Plant species richness and composition of heathland relics in north-western Belgium: evidence for a rescue-effect?

Aim To study the effects of the degree of fragmentation of heathland patches on their species richness and species composition, and to infer the ecological mechanisms behind the observed patterns. Location The heathland patches of the north-western part of Flanders, Belgium. During the last 200 years, the heathland area in this study area has been reduced from c. 10,000 to c. 40 ha, yielding c. 150 small and highly isolated relic fragments. Methods Different isolation measures were calculated for each of the 153 inventoried heathland patches. The influence of isolation, area and habitat diversity on species richness was investigated using correlation techniques. Community composition of the patches was tested for nestedness, and the mechanisms potentially underlying this pattern were determined. Results and main conclusions Both the analyses at the species richness and community composition level yielded evidence for a positive species-area relation. This relation was not caused by higher habitat heterogeneity in larger patches. Patch isolation, measured in different ways, however seemed much more important in explaining species richness and community composition than fragment area. Our results indicate that area effects are overcompensated by the rescue-effect: if a patch is close to other patches, species can disperse between them and prevent the species from going extinct. Species having a short living seed bank were also more sensitive to isolation than species with a long living seed bank, indicating that the latter most probably depend on the seed bank to survive periods when environmental conditions are harsh. Analogously to the spatial rescue-effect, the existence of a persistent seed bank may lead to a sort of temporal rescue-effect, where the extinction of a plant species is prevented through survival in the seed bank of a patch.