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The nature of the plant community: a reductionist view
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J. Bastow Wilson
Botany Department, University of Otago, P.O. Box 56, Dunedin, New Zealand.
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Andrew D.Q. Agnew
Institute of Biological Sciences, University of Wales Aberystwyth, SY23 3DA, U.K.
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Chapter 1: Plants are strange and wondrous things
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1 From plants to communities
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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 ................................................................................... 8
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 ...................................................................................................... 20
Concepts of the space occupied by one species .................................................................... 21
4.1 The niche ....................................................................................................................... 21
4.2 Guilds ............................................................................................................................. 24
4.3 Stratification .................................................................................................................. 26
Conclusion ............................................................................................................................ 27
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Our aim in this book is to explore the workings of plant communities and especially the
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forces that limit the coexistence of some species and promote the coexistence of others. We are
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searching for generalisations that can be applied to plant assemblages, working from the bottom
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up. We shall rarely discuss animals: this book is about plants. First we explain our view of
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vegetation and of the plants that comprise it.
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The landforms of the earth result from an underlying geological diversity, moulded by
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geomorphological forces and mostly clothed with vegetation. Even in arid climates, any
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scattering of plants intrudes and holds the human eye. Like the architectural heritage of the built
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environment, landscape has the power to be emotionally and spiritually uplifting, or depressing.
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Our reaction depends on our cultural history, our background experience and often current
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fashion. We, the authors, have been able to study vegetation during our full working lives, and it
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has been enormously rewarding and emotionally satisfying. Such studies are in some way a
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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
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world, behoving us to seek the processes behind the vegetation that we see, to search for general
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patterns, and to attempt the formulation of community-level theories.
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From the beginnings of plant ecology, some scientists have concentrated on describing
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the myriad of combinations in which species occur (e.g. Lawesson 2004). Others have used a
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reductionist approach, examining a process by which species A affects species B, but have sought
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no deeper generalisations. Yet others have developed theories into which they hope the world
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will fit (see Bio 2000). Such is the complexity of plant communities that, whether the theories
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have been primarily deductive (e.g. MacArthur 1969) or empirical (e.g. Grime 1979), all have
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basically failed. This book is an attempt to move reality and theories closer.
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There are plenty of theories to test, some more trivial than others, but it seems none have
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reliable truth. Suppose we take a group of students into the field, tell them that there is a ‘theory’
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that species richness is higher in ecotones (boundaries) and have them sample. Will they find
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that? Probably not. Suppose we tell them of the opposite ‘theory’ that can be found in the
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literature – species richness is lower in ecotones – will they take community ecology seriously as
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a science? Suppose we draw out of the hat a theory on where species evenness will be high, or
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where the relative abundance distribution will be a particular shape; will the students find it?
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Probably not. The only reason that students put up with this ‘science’ is that they, like us, find
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being in the field more pleasant than being in the lab. Nevertheless, it is our duty as scientists to
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start solving these problems.
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We shall emphasise terrestrial vascular plants, because more is known about them, and
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most of the processes to be found are found in them. However, it is likely that many of the same
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principles apply to lower plants, down to macro-algae and plankton (Tilman 1981; Wilson et al.
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1995 %689; Steel et al. 2004), and we shall take examples from any group of plants when we
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fancy. Very rarely do we see a plant species persisting on its own even when we try to make it do
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so in a garden or farm, so this book is about plant communities. However, in keeping with our
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reductionist approach we start by examining the importance and nature of plants.
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The importance of plants
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Plants, as the dominant carbon fixers in the biosphere, control all ecosystems. The
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terrestrial part of the biosphere is overwhelmingly vascular plant cover. Plant communities have a
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global entropic effect. Visible light from the sun is intercepted by our planet, and is dissipated at
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longer wavelengths into space (D.H. Miller 1981). This represents a gain in entropy, i.e. a trend
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towards homogenization of the universe. The plant covering of the Earth increases, be it almost
Wilson & Agnew, chapter 1, Plants, page 3 of 28
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immeasurably, this entropy gain. It does this by fixing a tiny part of solar energy into organic
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matter and, through evolutionary processes, maximising the efficiency of its utilisation
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(Ulanowicz and Hannon 1987) so that even more energy is re-radiated as long wave radiation.
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This is quite temporary for an individual living plant, but forests hold a long-term store of energy
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as reduced carbon and terrestrial plant products can remain for longer in soil, peat and eventually
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in subfossil and fossil deposits. The result is the maintenance of the oxygenated atmospheric
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state, of no small importance to us all. In fact, the vegetation cover has multifarious feedbacks on
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the climate (Hayden 1998).
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Plant communities affect the rocks and soil too, exercising major geomorphological
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controls on the earth’s land surface and landforms. They intercept precipitation and wind, damp
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down environmental fluctuations, reduce erosional rates, affect soil formation and dominate
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geochemical cycles (Trudgill 1977). Local and regional hydrology are profoundly affected by
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vegetation through evapotranspiration, which reduces the amount of water available in soil and
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the catchment outflow. Plant cover may accrue wind- and water-borne deposits and thus build
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landscapes. Every plant affects the local environment in ways that are again multifarious (Eviner
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and Chapin 2003). This is the ‘reaction’ of Clements (1904; 1916) and Gleason (1927).
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Plants are also almost the sole basis for the food chain. Reichle et al. (1975) itemise the
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four essential parts of ecosystem function as: (1) energy input in photosynthesis (‘energy base’),
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(2) the capital of energy in photosynthetic biomass (‘reservoir of energy’), (3) cycling, especially
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of elements, and (4) the control of the rates of these and other processes by factors such as
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temperature and the availability of heterotrophs (‘rate regulation’). On land, green vascular plants
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comprise almost the whole of the energy base and the resevoir of energy, and they make major
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contributions to cycling and rate regulation.
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1.1 Features of all land plants that predetermine their natural history
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Terrestrial green plants are so familiar to us that we often lose our sense of wonder at
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them, even as their features become more extraordinary as our knowledge of biology deepens.
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We argue that:
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1. Land plants root in the soil to obtain mineral nutrients, water and anchorage. Therefore,
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they are sedentary, so defence from herbivores can be only by structure and chemistry,
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not by escape.
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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
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cannot be efficiently recycled. Therefore plants almost always have to grow by the
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replacement of modules, such as leaves. Discarded modules are a necessary byproduct,
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comprising litter.
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3. Because of modular growth, the number of cell divisions between generations (i.e.
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gamete-to-gamete) is indeterminate and large. In the process of module production
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somatic mutations can occur, so all ‘individuals’ are potentially genetic mosaics. The
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germ cells are defined only just before the meiotic process, so they include these somatic
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mutations. This contrasts with animals, where the germ cells are defined at an early stage
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and migrate to the gonads (Gilbert 1997) with few cell divisions between one generation
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and the next and hence little opportunity for somatic mutations to accumulate and be
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passed on.
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Another result of modular growth is movement. Motile animals move around but, having grown,
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usually stay within approximately the same adult body, replacing organs cell-by-cell or molecule-
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by-molecule until death. Plants are sedentary, but their organs and elements of their living
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transport system have a limited length of useful life and must be replaced by new ones (Larcher
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1980). The photosynthetic rate of a leaf is maximal early in its life and declines thereafter, so
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leaves and their supporting organs are generally replaced several times during the lifespan of a
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plant. These replacement leaves are formed distally on the stem, or on side branches. This means
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that plants can never persist in an unchanged physical space; they must grow and in the process
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explore and expand into new space. Even cacti must increase in size during their life (de Kroon
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and van Groenendael 1997). This remorseless renewal of all modules of growth, the discard of
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old dead plants as litter and exploration of new space results in disturbance to neighbours. In
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other words: plants move, animals don’t.
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Some colonial, sedentaryi animals are similar to plants in that they must grow to stay
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alive: some Urochordata (tunicates), corals and Porifera (sponges). As a result they have several
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similarities to plants. They have similar genetic characteristics. They filter water for carbon just
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as plants can be said to be filtering water and air. The sedentary tunicates and corals have
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exoskeletons somewhat resistant to decay and predation (tunicin and calcium carbonate
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respectively), comparable to the epidermis of plants. However, there are differences. The
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modules causing the mandatory growth of corals are not discarded in the way leaves are, though
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the xylem in the heartwood of trees is retained too. Although animals that accumulate calcium
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carbonate have profound effects on marine geomorphology and the biosphere, no animals on land
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have byproducts similar to the litter of dead waste parts produced by living plants (chapt. 2,
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sect. 2 below). In the arthropods there is a periodically-shed exoskeleton that includes cellulose-
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like material, but there is sufficient protein in the exoskeleton to make it readily decomposable
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and anyway the biomass of herbivores can never approach that of the primary producers and
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therefore cannot modify the environment of entire systems as can plants.
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The problem of the individual
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The problem of recognising individuals in plant populations is longstanding. It is reflected
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in discussions of the terms biotype, genet and ramet (Harper 1977) as well as more philosophical
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discussions of the nature of the plant individual (Firn 2004). In an annual with no vegetative
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reproduction it is clear what an individual is. In vegetatively-reproducing plants with ramets
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gradually becoming independent (Marshall 1996), perhaps with the clone then splitting into
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several discrete patches (Harberd 1962), ‘individual’ has no demographic meaning. The same
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issue arises with the apomictic offspring in genera such as Crepis (hawksbeard), Poa and
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Taraxacum (dandelion) that are potentially identical in genotype. Another problem with applying
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the animal ‘individual’ concept to plants is that whilst most animals are relatively constant in size
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at any particular age, individuals of one plant genotype can differ in biomass by several orders of
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magnitude (Harper 1977). There is some evidence that the root system of an individual genet or
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even ramet can differentiate between roots from its own parent and other individuals of its own or
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other species. The experimental evidence of Gersani et al. (2001) using Glycine max (soybean)
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plants, of Gruntman and Novoplansky (2004) in Buchloe dactyloides (buffalo grass) and the
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neurotransmission speculations of Baluska et al. (2004) are fascinating in this respect and need
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confirmation.
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Somatic mutations complicate the issue further. There can be mutations as Taraxacum
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plants reproduce (King and Schaal 1990) so the apomictic offspring need not be genetically
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identical. Somatic mutations can occur in vegetatively-reproducing plants and during growth
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(Gill et al. 1995). Using the plant cell sizes in the classic Strasburger's textbook of botany
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(Harder et al. 1965) with a conservative estimate of mean cambial cell length of 0.1 mm it is clear
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that there could be of the order of 220 cell divisions between separate sectors of growth in a tree,
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with a consequent probability of mitotic errors. Therefore, even an apparently ‘individual’ plant
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cannot reliably be taken as a single genotype, and has to be regarded as a colony of apical
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meristems, even a colony of apical meristem segments (Fig. 1.1). Every apex and therefore each
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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
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The modules of a physiological individual such as a tree also differ in their environment (e.g.
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light intensity) and often the cause of that variation (shade, in our example) can be the individual
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itself (self-shading). However, physiological interdependence between the modules overcomes
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this to some extent.
Litter
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Fig. 1.1: A stylised dicotyledonous plant as a colony of active and
inactive apices
We conclude that because of vegetative reproduction and apomixis, variation in size,
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somatic mutation and plasticity, the animal concept of ‘individual’ is not appropriate or useful in
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plants.
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The features we have been discussing make genetic change difficult. We must ask why
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plants need genetic change when they can change plastically. One answer to this paradox has
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been the controversial theory of ‘genetic assimilation’ (Pigliucci and Murren 2003): that plastic
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changes can become incorporated into the genotype. Bradshaw’s (1973) answer was that plants
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are genetically ‘sown into their winter underwear’, because their plastic response to an adverse
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environmental shock would be too slow. A third answer is that they do not actually become
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adapted genetically: Rapson and Wilson (1988; 1992) found that though significant genetic
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differences had developed in Agrostis capillaris (bent) in southern New Zealand since it was
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introduced in 1853, there was no sign that populations were adapted to the habitat they were
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growing in. Perhaps genetic conservatism is a result of duplication of alleles on chromosomes
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and of duplication of genomes (polyploidy). Of course, populations and eventually species do
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change sometimes, giving in some cases dramatic ecotypic adaptation and eventually leading to
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the 400,000 flowering plant species that we see today and the millions that rest in peace.
Wilson & Agnew, chapter 1, Plants, page 7 of 28
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Interaction with other trophic levels
Plants are mostly autotrophic, but they interact with all other trophic levels. Since our
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thrust is plant communities, we shall generally discuss this only so far as it mediates plant-plant
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interactions. Wardle (2002) has discussed interactions with decomposers. Plants meet and usually
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withstand challenges from herbivores and diseases, usually in two totally differing environments:
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the relatively humid soil below ground and the comparative aridity of sunlight above ground
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(chapts. 2 and 4). Many plants rely on animals for pollination and dispersal (chapts. 2 and 4).Top
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carnivores will have indirect effects. Mycorrhizae are crucial for many species, and will be
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discussed especially in chapter 2. In addition to their rôle in nutrition and water acquisition, it
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seems that vesicular-arbuscular mycorrhizae (VAM) can restrict the development of pathogen
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loads in their host (Larsen and Bødker 2001). Endophytic fungi and bacteria are also widespread
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and have a multiplicity of effects on plant growth. There is usually an extensive microflora in the
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phyllosphere and in the rhizosphere. Some plants form special relationships with ants, to which
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we shall refer. In some plants mites inhabit small pits in leaves (domatia), and apparently protect
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the plant against other herbivores or against pathogenic fungi (Grostal and O’Dowd 1994). This
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brief list of interactions is surely far from exhaustive.
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1.2 What is a plant community?
To test community theories we need communities. Unfortunately it is not possible to
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provide a definition of ‘community’ that includes areal extent, uniformity of environment,
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closeness to equilibrium, etc. All sorts of species mixtures exist, in all sorts of environments, and
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there are no discontinuities in the hierarchy of this variation. Furthermore, species mixtures are
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constantly changing. We believe the plethora of terms that have been applied to species mixtures
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(phytocoenose, association, nodum, etc.) are attempts to persuade vegetation ecologists that the
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study of this aspect of the natural world can yield general statements and predictive rules, but it
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cannot.
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How close can we get to defining ‘plant community’? A degree of repeatability between
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samples (i.e. quadrats) would be a useful restriction, but this again is difficult to prescribe (chapt.
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6, sect. 2 below). We need to specify scale at some point in our argument; Gleason (1936)
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suggested it should be one plant of a largest species but we do not feel able to insist on this..
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Mueller-Dombois and Ellenberg (1974) give a historical summary and agree that no rigid
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definition is possible. However, they distinguish between conceptual communities which are the
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abstract units of plant community classification and ‘concrete’ communities that are the actual
Wilson & Agnew, chapter 1, Plants, page 8 of 28
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plant species mixtures encountered in the field. We hope that all our discussions can be related to
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real, actual examples of plant communities, the concrete ones, for we are not persuaded of the
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relevance of conceptual communities. We could use the splendidly neutral and practical
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statement of Tansley and Chip (1926) that “A plant community may be defined as any naturally
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growing collection of plants which, for the purposes of the study of vegetation, can be usefully
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treated as an entity.”
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To include environmental relations, stability and change in the community, and spatial
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contiguity we here see the plant community as: Naturally generated plant stands where the
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environment of the individuals of one species potentially, predictably and persistently includes
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individuals of its own and usually a restricted number of other species. This excludes mixtures
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deliberately planted, such as a mixed shrubbery, but planted gardens and agricultural fields can
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contain a rich weed flora and are valid objects of study. Of course, indirect human intervention
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such as fertilisation and the release of grazers is quite acceptable: they often mimic perturbations
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in natural communities, and in any case it is fascinating to see how a mixture of species responds
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(e.g. Fuhlendorf and Smeins 1997; Silvertown 2006).
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We are trying to make sense of nature, starting with a vision about plants and plant
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communities, and looking for underlying predictability and repeatability so we can claim
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community ecology as a science. As the great Robert MacArthur (1972) said: “To do science is to
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search for repeated patterns”. The major difficulty for us is that we do not know what sort of
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pattern to look for (chapt. 5 below). One issue in dealing with samples of plant mixtures is the
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concept of phantom species. These are species present in the general area (“in the community”),
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potentially available in samples but not actually recorded. This may be a valid concern for animal
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communities where species at low density can be around and sometimes walk/swim/fly though
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the sample area/volume, but happened not to be there at the recording time. This is less relevant
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for plant communities and we follow Pielou’s (1990) suggestion: “a biological collection …
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should be treated as a universe in its own right”, rejecting the concept of phantom species as a
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figment of the theoretical ecologist’s imagination.
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2 The accession of species into mixtures
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We discuss the processes that initiate plant communities in six steps, in some
developmental order:
A. Speciation: Life has originated, and the species must have evolved.
Wilson & Agnew, chapter 1, Plants, page 9 of 28
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B. Biogeography: The species must be in the regional species pool.
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C. Dispersal: The species in the regional species pool must reach the particular site.
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D. Environmental filtering / ecesis: The species must be able to germinate/develop from its
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propagule and then grow to reproduction under the physical environmental conditions
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prevailing.
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E. Productivity and biotic filtering: The species must be able to ecise and reproduce under
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the general interference pressure from the other species present: competition etc. (chapt. 2
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below).
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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?
We shall deal only peripherally with sub-specific evolution and not at all with the
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evolution of species, but they are the first required taxonomic category above the plant. The
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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
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morphology and behaviour after the identification of a scrap (the use of ‘morphospecies’ does not
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allow this). This predictability was the basis for the development of the science of Botany in the
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eighteenth century, and our ability to describe the vegetation around us. Unlike with animals,
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plant taxonomists (Stace 1989) are happy to allow species with only incomplete restrictions to
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gene exchange. However, each species is required to have a distinct phenotype. It must therefore
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have a unique environmental tolerance and a unique reaction on the environment, even if the
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difference from other species is sometimes small.
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2.2 Step B, Biogeography: The species pool
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The plant community can, in the short term, comprise only species present in the region,
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which is the species pool (Fig. 1.2). The pool is difficult to define and quite as difficult to
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determine, because we never know the distances over which species have the ability to disperse
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or the frequency of dispersal events. However, different processes do occur on different spatial
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scales. The regional species distribution for many species comprises a metapopulation: a series of
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populations that are partly independent but connected by occasional migration events. In practice
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the metapopulations of many species will show similar distributions due to similar habitat
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requirements, giving a metacommunity (Holyoak et al. 2005). It is a nice distinction as to
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whether a disseminule arrives via long-distance dispersal or from the metacommunity hinterland,
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and in any case the resulting processes of establishment must be similar.
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Questions about the species pool are dependent on the time frame: how long are we
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prepared to wait for the species to arrive? Were time the only limitation to dispersal, disseminules
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from far and wide would arrive anywhere, 400,000 species, and clearly this does not happen.
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Continents have very different floras. Many European tree species have failed to occupy their
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potential ranges in spite of several thousand years in which to spread across the continent
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(Svenning and Skov 2004). The school of panbiogeography sees many present-day restrictions in
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distribution between and within land areas as a reflection of the geography millions of years ago
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(Fig. 1.3), and its analyses of species distributions that have repeatedly been borne out by
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subsequent geological discoveries (Heads 2005). Clements and Shelford (1939) agree that
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whereas migration of propagules is common, establishment of them is “altogether exceptional”.
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There are also restrictions on the scale of hundreds of years: Matlack (2005) modelled the
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distribution of species in eastern USA and concluded that the frequencies of species in the
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modern landscape was controlled by the time available for spread in the last 300 years, with
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vertebrate-dispersed species occupying considerably more of their potential geographical range
Wilson & Agnew, chapter 1, Plants, page 11 of 28
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than other species. It is often unclear on which timescale the distribution limitation has occurred;
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for example a gap in the distribution of Nothofagus spp. in the South Island of New Zealand (Fig.
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1.3) has variously been correlated with geological movements (Heads 1989), the last glaciation
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(Wardle 1980) and the current environment (Haase 1990). Therefore, the closest we can come to
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definition is to say that over realistic time spans most members of the area’s species pool could
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arrive, and we have to explain the restricted subset of species found in each plant community.
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The concept of the species pool has sometimes included only species suited to the
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environment of the habitat in question. In Europe, species have sometimes been excluded from
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the pool using their Ellenberg ecological-tolerance rating (Ellenberg 1974). These values,
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originally crude, have been progressively refined. Outside Europe, little information exists on
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species tolerances for whole floras. A confounding question is whether the species pool is
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defined before or after interference. If the species pool comprises only those species
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physiologically able to tolerate the physical conditions at the site, it would include many never
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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).
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interference assume that the species pool includes species that can tolerate the environment, but
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cannot stand interference. However, the species lists are often taken from post-interference
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communities and so the argument is circular. An example of this syndrome is when climate
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change models of future vegetation are based on physiological parameters derived from
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distributions (i.e. from the realised niches; sect. 4.1 below), and used in models as physiological
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parameters (e.g. Sykes and Prentice 1996).
Wilson & Agnew, chapter 1, Plants, page 12 of 28
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2.3 Step C, Dispersal
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Propagules
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Propagule types are various. Within the angiosperms, seeds can be produced sexually
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(after meiosis and fertilisation), apomictically (with no meiosis and no involvement by pollen) or
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by pseudogamy (pollen is needed for seed development, and fertilises the endosperm, but the
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embryo itself is produced apomictically). Vegetative reproduction can occur via bulbils, stolons,
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rhizomes, layering of branches (e.g. Salix cinerea, willow tree), root suckers, etc. There is no
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basic distinction between the apomictic seeds of Taraxacum spp. (dandelion), ‘vegetative
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reproduction’ such as the production of Kalanchoe daigremontiana plantlets from the leaf
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margin, the growth of an Elytrigia repens (couch grass) clone by rhizomes, the growth of a
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Populus tremuloides (aspen) clone by root suckers and the growth of an axillary bud on a tree
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branch to give new leaf modules. All replicate an original genotype but after many mitotic
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divisions which can accumulate errors.
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The immediate fate of these propagules is various. Bulbils and viviparous seeds both
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develop as plantlets on the parent. Ramets produced by stolon or rhizome are initially dependent
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on the parent, then for a period are physiologically independent unless a change occurs, such as
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defoliation or shading, when ramets subsidise each other (Marshall 1996), and then become fully
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independent as the connecting stolons/rhizomes wither. Seeds are usually dispersed by wind,
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water or animals, although a few plants produce hypogeal seeds (i.e. belowground). Tree and
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herb sectors behave similarly to clonal tillers with limited integration, except that there is a
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greater tendency for branches to overtop one another competitively (Novoplansky 1996).
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Migration
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Dispersal is the means by which species move around the landscape. The two critical
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considerations are the distance and frequency with which disseminules move outside their source
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habitat, which is negatively related to disseminule size, and their potential for establishment as a
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seedling in a new site amongst existing plants which is positively related to disseminule size
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(Salisbury 1942).
343
Plant dispersal usually has a long tail (Fig. 1.4), i.e. it is leptokurtic, and is often best
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fitted by a negative exponential function. That is, most dispersal of disseminules of every type is
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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.
70
Number of seeds
60
50
40
30
20
10
0
-80
-60
-40
-20
0
20
40
60
80
100
120
Distance (m)
355
Fig. 1.3. Fig. 1.4. The leptokurtic curve of dispersal: Juncus effusus seed rain around a single plant.
356
The population of dormant seeds in the soil or in aerial fruits – the ‘seed pool’ or ‘seed
357
bank’ – is a buffer against elimination of a species. This is important for an annual species in an
358
adverse year, and for species surviving through disasters such as fire. The seed pool is not only
359
local, it can have a metapopulation structure comparable to that of adults when seeds are moved
360
around by floods or large herbivores such as elephants. This can be seen as spatial mass effect
361
(chapt. 4, sect. 12 below).
362
2.4 Step D, Environmental filtering / ecesis
363
Propagule germination and establishment
364
The germination of a propagule starts phase of an invasion when it is the challenged by
365
the conditions in a new site (Fig. 1.2). Sometimes seeds germinate only when conditions arrive
366
that are more mesic than those tolerated by their parents, and these events may be rare in stressed
367
habitats. For example, many halophytes need unusually low salinity on a saltmarsh before they
368
germinate, when their more glycophytic seedlings can become established (Alexander and
Wilson & Agnew, chapter 1, Plants, page 14 of 28
369
Dunton 2002). Arid land species show very precise adaptations for effective dispersal and
370
germination in the highly variable rainfall patterns found in these habitats (Gutterman 2002). For
371
example, Pake and Venable (1996) found that different species of winter annual in the Sonoran
372
Desert tended to germinate in different years, and species tended to germinate more in years that
373
turned out to give them higher reproductive success.
374
Plants of all species need to pass through a juvenile phase before becoming reproductive.
375
This part of the challenge in occupying a new site is called ecesis (Clements 1904). As Clements
376
(1916) wrote: “Ecesis is the adjustment of the plant to a new home. It consists of three essential
377
processes, germination, growth, and reproduction. ... Ecesis comprises all the processes exhibited
378
by an invading germule from the time it enters a new area until it is thoroughly established there.
379
Hence it really includes competition and other types of interference, except in the case of
380
pioneers in bare areas.”. This definition is of course far too broad, because it seems to include the
381
whole life cycle. We need to separate out the first stages for our reductionist view, so we restrict
382
‘ecesis’ to post-germination survival, growth and establishment: the part of the species filtering
383
process that determines which species survive the initial dispersal and germination phases of
384
plant community establishment.
385
Invasion patterns
386
Invasion can be seen on all scales, from movement to and fro across 2 m within a decade
387
in links (Olff et al. 2000) to movement over thousands of years. The patterns of invasion seen are
388
much the same at any scale, and fall into three types. Phalanx invasion is dense and over a broad,
389
solid front. Guerrilla invasion comprises invasion by isolated individuals, which gradually fills in
390
the space available (Hutchings 1986). It is difficult to measure invasion processes because
391
a-priori assessment of habitat suitability is problematic. However, an example of an invasion
392
where at least part of the flora used guerrilla invasion is the advance of the herb flora of ancient
393
forest into adjacent secondary growth in Sweden, where Brunet et al. (2000) concluded that
394
distance (equivalent to time for dispersal) and the soil environment almost equally controlled
395
species composition. However, by far the most common seems to be a third type of invasion – a
396
combination of the former two – infiltration invasion in which there is occasional long-distance
397
dispersal (as in guerrilla invasion), followed by local short-range dispersal from these foci (as in
398
phalanx) (Fig. 1.5; Wilson and Lee 1989). This matches the leptokurtic / two-mechanism
399
dispersal typical of plant disseminules. It can be seen at a variety of scales, e.g. over kilometres
400
(Lee et al. 1991) to centimetres. A small-scale example occurs when most of a tussock grass’
Wilson & Agnew, chapter 1, Plants, page 15 of 28
401
tillers are produced within the leaf sheath, but a few are pushed greater distances by animal
402
hooves (Harberd 1962), an example of the leptokurtic / two-mechanism dispersal to which we
403
referred in section 2.3 above. Egler (1977) describes all these patterns, with details, though using
404
different terms.
Expanding
foci
Guerrilla
individuals
g clumps
405
406
407
408
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
409
environmental conditions, such as soil type, hydrology, climatic regimes and altitude, that
410
prevent the immigrants’ growing (Honnay et al. 2001). This has been called environmental
411
filtering or abiotic filtering (Weiher and Keddy 1995). It occurs largely during ecesis. The
412
existence of this filter is obvious. Sophisticated methods can be used to record the response
413
surface (e.g. Bio et al. 1998), but it remains what Warming (1909) described as “this easy task”.
414
In this book, we generally take the physical restrictions as given, and concentrate on those
415
community processes that control species composition. However, we need to return to this topic
416
in considering the niche.
417
Reaction
418
The physical environment is not strictly abiotic, for the receptor community can alter the
419
environment to create or close invasible sites, a process for which Clements (1904) coined the
420
term ‘reaction’: “By the term reaction is understood the effect which a plant or a community
421
exerts upon its habitat. … Direct reactions of importance are confined almost wholly to physical
422
factors” (Clements 1916). Any organism must cause reaction, though the environmental
423
modification varies from slight to major, and the causal species' autoresponse from negative
Wilson & Agnew, chapter 1, Plants, page 16 of 28
424
(facilitation: Clements 1916) to positive (switch: Wilson & Agnew 1992). Reaction is the basis of
425
almost all plant-plant interactions (Clements 1904). Since Clements, other terms have been used
426
for the same effect, such as ‘ecosystem engineer’ (Jones et al. 1994), and ‘niche construction’
427
(Laland et al. 1996). They seem to be later synonyms, but we shall sometimes use the latter when
428
discussing the niche.
429
Sometimes, the favourable microsites for establishment are gaps, but Ryser (1993) found
430
in a temperate calcareous grassland that the favourable microsites were those where the reaction
431
of established plants in the community provided shelter from frost; unvegetated gaps were not
432
colonised. Litter production is a common mode of reaction, forming the seedbed of an invader,
433
and enabling or inhibiting its germination (chapt. 2 below).
434
2.5 Step E, Productivity and biotic filtering
435
Communities develop a regime of carbon cycling, of which the autotrophic production is
436
our concern. We devote space to productivity here because it is easy to think rather loosely about
437
it. Productivity is “The potential rate of incorporation or generation of energy or organic
438
matter … per unit area …” (Lincoln et al. 1982), but there is a lot of complication behind this
439
definition:
440
441
442
443
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.
444
3. Later, at night, some of the fixed C is lost by dark respiration.
445
4. The remaining C is transported to sinks at sites of cell division (secondary cambium, root,
446
shoot apex, inflorescence, etc.), where it is incorporated into cell wall tissue, storage
447
carbohydrates or cytoplasm, with shared potential fates. In this process, it can be:
448
A. Lost by respiration whilst still incorporated in soluble C compounds, etc.
449
B. Lost to aphids on the way: Heizmann et al. (2001) wrote: “during midday depression
450
of photosynthesis, a high percentage of the total C delivery was provided to the
451
leaves by the transpiration stream (83 to 91%). Apparently, attack by phloem-feeding
452
aphids lowered the assimilate transport from roots to shoots; as a consequence the
453
portion of C available to the leaves from xylem transport amounted to only 12 to
454
16%.”
455
C. Leached from leaves as soluble C compounds (Czech and Kappen 1997).
Wilson & Agnew, chapter 1, Plants, page 17 of 28
456
457
458
459
460
D. Lost by roots as exudate of soluble C compounds (Kuzyakov and Siniakina 2001) and
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).
461
ii. Removed by allogenic or autogenic damage or abscised. The amount lost varies
462
from parts of leaves to tree branches: cell walls plus modified cell contents.
463
iii. Lost at the death of tissues (wood, bark), organs (roots, leaves flowers and fruit),
464
465
or the whole plant.
F. In living tissues, C in carbohydrate storage and cytoplasm can be lost by respiration as
466
the root becomes old or the leaf becomes shaded and/or old, or it can be translocated
467
with attendant respiratory costs. However, cell wall C cannot be lost this way,
468
because no autolysis of cellulose or lignin occurs within living plants. (This contrasts
469
with animal tissue, where all C is part of the labile pool except for some dermal
470
structures.)
471
In the face of this complexity there is no consensus as to what productivity is or how it should be
472
measured. Logic and simplicity would suggest that the real definition of productivity is the
473
amount of C that reaches the next trophic level, herbivores or decomposers. This top down
474
definition would comprise only E1-3 in the above schema. ‘Gross productivity’ can be measured
475
in the field through gas analysis (though this omits photorespiration: Step 2), and ‘net
476
productivity’ as this less all the later losses. Productivity is most often estimated by sequential
477
sampling. Suppose a habitat holding mature stable vegetation in an approximately steady state. If
478
we sample at one time and then resample the same area 12 months later, in many areas there
479
would be no change in the absence of climate change and apart from sampling error – neither
480
accumulation nor loss of biomass – so we would arrive at an estimate of zero productivity. This is
481
either accurate or misleadingly trivial, depending on your definition of productivity. However, all
482
systems show some seasonal development and change, and most estimates of plant productivity
483
rest on the successive sampling of harvest biomass (standing crop) during the season of maximal
484
growth (Perkins et al. 1978). This working approximation is a wild under-estimate of actual
485
productivity, yet has physical presence and ecological meaning. Actually, in much discussion of
486
productivity, e.g. in testing for a humped-back curve (Grime 1979), standing crop is used as a
487
substitute, and it is a very poor one.
Wilson & Agnew, chapter 1, Plants, page 18 of 28
488
The productivity potential of a site controls what plant community develops in three
489
ways. Firstly, the readiness of the soil surface to provide sites for invasion and thus augment the
490
community: often with greater productivity more litter will be available affecting the ecesis of
491
invaders (sect. 2.4 above; chapt. 2 below). Secondly, disturbance of the community through
492
herbivory and often fire: high productivity attracts herbivory, while pronounced dry seasons
493
between productive growing seasons favour fire. However, in general these first two factors
494
affect the rate of invasion more than the eventual fate of an invasion. The third aspect is the
495
competitive status of the community: greater productivity means more competition and more
496
difficulty for additional species to establish. For example, Cantero et al. (1999) concluded that the
497
diversity of short grasslands in Argentina was affected by surrounding species pools, while that
498
of tall grasslands with more competition was not. This is the interference filter, in which a species
499
is able to tolerate the physical environment of a site, but cannot grow well enough there to
500
withstand the general level of interference present there: competition, allelopathy, etc. It is an
501
effect not specially dependent on the identity of the associates. This is the classic distinction
502
between the fundamental and realised niche (sect. 4.1 below). It is clearly a major factor, as can
503
be seen by the ready cultivation of many species in botanic gardens outside their natural edaphic
504
and/or climatic range.
505
Organisms of other trophic levels affect ecesis and reproduction (sect. 1.1 above,
506
“Interaction with other trophic levels”). A species might be able to maintain a positive population
507
growth rate without heterotrophs, but be pushed into negative growth when pathogens or
508
herbivores take their toll. This could be environmentally dependent: a potentially fatal herbivore
509
might be absent because the environment is beyond its tolerance. On the other hand, a plant may
510
be unable to reproduce because a normally subventing pollinator is beyond its environmental
511
range, and hence absent. This would be a type of assembly rule (Step F), though in this book we
512
consider only plant-plant assembly rules as such.
513
2.6 Step F: Assembly rules and micro-evolution
514
Assembly rules are "restrictions on the observed patterns of species presence or
515
abundance that are based on the presence or abundance of one or other species or groups of
516
species …" (Wilson 1999 %chapter). We discuss them in chapter 5. It is clearly a simplification
517
to take species as fixed units and we do so only to limit the scope of this book. To glimpse into
518
the world of ecotypes and micro-evolution as they affect community structure we examine the
519
work of Turkington and Harper (1979), taking plants of Trifolium repens (white clover) from
Wilson & Agnew, chapter 1, Plants, page 19 of 28
520
patches of a field dominated by four different grass species. When they planted them into boxes
521
of a standardised soil sown with the four grass species, each T. repens genotype was the best
522
performer against the species from whose neighbourhood it had been taken in the field – an
523
amazingly neat result. They interpreted this as genetic coadaptation within the field. A
524
mechanism such as the quality of transmitted light is possible (Thompson and Harper 1988).
525
Aarssen and Turkington (1985 %605) performed a similar experiment in a pasture in
526
British Columbia, Canada, but using different patches/genotypes of Lolium perenne (ryegrass).
527
They obtained similar results for T. repens. However, they also examined variation within the
528
associated L. perenne and the results for it were the opposite – three of the four L. perenne
529
genotypes had their lowest competitive ability against the T. repens genotype with which they
530
had been growing in the field. This would tend to keep the grass/clover competitive abilities
531
balanced. There remains a fear that the effects were due to carry-over, i.e. maternal effects in the
532
vegetative material. Aarssen (1988) found that collecting seed rather than using vegetative
533
material (ramets) gave quite different results, which he attributed to screening of the gene pool
534
between seed and adult populations. Such screening has certainly been seen in heavy-metal
535
ecotypes, though under conditions where gene flow was high and the selective differentials
536
extreme. However, Turkington and Harper (1979) had used preconditioning periods of only 3
537
months. Evans and Turkington (1988) in Canada, collecting plants in a similar way to Turkington
538
and Harper (1979) – from below four different grass species – found morphological differences
539
between T. repens of the four origins after 4 months growth in a common garden which
540
disappeared after 27 months growth. Chanway et al. (1989) suggested that the difference between
541
T. repens material might be in the specific Rhizobium strains carried with it, not in the T. repens
542
itself. This could still be a force in structuring communities.
543
The Turkington and Harper (1979) result would have been small-scale character
544
displacement. It is almost impossible to prove that character displacement has occurred because
545
the evidence must involve comparisons between areas, and those areas might differ in other ways
546
(Strong 1983). However, some cases are suggestive. If found, character displacement would be
547
evidence that species interactions were a strong force in genetic selection, and therefore also in
548
ecological selection, implying deterministic community structure.
Wilson & Agnew, chapter 1, Plants, page 20 of 28
549
550
3 Geographical boundaries
The behaviour of a species at its distributional limit can be fascinating. Often the habitat
551
range of a species becomes more restricted towards its boundary. Pigott (1970) reported that near
552
the limit of Ilex aquifolium (holly) in Britain it becomes increasingly restricted to forest, and
553
Cirsium acaule (stemless thistle) at its northern limit becomes confined to southern (warm)
554
aspects. On the other hand, Diekmann and Lawesson (1999) found four potential examples where
555
species had wider ecological amplitudes towards their range margin in northern Sweden, and
556
suggested that there is such climatic stress in that region that a smaller flora is present and
557
important competitors are absent. Usually the plants are smaller and less fecund towards the
558
limit, leading to populations becoming smaller and absent from apparently suitable habitat
559
patches (Carey et al. 1995; Nantel and Gagnon 1999; Jump and Woodward 2003). Lower
560
fecundity also makes populations more sensitive to disturbance. For example, fire restricts
561
Canadian Abies balsamea (balsam fir; Sirois 1997) and Pinus resinosa (red pine; Flannigan and
562
Bergeron 1998) to islands and isolated populations at the northern edge of their ranges. In some
563
cases there is a sudden cut-off point in a species with no reduction in vigour near the limit
564
(Lactuca serriola, prickly lettuce; Carter and Prince 1985). This may not be exceptional. Griggs
565
(1914) made an early and careful observational study of Sugar Creek, in a “tension zone” in Ohio
566
where over 120 species have geographical boundaries, asking whether populations became sparse
567
or less fecund in this region. He found no consistency of behaviour, but most edge-of-range
568
species were abundant and flowered and fruited successfully up to the geographical limit, as in L.
569
serriola. Griggs could only hypothesise that competition sharpened boundaries to make them
570
abrupt. There can be many different reasons for a species’ failing to expand its distribution,
571
sometimes surprising ones. Pigott and Huntley (1981) found that the environmental filter for Tilia
572
cordata (linden) at the northern limit of its range in England was that the pollen tube could not
573
grow fast enough in the low temperature to reach the ovule, leaving a relictual population now
574
unable to reproduce by seed.
575
At present it seems that the behaviour of species at the margins of their ranges is complex
576
and unpredictable. For example, if the range limit were due to individuals being selectively
577
eliminated we might expect lower variances of morphological measurements at geographic
578
edges, but Wilson et al. (1991 %780) could find no such effect.
Wilson & Agnew, chapter 1, Plants, page 21 of 28
579
580
4 Concepts of the space occupied by one species
Our purpose in this book is to examine the way species fit together in a mixture. Two
581
powerful conceptual tools, developed for this purpose, are the niche and the guild.
582
4.1 The niche
583
The end point of a species' pilgrimage from the pool into a community is occupancy of a
584
niche. The niche is an old concept. Grinnell (1904) and Elton (1927) introduced the term, and
585
both used it to describe an area available within habitat space, broadly defined by physical and
586
trophic parameters. Hutchinson (1944) formalised this to “a region in n-dimensional hyperspace”
587
where the dimensions are all the environmental, resource or behavioural (e.g. phenology,
588
foraging) parameters that permit an organism to live.
589
Since Hutchinson’s overarching statement it has been tempting to regard species presence
590
as the only definition of the niche. Thus, Levins and Lewontin (1985) advocated that “ecological
591
niches are defined only by the organisms in them”. Olding-Smee et al. (2003 %book) believed
592
that for Hutchinson “a niche cannot exist without an occupant”. We see no reason to understand
593
Hutchinson thus. The crunch comes with the empty niche. Under the “the species is the niche”
594
concept “the idea of an ecological niche without an organism filling it loses all meaning” (Levins
595
and Lewontin 1985). However, the empty niche is a necessary concept in theory, especially in
596
relation to invasions. The absurdity of the “the species is the niche” is seen by observing
597
innovative invaders. Did no niche for a cactus exist in central Australia until Opuntia stricta
598
(prickly pear cactus) was introduced (Hosking et al. 1994)? Was there no niche for a cactus-
599
eating insect before the moth Cactoblastis cactorum was introduced for biological control of O.
600
stricta? Was there no niche below the saltmeadow in British estuaries until Spartina ×townsendii
601
/ anglica (cord grass) created itself by hybridisation in 1887? Was there no niche for an emergent
602
tree in Bonin Island shrublands until Pinus lutchuensis (a pine from elsewhere in Japan) was
603
introduced (Shimizu and Tabata 1985)? It seems better to regard all these as empty niches that
604
were later filled. To be sure, the identification of empty niches is very hard. There must be areas
605
of hyperspace that it is impossible for plants to fill: floating in the air, growing on the ice at the
606
South Pole or growing at 100 °C in hydrothermal steam vents?
607
Tilman (1997 %81) claimed to find evidence of empty niches. In 1991 he sowed seeds of
608
up to 54 species into native grassland at Cedar Creek. Many became established. However, this
609
did not cause extinctions among the species originally present in 1991: the proportion of those
Wilson & Agnew, chapter 1, Plants, page 22 of 28
610
lost was not correlated with the number of species added (r = +0.16, R2 = 2.6 %, not significant).
611
Even more interestingly, the total cover of those species present in 1991 did not decrease (r =
612
0.04, R2 = 0.16%, not significant). The R2 values are impressively low so we could conclude, as
613
Tilman did, that the added species occupied empty niches. There is a problem that the species
614
composition probably co-varied with the species richness. The use of ‘total cover’ is odd. If two
615
leaves of different species are vertically aligned, both count towards cover, and if two leaves of
616
the same species are horizontally aligned both count, but if two leaves of the same species are
617
vertically aligned only one counts. ‘Total cover’ is not a sensible concept. In this case, the ‘cover’
618
of each species was, unfortunately, guessed. Cardboard cutouts were used to guide the guessing
619
but we do not take aids to guessing as removing the fact that cover was guessed. Philip Grime’s
620
group at Sheffield always uses objective measurements: presence/absence, local frequency, point
621
quadrats or sorted biomass as appropriate. Why can’t everyone? We mention this issue because it
622
will repeatedly mar results that we report. We shall not shirk from pointing out when the data of
623
this type are used, nor shall we use euphemisms like “estimated by eye”, because we believe this
624
practice is a blot on our science (even if we have occasionally been guilty ourselves in evil places
625
far away and naughty times forever gone).
626
The second reason for rejecting the “the species is the niche” concept is that it takes
627
species presence in the field as the de facto description of its niche, but this is affected by
628
interaction with other biota (competition, herbivory etc), introducing an imponderable set of
629
variables over space and time. It is more useful to separate biotic variables as restricting the
630
occurrence of a species to its realised niche, whereas its environmental tolerance defines its
631
fundamental niche or physiological tolerance (Hutchinson 1957). This distinction was known to
632
Tansley (1917) and Gleason (1917), though dismissed by Clements (1907) as “merely
633
migration”. The niche width of a group of species is usually considerably narrower, and hence
634
their niche overlap is less, when they are grown in an experimental mixed community than when
635
they are grown alone in the same conditions. That is, their realised niche is smaller than their
636
fundamental niche. Silvertown et al. (1999 %61) found this re-analysing data of Ellenberg’s with
637
six grass species and a water table gradient: the mean niche overlap was considerably higher in
638
the 6-species mixture than comparing monocultures, and the niche modes spread out to a range of
639
5-100 cm depth-to-water-table in the mixture compared to a range of 20-35 cm among the
640
monocultures. However, definitions become difficult because of reaction: any organism must
Wilson & Agnew, chapter 1, Plants, page 23 of 28
641
alter its own environment and this may cause niche construction, potentially leading to realised
642
niches that are larger than the fundamental ones (Odling-Smee, et al. 2003).
643
The niche includes a species' developmental requirements (temperature etc.), its material
644
requirements (resources) and its relations with neighbouring species. A complete circumscription
645
of these is almost impossible, requiring knowledge of every aspect of the species' physiology and
646
life history, but two types of niche can be distinguished. The beta (β) niche is the range of
647
physical environmental conditions under which the fitness of a species is maintained (Alley
648
1985), e.g. its temperature tolerance, and therefore its potential geographical limits. It is related to
649
Chesson’s (in press) concept of ‘environment’ as a factor that does not form a feedback loop, i.e.
650
is not appreciably affected by the organisms themselves. The alpha (α) niche represents the
651
resources used within a community/site, the “‘profession’ or functional role” (Alley 1985), e.g.
652
different rooting depths. Many methods of analysis, e.g. the calculation of niche width and
653
overlap, can be used for both alpha and beta niches, and there are areas of character overlap.
654
However, when we use the niche concept we generally need either one or the other. Much
655
ecological discussion has been confused by failing to take the distinction into account.
656
657
658
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,
659
water source), persistence (storage organs, wood), autogenic disturbance (through litter
660
and physical environmental effects), heat budget (convective, transpirative, radiative),
661
physical defence against herbivores (glands, hairs, thorns), pollination and dispersal
662
biology. An example of these factors is synusiae in forest, such as epiphytes and lianas,
663
and indeed stratification in almost all communities. Another example is the parasitic
664
habit.
665
2. Growth phenology as the plant's response to environmental signals comprises the
666
seasonality of growth and reproduction (pollination and dispersal). Examples are the
667
progression of flowering in temperate vegetation and leaf flushes in tropical forests.
668
3. The chemical functioning of a plant ultimately controls everything, but we may list as
669
examples phototype (C3, C4 or CAM), light requirement, mycotrophy, P sources via root
670
phosphatase exudate, N source (N2, NH4 or NO3) and chemical defence against herbivore
671
and pathogen challenge.
Wilson & Agnew, chapter 1, Plants, page 24 of 28
672
4. Any of the above niche axes can influence plant/plant interactions, through interference or
673
subvention by neighbours (chapt. 2 below), for example in the morphological pre-emption
674
of soil resources, though the overgrowth of competitors and through the toxic chemical
675
countering of competition known as allelopathy.
676
677
5. Additional resources are gathered by the community. This is reaction causing niche
construction.
678
Beta niche axes are the environmental features of the locality and its biota, that is to say the
679
habitat. Complexities between factors are more apparent than with alpha axes. Aspects are:
680
1. Climate delivers solar insolation, water availability, CO2 (though the latter is rarely local),
681
nutrients (when ombrotrophic), some pollination vectors and rate regulators such as
682
temperature. These interact with the chemistry below. Climate also delivers exposure to
683
atmospheric humidity, wind, aeration and snow. This affects morphology, for example as
684
a partial determinant of the Raunkiaer life form (the life form can also differ between
685
alpha niches, for example in forest stratification).
686
2. Chemical features of soils (calcareous versus non-calcareous, pH, salinity, etc.) affect
687
system function (nutrient availability, cycling); these overlap with geomorphology below.
688
In the short term, mineral nutrients are often dominant.
689
3. Geomorphology delivers allogenic disturbance and soil substrate.
690
4. Biota deliver allogenic disturbance, generalised herbivore pressure and animal pollinators.
691
There can sometimes be overlap between the concepts of alpha-niche and beta-niche in terms of
692
characters: e.g. low growth is a feature of the ground alpha-niche in a forest but also of arctic
693
plants. However, the effects are opposite: species of the same beta niche will tend to co-occur
694
because they have the same environmental tolerances; species of the same alpha niche will have
695
no such tendency to co-occur, and if competitive exclusion is operating they will tend not to co-
696
occur (Wilson, submitted).
697
4.2 Guilds
698
The ecological term ‘guild’ was coined by Drude (1885) as the German
699
‘Artengenossenschaften’ to refer to a group of species moving from one region to another, such
700
as exotic species. It was used thus by Clements (1904; 1905) and Wilson (1989 %223). Perhaps
701
independently, Schimper (1898; 1903) used the term ‘Artengenossenschaften’ / ‘guild’ to mean a
702
synusia (e.g. stratum) in a forest. Tansley (1920) used it in the same way, writing of “guilds of
703
the same dependent life-form, such for instance as lianes”. Root (1967) ignored these established
Wilson & Agnew, chapter 1, Plants, page 25 of 28
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usages and with animal assemblages in mind re-defined the guild as a “group of species using
705
similar resources in a similar way”. This is not directly useful for plants, since almost all use the
706
same resources (the sun’s energy, water, CO2, N, P, K and minor elements). The guild is a
707
category that is intended to be ecological rather than taxonomic, and Wilson (1999) defined it as:
708
“a group of species that are similar in some way that is ecologically relevant, or might be”. It is
709
unusual to find “Or might be” in a scientific definition; it is necessary here because we hardly
710
ever known at the beginning of an investigation whether the guilds we are using are the real ones,
711
and often not at the end (but see the discussion of intrinsic guilds: chapt. 5, sect. 7.6 below). In
712
spite of the lack of precedence of Root’s usage, and the impossibility of applying it strictly to
713
plants, a similar usage can have value: a guild as a group of species that occupy similar niches.
714
Wilson (1999) pointed out that there are two basic types of guilds, corresponding to the
715
distinction between alpha-niches and beta-niches. Again, the outcomes are opposite: species that
716
are in the same beta-guild and therefore have similar environmental tolerances will generally co-
717
occur; species that are in the same alpha-guild and therefore use similar resources will tend
718
exclude each other. The species within one alpha-guild are similar in their resource use. For
719
example, within northern European forests, species that are within the same alpha-guild might be
720
the trees Tilia cordata (linden), Quercus petraea (sessile oak) and Fagus sylvatica (beech). They
721
are using similar resources: the light at the top of the canopy during the summer half-year, as well
722
as nutrients and water from the full profile of the soil. Conversely, if species are present in one
723
community that are in different alpha-guilds, they might be able to partition resources within a
724
community, so a community might tend to comprise species from several alpha-guilds. For
725
example, Tilia cordata, the hemi-parasite Viscum album (mistletoe), the liana Hedera helix (ivy)
726
and the ground herb Mercurialis perennis (dog’s mercury) would be in different alpha-guilds
727
because they use different light/support/nutritional resources, and if we found them together we
728
might see it as alpha-niche differentiation.
729
The species in one beta-guild are similar in their ecophysiology and therefore their
730
tolerance (across space or time) of environmental conditions, such as the “guilds of edaphic and
731
topographical specialists” of Hubbell and Foster (1986 %314). After a species pool has passed
732
through an environmental filter, the remaining species will be a beta guild; they have overlapping
733
beta-niches. For example, all sub-arctic saltmarsh species would be in the same beta-guild
734
because they occur in the same climatic and soil conditions. An example of species occurring in
735
different beta-guilds might be the temperate, mesic tree Tilia cordata, the subalpine Pinus
Wilson & Agnew, chapter 1, Plants, page 26 of 28
736
contorta (lodgepole pine), arid land trees/shrubs of Prosopis spp. (mesquite), the tropical
737
Cinchona officinalis (quinine) and a species of mangrove. They occur in different environmental
738
conditions (climate and/or soil), so they are necessarily found apart in space or time since we
739
cannot find different external environmental conditions simultaneously at one spot. Díaz et al.
740
(1998) recorded abundances of ‘plant functional types’ (PFTs) of 100 species along a climatic
741
gradient in Argentina and found that vegetative traits differed between climatic zones,
742
demonstrating that beta-guilds are filtered out from the available species pool. The species within
743
each zone will almost certainly belong to different alpha-guilds.
744
The concept of ‘functional type’ (used above by Diaz et al. 1988) and that of the ‘guild’
745
can be essentially identical (Wilson 1999; Blondel 2003). The current use of PFTs as the
746
predicted variate in models assumes that we know the characters of the types are trying to
747
summarise. In spite of the term ‘functional’ which implies alpha-guilds, most workers have
748
apparently intended to create beta-guilds. However, the characters they have chosen have often
749
been alpha-niche ones. For example, Kleyer (2002) formed guilds (‘functional types’) “to relate
750
unique PFTs to landscape specific habitat factors and to generalize syndrome-environment
751
relations across landscapes” and used characters such as annual versus biennial versus perennial,
752
plant height, regeneration from detached shoots, having leptophyllous leaves, longevity of seed
753
pool that are as likely to occur within a community. A distinction between ‘response’ and ‘effect’
754
guilds obscures the issue, because there is far more to the alpha-niche of a species than its
755
reaction (effect) on the environment. This situation has arisen from a failure to consider the
756
purpose of the guilds being formed, what type of guilds they will therefore be – alpha or beta –
757
and what characters are therefore appropriate.
758
4.3 Stratification
759
The most obvious alpha-guilds in plant communities are the guilds of Schimper (1898;
760
1903), synusiae (Fig. 1.6). Almost all plant communities are structured vertically. Aboveground,
761
the greater the vegetation cover, the more uniform and predictable is the vertical change in
762
microclimate. Highly structured forests have a stratum of separated, emergent trees, a more
763
continuous upper canopy, then sub-canopy trees, shrubs, tall herbs, creeping herbs and
764
bryophytes, lianas and epiphytes (including lichens, bryophytes and higher plants). This
765
represents specialisation to the attenuation of light, water, CO2 and nutrient resources. All this is
766
accepted for forests, but there is also complex stratification in grasslands, for example in the wet
767
grasslands of Tierra del Fuego (Díaz Barradas et al. 2001) and even in lawns (Roxburgh et al.
Wilson & Agnew, chapter 1, Plants, page 27 of 28
768
1993 %699). Naturally all stratification by primary producers is echoed by stratification in
769
consumer communities.
Fig. 1.5: Stratification: profile of a rain forest in British Guiana. From Richards (1964).
770
771
We might expect that similar patterning is happening below ground because litter is
772
deposited on the soil surface eventually adding to the water-holding capacity and mineral nutrient
773
status of the upper soil. Most water arrives at the soil surface and percolates down, acidified by
774
organic acids and CO2, hydrolising the mineral fragments in the soil and most importantly
775
releasing phosphate. Plant roots and respiration can affect this, for example releasing acids.
776
Water in deep soil, including artesian water, is available to deep roots and may rise up by
777
capillarity and hydraulic lift. This can lead to stratification of root systems. Succulents of New
778
and Old World deserts have surface roots adapted for the uptake in ephemeral rainstorms
779
(Whitford 2002). Dodd et al. (1984) surveyed 43 woody species from the open woodland in SW
780
Australia, and Timberlake and Calvert (1993) 96 shrubs and trees of Zimbabwe woodlands, both
781
finding that there were indeed species with consistently shallow systems and others with deep
782
taproots. Most species had both lateral superficial roots and descending taproots, but herbs can be
783
shallow-rooted and in herbaceous or mixed herbaceous/woody communities there can be
784
considerable stratification (Weaver and Clements 1929 %p213; Cody 1986 %381).
785
5 Conclusion
786
787
This opening chapter has described the basic material of plant communities: the plants
themselves. We argued that the characteristics of plants are that they are colonies of modules that
Wilson & Agnew, chapter 1, Plants, page 28 of 28
788
must constantly be replaced causing movement in space, they are potentially genetic mosaics and
789
they are plastic. As a consequence, the term ‘individual’ usually has no meaning for plants. They
790
also have features that make their evolution different from that of animals. The huge majority of
791
stands have more than one plant species, and we have outlined the basic processes through which
792
multi-species communities establish and develop. The occupancy of a niche is axiomatic in a
793
species’ presence in a community, and the interactions between pool, dispersal and niche are all
794
important in this process (Fig. 1.2). There are recent demonstrations of this at a large scale in the
795
Palm floras of Ecuador and Peru (Vormisto et al. 2004), and at an intermediate scale in the
796
Netherlands (Ozinga et al. 2005). Our conclusion, which we hope the reader shares, is that there
797
is enormous complexity in the life of plants in spite of the simplicity implied in their common
798
sedentary habit and modular structure, and their almost universal trophic function. Our basic
799
concern in this book is to examine how the species fit together to form communities and basic
800
concepts for this are the niche and the guild. In the next chapter we examine the processes
801
involved when one species interacts with another, starting community development.
802
ILLUSTRATIONS
803
Fig. 1.1: The plant as a colony of active and inactive apices
804
Fig. 1.2: Pathways from the species pool to community entry
805
Fig. 1.3: Disjunct distribution of Kelleria laxa in South Island, New Zealand, interpreted as an
806
originally contiguous distribution torn apart by movement along the Alpine Fault 2-10
807
million years ago. From Heads (1989).
808
Fig. 1.4: Infiltration invasion by Olearia lyallii in the Auckland Islands. After Lee et al. (1991).
809
Fig. 1.5: The leptokurtic curve of dispersal: Juncus effusus seed rain around a single plant.
810
Fig. 1.6: Stratification: profile of a rain forest in Guiana: From Richards (1964).
i
‘sessile’ in zoological terminology