Metapopulation Dynamics of the Tephritid Fly Urophora cardui: An Evaluation of Incidence-Function Model Assumptions with Field Data

Abstract

The analysis of metapopulation systems is to a great extent based on presence/absence patterns. It is possible to extract the rates of extinction and colonization from these patterns making several assumptions : (i) the time-scale of within-patch dynamics is considerably shorter than that of between-patch dynamics, (ii) the probability of extinction depends on patch area, whereby patch area is used as a surrogate of population size, (iii) the probability of colonization depends on isolation of patches. Field data on the metapopulation dynamics of a tephritid species together with habitat parameters were used to test the validity of the assumptions above. U. cardui is a monophagous stem gall former on the creeping thistle Cirsium arvense, forming a spatially structured population system due to the patchy distribution of the host plant with frequent extinctions of local populations and recolonization of patches. Gall numbers of U. cardui proved to be proportional to patch area. Thus, patch area is a reliable predictor for population size. The size of newly founded populations was 50% below the population size expected from patch area. The extinction probability of local populations was predominantly a function of population size and therefore patch area. The colonization probability of empty host plant patches was greater during years with low larval mortality and increased with patch size and neighbourhood density. Within the studied population system isolation was of minor importance for the colonization of empty patches because the distance between patches is shorter than the mean dispersal distance of the tephritid species. The colonization rate depends mainly on the number of dispersing individuals and hence on the size of existing populations. Between- and within-patch dynamics operate on similar, short time-scales. Nevertheless, metapopulation dynamics can occur as long as there are local breeding populations and a high risk of extinction of local populations. Presupposumption is that the species-specific dispersal ability by far exceeds the distance of patches.