THE INFLUENCE OF HABITAT HETEROGENEITY ON HOST-PATHOGEN POPULATION-DYNAMICS

The influence of spatial heterogeneity on the population dynamics of a naturally occurring invertebrate host-pathogen system was experimenta lly investigated. At ten week intervals over a two year period, I quan tified the spatial distribution of natural populations of the terrestr ial isopod crustacean Porcellio scaber infected with the isopod irides cent virus (IIV). During the seasonally dry periods of summer and earl y fall in central California, isopod populations were highly aggregate d and the degree of patchiness and distance between inhabited patches was greatest. Coincident with increased patchiness and patch spacing w as an increase in isopod density within patches. During the wet season s of winter and spring, isopod population patchiness, inter-patch spac ing, and within-patch density was low. Seasonal changes in virus preva lence were negatively correlated with within-patch density, patchiness , and inter-patch spacing. The influence of the spatial distribution o f isopods on virus prevalence was also tested in field experiments. Th e virus was introduced into arrays of artificial habitat patches colon ized by isopods in which inter-patch distance was varied. The prevalen ce of resulting infections was monitored at weekly intervals. In addit ion, dispersal rates between artificial patches and natural patches we re quantified and compared. The results showed that isopods in treatme nts with the smallest inter-patch spacing had the highest virus preval ence, with generally lower prevalence among isopods in more widely spa ced patches. The spacing of experimental patches significantly affecte d virus prevalence, although the experiments did not resolve a clear r elationship between patch spacing and virus prevalence. Rates of dispe rsal between patches decreased with increased patch spacing, and these rates did not differ significantly from dispersal between natural pat ches. The results suggest that rates of dispersal between isopod subpo pulations may be an important component of the infection dynamics in t his system. I discuss the consequences of these findings for host-path ogen dynamics in fragmented habitats, and for other ecological interac tions in spatially heterogeneous habitats.