ECOLOGICAL AND GENETIC SPATIAL PATTERNS OF UROPHORA-CARDUI (DIPTERA, TEPHRITIDAE) AS EVIDENCE FOR POPULATION-STRUCTURE AND BIOGEOGRAPHICAL PROCESSES

1. Urophora cardui is a phytophagous tephritid species that induces conspicuous stem galls on Cirsium arvense. Using these galls it is simple to map the distribution of U. cardui. 2. U. cardui has distributional gaps in Europe. Previous authors ascribed these gaps to an unfinished re-immigration since the Pleistocene. We investigated the ecological and genetic spatial pattern in an area in north-eastern Bavaria, where this fly species is known to have invaded part of such a gap. 3. The mean annual rate of range expansion of U. cardui into this distributional gap is about 2 km per year, which is much higher than postulated by some previous studies and consistent with results of others. 4. Between 50 and 70% of host-plant patches are occupied by U. cardui within the colonized part of the study area. Subpopulation sizes are often quite small and mortality rates of subpopulations caused by parasitoids may reach 100%. Mean rates of parasitization range from 28% in 1987 to 77% in 1991. 5. Extinctions of U. cardui subpopulations and colonizations of empty patches are common within the study area and both reached values around 30% during the years of investigation. 6. The pattern of occurrence of U. cardui at host-plant patches is only modestly correlated with the recorded habitat parameters. The probability of occurrence increases with patch size. In one year we found a negative influence of altitude. Gall density shows a positive spatial autocorrelation with the density of neighbouring subpopulations up to a range of about 2 km. 7. Out of 18 scorable loci, only AAT and ACOH proved to be polymorphic and useful for population genetic distinctions. Contrary to the gall density data, we found no spatial patterns of allele frequencies. High rates of gene flow with long-range dispersal of single individuals may prevent local genetic differentiation. Nevertheless, we found a correlation of single allele frequencies with altitude, which is matched by observations on a biogeographical scale. 8. We interpret the U. cardui system as a metapopulation, where stressful weather climate and parasitoids cause local extinctions and affect colonization rate. 9. Our data on the rate of range expansion and the influence of climatic factors suggest that the actual distributional gaps could be caused by temporary fluctuations of U. cardui based on climatic fluctuations in historical time rather than on climatic fluctuations during the Pleistocene.