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Interactions and coexistence of species in pasture community evolution Aarssen, Lonnie William


Recent studies have demonstrated intraspecific differentiation in plants associated with specialization in response to biotic interactions (e.g. competition), but the selection mechanisms involved remain largely undetermined. The common assumption that species coexistence in nature can be generally explained by processes of natural selection for niche divergence does not have strong empirical support for plants. This problem is addressed in a study of vegetation patterns and species interactions in three different aged pastures. Ordination of time-series percentage cover surveys showed a trend of increasing community 'constancy' in older pastures and suggested a developmental relationship -amongst the pastures. Soil analyses showed little correlation with species cover. Temporal patterns of fine-scale association between species was studied using contact sampling. Interspecific associations in younger communities were predominantly temporary in nature, and older. communities had more associations which persisted essentially unchanged. This data formed the basis of a qualitative model of pasture community evolution which attributes within community temporal changes to the selective forces accruing from biotic interactions. Competition experiments were set up for investigating reciprocal responses between clones of individuals which were in immediate proximity to one another as actual neighbours in the fields. For each of the 3 pastures, five of the most abundant species were used in a diallel design and three different species pairs were studied in a replacement series design. Results suggested that competitive relations between particular species may change with increasing pasture age either in the direction of increasing niche differentiation, more balanced competitive abilities, or towards competitive exclusion. Samples of Lolium perenne and Trifolium repens clones collected as neighbouring pairs from different localities in the oldest pasture were tested for their ability to grow in the presence of each other in a reciprocal transplant design. Natural neighbouring genet pairs had the most equitable component contribution to the total yield (i.e. highest combining ability) but did not differ in total yield from pairs of non-natural neighbours. Results from these studies suggest that selection in response to competition and other interactions among neighbours (e.g. beneficence) may result in two alternative types of 'combining ability' in plants: 1) ecological combining ability (niche differentiation), and 2) competitive combining ability (balanced competitive abilities). Most evidence was found for 2). In this mechanism, selection reduces the differential in competitive abilities of inferior and superior components for resources on which they both make demands. These findings are discussed in relation to contemporary theoretical considerations of natural selection operating in systems of competition. A general evolutionary theory of coexistence is offered based on a proposed distinction between the concepts of fundamental niche and competitive ability.

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