The spatial population dynamics of insects exploiting a patchy food resource: a model study of local persistence.
The density-dependent persistence of four tephritid flies on patches of their host vegetation (Urophora jaceana and Chaetorellia jaceae on Centaurea nigra and Tephritis bardanae and Cerajocera tussilaginis [Terellia tussilaginis] on Arctium minus) was studied in England, UK, in 1990-92 using a semi-analytic model. The model takes into account density dependence, environmental stochasticity and immigration. Basic statistical methods were used to parameterize the model for data collected. It was assumed that the extinction process was due mainly to a fluctuation of the amount of resources. The model was used to predict local extinction probability. For all species, these predictions were compared with, and found to be in reasonable agreement with, the observed levels of local extinction. The model can also be used to estimate the effects of increasing fragmentation on a given fly species. The occupancy of larger patches was strongly dependent on initial conditions. Initially-occupied large patches could last a long time, irrespective of isolation, but unoccupied ones must be colonized first, requiring that the patch be reasonably close to the source of dispersers. Small patches, irrespective of initial conditions, would, on average, only be occupied if they lay close to a large 'mainland' patch or group of patches; that is, their isolation was less than the 'dispersal distance' for that species. On the scale of the field system studied, immigration was the dominant factor determining persistence. The rate of immigration appeared to be proportional to the area of a patch, but the isolation of almost all patches (distance to the next patch) was significantly less than the measured dispersal distance of all the species studied.