Local extinction processes rather than edge effects affect ground beetle assemblages from fragmented and urbanised old beech forests

Local extinction of specialist species due to fragmentation is one of the major causes of biodiversity loss. Increased extinction rates in smaller fragments are expected to result from both smaller local population sizes, which increase the effect of environmental or demographic stochasticity, and increased edge effects. The relative effect sizes of these two factors are still poorly investigated, however. We attempt to disentangle these effects on ground beetle communities of temperate broadleaved woodland fragments situated in one of the most urbanised regions in Belgium. Assemblages were sampled along transects that extended from 30m outside to 100m inside both small and large historical forest fragments. Although species assemblages within the forest were highly distinct compared to those sampled outside the forest, species turnover along these transects was less pronounced within forest fragments indicating only weak edge effects. The magnitude of edge effects did not differ significantly between large and small fragments. Nevertheless, larger differences in species composition were observed with respect to fragment size, wherein highly specialised species persisted only in the largest fragment. In summary, increased local extinction processes in smaller fragments, which led to a strong reduction in specialised and wingless forest species, appeared to be the most important factor that drives changes in species composition in this historical and fragmented woodland complex.