METAPOPULATION DYNAMICS OF THE TEPHRITID FLY UROPHORA-CARDUI - AN EVALUATION OF INCIDENCE-FUNCTION MODEL ASSUMPTIONS WITH FIELD DATA

1. The analysis of metapopulation systems is to a great extent based o n presence/absence patterns. It is possible to extract the rates of ex tinction and colonization from these patterns making several assumptio ns: (i) the time-scale of within-patch dynamics is considerably shorte r than that of between-patch dynamics, (ii) the probability of extinct ion depends on patch area, whereby patch area is used as a surrogate o f population size, (iii) the probability of colonization depends on is olation of patches. 2. Field data on the metapopulation dynamics of a tephritid species together with habitat parameters were used to test t he validity of the assumptions above. U. cardui is a monophagous stem gall former on the creeping thistle Cirsium arvense, forming a spatial ly structured population system due to the patchy distribution of the host plant with frequent extinctions of local populations and recoloni zation of patches. 3. Gall numbers of U. cardui proved to be proportio nal to patch area. Thus, patch area is a reliable predictor for popula tion size. The size of newly founded populations was 50% below the pop ulation size expected from patch area. 4. The extinction probability o f local populations was predominantly a function of population size an d therefore patch area. The colonization probability of empty host pla nt patches was greater during years with low larval mortality and incr eased with patch size and neighbourhood density. 5. Within the studied population system isolation was of minor importance for the colonizat ion of empty patches because the distance between patches is shorter t han the mean dispersal distance of the tephritid species. The coloniza tion rate depends mainly on the number of dispersing individuals and h ence on the size of existing populations. 6. Between- and within-patch dynamics operate on similar, short time-scales. Nevertheless, metapop ulation dynamics can occur as long as there are local breeding populat ions and a high risk of extinction of local populations. Presupposumpt ion is that the species-specific dispersal ability by far exceeds the distance of patches.