The British Isles have been invaded by 12 alien cynipid gallwasps over the past 150 years. The first 4 of these species have been studied in depth and represent a model system in phytophagous insect community structure. In this thesis, I extend this research programme to incorporate 8 further invaders. I examine recent changes in the distribution of invading oak gallwasps in Britain and spatial patterns in the composition of the associated communities of phytophagous cynipid inquilines and parasitoids. I use fully quantitative webs to assess the diversity and strength of trophic interactions between native and invading species and assess the potential for apparent competition between gallwasps mediated by shared natural enemies. Of the first 4 invaders to be studied, 3 have expanded their range since 1991/2. Three of these 4 species are now well established in Scotland, while Andricus corruptrix remains confined to England. Four new invaders (A. aries, A. lucidus, A. grossulariae, Aphelonyx cerricola) are established in southern England and are spreading. Rates of range expansion vary across species (between means of 3.3 and 24.4 km per year), and may be correlated with variation in lifecycles and abundance. The four newest invaders (Neuroterus saliens, Plagiotrochus australis, P. coriaceus, P. quercusilicis) are currently restricted to their sites of first record. Previous studies on one of the early invaders, Andricus quercuscalicis, identified south to north and east to west declines in community species richness and in the abundance of specific parasitoid species. I find that: 1) Parasitoid associations with the asexual galls of A. quercuscalicis track inquiline recruitment to this host. 2) The longitudinal and latitudinal gradients in parasitoid species richness demonstrated in previous work are no longer apparent, suggesting that younger northern communities may be converging on their older southern counterparts. 3) Inquilines show increasing survivorship with distance from the original centre of their distribution in south east England, suggesting at least temporary exploitation of enemy-free space. 4) The recently invading Andricus and Aphelonyx species have all rapidly recruited parasitoids and inquilines. Fully quantitative webs were constructed for 4 sites in England and Scotland incorporating both native and invading cynipids. I tested the hypothesis that newly arriving gallwasp generations would fall within food web compartments based on their host oaks and location on the tree as demonstrated in previous work. Counter to this hypothesis, parasitoids attacking one of the newest invaders (A. grossulariae) break down host tree-associated compartmentalisation. Where A. grossulariae has yet to become established, host-based compartmentalisation remains pronounced. Despite extensive sharing of parasitoid species, I found only one strong indirect interaction between species (both aliens) and no evidence for widespread apparent competition. Spatial density dependent predation on an appropriate scale can stabilise population dynamics. I quantified predation by blue tits (Parus caeruleus) of spring generation bud galls on Turkey oak (Q. cerris) at three spatial scales (shoots within branches, branches within trees, trees within sites). I found significant levels of bird predation, with most variation occurring between trees rather than between shoots within branches. Spatial density dependence was detected at sites in southern England, primarily at the level of trees within a site. Relationships at finer spatial scales were far more variable in magnitude and sign. My results suggest that blue tits forage primarily at the level of trees. This thesis presents comprehensive new data on the establishment and spread of 12 invading cynipid species and on their interactions with native communities. The results further understanding of both spatial and temporal aspects of natural enemy recruitment to invading species. In particular, it is clear that individual invading species can significantly modify trophic linkage between established food web compartments. Finally, my data emphasise the significant (but often unstudied) contribution of highly mobile vertebrate predators to otherwise closed ecological microcosms.

Oak-dominated forest has declined in the eastern United States as shade-tolerant species (e.g., maple [Acer spp.]) replace oaks (Quercus spp.), sparking concern among ecologists regarding species that consume acorns. My goal was to describe how increasing mesophication of oak forests may affect consumers in higher trophic levels. I investigated relationships among forest composition, mast production, small-mammal density, and carnivore occurrence in 8 stands representing 4 forest types (upland oak, mixed-mesophytic, non-oak, and bottomland oak) in southern Illinois. I calculated tree-species richness, basal area, and other measures of forest composition using 3 0.04-ha plots/stand. In June-August 2009 and 2010, I live-trapped small mammals for 16,236 trap-nights with trapping webs to estimate population density of mice (Peromyscus spp.). I collected mast seeds during October-November 2009 and 2010 and calculated average dry biomass (g/m2) for each species and stand. During winter 2009-2011, I photographed carnivores using baited camera traps and combined these data with those from stands used in a concomitant large-scale carnivore survey. I regressed mast biomass, Peromyscus density, Peromyscus survival, and carnivore occurrence on measures of forest composition and hard-mast biomass. Peromyscus summer density was not related to % hard-mast basal area, nor to hard-mast biomass from the previous autumn. Survival of Peromyscus in 2010 displayed a significant positive relationship with hard-mast biomass in 2009 (F1,6 = 7.85, P = 0.04, r2 = 0.61). Logistic regressions of carnivore occurrence on Peromyscus density were not significant. Coyote (Canis latrans) occurrence at my sites and additional sites during January-April 2010 decreased with increasing % hard-mast basal area (x2 = 4.64, df = 1, P = 0.03). Bobcat (Lynx rufus) and gray fox (Urocyon cinereoargenteus) occurrence showed no relationship with % hard-mast basal area. Many other studies have demonstrated links of several species to oak forest, but the scale of this study may have been too small to detect effects of mesophication. Alternatively, small-mammal density may be influenced by invertebrate prey abundance or weather conditions. The landscape matrix of oak-hickory forest may also act to homogenize Peromyscus density across my study sites. Longer-term research could help clarify relationships among trophic levels. It is important for managers to consider techniques that may conserve oak forest.