Quantitative webs as a means of assessing the impact of alien insects.

1. We use quantitative linkage webs to investigate the impact of alien gall wasps on community structure. Britain has been invaded by four alien species of cynipid gallwasp, Andricus corruptrix, A. lignicola, A. kollari and A. quercuscalicis, over the last 150 years. To date, Britain can be divided into four zones from the north to the south with one, two, three and four invading species established in each zone. 2. The four species are naturalized in their new ranges and are locally the most abundant cynipid species, especially in their spring (sexual) generations. Like the native cynipid species they showed dramatic changes (up to three orders of magnitude) in density between generations, and the dominance structure of alien and native host species changed radically from generation to generation. 3. All four invading cynipid species were attacked by native parasitoid species. Using quantified linkage webs, we assess the contribution made by individual host gall species to each parasitoids population size. Although the parasitoid species have been described as broadly polyphagous, suggesting that the aliens should be richly linked with the native cynipid communities, we found that the galls of the invading species have become the main, and in a few cases the sole, contributors to local parasitoid populations, indicating major host shifts by the parasitoid species. 4. Within generations we found very little overlap among the parasitoid and inquiline communities associated with native and alien galls within generations. Similarly, the quantification of indirect interactions among cynipids between generations suggests that parasitoids and inquilines are not main factors in the dynamics of local cynipid communities. The recruitment of parasitoids and inquilines by the invading cynipid species is therefore unlikely to have a strong affect on native cynipid species.