Habitat heterogeneity, species diversity and null models

The habitat heterogeneity hypothesis states that an increase in habitat heterogeneity leads to an increase in species diversity. We tested this hypothesis for a community of small mammals in the semiarid, sand‐shinnery‐oak ecosystem of the southwestern United States. We used indices of differentiation diversity to quantify differences between two habitat types (blowouts in a sand‐shinnery‐oak matrix) in terms of species diversity. The Wilson‐Shmida index (β T ) considers species composition only, whereas the Morisita‐Horn index (C mH ) also takes species abundances into account. We constructed null models to test the hypothesis that differentiation diversity between habitat types is greater than that produced by stochastic processes. Two models were constructed, one based on the random placement of species and one based on the random placement of individuals. No evidence supported the hypothesis that habitat heterogeneity enhances diversity of a landscape by increasing the number of species in an area. Indeed, paired habitats were more similar than chance alone would dictate in terms of species identities. In contrast, habitat heterogeneity affects diversity by significantly altering the relative proportions of species in contrasting habitat types. Because seeds differentially accumulate at the interface between blowouts and matrix, the high productivity of the edge may actually homogenize habitat types in terms of species richness. Nonetheless, blowouts might best be considered to be microhabitats which enhance or complement the value of the matrix even though the species which use either habitat type are identical.