Environmental heterogeneity hypothesis
Why do species coexist? It is well accepted that species with the same habitat requirements cannot coexist (Hardin 1960). However, the large number of autotrophic plant species that co-occur in certain ecosystems and that rely on the same limited set of abiotic resources (light, water, nutrients, space) seem to contradict this theory. Hence, many ecologists are preoccupied with this contradiction (Chesson 2000, Wright 2002, Tokeshi 2009, Angert et al. 2009). Some argue that competitive exclusion may be slowed down by top down or bottom up control-mechanisms or disturbance events (Connell 1978). Others have suggested alternative mechanisms where competitive exclusion can be avoided by environmental heterogeneity of habitats, which increases the number of available niches to which different species may be adapted (Hutchinson 1957, MacArthur and MacArthur 1961, Macarthur and Levins 1967, Silvertown 2004, Sedio and Ostling 2013). This “environmental heterogeneity hypothesis” as a promoter of species coexistence and hence a driver for species diversity in ecosystems is now a widely accepted theory (Chesson 2000, Allouche et al. 2012, Stein et al. 2014). Many studies testing this hypothesis have investigated open land habitats (Morzaria-Luna et al. 2004, Lundholm 2009, Bergholz et al. 2017), or focused on landscape level measures of heterogeneity in abiotic or biotic conditions (see studies in Stein et al. 2014). In forest habitats, which show a rather high within-habitat heterogeneity due to the complex canopy architecture of trees, studies have included vegetation structure, dead wood occurrence, management regimes, wind throws and other disturbances, differences in overstory species richness, or abiotic conditions as proxies for heterogeneity and many different groups of organism ranging from birds to plants as biodiversity variables (e.g. MacArthur and MacArthur 1961, Richard et al. 2000, Taboada et al. 2008, Tamme et al. 2010, Bartels and Chen 2010). The effect of environmental heterogeneity on diversity will also depend on the spatial scale investigated, which could range from centimetres, metres (i.e. patch scale, e.g. safe sites for seed germination, or sun flecks in the forest understory) to kilometres (i.e. landscape scale, e.g. altitude and climatic belts).