The Relationship Between Competition and Morphology. II. Experiments on Co-Occurring Dytiscid Beetles

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Abstract
(1) We performed field experiments at two sites to test for interspecific competition and whether morphologically similar species compete more intensely in assemblages of dytiscid beetles. (2) In a small acidic ditch where co-occurring species showed patterns of morphology consistent with random assembly of species, feeding rate was unaffected by manipulation of density of adults, and was unrelated to the presence of potential competitors, regardless of body size. Field data indicated that adult Hydroporus at this site were not food limited. (3) Competition for food or cannibalism imposed density-dependent survival on larvae of Hydroporus across the natural range of densities. Density-dependent survival among larvae probably kept populations of adults sufficiently low for competition among adults to be minimal. (4) In a large well-buffered canal where co-occurring dytiscids are more widely and regularly dispersed in morphological space than expected from random assembly of species, increasing density of adults reduced feeding rate; however, there was no effect of differences in body size on intensity of competition. Adults at this site did not appear to be food-limited at natural densities. The observed non-random pattern of morphologies does not seem to be a result of interspecific competition.