EFFECTS OF DISTURBANCE ON SPECIES-DIVERSITY - A MULTITROPHIC PERSPECTIVE

Models of the effects of disturbance on ecological communities have la rgely considered communities of competing species at a single trophic level. In contrast, most real communities have multiple interacting tr ophic levels. I explored several versions of simple single- and multit rophic models to determine whether predictions of the intermediate dis turbance hypothesis (IDH), derived from considering only a single trop hic 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, b asal species in a food web tended to follow the IDH, whereas competito rs at top trophic levels did not. Additional analyses indicated that o utside 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 m ultiple trophic level situations. Model predictions matched the result s of empirical studies to date: the IDH has generally been supported f or species competing for nondynamic basal resources but not for mobile aquatic invertebrates at higher trophic levels. The model analysis al so verified basic predictions of verbal models addressing the effects of physical stress. Three different aspects of disturbance and their c ontributions to species coexistence were identified: changes in averag e mortality rates, changes in temporal variability, and changes in spa tial heterogeneity. The results indicate that the IDH should be applie d with caution to real multitrophic communities.