Plant coexistence and the niche

a Diversity: the number of species found in a local community. a Niche: the region of the realized niche of a species corresponding to species diversity at the local community (or a-diversity) scale at which interactions among species occur. Niche differences at this scale should facilitate coexistence. Character displacement: the evolutionary accentuation of trait differences between sympatric species caused by competition for resources. Community structure: regularity in the membership (presence or absence) or abundance of species in a community with respect to random expectation based upon an appropriate null model of trait or taxonomic distribution. Fundamental niche: the region of its niche that a species is able to occupy in the absence of interspecific competition and natural enemies. Guild: a group of functionally similar or taxonomically related species that exploit a common resource and, hence, are expected to experience greater interspecific competition among guild members. Niche: an n-dimensional hypervolume [74] defined by axes of resource use and/or environmental conditions and within which populations of a species are able to maintain a long-term average net reproductive rate that is R1. Niche axis: a dimension in n-dimensional niche space along which species can show segregated distributions. Niche dimension: used interchangeably with niche axis. Niche shift: a change in the mode and/or a reduction in the variance of the distribution of a species (or of its resource use) along a niche axis caused by the presence of competitors. Niche space: the n-dimensional envelope containing all the niches present in a community of species. Realized niche: the region of its niche that a species is able to occupy in the presence of interspecific competition and natural enemies. Regeneration niche: the component of the niche of a plant that is concerned with processes such as seed production and germination and by which one mature individual is replaced by another [69]. How large numbers of competing plant species manage to coexist is a major unresolved question in community ecology. The classical explanation, that each species occupies its own niche, seems at first unlikely because most plants require the same set of resources and acquire these in a limited number of ways. However, recent studies, although few in number and incomplete in many ways, do suggest that plants segregate along various environmental niche axes, including gradients of light, soil moisture and root depth, and that partitioning of soil nutrients occurs, possibly through the mediation of microbial symbionts, some of which are more species specific than was previously thought. Although it is unlikely that niche separation along environmental axes is the only mechanism of coexistence in any large community, the evidence now suggests that it plays a more significant role than has been previously appreciated. More research into the consequences of various known tradeoffs is likely to uncover further cases of niche separation facilitating coexistence.