Co-existence of divergent communities at natural boundaries: spider (Arachnida: Araneae) diversity across an alpine timberline

1. Habitat boundaries can contain high biodiversity because they potentially combine species from two habitat types plus additional boundary specialists. However, most research on animal communities is focused on uniform habitats. 2. Here, we assessed the degree to which the community change at a habitat edge is determined by the broad-scale spatial transition from one habitat to the other, or by fine-scale environmental influences. We compared the distribution of ground-dwelling spider species from open land to forest with the distribution around stand-alone trees at the boundary, the alpine timberline (Grisons, Switzerland). 3. Our results showed that spiders were more strongly influenced by local environmental conditions (40% of explained variation) than by the spatial position within the ecotone (24.5% of explained variation, with 15.6% overlap between the two). Spider communities differentiated according to light availability and corresponding changes in the ground vegetation. 4. Since the small area around a single tree at the studied timberline offered a similar broad spectrum of environmental conditions as the open land and forest together, it provided both habitats for species from the adjoining open land and forest as well as for some possible timberline specialists. 5. Accordingly, natural habitat boundaries may maintain very contrasting communities by providing a wide range of habitat conditions.