Download 1 From plants to communities - Biology Department | UNC Chapel Hill

The nature of the plant community: a reductionist view
1
2
3
J. Bastow Wilson
Botany Department, University of Otago, P.O. Box 56, Dunedin, New Zealand.
4
5
Andrew D.Q. Agnew
Institute of Biological Sciences, University of Wales Aberystwyth, SY23 3DA, U.K.
6
Chapter 1: Plants are strange and wondrous things
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
1
24
1 From plants to communities
2
3
4
5
From plants to communities ................................................................................................... 1
1.1 Features of all land plants that predetermine their natural history .................................. 3
1.2 What is a plant community? ............................................................................................ 7
The accession of species into mixtures ................................................................................... 9
2.1 Step A, Speciation: What is a species? ............................................................................ 9
2.2 Step B, Biogeography: The species pool ....................................................................... 10
2.3 Step C, Dispersal ........................................................................................................... 12
2.4 Step D, Environmental filtering / ecesis ........................................................................ 13
2.5 Step E, Interference filtering (mainly competition) ...................................................... 16
2.6 Step F: Assembly rules .................................................................................................. 18
Geographical boundaries ...................................................................................................... 19
Concepts of the space occupied by one species .................................................................... 20
4.1 The niche ....................................................................................................................... 20
4.2 Guilds ............................................................................................................................. 24
4.3 Stratification .................................................................................................................. 26
Conclusion ............................................................................................................................ 27
25
Our aim in this book is to explore the workings of plant communities and especially the
26
forces that limit the coexistence of some species and promote the coexistence of others. We are
27
searching for generalisations that can be applied to plant assemblages, working from the bottom
28
up. We shall only rarely discuss animals: this book is about plants. First we explain our view of
29
vegetation and of the plants that comprise it.
30
The landforms of the earth result from an underlying geological diversity, moulded by
31
geomorphological forces and mostly clothed with vegetation. Even in arid climates, any
32
scattering of plants intrudes and holds the human eye. Like the architectural heritage of the built
33
environment, landscape has the power to be emotionally and spiritually uplifting, or depressing.
34
Our reaction depends on our cultural history, our background experience and often current
35
fashion. We, the authors, have been able to study vegetation during our full working lives, and it
36
has been enormously rewarding and emotionally satisfying. Such studies are in some way a
37
homage to nature and to God. However, we also enjoy the application of science to the natural
Wilson & Agnew, chapter 1, Plants, page 2 of 28
38
world, behoving us to seek the processes behind the vegetation that we see, to search for general
39
patterns, and to attempt the formulation of community-level theories.
40
From the beginnings of plant ecology, some scientists have concentrated on describing
41
the myriad of combinations in which species occur (e.g. Lawesson 2004). Others have used a
42
reductionist approach, examining a process by which species A affects species B, but have sought
43
no deeper generalisations. Yet others have developed theories into which they hope the world
44
will fit (see Bio 2000). Such is the complexity of plant communities that, whether the theories
45
have been primarily deductive (e.g. MacArthur 1969) or empirical (e.g. Grime 1979), all have
46
basically failed. This book is an attempt to move reality and theories closer.
47
There are plenty of theories to test, some more trivial than others, but it seems none have
48
reliable truth. Suppose we take a group of students into the field, tell them that there is a ‘theory’
49
that species richness is higher in ecotones (boundaries) and have them sample. Will they find
50
that? Probably not. Suppose we tell them of the opposite ‘theory’ that can be found in the
51
literature – species richness is lower in ecotones – will they take community ecology seriously as
52
a science? Suppose we draw out of the hat a theory on where species evenness will be high, or
53
where the relative abundance distribution will be a particular shape; will the students find it?
54
Probably not. The only reason that students put up with this ‘science’ is that they, like us, find
55
being in the field more pleasant than being in the lab. Nevertheless, it is our duty as scientists to
56
start solving these problems.
57
We shall emphasise terrestrial vascular plants, because more is known about them, and
58
most of the processes to be found are found in them. However, it is likely that many of the same
59
principles apply to lower plants, down to macro-algae and plankton (Tilman 1981; J.B. Wilson et
60
al. 1995b; Steel et al. 2004), and we shall take examples from any group of plants when we
61
fancy. Very rarely do we see a plant species persisting on its own even when we try to make it do
62
so in a garden or farm, so this book is about plant communities. However, in keeping with our
63
reductionist approach we start by examining the importance and nature of plants.
64
The importance of plants
65
Plants, as the dominant carbon fixers in the biosphere, control all ecosystems. The
66
terrestrial part of the biosphere is overwhelmingly vascular plant cover. Plant communities have a
67
global entropic effect. Visible light from the sun is intercepted by our planet, and is dissipated at
68
longer wavelengths into space (D.H. Miller 1981). This represents a gain in entropy, i.e. a trend
69
towards homogenization of the universe. The plant covering of the Earth increases, be it almost
Wilson & Agnew, chapter 1, Plants, page 3 of 28
70
immeasurably, this entropy gain. It does this by fixing a tiny part of solar energy into organic
71
matter and, through evolutionary processes, maximising the efficiency of its utilisation
72
(Ulanowicz and Hannon 1987) so that even more energy is re-radiated as long wave radiation.
73
This is quite temporary for an individual living plant, but forests hold a long-term store of energy
74
as reduced carbon and terrestrial plant products can remain for longer in soil, peat and eventually
75
in subfossil and fossil deposits. The result is the maintenance of the oxygenated atmospheric
76
state, of no small importance to us all. In fact, the vegetation cover has multifarious feedbacks on
77
the climate (Hayden 1998).
78
Plant communities affect the rocks and soil too, exercising major geomorphological
79
controls on the earth’s land surface and landforms. They intercept precipitation and wind, damp
80
down environmental fluctuations, reduce erosional rates, affect soil formation and dominate
81
geochemical cycles (Trudgill 1977). Local and regional hydrology are profoundly affected by
82
vegetation through evapotranspiration, which reduces the amount of water available in soil and
83
the catchment outflow. Plant cover may accrue wind- and water-borne deposits and thus build
84
landscapes. Every plant affects the local environment in ways that are again multifarious (Eviner
85
and Chapin 2003). This is the ‘reaction’ of Clements (1904; 1916) and Gleason (1927).
86
Plants are also almost the sole basis for the food chain. Reichle et al. (1975) itemise the
87
four essential parts of ecosystem function as: (1) energy input in photosynthesis (‘energy base’),
88
(2) the capital of energy in photosynthetic biomass (‘reservoir of energy’), (3) cycling, especially
89
of elements, and (4) the control of the rates of these and other processes by factors such as
90
temperature and the availability of heterotrophs (‘rate regulation’). On land, green vascular plants
91
comprise almost the whole of the energy base and the resevoir of energy, and they make major
92
contributions to cycling and rate regulation.
93
1.1 Features of all land plants that predetermine their natural history
94
Terrestrial green plants are so familiar to us that we often lose our sense of wonder at
95
them, even as their features become more extraordinary as our knowledge of biology deepens.
96
We argue that:
97
1. Land plants root in the soil to obtain mineral nutrients, water and anchorage. Therefore,
98
they are sedentary, so defence from herbivores can be only by structure and chemistry,
99
not by escape.
100
101
2. This puts a selective premium on cell walls that are low in food value to herbivores,
basically cellulose which is also strong enough to support cell turgor. However, cellulose
Wilson & Agnew, chapter 1, Plants, page 4 of 28
102
cannot be efficiently recycled. Therefore plants almost always have to grow by the
103
replacement of modules, such as leaves. Discarded modules are a necessary byproduct,
104
comprising litter.
105
3. Because of modular growth, the number of cell divisions between generations (i.e.
106
gamete-to-gamete) is indeterminate and large. In the process of module production
107
somatic mutations can occur, so all ‘individuals’ are potentially genetic mosaics. The
108
germ cells are defined only just before the meiotic process, so they include these somatic
109
mutations. This contrasts with animals, where the germ cells are defined at an early stage
110
and migrate to the gonads (Gilbert 1997) with few cell divisions between one generation
111
and the next and hence little opportunity for somatic mutations to accumulate and be
112
passed on.
113
Another result of modular growth is movement. Motile animals move around but, having grown,
114
usually stay within approximately the same adult body, replacing organs cell-by-cell or molecule-
115
by-molecule until death. Plants are sedentary, but their organs and elements of their living
116
transport system have a limited length of useful life and must be replaced by new ones (Larcher
117
1980). The photosynthetic rate of a leaf is maximal early in its life and declines thereafter, so
118
leaves and their supporting organs are generally replaced several times during the lifespan of a
119
plant. These replacement leaves are formed distally on the stem, or on side branches. This means
120
that plants can never persist in an unchanged physical space; they must grow and in the process
121
explore and expand into new space. Even cacti must increase in size during their life (de Kroon
122
and van Groenendael 1997). This remorseless renewal of all modules of growth, the discard of
123
old dead plants as litter and exploration of new space results in disturbance to neighbours. In
124
other words: plants move, animals don’t.
125
Some colonial, sedentary1 animals are similar to plants in that they must grow to stay
126
alive: some Urochordata (tunicates), corals and Porifera (sponges). As a result they have several
127
similarities to plants. They have similar genetic characteristics. They filter water for carbon just
128
as plants can be said to be filtering water and air. The sedentary tunicates and corals have
129
exoskeletons somewhat resistant to decay and predation (tunicin and calcium carbonate
130
respectively), comparable to the epidermis of plants. However, there are differences. The
131
modules causing the mandatory growth of corals are not discarded in the way leaves are, though
132
the xylem in the heartwood of trees is retained too. Although animals that accumulate calcium
133
carbonate have profound effects on marine geomorphology and the biosphere, no animals on land
Wilson & Agnew, chapter 1, Plants, page 5 of 28
134
have byproducts similar to the litter of dead waste parts produced by living plants (chap. 2,
135
sect. 5). In the arthropods there is a periodically-shed exoskeleton that includes cellulose-like
136
material, but there is sufficient protein in the exoskeleton to make it readily decomposable and
137
anyway the biomass of herbivores can never approach that of the primary producers and therefore
138
cannot modify the environment of entire systems as can plants.
139
The problem of the individual
140
The problem of recognising individuals in plant populations is longstanding. It is reflected
141
in discussions of the terms biotype, genet and ramet (Harper 1977) as well as more philosophical
142
discussions of the nature of the plant individual (Firn 2004). In an annual with no vegetative
143
reproduction it is clear what an individual is. In vegetatively-reproducing plants with ramets
144
gradually becoming independent (Marshall 1996), perhaps with the clone then splitting into
145
several discrete patches (Harberd 1962), ‘individual’ has no demographic meaning. The same
146
issue arises with the apomictic offspring in genera such as Crepis (hawksbeard), Poa and
147
Taraxacum (dandelion) that are potentially identical in genotype (this does occur in some parts of
148
the animal kingdom). Another problem with applying the animal ‘individual’ concept to plants is
149
that whilst most animals are relatively constant in size at any particular age, individuals of one
150
plant genotype can differ in biomass by several orders of magnitude (Harper 1977). There is
151
some evidence that the root system of an individual genet or even ramet can differentiate between
152
roots from its own parent and other individuals of its own or other species. The experimental
153
evidence of Gersani et al. (2001) using Glycine max (soybean) plants, of Gruntman and
154
Novoplansky (2004) in Buchloe dactyloides (buffalo grass) and the neurotransmission
155
speculations of Baluska et al. (2004) are fascinating in this respect and need confirmation.
156
Somatic mutations complicate the issue further. There can be mutations as Taraxacum
157
plants reproduce (King and Schaal 1990) so the apomictic offspring need not be genetically
158
identical. Somatic mutations can occur in vegetatively-reproducing plants and during growth
159
(Gill et al. 1995). Using the plant cell sizes in the classic Strasburger's textbook of botany
160
(Harder et al. 1965) with a conservative estimate of mean cambial cell length of 0.1 mm it is clear
161
that there could be of the order of 220 cell divisions between separate sectors of growth in a tree,
162
with a consequent probability of mitotic errors. Therefore, even an apparently ‘individual’ plant
163
cannot reliably be taken as a single genotype, and has to be regarded as a colony of apical
164
meristems, even a colony of apical meristem segments (Fig. 1.1). Every apex and therefore each
165
flower can be genetically unique, or perhaps every sector within an apex (Newbury et al. 2000).
Wilson & Agnew, chapter 1, Plants, page 6 of 28
166
The modules of a physiological individual such as a tree also differ in their environment (e.g.
167
light intensity) and often the cause of that variation (shade, in our example) can be the individual
168
itself (self-shading). However, physiological interdependence between the modules overcomes
169
this to some extent.
Litter
171
Fig. 1.1: A stylised dicotyledonous plant as a colony of active and
inactive apices
Kikvidze et al. (2005) describe most of the species of a community as not being “clonal”.
172
But what is clonal? Are all grasses clonal because they tiller, are oak trees clonal because they are
173
colonies of modules, are dandelions clonal because they are apomictic? We conclude that
174
because of vegetative reproduction and apomixis, variation in size, somatic mutation and
175
plasticity, the animal concept of ‘individual’ is not appropriate or useful in plants.
170
176
The features we have been discussing make genetic change difficult. We must ask why
177
plants need genetic change when they can change plastically. One answer to this paradox has
178
been the controversial theory of ‘genetic assimilation’ (Pigliucci and Murren 2003): that plastic
179
changes can become incorporated into the genotype. Bradshaw’s (1965) answer was that plants
180
are genetically ‘sown into their winter underwear’, because their plastic response to an adverse
181
environmental shock would be too slow. A third answer is that they do not actually become
182
adapted genetically: Rapson and Wilson (1988; 1992) found that though significant genetic
183
differences had developed in Agrostis capillaris (bent) in southern New Zealand since it was
184
introduced in 1853, there was no sign that populations were adapted to the habitat they were
185
growing in. Perhaps genetic conservatism is a result of duplication of alleles on chromosomes
186
and of duplication of genomes (polyploidy). Of course, populations and eventually species do
Wilson & Agnew, chapter 1, Plants, page 7 of 28
187
change sometimes, giving in some cases dramatic ecotypic adaptation and eventually leading to
188
the 400,000 flowering plant species that we see today and the millions that rest in peace.
189
Interaction with other trophic levels
Plants are mostly autotrophic, but they interact with all other trophic levels. Since our
190
191
thrust is plant communities, we shall generally discuss this only so far as it mediates plant-plant
192
interactions. D.A. Wardle (2002) has discussed interactions with decomposers. Plants meet and
193
usually withstand challenges from herbivores and diseases, usually in two totally differing
194
environments: the relatively humid soil below ground and the comparative aridity of sunlight
195
above ground (chapters 2 and 4). Many plants rely on animals for pollination and dispersal
196
(chapters 2 and 4).Top carnivores will have indirect effects. Mycorrhizae are crucial for many
197
species, and will be discussed especially in chapter 2. In addition to their rôle in nutrition and
198
water acquisition, it seems that vesicular-arbuscular mycorrhizae (VAM) can restrict the
199
development of pathogen loads in their host (Larsen and Bødker 2001). Endophytic fungi and
200
bacteria are also widespread and have a multiplicity of effects on plant growth. There is usually
201
an extensive microflora in the phyllosphere and in the rhizosphere. Some plants form special
202
relationships with ants, to which we shall refer. In some plants mites inhabit small pits in leaves
203
(domatia), and apparently protect the plant against other herbivores or against pathogenic fungi
204
(Grostal and O’Dowd 1994). This brief list of interactions is surely far from exhaustive.
205
1.2 What is a plant community?
To test community theories we need communities. Unfortunately it is not possible to
206
207
provide a definition of ‘community’ that includes areal extent, uniformity of environment,
208
closeness to equilibrium, etc. All sorts of species mixtures exist, in all sorts of environments, and
209
there are no discontinuities in the hierarchy of this variation. Furthermore, species mixtures are
210
constantly changing. We believe the plethora of terms that have been applied to species mixtures
211
(phytocoenose, association, nodum, etc.) are attempts to persuade vegetation ecologists that the
212
study of this aspect of the natural world can yield general statements and predictive rules, but it
213
cannot.
214
How close can we get to defining ‘plant community’? A degree of repeatability between
215
samples (i.e. quadrats) would be a useful restriction, but this again is difficult to prescribe (chap.
216
6, sect. 2). We need to specify scale at some point in our argument; Gleason (1936) suggested it
217
should be one plant of a largest species but we do not feel able to insist on this.. Mueller-
Wilson & Agnew, chapter 1, Plants, page 8 of 28
218
Dombois and Ellenberg (1974) give a historical summary and agree that no rigid definition is
219
possible. However, they distinguish between conceptual communities which are the abstract units
220
of plant community classification and ‘concrete’ communities that are the actual plant species
221
mixtures encountered in the field. We hope that all our discussions can be related to real, actual
222
examples of plant communities, the concrete ones, for we are not persuaded of the relevance of
223
conceptual communities. We could use the splendidly neutral and practical statement of Tansley
224
and Chip (1926) that “A plant community may be defined as any naturally growing collection of
225
plants which, for the purposes of the study of vegetation, can be usefully treated as an entity.”
226
To include environmental relations, stability and change in the community, and spatial
227
contiguity we here see the plant community as: Naturally generated plant stands where the
228
environment of the individuals of one species potentially, predictably and persistently includes
229
individuals of its own and usually a restricted number of other species. This excludes mixtures
230
deliberately planted, such as a mixed shrubbery, but planted gardens and agricultural fields can
231
contain a rich weed flora and are valid objects of study. Of course, indirect human intervention
232
such as fertilisation and the release of grazers is quite acceptable: they often mimic perturbations
233
in natural communities, and in any case it is fascinating to see how a mixture of species responds
234
(e.g. Fuhlendorf and Smeins 1997; Silvertown et al. 2006).
235
We are trying to make sense of nature, starting with a vision about plants and plant
236
communities, and looking for underlying predictability and repeatability so we can claim
237
community ecology as a science. As the great Robert MacArthur (1972) said: “To do science is to
238
search for repeated patterns”. The major difficulty for us is that we do not know what sort of
239
pattern to look for (chapter 5). One issue in dealing with samples of plant mixtures is the concept
240
of phantom species. These are species present in the general area (“in the community”),
241
potentially available in samples but not actually recorded. This may be a valid concern for animal
242
communities where species at low density can be around and sometimes walk/swim/fly though
243
the sample area/volume, but happened not to be there at the recording time. This is less relevant
244
for plant communities and we follow Pielou’s (1990) suggestion: “a biological collection …
245
should be treated as a universe in its own right”, rejecting the concept of phantom species as a
246
figment of the theoretical ecologist’s imagination.
Wilson & Agnew, chapter 1, Plants, page 9 of 28
247
248
249
2 The accession of species into mixtures
We discuss the processes that initiate plant communities in six steps, in some
developmental order:
250
A. Speciation: Life has originated, and the species must have evolved.
251
B. Biogeography: The species must be in the regional species pool.
252
C. Dispersal: The species in the regional species pool must reach the particular site.
253
D. Environmental filtering / ecesis: The species must be able to germinate/develop from its
254
propagule and then grow to reproduction under the physical environmental conditions
255
prevailing.
256
257
258
259
260
261
E. Productivity and biotic filtering: The species must be able to ecise and reproduce under
the general pressure of interference from the other species present (chapter 2).
F. Assembly rules: The species must withstand restrictions from the particular species or
types of species present (chapter 5).
2.1 Step A, Speciation: What is a species?
We shall deal only peripherally with sub-specific evolution and not at all with the
262
evolution of species, but they are the first required taxonomic category above the plant. The
263
recognition, description and diagnosis of species allow us to predict much of a plant’s
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.
Wilson & Agnew, chapter 1, Plants, page 10 of 28
264
morphology and behaviour after the identification of a scrap (the use of ‘morphospecies’ does not
265
allow this). This predictability was the basis for the development of the science of Botany in the
266
eighteenth century, and our ability to describe the vegetation around us. Unlike with animals,
267
plant taxonomists (Stace 1989) are happy to allow species with only incomplete restrictions to
268
gene exchange. However, each species is required to have a distinct phenotype. It must therefore
269
have a unique environmental tolerance and a unique reaction on the environment, even if the
270
difference from other species is sometimes small.
271
2.2 Step B, Biogeography: The species pool
272
The plant community can, in the short term, comprise only species present in the region,
273
which is the species pool (Fig. 1.2). The pool is difficult to define and quite as difficult to
274
determine, because we never know the distances over which species have the ability to disperse
275
or the frequency of dispersal events. However, different processes do occur on different spatial
276
scales. The regional species distribution for many species comprises a metapopulation: a series of
277
populations that are partly independent but connected by occasional migration events. In practice
278
the metapopulations of many species will show similar distributions due to similar habitat
279
requirements, giving a metacommunity (Holyoak et al. 2005). It is a nice distinction as to
280
whether a disseminule arrives via long-distance dispersal or from the metacommunity hinterland,
281
and in any case the resulting processes of establishment must be similar.
282
Questions about the species pool are dependent on the time frame: how long are we
283
prepared to wait for the species to arrive? Were time the only limitation to dispersal, disseminules
284
from far and wide would arrive anywhere, 400,000 species, and clearly this does not happen.
285
Continents have very different floras. Many European tree species have failed to occupy their
286
potential ranges in spite of several thousand years in which to spread across the continent
287
(Svenning and Skov 2004). The school of panbiogeography sees many present-day restrictions in
288
distribution between and within land areas as a reflection of the geography millions of years ago
289
(Fig. 1.3), and its analyses of species distributions that have repeatedly been borne out by
290
subsequent geological discoveries (Heads 2005). Clements and Shelford (1939) agree that
291
whereas migration of propagules is common, establishment of them is “altogether exceptional”.
292
There are also restrictions on the scale of hundreds of years: Matlack (2005) modelled the
293
distribution of species in eastern USA and concluded that the frequencies of species in the
294
modern landscape was controlled by the time available for spread in the last 300 years, with
295
vertebrate-dispersed species occupying considerably more of their potential geographical range
Wilson & Agnew, chapter 1, Plants, page 11 of 28
296
than other species. It is often unclear on which timescale the distribution limitation has occurred;
297
for example a gap in the distribution of Nothofagus spp. in the South Island of New Zealand (Fig.
298
1.3) has variously been correlated with geological movements (Heads 1989), the last glaciation
299
(P. Wardle 1964) and the current environment (Haase 1990). Therefore, the closest we can come
300
to definition is to say that over realistic time spans most members of the area’s species pool could
301
arrive, and we have to explain the restricted subset of species found in each plant community.
302
The concept of the species pool has sometimes included only species suited to the
303
environment of the habitat in question. In Europe, species have sometimes been excluded from
304
the pool using their Ellenberg ecological-tolerance rating (Ellenberg 1974). These values,
305
originally crude, have been progressively refined. Outside Europe, little information exists on
306
species tolerances for whole floras. A confounding question is whether the species pool is
307
defined before or interference. If the species pool comprises only those species physiologically
308
able to tolerate the physical conditions at the site, it would include many never found there,
309
because of interference (Steps E and F). Many reports concerning filtering and interference
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 from Heads (1989) and the
‘beech gap’ (
) from Wardle (1964).
310
assume that the species pool includes species that can tolerate the environment, but cannot stand
311
interference. However, the species lists are often taken from post-interference communities and
312
so the argument is circular. An example of this syndrome is when climate change models of
313
future vegetation are based on physiological parameters derived from distributions (i.e. from the
314
realised niches; section 4.1), and used in models as physiological parameters (e.g. Sykes and
315
Prentice 1996).
Wilson & Agnew, chapter 1, Plants, page 12 of 28
316
2.3 Step C, Dispersal
317
Propagules
318
Propagule types are various. Within the angiosperms, seeds can be produced sexually
319
(after meiosis and fertilisation), apomictically (with no meiosis and no involvement by pollen) or
320
by pseudogamy (pollen is needed for seed development, and fertilises the endosperm, but the
321
embryo itself is produced apomictically). Vegetative reproduction can occur via bulbils, stolons,
322
rhizomes, layering of branches (e.g. Salix cinerea, willow tree), root suckers, etc. There is no
323
basic distinction between the apomictic seeds of Taraxacum spp. (dandelion), ‘vegetative
324
reproduction’ such as the production of Kalanchoe daigremontiana plantlets from the leaf
325
margin, the growth of an Elytrigia repens (couch grass) clone by rhizomes, the growth of a
326
Populus tremuloides (aspen) clone by root suckers and the growth of an axillary bud on a tree
327
branch to give new leaf modules. All replicate an original genotype but after many mitotic
328
divisions which can accumulate errors.
329
The immediate fate of these propagules is various. Bulbils and viviparous seeds both
330
develop as plantlets on the parent. Ramets produced by stolon or rhizome are initially dependent
331
on the parent, then for a period are physiologically independent unless a change occurs, such as
332
defoliation or shading, when ramets subsidise each other (Marshall 1996), and then become fully
333
independent as the connecting stolons/rhizomes wither. Seeds are usually dispersed by wind,
334
water or animals, although a few plants produce hypogeal seeds (i.e. belowground). Tree and
335
herb sectors behave similarly to clonal tillers with limited integration, except that there is a
336
greater tendency for branches to overtop one another competitively (Novoplansky 1996).
337
Migration
338
Dispersal is the means by which species move around the landscape. The two critical
339
considerations are the distance and frequency with which disseminules move outside their source
340
habitat, which is negatively related to disseminule size, and their potential for establishment as a
341
seedling in a new site amongst existing plants which is positively related to disseminule size
342
(Salisbury 1942).
343
Plant dispersal usually has a long tail (Fig. 1.4), i.e. it is leptokurtic, and is often best
344
fitted by a negative exponential function. That is, most dispersal of disseminules of every type is
345
surprisingly short-distance, with rare long-distance events, as Carey and Watkinson (1993) found
346
for mechanical scatter of the seeds of an annual festucoid grass and Matlack (2005) for dispersal
347
of ingested seeds. The reason is probably that most species are dispersed by two or more
Wilson & Agnew, chapter 1, Plants, page 13 of 28
348
mechanisms: for example Agnew and Flux (1970) found in the Rift Valley, Kenya, that though
349
many grass disseminules had a large wing apparently adapted for wind dispersal, the longer
350
distance dispersal seemed to occur when the fruit became entangled in the coats of Lepus
351
capensis (hares). Occasionally, the direction can be towards suitable habitat, for example ants
352
dispersing seeds along their runways (Huxley and Cutler 1991). In general, the number of
353
disseminules arriving (the ‘propagule pressure’) will not matter: a smaller number will delay an
354
invasion but will not prevent it. The exception is when an Allee effect is operating.
355
356
357
358
359
360
361
362
363
364
Fig.
1.3. Fig. 1.4. Dispersal of fruit of Vulpia fasciculata. From Carey and Watkinson (1993).
365
366
The population of dormant seeds in the soil or in aerial fruits – the ‘seed pool’ or ‘seed
367
bank’ – is a buffer against elimination of a species. This is important for an annual species in an
368
adverse year, and for species surviving through disasters such as fire. The seed pool is not only
369
local, it can have a metapopulation structure comparable to that of adults when seeds are moved
370
around by floods or large herbivores such as elephants. This can be seen as spatial mass effect
371
(chap. 4, sect. 12).
372
2.4 Step D, Environmental filtering / ecesis
373
Propagule germination and establishment
374
The germination of a propagule starts phase of an invasion when it is the challenged by
375
the conditions in a new site (Fig. 1.2). Sometimes seeds germinate only when conditions arrive
376
that are more mesic than those tolerated by their parents, and these events may be rare in stressed
377
habitats. For example, many halophytes need unusually low salinity on a saltmarsh before they
378
germinate, when their more glycophytic seedlings can become established (Alexander and
Wilson & Agnew, chapter 1, Plants, page 14 of 28
379
Dunton 2002). Arid land species show very precise adaptations for effective dispersal and
380
germination in the highly variable rainfall patterns found in these habitats (Gutterman 2002). For
381
example, Pake and Venable (1996) found that different species of winter annual in the Sonoran
382
Desert tended to germinate in different years, and species tended to germinate more in years that
383
turned out to give them higher reproductive success.
384
Plants of all species need to pass through a juvenile phase before becoming reproductive.
385
This part of the challenge in occupying a new site is called ecesis (Clements 1904). As Clements
386
(1916) wrote: “Ecesis is the adjustment of the plant to a new home. It consists of three essential
387
processes, germination, growth, and reproduction. ... Ecesis comprises all the processes exhibited
388
by an invading germule from the time it enters a new area until it is thoroughly established there.
389
Hence it really includes competition, except in the case of pioneers in bare areas.”. This
390
definition is of course far too broad, because it seems to include the whole life cycle. We need to
391
separate out the first stages for our reductionist view, so we restrict ‘ecesis’ to post-germination
392
survival, growth and establishment: the part of the species filtering process that determines which
393
species survive the initial dispersal and germination phases of plant community establishment.
394
Invasion patterns
395
Invasion can be seen on all scales, from movement to and fro across 2 m within a decade
396
in links (Olff et al. 2000) to movement over thousands of years. The patterns of invasion seen are
397
much the same at any scale, and fall into three types. Phalanx invasion is dense and over a broad,
398
solid front. Guerrilla invasion comprises invasion by isolated individuals, which gradually fills in
399
the space available (Hutchings 1986). It is difficult to measure invasion processes because
400
a-priori assessment of habitat suitability is problematic. However, an example of an invasion
401
where at least part of the flora used guerrilla invasion is the advance of the herb flora of ancient
402
forest into adjacent secondary growth in Sweden, where Brunet et al. (2000) concluded that
403
distance (equivalent to time for dispersal) and the soil environment almost equally controlled
404
species composition. However, by far the most common seems to be a third type of invasion – a
405
combination of the former two – infiltration invasion in which there is occasional long-distance
406
dispersal (as in guerrilla invasion), followed by local short-range dispersal from these foci (as in
407
phalanx) (Fig. 1.5; J.B. Wilson and Lee 1989). This matches the leptokurtic / two-mechanism
408
dispersal typical of plant disseminules. It can be seen at a variety of scales, e.g. over kilometres
409
(Lee et al. 1991) to centimetres. A small-scale example occurs when most of a tussock grass’
410
tillers are produced within the leaf sheath, but a few are pushed greater distances by animal
Wilson & Agnew, chapter 1, Plants, page 15 of 28
411
hooves (Harberd 1962), an example of the leptokurtic / two-mechanism dispersal to which we
412
referred in section 2.3. Egler (1977) describes all these patterns, with details, though using
413
different terms.
Expanding
foci
Guerrilla
individuals
g clumps
414
415
416
417
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
418
environmental conditions, such as soil type, hydrology, climatic regimes and altitude, that
419
prevent the immigrants’ growing (Honnay et al. 2001). This has been called environmental
420
filtering or abiotic filtering (Weiher and Keddy 1995). It occurs largely during ecesis. The
421
existence of this filter is obvious. Sophisticated methods can be used to record the response
422
surface (e.g. Bio et al. 1998), but it remains what Warming (1909) described as “this easy task”.
423
In this book, we generally take the physical restrictions as given, and concentrate on those
424
community processes that control species composition. However, we need to return to this topic
425
in considering the niche.
426
Reaction
427
The physical environment is not strictly abiotic, for the receptor community can alter the
428
environment to create or close invasible sites, a process for which Clements (1904) coined the
429
term ‘reaction’: “By the term reaction is understood the effect which a plant or a community
430
exerts upon its habitat. … Direct reactions of importance are confined almost wholly to physical
431
factors” (Clements 1916). Any organism must cause reaction, though the environmental
432
modification varies from slight to major, and the causal species' autoresponse from negative
433
(facilitation: Clements 1916) to positive (switch: J.B. Wilson & Agnew 1992). Reaction is the
Wilson & Agnew, chapter 1, Plants, page 16 of 28
434
basis of almost all plant-plant interactions (Clements 1904). Since Clements, other terms have
435
been used for the same effect, such as ‘ecosystem engineer’ (Jones et al. 1994), and ‘niche
436
construction’ (Odling-Smee 1988). They seem to be later synonyms, but we shall sometimes use
437
the latter when discussing the niche.
438
Sometimes, the favourable microsites for establishment are gaps, but Ryser (1993) found
439
in a temperate calcareous grassland that the favourable microsites were those where the reaction
440
of established plants in the community provided shelter from frost; unvegetated gaps were not
441
colonised. Litter production is a common mode of reaction, forming the seedbed of an invader,
442
and enabling or inhibiting its germination. We can see litter as part of the extended phenotype of
443
a plant (chapter 2).
444
2.5 Step E, Productivity and biotic filtering
445
Communities develop a regime of carbon cycling, of which the autotrophic production is
446
our concern. We devote space to productivity here because it is easy to think rather loosely about
447
it. Productivity is “The potential rate of incorporation or generation of energy or organic
448
matter … per unit area …” (Lincoln et al. 1982), but there is a lot of complication behind this
449
definition:
450
451
452
453
1. Carbon is fixed from CO2 by the C3, C4 or CAM mechanisms. 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.
454
3. Later, at night, some of the fixed C is lost by dark respiration.
455
4. The remaining C is transported to sinks at sites of cell division (secondary cambium, root,
456
shoot apex, inflorescence, etc.), where it is incorporated into cell wall tissue, storage
457
carbohydrates or cytoplasm, with shared potential fates. In this process, it can be:
458
A. Lost by respiration whilst still incorporated in soluble C compounds, etc.
459
B. Lost to aphids feeding on the phloem sap (Heizmann et al. 2001).
460
C. Leached from leaves as soluble C compounds (Czech and Kappen 1997).
461
D. Lost by roots as exudate of soluble C compounds (Kuzyakov and Siniakina 2001) and
462
463
464
465
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).
Wilson & Agnew, chapter 1, Plants, page 17 of 28
466
ii. Removed by allogenic or autogenic damage or abscised. The amount lost varies
467
from parts of leaves to tree branches: cell walls plus modified cell contents.
468
iii. Lost at the death of tissues (wood, bark), organs (roots, leaves flowers and fruit),
469
470
or the whole plant.
F. In living tissues, C in carbohydrate storage and cytoplasm can be lost by respiration as
471
the root becomes old or the leaf becomes shaded and/or old, or it can be translocated
472
with attendant respiratory costs. However, cell wall C cannot be lost this way,
473
because no autolysis of cellulose or lignin occurs within living plants. (This contrasts
474
with animal tissue, where all C is part of the labile pool except for some dermal
475
structures.)
476
In the face of this complexity there is no consensus as to what productivity is or how it should be
477
measured. Logic and simplicity would suggest that the real definition of productivity is the
478
amount of C that reaches the next trophic level, herbivores or decomposers. This top down
479
definition would comprise only E1-3 in the above schema. ‘Gross productivity’ can be measured
480
in the field through gas analysis (though this omits photorespiration: Step 2), and ‘net
481
productivity’ as this less all the later losses. Productivity is most often estimated by sequential
482
sampling. Suppose a habitat holding mature stable vegetation in an approximately steady state. If
483
we sample at one time and then resample the same area 12 months later, in many areas there
484
would be no change in the absence of climate change and apart from sampling error – neither
485
accumulation nor loss of biomass – so we would arrive at an estimate of zero productivity. This is
486
either accurate or misleadingly trivial, depending on your definition of productivity. However, all
487
systems show some seasonal development and change, and most estimates of plant productivity
488
rest on the successive sampling of harvest biomass (standing crop) during the season of maximal
489
growth (Perkins et al. 1978). This working approximation is a wild under-estimate of actual
490
productivity, yet has physical presence and ecological meaning. Actually, in much discussion of
491
productivity, e.g. in testing for a humped-back curve (Grime 1979), standing crop is used as a
492
substitute, and it is a very poor one.
493
The productivity potential of a site controls what plant community develops in three
494
ways. Firstly, the readiness of the soil surface to provide sites for invasion and thus augment the
495
community: often with greater productivity more litter will be available affecting the ecesis of
496
invaders (section 2.4 above; chap. 2, sect. 5.4). Secondly, disturbance of the community through
497
herbivory and often fire: high productivity attracts herbivory, while pronounced dry seasons
Wilson & Agnew, chapter 1, Plants, page 18 of 28
498
between productive growing seasons favour fire. However, in general these first two factors
499
affect the rate of invasion more than the eventual fate of an invasion. The third aspect is the
500
competitive status of the community: greater productivity means more competition and more
501
difficulty for additional species to establish. For example, Cantero et al. (1999) concluded that the
502
diversity of short grasslands in Argentina was affected by surrounding species pools, while that
503
of tall grasslands with more competition was not. This is the interference filter, in which a species
504
is able to tolerate the physical environment of a site, but cannot grow well enough there to
505
withstand the general level of interference present there: competition, allelopathy, etc. It is an
506
effect not specially dependent on the identity of the associates. This is the classic distinction
507
between the fundamental and realised niche (section 4.1). It is clearly a major factor, as can be
508
seen by the ready cultivation of many species in botanic gardens outside their natural edaphic
509
and/or climatic range.
510
Organisms of other trophic levels affect ecesis and reproduction (section 1.1, “Interaction
511
with other trophic levels”). A species might be able to maintain a positive population growth rate
512
without heterotrophs, but be pushed into negative growth when pathogens or herbivores take their
513
toll. This could be environmentally dependent: a potentially fatal herbivore might be absent
514
because the environment is beyond its tolerance. On the other hand, a plant may be unable to
515
reproduce because a normally subventing pollinator is beyond its environmental range, and hence
516
absent. This would be a type of assembly rule (Step F), though in this book we consider only
517
plant-plant assembly rules as such.
518
2.6 Step F: Assembly rules and micro-evolution
519
Assembly rules are "restrictions on the observed patterns of species presence or
520
abundance that are based on the presence or abundance of one or other species or groups of
521
species …" (J.B. Wilson 1999a). We discuss them in chapter 5. It is clearly a simplification to
522
take species as fixed units and we do so only to limit the scope of this book. To glimpse into the
523
world of ecotypes and micro-evolution as they affect community structure we examine the work
524
of Turkington and Harper (1979b), taking plants of Trifolium repens (white clover) from patches
525
of a field dominated by four different grass species. When they planted them into boxes of a
526
standardised soil sown with the four grass species, each T. repens genotype was the best
527
performer against the species from whose neighbourhood it had been taken in the field – an
528
amazingly neat result. They interpreted this as genetic coadaptation within the field. A
529
mechanism such as the quality of transmitted light is possible (Thompson and Harper 1988).
Wilson & Agnew, chapter 1, Plants, page 19 of 28
530
Aarssen and Turkington (1985b) performed a similar experiment in a pasture in British
531
Columbia, Canada, but using different patches/genotypes of Lolium perenne (ryegrass). They
532
obtained similar results for T. repens. However, they also examined variation within the
533
associated L. perenne and the results for it were the opposite – three of the four L. perenne
534
genotypes had their lowest competitive ability against the T. repens genotype with which they
535
had been growing in the field. This would tend to keep the grass/clover competitive abilities
536
balanced. There remains a fear that the effects were due to carry-over, i.e. maternal effects in the
537
vegetative material. Aarssen (1988) found that collecting seed rather than using vegetative
538
material (ramets) gave quite different results, which he attributed to screening of the gene pool
539
between seed and adult populations. Such screening has certainly been seen in heavy-metal
540
ecotypes, though under conditions where gene flow was high and the selective differentials
541
extreme. However, Turkington and Harper (1979b) had used preconditioning periods of only 3
542
months. Evans and Turkington (1988) in Canada, collecting plants in a similar way to Turkington
543
and Harper (1979b) – from below four different grass species – found morphological differences
544
between T. repens of the four origins after 4 months growth in a common garden which
545
disappeared after 27 months growth. Chanway et al. (1989) suggested that the difference between
546
T. repens material might be in the specific Rhizobium strains carried with it, not in the T. repens
547
itself. This could still be a force in structuring communities.
548
The Turkington and Harper (1979b) result would have been small-scale character
549
displacement. It is almost impossible to prove that character displacement has occurred because
550
the evidence must involve comparisons between areas, and those areas might differ in other ways
551
(Strong 1983). However, some cases are suggestive. If found, character displacement would be
552
evidence that species interactions were a strong force in genetic selection, and therefore also in
553
ecological selection, implying deterministic community structure.
554
3 Geographical boundaries
555
The behaviour of a species at its distributional limit can be fascinating. Often the habitat
556
range of a species becomes more restricted towards its boundary. Pigott (1970) reported that near
557
the eastern limit of Ilex aquifolium (holly) in Europe it becomes increasingly restricted to forest,
558
and Cirsium acaule (stemless thistle) at its northern limit in England becomes confined to
559
southern (warm) aspects. On the other hand, Diekmann and Lawesson (1999) found four
560
potential examples where species had wider ecological amplitudes towards their range margin in
Wilson & Agnew, chapter 1, Plants, page 20 of 28
561
northern Sweden, and suggested that there is such climatic stress in that region that a smaller
562
flora is present and important competitors are absent. Usually the plants are smaller and less
563
fecund towards the limit, leading to populations becoming smaller and absent from apparently
564
suitable habitat patches (Carey et al. 1995; Nantel and Gagnon 1999; Jump and Woodward
565
2003). Lower fecundity also makes populations more sensitive to disturbance. For example, fire
566
restricts Canadian Abies balsamea (balsam fir; Sirois 1997) and Pinus resinosa (red pine;
567
Flannigan and Bergeron 1998) to islands and isolated populations at the northern edge of their
568
ranges. In some cases there is a sudden cut-off point in a species with no reduction in vigour near
569
the limit (Lactuca serriola, prickly lettuce; Carter and Prince 1985). This may not be exceptional.
570
Griggs (1914) made an early and careful observational study of Sugar Creek, in a “tension zone”
571
in Ohio where over 120 species have geographical boundaries, asking whether populations
572
became sparse or less fecund in this region. He found no consistency of behaviour, but most
573
edge-of-range species were abundant and flowered and fruited successfully up to the
574
geographical limit, as in L. serriola. Griggs could only hypothesise that competition sharpened
575
boundaries to make them abrupt. There can be many different reasons for a species’ failing to
576
expand its distribution, sometimes surprising ones. Pigott and Huntley (1981) found that the
577
environmental filter for Tilia cordata (linden) at the northern limit of its range in England was
578
that the pollen tube could not grow fast enough in the low temperature to reach the ovule, leaving
579
a relictual population now unable to reproduce by seed.
580
At present it seems that the behaviour of species at the margins of their ranges is complex
581
and unpredictable. For example, if the range limit were due to individuals being selectively
582
eliminated we might expect lower variances of morphological measurements at geographic
583
edges, but J.B. Wilson et al. (1991) could find no such effect.
584
4 Concepts of the space occupied by one species
585
Our purpose in this book is to examine the way species fit together in a mixture. Two
586
powerful conceptual tools, developed for this purpose, are the niche and the guild.
587
4.1 The niche
588
The end point of a species' pilgrimage from the pool into a community is occupancy of a
589
niche. The niche is an old concept. Grinnell (1904) and Elton (1927) introduced the term, and
590
both used it to describe an area available within habitat space, broadly defined by physical and
591
trophic parameters. Hutchinson (1944) formalised this to “a region in n-dimensional hyperspace”
Wilson & Agnew, chapter 1, Plants, page 21 of 28
592
where the dimensions are all the environmental, resource or behavioural (e.g. phenology,
593
foraging) parameters that permit an organism to live.
594
Since Hutchinson’s overarching statement it has been tempting to regard species presence
595
as the only definition of the niche. Thus, Levins and Lewontin (1985) advocated that “ecological
596
niches are defined only by the organisms in them”. Odling-Smee et al. (2003) believed that for
597
Hutchinson “a niche cannot exist without an occupant”. We see no reason to understand
598
Hutchinson thus. The crunch comes with the empty niche. Under the “the species is the niche”
599
concept “the idea of an ecological niche without an organism filling it loses all meaning” (Levins
600
and Lewontin 1985). However, the empty niche is a necessary concept in theory, especially in
601
relation to invasions. The absurdity of the “the species is the niche” is seen by observing
602
innovative invaders. Did no niche for a cactus exist in central Australia until Opuntia stricta
603
(prickly pear cactus) was introduced (Hosking et al. 1994)? Was there no niche for a cactus-
604
eating insect before the moth Cactoblastis cactorum was introduced for biological control of O.
605
stricta? Was there no niche below the saltmeadow in British estuaries until Spartina ×townsendii
606
/ anglica (cord grass) created itself by hybridisation in 1887? Was there no niche for an emergent
607
tree in Bonin Island shrublands until Pinus lutchuensis (a pine from elsewhere in Japan) was
608
introduced (Shimizu and Tabata 1985)? It seems better to regard all these as empty niches that
609
were later filled. To be sure, the identification of empty niches is very hard. There must be areas
610
of hyperspace that it is impossible for plants to fill: floating in the air, growing on the ice at the
611
South Pole or growing at 100 °C in hydrothermal steam vents?
612
Tilman (1997) claimed to find evidence of empty niches. In 1991 he sowed seeds of up to
613
54 species into native grassland at Cedar Creek. Many became established. However, this did not
614
cause extinctions among the species originally present in 1991: the proportion of those lost was
615
not correlated with the number of species added (r = +0.16, R2 = 2.6 %, not significant). Even
616
more interestingly, the total cover of those species present in 1991 did not decrease (r = 0.04, R2
617
= 0.16 %, not significant). The R2 values are impressively low so we could conclude, as Tilman
618
did, that the added species occupied empty niches. There is a problem that the species
619
composition probably co-varied with the species richness. The use of ‘total cover’ is odd. If two
620
leaves of different species are vertically aligned, both count towards cover, and if two leaves of
621
the same species are horizontally aligned both count, but if two leaves of the same species are
622
vertically aligned only one counts. ‘Total cover’ is not a sensible concept. In this case, the ‘cover’
623
of each species was, unfortunately, guessed. Cardboard cutouts were used to guide the guessing
Wilson & Agnew, chapter 1, Plants, page 22 of 28
624
but we do not take aids to guessing as removing the fact that cover was guessed. Philip Grime’s
625
group at Sheffield always uses objective measurements: presence/absence, local frequency, point
626
quadrats or sorted biomass as appropriate. Why can’t everyone? We mention this issue because it
627
will repeatedly mar results that we report. We shall not shirk from pointing out when the data of
628
this type are used, nor shall we use euphemisms like “estimated by eye”, because we believe this
629
practice is a blot on our science (even if we have occasionally been guilty ourselves in evil places
630
far away and naughty times forever gone).
631
The second reason for rejecting the “the species is the niche” concept is that it takes
632
species presence in the field as the de facto description of its niche, but this is affected by
633
interaction with other biota (competition, herbivory etc), introducing an imponderable set of
634
variables over space and time. It is more useful to separate biotic variables as restricting the
635
occurrence of a species to its realised niche, whereas its environmental tolerance defines its
636
fundamental niche or physiological tolerance (Hutchinson 1957). This distinction was known to
637
Tansley (1917) and Gleason (1917), though dismissed by Clements (1907) as “merely
638
migration”. The niche width of a group of species is usually considerably narrower, and hence
639
their niche overlap is less, when they are grown in an experimental mixed community than when
640
they are grown alone in the same conditions. That is, their realised niche is smaller than their
641
fundamental niche. Silvertown et al. (1999) found this re-analysing data of Ellenberg’s with six
642
grass species and a water table gradient: the mean niche overlap was considerably higher in the 6-
643
species mixture than comparing monocultures, and the niche modes spread out to a range of 5-
644
100 cm depth-to-water-table in the mixture compared to a range of 20-35 cm among the
645
monocultures. However, definitions become difficult because of reaction: any organism must
646
alter its own environment and this may cause niche construction, potentially leading to realised
647
niches that are larger than the fundamental ones (Odling-Smee et al. 2003).
648
The niche includes a species' developmental requirements (temperature etc.), its material
649
requirements (resources) and its relations with neighbouring species. A complete circumscription
650
of these is almost impossible, requiring knowledge of every aspect of the species' physiology and
651
life history, but two types of niche can be distinguished. The beta (β) niche is the range of
652
physical environmental conditions under which the fitness of a species is maintained (Alley
653
1985), e.g. its temperature tolerance, and therefore its potential geographical limits. It is related to
654
Chesson’s (in press) concept of ‘environment’ as a factor that does not form a feedback loop, i.e.
655
is not appreciably affected by the organisms themselves. The alpha (α) niche represents the
Wilson & Agnew, chapter 1, Plants, page 23 of 28
656
resources used within a community/site, the “‘profession’ or functional role” (Alley 1985), e.g.
657
different rooting depths. Many methods of analysis, e.g. the calculation of niche width and
658
overlap, can be used for both alpha and beta niches, and there are areas of character overlap.
659
However, when we use the niche concept we generally need either one or the other. Much
660
ecological discussion has been confused by failing to take the distinction into account.
661
662
663
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,
664
water source), persistence (storage organs, wood), autogenic disturbance (through litter
665
and physical environmental effects), heat budget (convective, transpirative, radiative),
666
physical defence against herbivores (glands, hairs, thorns), pollination and dispersal
667
biology. An example of these factors is synusiae in forest, such as epiphytes and lianas,
668
and indeed stratification in almost all communities. Another example is the parasitic
669
habit.
670
2. Growth phenology as the plant's response to environmental signals comprises the
671
seasonality of growth and reproduction (pollination and dispersal). Examples are the
672
progression of flowering in temperate vegetation and leaf flushes in tropical forests.
673
3. The chemical functioning of a plant ultimately controls everything, but we may list as
674
examples phototype (C3, C4 or CAM), light requirement, mycotrophy, P sources via root
675
phosphatase exudate, N source (N2, NH4 or NO3) and chemical defence against herbivore
676
and pathogen challenge.
677
4. Any of the above niche axes can be influenced by interference between neighbouring
678
plants (chapter 2), e.g. in the morphological pre-emption of soil resources, though the
679
overgrowth of competitors, through allelopathy and through positive effects.
680
681
5. Additional resources are gathered by the community. This is reaction causing niche
construction.
682
Beta niche axes are the environmental features of the locality and its biota, that is to say the
683
habitat. Complexities between factors are more apparent than with alpha axes. Aspects are:
684
1. Climate delivers solar insolation, water availability, CO2 (though the latter is rarely local),
685
nutrients (when ombrotrophic), some pollination vectors and rate regulators such as
686
temperature. These interact with the chemistry below. Climate also delivers exposure to
687
atmospheric humidity, wind, aeration and snow. This affects morphology, for example as
Wilson & Agnew, chapter 1, Plants, page 24 of 28
688
a partial determinant of the Raunkiaer life form (the life form can also differ between
689
alpha niches, for example in forest stratification).
690
2. Chemical features of soils (calcareous versus non-calcareous, pH, salinity, etc.) affect
691
system function (nutrient availability, cycling); these overlap with geomorphology below.
692
In the short term, mineral nutrients are often dominant.
693
3. Geomorphology delivers allogenic disturbance and soil substrate.
694
4. Biota deliver allogenic disturbance, generalised herbivore pressure and animal pollinators.
695
There can sometimes be overlap between the concepts of alpha-niche and beta-niche in terms of
696
characters: e.g. low growth is a feature of the ground alpha-niche in a forest but also of arctic
697
plants. However, the effects are opposite: species of the same beta niche will tend to co-occur
698
because they have the same environmental tolerances; species of the same alpha niche will have
699
no such tendency to co-occur, and if competitive exclusion is operating they will tend not to co-
700
occur (J.B. Wilson, submitted).
701
4.2 Guilds
702
The ecological term ‘guild’ was coined by Drude (1885) as the German
703
‘Artengenossenschaften’ to refer to a group of species moving from one region to another, such
704
as exotic species. It was used thus by Clements (1904; 1905) and J.B. Wilson (1989a). Perhaps
705
independently, Schimper (1898; 1903) used the term ‘Artengenossenschaften’ / ‘guild’ to mean a
706
synusia (e.g. stratum) in a forest. Tansley (1920) used it in the same way, writing of “guilds of
707
the same dependent life-form, such for instance as lianes”. Root (1967) ignored these established
708
usages and with animal assemblages in mind re-defined the guild as a “group of species using
709
similar resources in a similar way”. This is not directly useful for plants, since almost all use the
710
same resources (the sun’s energy, water, CO2, N, P, K and minor elements). The guild is a
711
category that is intended to be ecological rather than taxonomic, and J.B. Wilson (1999b) defined
712
it as: “a group of species that are similar in some way that is ecologically relevant, or might be”.
713
It is unusual to find “Or might be” in a scientific definition; it is necessary here because we
714
hardly ever known at the beginning of an investigation whether the guilds we are using are the
715
real ones, and often not at the end (but see the discussion of intrinsic guilds: chap. 5, sect. 7.6). In
716
spite of the lack of precedence of Root’s usage, and the impossibility of applying it strictly to
717
plants, a similar usage can have value: a guild as a group of species that occupy similar niches.
718
719
J.B. Wilson (1999b) pointed out that there are two basic types of guilds, corresponding to
the distinction between alpha-niches and beta-niches. Again, the outcomes are opposite: species
Wilson & Agnew, chapter 1, Plants, page 25 of 28
720
that are in the same beta-guild and therefore have similar environmental tolerances will generally
721
co-occur; species that are in the same alpha-guild and therefore use similar resources will tend
722
exclude each other. The species within one alpha-guild are similar in their resource use. For
723
example, within northern European forests, species that are within the same alpha-guild might be
724
the trees Tilia cordata (linden), Quercus petraea (sessile oak) and Fagus sylvatica (beech). They
725
are using similar resources: the light at the top of the canopy during the summer half-year, as well
726
as nutrients and water from the full profile of the soil. Conversely, if species are present in one
727
community that are in different alpha-guilds, they might be able to partition resources within a
728
community, so a community might tend to comprise species from several alpha-guilds. For
729
example, Tilia cordata, the hemi-parasite Viscum album (mistletoe), the liana Hedera helix (ivy)
730
and the ground herb Mercurialis perennis (dog’s mercury) would be in different alpha-guilds
731
because they use different light/support/nutritional resources, and if we found them together we
732
might see it as alpha-niche differentiation.
733
The species in one beta-guild are similar in their ecophysiology and therefore their
734
tolerance (across space or time) of environmental conditions, such as the “guilds of edaphic and
735
topographical specialists” of Hubbell and Foster (1986). After a species pool has passed through
736
an environmental filter, the remaining species will be a beta guild; they have overlapping beta-
737
niches. For example, all sub-arctic saltmarsh species would be in the same beta-guild because
738
they occur in the same climatic and soil conditions. An example of species occurring in different
739
beta-guilds might be the temperate, mesic tree Tilia cordata, the subalpine Pinus contorta
740
(lodgepole pine), arid land trees/shrubs of Prosopis spp. (mesquite), the tropical Cinchona
741
officinalis (quinine) and a species of mangrove. They occur in different environmental conditions
742
(climate and/or soil), so they are necessarily found apart in space or time since we cannot find
743
different external environmental conditions simultaneously at one spot. Díaz et al. (1998)
744
recorded abundances of ‘plant functional types’ (PFTs) of 100 species along a climatic gradient
745
in Argentina and found that vegetative traits differed between climatic zones, demonstrating that
746
beta-guilds are filtered out from the available species pool. The species within each zone will
747
almost certainly belong to different alpha-guilds.
748
The concept of ‘functional type’ (used above by Diaz et al. 1998) and that of the ‘guild’
749
can be essentially identical (J.B. Wilson 1999b; Blondel 2003). The current use of PFTs as the
750
predicted variate in models assumes that we know the characters of the types are trying to
751
summarise. In spite of the term ‘functional’ which implies alpha-guilds, most workers have
Wilson & Agnew, chapter 1, Plants, page 26 of 28
752
apparently intended to create beta-guilds. However, the characters they have chosen have often
753
been alpha-niche ones. For example, Kleyer (2002) formed guilds (‘functional types’) “to relate
754
unique PFTs to landscape specific habitat factors and to generalize syndrome-environment
755
relations across landscapes” and used characters such as annual versus biennial versus perennial,
756
plant height, regeneration from detached shoots, having leptophyllous leaves, longevity of seed
757
pool that are as likely to occur within a community. A distinction between ‘response’ and ‘effect’
758
guilds obscures the issue, because there is far more to the alpha-niche of a species than its
759
reaction (effect) on the environment. This situation has arisen from a failure to consider the
760
purpose of the guilds being formed, what type of guilds they will therefore be – alpha or beta –
761
and what characters are therefore appropriate.
762
4.3 Stratification
763
The most obvious alpha-guilds in plant communities are the guilds of Schimper (1898;
764
1903), synusiae (Fig. 1.6). Almost all plant communities are structured vertically. Aboveground,
765
the greater the vegetation cover, the more uniform and predictable is the vertical change in
766
microclimate. Highly structured forests have a stratum of separated, emergent trees, a more
767
continuous upper canopy, then sub-canopy trees, shrubs, tall herbs, creeping herbs and
768
bryophytes, lianas and epiphytes (including lichens, bryophytes and higher plants). This
769
represents specialisation to the attenuation of light, water, CO2 and nutrient resources. All this is
770
accepted for forests, but there is also complex stratification in grasslands, for example in the wet
771
grasslands of Tierra del Fuego (Díaz Barradas et al. 2001) and even in lawns (Roxburgh et al.
772
1993). Naturally all stratification by primary producers is echoed by stratification in consumer
Wilson & Agnew, chapter 1, Plants, page 27 of 28
773
communities.
Fig. 1.5: Stratification: profile of a rain forest in British Guiana. From Richards (1964).
774
775
We might expect that similar patterning is happening below ground because litter is
776
deposited on the soil surface eventually adding to the water-holding capacity and mineral nutrient
777
status of the upper soil. Most water arrives at the soil surface and percolates down, acidified by
778
organic acids and CO2, hydrolising the mineral fragments in the soil and most importantly
779
releasing phosphate. Plant roots and respiration can affect this, for example releasing acids.
780
Water in deep soil, including artesian water, is available to deep roots and may rise up by
781
capillarity and hydraulic lift. This can lead to stratification of root systems. Succulents of New
782
and Old World deserts have surface roots adapted for the uptake in ephemeral rainstorms
783
(Whitford 2002). Dodd et al. (1984) surveyed 43 woody species from the open woodland in SW
784
Australia, and Timberlake and Calvert (1993) 96 shrubs and trees of Zimbabwe woodlands, both
785
finding that there were indeed species with consistently shallow systems and others with deep
786
taproots. Most species had both lateral superficial roots and descending taproots, but herbs can be
787
shallow-rooted and in herbaceous or mixed herbaceous/woody communities there can be
788
considerable stratification (Weaver and Clements 1929 p. 213; Cody 1986).
789
5 Conclusion
790
This opening chapter has described the basic material of plant communities: the plants
791
themselves. We argued that plants are colonies of modules. The animal concept of ‘individual’
792
has no meaning for plants because: (a) it is often arbitrary when one set of modules becomes two,
Wilson & Agnew, chapter 1, Plants, page 28 of 28
793
notably in vegetatively-reproducing and apomictic plants, (b) the opportunity for somatic
794
mutation means that plants are potentially genetic mosaics and (c) plants are hugely plastic. There
795
is therefore no basic demographic or genetic difference between growth, vegetative reproduction,
796
apomictic reproduction and sexual reproduction; all are an increase in the number of modules.
797
Sexual reproduction differs genetically only in how variation is achieved. Even modules can be
798
plastic in size so the best measure of growth and abundance is biomass, or better still calorific
799
value. The disposable photosynthetic modules of plants must constantly be replaced, so a plant
800
moves in space and its extended phenotype moves through litter production. The contrast with
801
animals led us to the conclusion that “Plants move, animals don’t”. Plants also have features that
802
make their evolution different from that of animals.
The huge majority of plant stands have more than one species, and we have outlined the
803
804
basic processes through which multi-species communities establish and develop. The interactions
805
between pool, dispersal and niche are all important in this process (Fig. 1.2). There are recent
806
demonstrations of this at a large scale in the palm floras of Ecuador and Peru (Vormisto et al.
807
2004), and at an intermediate scale in the Netherlands (Ozinga et al. 2005). Our conclusion,
808
which we hope the reader shares, is that there is enormous complexity in the life of plants in spite
809
of the simplicity implied in their common sedentary habit and modular structure, and their almost
810
universal trophic function. Our basic concern in this book is to examine how the species fit
811
together to form communities and basic concepts for this are the niche and the guild. In the next
812
chapter we examine the processes involved when one species interacts with another, starting
813
community development.
814
ILLUSTRATIONS
815
Fig. 1.1: The plant as a colony of active and inactive apices
816
Fig. 1.2: Pathways from the species pool to community entry
817
Fig. 1.3: Disjunct distribution of Kelleria laxa in South Island, New Zealand, interpreted as an
818
originally contiguous distribution torn apart by movement along the Alpine Fault 2-10
819
million years ago and the ‘beech gap’. From Heads (1989).
820
Fig. 1.4: Dispersal of fruit of Vulpia fasciculata. From Carey and Watkinson (1993).
821
Fig. 1.5: Infiltration invasion by Olearia lyallii in the Auckland Islands. After Lee et al. (1991).
822
Fig. 1.6: Stratification: profile of a rain forest in Guiana: From Richards (1964).
1
‘sessile’ in zoological terminology