Species associated with transient habitats need efficient dispersal strategies to ensure their regional survival. Using a spatially explicit metapopulation model, we studied the effect of the dispersal range on the persistence of a metapopulation as a function of the local population and landscape dynamics (including habitat patch destruction and subsequent regeneration). Our results show that the impact of the dispersal range depends on both the local population and patch growth. This is due to interactions between dispersal and the dynamics of patches and populations via the number of potential dispersers. In general, long‐range dispersal had a positive effect on persistence in a dynamic landscape compared to short‐range dispersal. Long‐range dispersal increases the number of couplings between the patches and thus the colonisation of regenerated patches. However, long‐range dispersal lost its advantage for long‐term persistence when the number of potential dispersers was low due to small population growth rates and/or small patch growth rates. Its advantage also disappeared with complex local population dynamics and in a landscape with clumped patch distribution.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary 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. 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 2km. 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 longrange 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.

The European corn borer, Ostrinia nubilalis (Hübner), is one of the most important insect pests in corn, Zea mays L. Transgenic corn cultivars expressing Bacillus thuringiensis (Bt) toxin provide a promising crop protection strategy against European corn borer; however, management is needed to avoid resistance development of the target pest species. The aim of this work was to establish the baseline susceptibility of different European corn borer populations in Germany to be able to forecast a possible development of resistance at an early stage. To standardize test procedures for future resistance management, the efficiency of Cry1Ab toxins from different suppliers and different production was assessed. Furthermore, two different test methods, surface treatment and the incorporation method, were compared with regard to their practicability and efficiency. Neither method provided significant differences in the baseline susceptibility of populations from different German regions. Overall, the data suggested little differentiation among German populations in terms of their susceptibility to Bt toxin and their genetic background. Future monitoring could therefore use a single European corn borer population as a representative for southwestern Germany. However, toxins from different suppliers and different production batches produced a vast range of LC50 values. Changes because of different toxin batches may be mistaken as a change in baseline susceptibility or even as the start of a resistance development. Thus, it is important throughout insect resistance management that the same toxin batches will be available for baseline susceptibility bioassays and for future tests.

Summary 1.The holly leaf-miner Phytomyza ilicis Curtis shows density patterns typical for noneruptive leaf-miners, with stable mean densities, but consistently different infestation rates on individual trees. 2. To identify the mechanisms responsible for these patterns the population dynamics of P. ilicis were studied during an 8-year period on 92 host trees in 15 holly patches. The patterns of oviposition, mining and survival and the dominant sources and rates of mortality were monitored. The impact of tree and patch parameters on leaf-miner population dynamics was analysed, and the dispersal ability of P. ilicis was evaluated experimentally. 3. Infestation differences among trees were due to host selection, which determined egg numbers on individual trees and was explained partly by holly variety and leaf availability. 4. Egg distributions on medium to highly infested host trees were aggregated, suggesting limits in the availability of young leaves for oviposition. 5. Due to a suite of mortality factors mean survival rates of P. ilicis were below 10% and did not show a relationship with host choice. 6. There was strongly density-dependent pre-mine mortality most probably caused by intraspecific competition in the leaf midrib. This lead to the formation of a single mine on most leaves. There was a weak density-dependent effect of mine density on miscellaneous larval death and on the attack rates of the larval parasitoid Chrysocaris gemma. 7. In the holly leaf-miner system phenology effects set a population ceiling delimited by the availability of oviposition sites. The resulting egg distribution determined the impact of density-dependent intraspecific competition in the leaf midrib. The incidence of larval competition was therefore not a function of density per se, but density-relative to the availability of a limiting resource, namely suitable young leaves for oviposition.

In many clonal species, seedling establishment is restricted to early successional stages when recruitment is still possible. Then, one expects that adapted genotypes become dominant and genotypic and genetic diversity should decrease with time. We investigated genotypic and genetic diversity within recently founded and established populations of the common weed Cirsium arvense. We used highly polymorphic amplified fragments length polymorphism (AFLP) markers. All populations were multiclonal and highly diverse (the proportion of distinguishable genotypes was 0:73 6 0:25 [mean 6 SD]). Clonal evenness was variable and ranged from 0.2 to 1. Independent of successional stage, we found on the small geographic scale of our study (<5 km) a considerable differentiation between populations (F SC ¼ 0:63). This amount of differentiation was similar between founder and established populations and could result from selection in the early stage of succession as well as founder effects. Contrary to the general expectation, genotypic and genetic diversity were maintained through time, and molecular variance did not differ between successional stages (1:9 6 0:89 vs. 2:5 6 1:41). We suggest that this pattern is a consequence of the particular reproductive system of C. arvense that combines clonality with dioecy. The combination of clonal reproduction with the recruitment of sexually outcrossed seedlings in the first years allows the species to perform efficient colonizations even with founder effects, to undergo selection without loss of diversity, and to persist locally. This strategy appears to be very efficient in C. arvense and may have contributed to the worldwide success of this species.