Animal species diversity driven by habitat heterogeneity/diversity: the importance of keystone structures

The ‘habitat heterogeneity hypothesis’ is one of the cornerstones of ecology (e.g. Simpson, 1949; MacArthur & Wilson, 1967; Lack, 1969). It assumes that structurally complex habitats may provide more niches and diverse ways of exploiting the environmental resources and thus increase species diversity (Bazzaz, 1975). In most habitats, plant communities determine the physical structure of the environment, and therefore, have a considerable influence on the distributions and interactions of animal species (reviews in Lawton, 1983; McCoy & Bell, 1991). For example, for bird species diversity in forests, MacArthur & MacArthur (1961) evidenced that the physical structure of a plant community, i.e. how the foliage is distributed vertically, may be more important than the actual composition of plant species. Although positive relationships between vegetation-shaped habitat heterogeneity and animal species diversity are well documented on both local and regional scales (Davidowitz & Rosenzweig, 1998), empirical and theoretical studies have yielded contradictory results. Depending on the taxonomic group, the structural parameter of the vegetation and the Institute of Biochemistry and Biology, Plant Ecology and Nature Conservation, University of Potsdam, Potsdam, Germany, Department of Biology, Romberg Tiburon Center, San Francisco State University, Tiburon, CA, USA and Department of Ecological Modelling, Center for Environmental Research LeipzigHalle, Leipzig, Germany.