Effects of disturbance on species diversity: a multitrophic perspective

Models of the effects of disturbance on ecological communities have largely considered communities of competing species at a single trophic level. In contrast, most real communities have multiple interacting trophic levels. I explored several versions of simple single- and multitrophic models to determine whether predictions of the intermediate disturbance hypothesis (IDH), derived from considering only a single trophic level, apply to multitrophic situations. The IDH was predicted by models of competing species at a single trophic level but did not hold in many situations with more natural trophic structure. In general, basal species in a food web tended to follow the IDH, whereas competitors at top trophic levels did not. Additional analyses indicated that outside immigration interacted with trophic structure to produce widely differing predictions about the consequences of disturbance and that density-dependent disturbance events could recapture the IDH in some multiple trophic level situations. Model predictions matched the results of empirical studies to date: the IDH has generally been supported for species competing for nondynamic basal resources but not for mobile aquatic invertebrates at higher trophic levels. The model analysis also verified basic predictions of verbal models addressing the effects of physical stress. Three different aspects of disturbance and their contributions to species coexistence were identified: changes in average mortality rates, changes in temporal variability, and changes in spatial heterogeneity. The results indicate that the IDH should be applied with caution to real multitrophic communities.