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