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Chapter Seven - Traits, states and rates: understanding coexistence in forests
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- By Drew W. Purves, Microsoft Research Cambridge, Mark C. Vanderwel, Microsoft Research Cambridge
- Edited by David A. Coomes, University of Cambridge, David F. R. P. Burslem, University of Aberdeen, William D. Simonson, University of Cambridge
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- Book:
- Forests and Global Change
- Published online:
- 05 June 2014
- Print publication:
- 20 February 2014, pp 161-194
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Summary
Introduction: why do tree species coexist?
The question of why there is more than one plant species on Earth is probably not one for ecology. Rather, it would appear to us at least that it is up to systems biology and evolutionary biology to explain why the enormous variation in structure and function exhibited by individual plants – a variation that makes sense given the huge range of physical environments that they occupy – occurs primarily as species-to-species variation, rather than as variation among ecotypes via local adaptation, or variation among individuals via phenotypic plasticity. However, given that plant species are so very different, the question of why we appear to observe the long-term co-occurrence of multiple species in the same region certainly is a question for ecology, so much so that the paradox of coexistence has remained central to community ecology for decades (e.g. Gause 1934; Grubb 1977; Hutchinson 1961; MacArthur 1970).
An important recent development has been the realisation, thanks to neutral theory, that the long-term co-occurrence of multiple taxonomic species is not, by itself, a paradox at all (Chave 2004; Hubbell 2001). We now know that it could take an enormous amount of time for a mixed community to drift to monodominance in any one region, if species were indistinguishable in terms of their traits. But this still leaves the challenge of explaining why we observe the long-term co-occurrence of species that are measurably different in traits that obviously affect fitness, such as growth, mortality and reproductive rates (see Purves & Turnbull 2010). Theoretical ecology has provided one kind of answer to this question, by identifying a suite of fundamental mechanisms that can maintain the coexistence of multiple species (Chesson 2000a). Although it is likely that there are new mechanisms still to be discovered, theoretical ecologists are almost entirely agreed that coexistence requires some form of negative feedback: if one species becomes too dominant, its performance declines, which in turn reduces its abundance; the opposite occurs for species that drift to abundances that are too low (Chesson 2000a; and for forests see Dislich, Johst & Huth 2010). In the presence of such negative feedbacks, communities can exhibit stable coexistence of multiple species, where the community exhibits a typical mixture of species (or mixture of traits) that it tends to return to after perturbations.
5 - Ecological drift in niche-structured communities: neutral pattern does not imply neutral process
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- By Drew W. Purves, Princeton University, Stephen W. Pacala, Princeton University
- Edited by David Burslem, University of Aberdeen, Michelle Pinard, University of Aberdeen, Sue Hartley, University of Sussex
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- Book:
- Biotic Interactions in the Tropics
- Published online:
- 25 August 2009
- Print publication:
- 08 September 2005, pp 107-138
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Summary
Introduction
The neutral vs. structure debate
We can define a neutral community as one in which all species, and so all individuals, are equivalent, in the sense that they are interchangeable at all times and under all conditions. In contrast, we can define a structured community as one in which species are not equivalent, and species-specific differences affect the population dynamics, and therefore the behaviour, of the community.
This distinction is an important one, because in a neutral community the biodiversity, as measured by species richness and abundance patterns, has nothing to do with the biogeochemical functioning of the community (e.g. carbon fixation and nutrient-cycling). In fact, in a truly neutral community one could eliminate all but one species without affecting the biogeochemical functioning of the community at all.
In contrast, much of the species-specific variation in biological traits observed in reality (see below) has direct relevance for the functioning of the community. For example, the short-term carbon uptake of a forest depends on the growth rates of the individual trees, and the long-term carbon storage depends on adult life-span and wood density, and there is wide species-specific variation in these traits. In niche-structured communities, the biodiversity and functioning are intimately linked, and some combination of at least some species is required to maintain the functioning of the community.