Impact of environmental change on ecological communities.

The impacts of environmental change on ecological communities are poorly understood relative to impacts on species. Impacts on inter-community (beta-) diversity are particularly neglected. As a result, our ability to forecast the impacts of environmental change on communities, and on individual species constrained by those communities, is seriously limited. However, as conservation efforts increasingly emphasise broad-scale approaches in terms of multi-species coverage and spatial scale, it is imperative that understanding ofbiodiversity change at these scales is enhanced so that conservation can be based on appropriate scientific evidence. Within this thesis I aimed to conduct multi-species analyses over multi-decada1 temporal scales at the spatial meso-sca1e to improve our understanding of such issues in both terrestrial and marine ecosystems. I discussed my findings in the theoretical context of G1easonian and C1ementsian views of species distributions (i.e. limitations to individualistic species responses) and their impact at the community scale. The temporal extent provided the opportunity to empirically test emerging concepts, including non-analogous communities, biotic homogenization, metacommunities and climate tracking responses at an appropriate meso-sca1e. Although no evidence for nonanalogous communities was found, biotic homogenization was supported and appeared to be caused by increased nitrogen and decreased light availability. An intertidal assemblage also converged but appeared to be driven by a reduced sea surface temperature gradient. Woodland plant metacommunity structure was demonstrated to be C1ementsian for woodland plants despite experiencing biodiversity loss. Hydrodynamic features were demonstrated to act as meso-sca1e dispersal barriers that limited intertidal invertebrates in tracking of climate (sea surface temperature) over the last 20 years. These barriers appear to alter when modelled under a scenario of sea level rise. Overall, results suggest that species are responding individualistically but that these responses are bounded by extrinsic constraints