Parasites detect host spatial pattern and density: a field experimental analysis.

A common assumption in mathematical models of parasitism is that the susceptibility to parasitism of an individual host increases both with host density and the degree of host spatial aggregation. To determine whether this assumption is correct in nature, we developed a factorial field experiment with the parasitic marine isopod Hemioniscus balani and its barnacle host Chthamalus dalli. Our factorial design enabled evaluation of the separate effects on parasitism of the two factors (host density and host spatial pattern) and also to assess the host density-spatial pattern interaction effect. Both host density and spatial aggregation were found to lead to increased parasitism, and the interaction effect was nonsignificant. These findings are the first experimental field demonstration that these processes occur in nature, as widely assumed in ecological theory.