The spatial population dynamics of insects exploiting a patchy food resource : I. Population extinctions and regulation.

The population dynamics of insects in a spatially fragmented environment were studied by examining three main aspects of their ecology, namely, rates of local population extinction, density dependence in population change, and movements between populations. Ten phytophagous insects 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 over 5 km2 of arable farmland. The results were used to test the relative importance of immigration and of local population regulation to population persistence. This paper describes the rates of local extinction of the various species and the density-dependent factors operating on their populations. Local extinction was found to be a regular feature of these populations, especially on the smaller patches of the plants. Density dependence was identified in the changes in population sizes between generations of eight species (five herbivores and three parasitoids). Parasitoids appeared to be incapable of regulating their host populations, in the four host species for which we have data. Populations of all species were, however, limited by their food resources, and their numbers tracked changes, both spatially and temporally, in resource availability. The implications of these findings to population theory are discussed.