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.