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Journal Article

Competitive Ability and Species Coexistence: A 'Plant's-Eye' View

Lonnie W. Aarssen
Oikos
Vol. 56, No. 3 (Nov., 1989), pp. 386-401
Published by: Wiley on behalf of Nordic Society Oikos
DOI: 10.2307/3565625
Stable URL: http://www.jstor.org/stable/3565625
Page Count: 16
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Competitive Ability and Species Coexistence: A 'Plant's-Eye' View
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Abstract

Plant species coexist by avoiding competitive exclusion. This paper develops a 'plant's-eye' view of avoiding competitive exclusion which interprets coexistence at the species level as a consequence of ongoing selection resulting from genetically-based differences in competitive abilities within local neighborhoods. That competitive ability may change as a consequence of selection is supported by results from a multi-generation competition experiment involving Senecio vulgaris and Phleum pratense. The potential evolutionary consequences of such selection operating within a community of several species is then explored. The approach developed here assumes that resource supply/demand ratio is sufficiently low and that the extent to which demands are made on the same resource units is sufficiently great for plants of different species to compete intensely. It also assumes however that, several genetically variable attributes combine to define the competitive ability of a plant and that the resulting genotypic variability in competitive ability is no greater between species than within species. This forms the basis of the competitive combining ability hypothesis for species coexistence in which competitive exclusion is avoided at the whole population level because no population contains even one genotype that is competitively superior to all other genotypes belonging to any other coexisting population. One mechanism under this hypothesis assumes that competitive abilities are intransitive at the genotype level. If this intransitive network spans across taxonomic boundaries, then those genotypes with the best competitive abilities within local neighbourhoods are just as likely to belong to one species as to another. The major implication of this hypothesis relates to its role in helping to explain the conflicting truisms that competition within plant communities is intense and should therefore have important evolutionary consequences, but that plant species coexist with apparently little differentiation permitting interaction avoidance.

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