Diverse effects of forest fragmentation on a number of animal species.

Since 1990 we have initiated a number of studies on the population ecology, population genetics and social organization of several animal species in forest fragments of varying sizes. Study species include squirrels, small passerines and butterflies. Study sites are forest fragments between 1 and 40 ha, and study plots in `'continuous'' forest of 100 to 1500 ha. Our results so far suggest that forest fragmentation has more profound effects on dispersal patterns and population structure than on reproductive output and survival. Effects on dispersal include at least two aspects, which are not necessarily found in the same species. First, immigration deficits may be caused by low disperser success and/or source-sink effects. Second, reduced gene flow among local populations may result in genetic differentiation and/or loss of genetic variation. Negative consequences of the latter are not obvious, but indirect effects may be important e.g. for local adaptation. We conclude that studies on population structure in fragmented habitats should include quantitative (dispersal distance, immi/emigration, genetic structure) as well as qualitative aspects (timing of dispersal, habitat selection).