HOST-PARASITOID SPATIAL MODELS - THE INTERPLAY OF DEMOGRAPHIC STOCHASTICITY AND DYNAMICS

Host-parasitoid metapopulation models have typically been deterministi c models formulated with population numbers as a continuous variable. Spatial heterogeneity in local population abundance is a typical (and often essential) feature of these models and means that, even when ave rage population density is high, some patches have small population si zes. In addition, large temporal population fluctuations are character istic of many of these models. and this also results in periodically s mall local population sizes. Whenever population abundances are small, demographic stochasticity can become important in several ways. To in vestigate this problem, we have reformulated a deterministic, host-par asitoid metapopulation as an integer-based model in which encounters b etween hosts and parasitoids, and the fecundity of individuals are mod elled as stochastic processes. This has several important consequences : (1) stochastic fluctuations at small population sizes tend to be amp lified by the dynamics to cause massive population variability, i.e. t he demographic stochasticity has a destabilizing effect; (2) the spati al patterns of local abundance observed in the deterministic counterpa rt are largely maintained (although the area of 'spatial chaos' is ext ended); (3) at small population sizes, dispersal by discrete individua ls leads to a smaller fraction of new patches being colonized, so that parasitoids with small dispersal rates have a greater tendency for ex tinction and higher dispersal rates have a larger competitive advantag e; and (4) competing parasitoids that could coexist in the determinist ic model due to spatial segregation cannot now coexist for any combina tion of parameters.