Ecological Combining Ability and Competitive Combining Ability in Plants: Toward a General Evolutionary Theory of Coexistence in Systems of Competition
The common assumption that species coexistence in nature can be generally explained by processes of natural selection for niche differentiation does not have strong empirical support for plants. The meaning of competitive ability has traditionally been embedded within an Eltonian view of the niche. In particular reference to plants, however, a true distinction between the meanings of niche and competitive ability is established based on the premise that for one species to competitively exclude another, two distinct conditions must be satisfied: (1) There must be a sufficient degree of overlap in the niche requirements of the two species; and (2) one species must be sufficiently superior at reducing the availability of resources to the other. Following these considerations an evolutionary theory of coexistence is proposed involving two alternative mechanisms based on a broader interpretation of the operation of natural selection in systems of competition than previously recognized. Competitive exclusion may be avoided either if natural selection results in niche differentiation (selection for ecological combining ability), or if reciprocal selection maintains a balance of competitive abilities (selection for competitive combining ability). Competitive combining ability, although previously overlooked, has wide applicability to plants for which coexistence frequently is explained incompletely by ecological combining ability. The main argument is as follows: All biological attributes of a species will play a part in determining its niche requirements on the one hand, and its relative competitive ability for those requirements on the other. Given the assumption that niche requirements generally overlap broadly in plants, I propose that coexistence is permitted because there are numerous possible permutations and combinations of biological attributes in plants which are roughly equivalent in the overall competitive power which they confer. In selection for competitive combining ability, these attribute complexes are continually adjusted in an ongoing process of coevolution in which local neighborhoods are constantly engaged in a fine-tuning process that alters the way members respond to one another. Consequently, natural selection ultimately maintains a balance of overall relative competitive abilities between two species for essentially the same contested resources. I discuss the relevance of this mechanism and its relationship to other theories and concepts associated with coexistence in contexts of competition. It is proposed that, in nature, selection for ecological combining ability and selection for competitive combining ability operate in concert, and this provides reconciliation of the contradiction in theory between convergent adaptation to a common habitat and divergent adaptation to other members of the community.