy during the summer half-year, as well 911 as nutrients and water from the full profile of the soil. Conversely, if species are present in one 912 community that are in different alpha-guilds, they might be able to partition resources within a 913 community, so a community might tend to comprise species from several alpha-guilds. For 914 example, Tilia cordata, the hemi-parasite Viscum album (mistletoe), the liana Hedera helix (ivy) 915 and the ground herb Mercurialis perennis (dog’s mercury) would be in different alpha-guilds 916 because they use different light/support/nutritional resources, and if we found them together we 917 might see it as alpha-niche differentiation. 918 The species in one beta-guild are similar in their ecophysiology and therefore their 919 tolerance (across space or time) of environmental conditions, such as the “guilds of edaphic and 920 topographical specialists” of Hubbell and Foster (1986 %314). After a species pool has passed Wilson & Agnew, chapter 1, Plants, page 32 of 34 921 through an environmental filter, the remaining species will be a beta guild; they have overlapping 922 beta-niches. For example, all sub-arctic saltmarsh species would be in the same beta-guild 923 because they occur in the same climatic and soil conditions. An example of species occurring in 924 different beta-guilds might be the temperate, mesic tree Tilia cordata, the subalpine Pinus 925 contorta (lodgepole pine), arid land trees/shrubs of Prosopis spp. (mesquite), the tropical 926 Cinchona officinalis (quinine) and a species of mangrove. They occur in different environmental 927 conditions (climate and/or soil), so they are necessarily found apart in space or time since we 928 cannot find different external environmental conditions simultaneously at one spot. Díaz et al. 929 (1998) recorded abundances of ‘plant functional types’ (PFTs) of 100 species along a climatic 930 gradient in Argentina and found that vegetative traits differed between climatic zones, 931 demonstrating that beta-guilds are filtered out from the available species pool. The species within 932 each zone will almost certainly belong to different alpha-guilds. 933 The concept of ‘functional type’ (used above by Diaz et al. 1988) and that of the ‘guild’ 934 can be essentially identical (Wilson 1999; Blondel 2003). The current use of PFTs as the 935 predicted variate in models assumes that we know the characters of the types are trying to 936 summarise. In spite of the term ‘functional’ which implies alpha-guilds, most workers have 937 apparently intended to create beta-guilds. However, the characters they have chosen have often 938 been alpha-niche ones. For example, Kleyer (2002) formed guilds (‘functional types’) “to relate 939 unique PFTs to landscape specific habitat factors and to generalize syndrome-environment 940 relations across landscapes” and used characters such as annual versus biennial versus perennial, 941 plant height, regeneration from detached shoots, having leptophyllous leaves, longevity of seed 942 pool that are as likely to occur within a community. A distinction between ‘response’ and ‘effect’ 943 guilds obscures the issue, because there is far more to the alpha-niche of a species than its 944 reaction (effect) on the environment. This situation has arisen from a failure to consider the 945 purpose of the guilds being formed, what type of guilds they will therefore be – alpha or beta – 946 and what characters are therefore appropriate. 947 4.3 Stratification 948 The most obvious alpha-guilds in plant communities are the guilds of Schimper (1898; 949 1903), synusiae (Fig. 1.6). Almost all plant communities are structured vertically. Aboveground, 950 the greater the vegetation cover, the more uniform and predictable is the vertical change in 951 microclimate. Highly structured forests have a stratum of separated, emergent trees, a more 952 continuous upper canopy, then sub-canopy trees, shrubs, tall herbs, creeping herbs and Wilson & Agnew, chapter 1, Plants, page 33 of 34 953 bryophytes, lianas and epiphytes (including lichens, bryophytes and higher plants). This 954 represents specialisation to the attenuation of light, water, CO2 and nutrient resources. All this is 955 accepted for forests, but there is also complex stratification in grasslands, for example in the wet 956 grasslands of Tierra del Fuego (Díaz Barradas et al. 2001) and even in lawns (Roxburgh et al. 957 1993 %699). Naturally all stratification by primary producers is echoed by stratification in 958 consumer communities. Fig. 1.5: Stratification: profile of a rain forest in British Guiana. From Richards (1964). 959 960 We might expect that similar patterning is happening below ground because litter is 961 deposited on the soil surface eventually adding to the water-holding capacity and mineral nutrient 962 status of the upper soil. Most water arrives at the soil surface and percolates down, acidified by 963 organic acids and CO2, hydrolising the mineral fragments in the soil and most importantly 964 releasing phosphate. Plant roots and respiration can affect this, for example releasing acids. 965 Water in deep soil, including artesian water, is available to deep roots and may rise up by 966 capillarity and hydraulic lift. This can lead to stratification of root systems. Succulents of New 967 and Old World deserts have surface roots adapted for the uptake in ephemeral rainstorms 968 (Whitford 2002). Dodd et al. (1984) surveyed 43 woody species from the open woodland in SW 969 Australia, and Timberlake and Calvert (1993) 96 shrubs and trees of Zimbabwe woodlands, both 970 finding that there were indeed species with consistently shallow systems and others with deep 971 taproots. Most species had both lateral superficial roots and descending taproots, but herbs can be 972 shallow-rooted and in herbaceous or mixed herbaceous/woody communities there can be 973 considerable stratification (Weaver and Clements 1929 %p213; Cody 1986 %381). Wilson & Agnew, chapter 1, Plants, page 34 of 34 974 5 Conclusion This opening chapter has described the basic material of plant communities: the plants 975 976 themselves. We argued that the characteristics of plants are that they are colonies of modules that 977 must constantly be replaced causing movement in space, they are potentially genetic mosaics and 978 they are plastic. As a consequence, the term ‘individual’ usually has no meaning for plants. [Is 979 this true? In many cases individuals are discrete and meaningful ] They also have features that 980 make their evolution different from that of animals. The huge majority of stands have more than 981 one plant species, and we have outlined the basic processes through which multi-species 982 communities establish and develop. The occupancy of a niche is axiomatic in a species’ presence 983 in a community, and the interactions between pool, dispersal and niche are all important in this 984 process (Fig. 1.2). There are recent demonstrations of this at a large scale in the Palm floras of 985 Ecuador and Peru (Vormisto et al. 2004), and at an intermediate scale in the Netherlands (Ozinga 986 et al. 2005). Our conclusion, which we hope the reader shares, is that there is enormous 987 complexity in the life of plants in spite of the simplicity implied in their common sedentary habit 988 and modular structure, and their almost universal trophic function. Our basic concern in this book 989 is to examine how the species fit together to form communities and basic concepts for this are the 990 niche and the guild. In the next chapter we examine the processes involved when one species 991 interacts with another, starting community development. 992 ILLUSTRATIONS 993 Fig. 1.1: The plant as a colony of active and inactive apices 994 Fig. 1.2: Pathways from the species pool to community entry 995 Fig. 1.3: Disjunct distribution of Kelleria laxa in South Island, New Zealand, interpreted as an 996 originally contiguous distribution torn apart by movement along the Alpine Fault 2-10 997 million years ago. From Heads (1989). 998 Fig. 1.4: Infiltration invasion by Olearia lyallii in the Auckland Islands. After Lee et al. (1991). 999 Fig. 1.5: The leptokurtic curve of dispersal: Juncus effusus seed rain around a single plant. 1000 Fig. 1.6: Stratification: profile of a rain forest in Guiana: From Richards (1964). i ‘sessile’ in zoological terminology