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).