THE SPATIAL POPULATION-DYNAMICS OF INSECTS EXPLOITING A PATCHY FOOD RESOURCE .1. POPULATION EXTINCTIONS AND REGULATION

The population dynamics of insects in a spatially fragmented environme nt were studied by examining three main aspects of their ecology, name ly, rates of local population extinction, density dependence in popula tion change, and movements between populations. Ten phytophagous insec ts and seven parasitoids inhabiting the flowerheads of two herbaceous plants, Centaurea nigra and Arctium minus, were studied by monitoring their populations on more than 50 patches of each plant, scattered ove r 5 km(2) of arable farmland. The results were used to test the relati ve importance of immigration and of local population regulation to pop ulation persistence. This paper describes the rates of local extinctio n of the various species and the density-dependent factors operating o n their populations. Local extinction was found to be a regular featur e of these populations, especially on the smaller patches of the plant s. Density dependence was identified in the changes in population size s between generations of eight species (five herbivores and three para sitoids). Parasitoids appeared to be incapable of regulating their hos t populations, in the four host species for which we have data. Popula tions of all species were, however, limited by their food resources, a nd their numbers tracked changes, both spatially and temporally, in re source availability. The implications of these findings to population theory are discussed.