Spatial heterogeneity and the population dynamics of a host- parasitoid system

The cabbage root fly Delia radicum at Silwood Park is attacked by both generalist and specialist natural enemies. This paper uses spatial and temporal census data to examine the relative contributions of these natural enemies to the population dynamics of D. radicum. Three particular cases of the model are analysed: 1) only the generalist, Aleochara bilineata, is present; 2) only the specialist, Trybliographa rapae, is present; and 3) both species of natural enemy are present. T. rapae can promote stability by virtue of its spatial response to host density, but only within a very narrow range of values of the host's rate of increase. A. bilineata acts as a simple, between-generation density-dependent factor and thus, in a straightforward way, tends to regulate the host population. When the two enemies act together a further property appears, provided that the survivorship of T. rapae is sufficiently high; the interplay of the two natural enemies can now lead to alternative, three-species stable states. -from Authors