"Active" passive sampling in two species of Lepomis from Par Pond, South Carolina, United States: a case study of infracommunity nestedness.

The influence of ontological diet shifts on infracommunity nestedness was examined by comparing the infracommunity structure of Lepomis gulosus and Lepomis macrochirus at 2 localities in Par Pond, South Carolina, United States. Fill-constrained, occurrence-constrained, and abundance-constrained null models were used to evaluate the degree of nestedness. The presence-absence matrices from all 4 component communities had significantly fewer discrepancies than those produced by the fill-constrained model, and none had significantly fewer discrepancies than those produced by the occurrence-constrained model. Only the presence-absence matrix for the infracommunities of L. gulosus from the Cold Dam locality had significantly fewer discrepancies than those produced by the abundance-constrained null model. The nestedness of the 4 samples could not be distinguished from that expected under a hypothesis of passive sampling. A positive correlation between host size and total parasite abundance indicates the passive mechanism has a deterministic basis. Thus, even in the absence of habitat or diet shifts, nestedness can arise in infracommunities of freshwater fishes when older, larger fish have sampled more individuals from an uneven distribution of infective stages.