Insect Conservation and Diversity (2009) 2, 36–44 © 2009 The Authors 36 Journal compilation © 2009 The Royal Entomological Society Blackwell Publishing Ltd Co-existence of divergent communities at natural boundaries: spider (Arachnida: Araneae) diversity across an alpine timberline PATRICK MUFF, 1 CHRISTIAN KROPF, 1 HOLGER FRICK, 1 WOLFGANG NENTWIG 2 and MARTIN H. SCHMIDT-ENTLING 2 1 Department of Invertebrates, Natural History Museum Bern, Bernastrasse 15, Bern, Switzerland, and 2 Zoological Institute, University of Bern, Baltzerstrasse 6, Bern, Switzerland Abstract. 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 environ- mental 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 pro- vided 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. Key words. Arthropods, dwarf-shrub heath, ecotone, environment and space, forest, light, microspatial distribution, pitfall traps, Swiss Central Alps, vegetation structure. Introduction Spatial patterning of landscapes and its influence on the abundance and distribution of organisms remains one of the most fundamental issues in ecology (Ricklefs, 2004; Rahbek, 2005). Ecosystem management requires an understanding of the factors that drive species diversity. Landscapes exist as mosaics of numerous different patch types that interact with each other. Hence, knowledge about the ecology of habitat edges is critical for understanding which resources and interactions determine the distribution of organisms (Ries et al., 2004). As land-use pressure often reduces the area of boundary habitats, their value in the conservation of habitats and species should be investigated (Harrison & Bruna, 1999; Brooks, 2000; Channell & Lomolino, 2000). Edges can be defined as ecotones between two plant communities, which are the transition zones between these communities (Holland et al., 1991). The specific characteristics and value of the ecotonal flora and fauna have been known for a long time as ‘edge effects’, which are manifested as high diversity in microhabitats, a change in abiotic factors and species interactions and elevated species richness (Odum, 1971; Matlack, 1993; Murcia, 1995; Laurance et al., 2002). Recently, ecotones were even found to be sources of speciation (Smith et al., 1997; Schilthuizen, 2000). However, our knowledge about biotic patterns and processes across natural boundaries and their extrapolation to landscapes Correspondence: P. Muff, Department of Invertebrates, Natural History Museum Bern, Bernastrasse 15, CH-3005 Bern, Switzerland. E-mail: patrick.muff@gmail.com