Parasite-Mediated Competition: Some Predictions and Tests

Abstract

Two new hypotheses on parasite-mediated competition are developed from well-defined patterns in nature involving the positive relationship of parasitic species number and host area, the negative relationship of population density and body mass, and the threshold theorem in epidemiology. The geographic-range hypothesis predicts that species with larger geographic ranges, carrying more parasites as "biological warfare" agents, usually displace species with smaller ranges. The body-size hypothesis predicts that smaller species, with higher density and with a higher intrinsic rate of population increase, sustain more parasitic species than do larger species and, counter to much competition theory, exclude larger species more frequently. These hypotheses are tested using all suspected cases of parasite-mediated competition for which the relevant data are available. Nine cases of 11 support the geographic-range hypothesis. For species of different size, 12 cases of 15 support the body-size hypothesis. No examples run counter to both hypotheses. More examples and more-detailed studies of the examples in current use are needed before these patterns can be considered firmly established.