Factors Influencing Invertebrate Size Distributions in Prairie Potholes and Implications for Coexisting Duck Species

Species of dabbling ducks which co-occur on, and obtain invertebrates from potholes in prairie Canada do not display habitat- and diet-niche complementarity as predicted by competition theory. We hypothesized that this was because some aspect of the diet niche of ducks was not independent of the habitat niche. Because dabbling ducks partition invertebrate foods on the basis of size, we tested whether invertebrate size distributions in potholes were influenced by microhabitat structure. Activity traps were used to sample aquatic invertebrates weekly from late May to early July at 50 sites over a gradient of water depth and emergent and submergent vegetation density. Presence of submergent vegetation correlated significantly with invertebrate size; emergent vegetation density and water depth accounted significantly for additional variation in invertebrate size. Invertebrate size also decreased with time over all sites. These patterns 1) are consistent with a general theory of the distribution of particle sizes in aquatic environments and 2) point out the problem of testing for patterns of niche complementarity among coexisting predators of invertebrates when some characteristics of the invertebrate resource by which predators partition it, like size, is not independent of a habitat gradient which the predators also partition.