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 co nspicuous stem galls on Cirsium arvense. Using these galls it is simpl e to map the distribution of U. cardui. 2. U. cardui has distributiona l gaps in Europe. Previous authors ascribed these gaps to an unfinishe d re-immigration since the Pleistocene. We investigated the ecological and genetic spatial pattern in an area in north-eastern Bavaria, wher e 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 distributi onal gap is about 2 km per year, which is much higher than postulated by some previous studies and consistent with results of others. 4. Bet ween 50 and 70% of host-plant patches are occupied by U. cardui within the colonized part of the study area. Subpopulation sizes are often q uite small and mortality rates of subpopulations caused by parasitoids may reach 100%. Mean rates of parasitization range from 28% in 1987 t o 77% in 1991. 5. Extinctions of U. cardui subpopulations and coloniza tions of empty patches are common within the study area and both reach ed values around 30% during the years of investigation. 6. The pattern of occurrence of U. cardui at host-plant patches is only modestly cor related with the recorded habitat parameters. The probability of occur rence increases with patch size. In one year we found a negative influ ence of altitude. Gall density shows a positive spatial autocorrelatio n with the density of neighbouring subpopulations up to a range of abo ut 2 km. 7. Out of 18 scorable loci, only AAT and ACOH proved to be po lymorphic and useful for population genetic distinctions. Contrary to the gall density data, we found no spatial patterns of allele frequenc ies. High rates of gene flow with long-range dispersal of single indiv iduals may prevent local genetic differentiation. Nevertheless, we fou nd a correlation of single allele frequencies with altitude, which is matched by observations on a biogeographical scale. 8. We interpret th e U. cardui system as a metapopulation, where stressful weather climat e and parasitoids cause local extinctions and affect colonization rate . 9. Our data on the rate of range expansion and the influence of clim atic factors suggest that the actual distributional gaps could be caus ed by temporary fluctuations of U. cardui based on climatic fluctuatio ns in historical time rather than on climatic fluctuations during the Pleistocene.