Spatial heterogeneity, population "regulation" and local extinction in simulated host-parasitoid interactions.

Simulation models have recently been used to suggest that spatial heterogeneity, acting on small spatial scales within local populations, may allow parasitoids and other natural enemies to "regulate" host or prey populations in ways that would not be detected by conventional (k-factor) analyses of life table data. However, additional study of these models suggests that local extinction may be a frequent event in the simulated interactions. The "spreading of risk" concept appears more applicable to the simulated populations than a classical view emphasizing tight regulation around stable equilibrium points. The spreading of risk viewpoint also appears to shed additional light on questions raised in the recent debate between Dempster (1983); Hassel (1985); Dempster and Pollard (1986), concerning the modeling of spatial heterogeneity and "regulation" in temperate-zone insect populations.