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This research was conducted to test two predictions about niche and competitive relationships in the structure of plant communities. It has been demonstrated that early successional species of plants have broad, overlapping niche occupation on many gradients whereas later succes- sional species show more niche differentiation. We predicted that, if the differences in resource use were for contested resources in limited supply, the more niche-differentiated species should: (1) experience less biomass reduction in pairwise interspecific competition than should broadly overlap- ping species of early succession, and (2) be able to convert relatively more of the available resources into plant matter as a heterospecific assemblage than can the nonspecific stands from that assemblage. Six species from each of early (annual), mid- (early perennial), and late- (prairie) successional plant communities were grown in containers singly, in con- and heterospecific pairs, and in pure and mixed stands. Competitive interactions were assessed by comparing biomass production of species under the various treatment conditions. The late-successional community, in which we have previously shown more niche separation on several axes, showed a significantly higher mean ratio for the mass of individuals grown in heterospecific pairs divided by the mass of individuals of the same species grown in nonspecific pairs than did the early successional community. All heterospecific pairs in the prairie and early perennial communities had a winner (ratio of mass in heterospecific pair to mass in nonspecific pair > 1.0) and a loser (ratio < 1.0), while for most pairs in the annual community both competitors had ratios of < 1.0. Annuals showed the most reduction in total mass and prairie species the least when the mass of an individual grown in a heterospecific pair was compared to the mass of an individual of the same species grown by itself. Therefore, there was less competitive reduction in the more niche-differentiated community, the prairie. There was some evidence that mixtures of the more niche-differentiated late-successional species may be able to convert relatively more of the available resources into plant matter. The prairie assemblage had a slightly greater ratio of mixed stand to pure stand biomass with more variation among species than did the annual assemblage. There was also a tendency for the pairs of prairie species to have higher relative and absolute yields in mixture than in nonspecific pairs. Thus members of the late-successional community experienced less competitive reduction in the presence of heterospecific neighbors than did species of early succession. This is strong evidence that niche reduction and separation are effective in reducing competition within a community and that selection to reduce competition could have been more important in the evolution of late-successional species than for early successional species.

Alpha-niche parameters involving pollination resources were examined for plants in communities of 3 different successional ages. Niche breadths and overlaps were compared in early successional annual, early perennial, and prairie communities. Niche axes examined were seasonal time of flowering, species of flower visitor, daily time of visit, and 2-dimensional parameter, species of visitor x daily time. Seasonal flowering was clumped in all communities. It occurred fairly synchronously in midsum- mer in the annual community and in the fall in the perennial community. Flowering in the winter annual and prairie communities occurred at 3 distinct seasonal times; spring, summer and fall. In this annual community, the different flowering times reflected varied life history strategies demonstrating adaptations mainly to the physical environment. In the prairie, the same 3 seasonal flowering aggre- gations were most likely formed through adaptation to the biotic environment. The prairie species had smaller seasonal flowering niche breadths, but showed no reduction in seasonal niche overlap. In fact, there was some evidence for convergence in flowering time of species attracting similar vector species. There was little difference in daily time of flowering for most species pairs in all communities. Only in early and midsummer was there evidence for niche separation in daily time of visitation. In early spring and fall, mean overlaps between species were > in comparable random data sets, sug- gesting convergence in daily time of insect visitation. The relative proportions of general types of visitor differed among the communities. There were tendencies for annuals to be visited more by halictid bees and syrphid flies and there were strong tendencies for prairie species to be visited more by apid and megachilid bees and by lepidopterans. Because the latter groups are considered to be more effective pollinators, significantly more of the visits could have effected cross-pollination in the prairie than in annual communities. Niche overlaps of species of visitor were significantly lower than random for all communities, indicating some niche differentiation on that axis. The mean similarities of species of visitor and the 2-dimensional visitor species x daily time for the prairie plants were significantly lower than the means for the annuals, and showed a tendency to be lower than the means of the early perennials. Niche breadths on the same 2 parameters were also lower in the prairie. Therefore, species of the late successional community, the tallgrass prairie, showed more niche specialization and separation than did species of early successional communities. We conclude that a-scale differentiation on these pollination parameters does occur between plant species, and the degree to which it occurs increases with the successional age of the community. These results support the hypothesis that selection to reduce competition has resulted in greater niche differentiation in late successional communities.