The spread of non-native species is a global threat and the rate at which biological invasions occur is likely to increase in the future. This thesis examines the implications of the invasive signal crayfish, Pacifastacus leniusculus (Dana), for instream communities and abiotic processes within lowland rivers in England. The potential effects on lotic macroinvertebrate communities and fine sediment dynamics are investigated at a range of spatial and temporal scales, from the examination of national long-term datasets through to short-term detailed mesocosm experiments. Interrogation of macroinvertebrate community data from three English regions was undertaken to understand the temporal and spatial extent of signal crayfish effects. Invasive crayfish had significant long-term and persistent effects on resident macroinvertebrate communities regardless of the lithology or other environmental characteristics of the region. The resultant modifications to community composition had repercussions for several widely employed freshwater biomonitoring tools which employ faunal abundance in their derivation. A reach-scale field study was undertaken at two sites, one invaded by crayfish and one which did not support populations of crayfish (control), to assess the potential contribution of signal crayfish for fine sediment dynamics (ingress, fluxes and ultimately the overall implications for sediment budgets). Reach-scale field experiments examining the effect of crayfish invasion on resident macroinvertebrate communities over a 126-day period indicated that once crayfish populations are well-established their effects are persistent. However, separating the effects of crayfish from wider macroinvertebrate community dynamics and life-history characteristics of the invader and resident taxa means that attributing the effects is far from clear. The thesis presents the results of a series of novel mesocosm experiments which examined the dynamic and two-way interactions of predator-prey relationships and potential zoogeomorphological effects of signal crayfish and the freshwater shrimp, Gammarus pulex. Signal crayfish had a significant effect on the mass of fine sediment infiltrating into the gravel matrix associated with foraging and predatory activity; however this was strongly controlled by prey availability. Finally, through the development of conceptual models, the thesis illustrates the need for greater integration of ecological and geomorphological theories, at relevant environmental scales (temporal and spatial) to achieve truly interdisciplinary research.

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