Coevolution of contrary choices in host-parasitoid systems

We investigate patch selection strategies of hosts and parasitoids in heter ogeneous environments. Previous theoretical work showed that when host trai ts vary among patches, coevolved populations of hosts and parasitoids make congruent choices (i.e., hosts and parasitoids preferentially select the sa me patches) and exhibit direct density dependence in the distribution of pe rcent parasitism. However, host-parasitoid systems in the field show a rang e of patterns in percent parasitism, while behavioral studies indicate that hosts and parasitoids can exhibit contrary choices (i.e., hosts avoid parc hes favored by the parasitoid). We extend previous theory by permitting lif e-history traits of the parasitoid as well as the host to vary among patche s. Our analysis implies that in coevolutionarily stable populations, hosts preferentially select patches that intrinsically support higher host equili brium numbers (i.e., the equilibrium number achieved by hosts when both pop ulations are confined to a single patch) and that parasitoids preferentiall y select patches that intrinsically support higher parasitoid equilibrium n umbers (i.e., the equilibrium number achieved by the parasitoids when both populations are confined to a patch). Using this result, we show how variat ion in life-history traits among patches leads to contrary or congruent cho ices or leads to direct density dependence, inverse density dependence, or density independence in the distribution of percent parasitism. In addition , we determine when populations playing the coevolutionarily stable strateg ies are ecologically stable. Our analysis shows that heterogeneous environm ents containing patches where the intrinsic rate of growth of the host and the survivorship rate of the parasitoid are low result in the coevolved pop ulations exhibiting contrary choices and, as a result, promote ecological s tability.