EXPERIMENTAL-ANALYSIS OF INTERMEDIATE DISTURBANCE AND INITIAL FLORISTIC COMPOSITION - DECOUPLING CAUSE AND EFFECT

The intermediate disturbance hypothesis predicts that richness will be highest in communities with moderate levels of disturbance and at int ermediate rime spans following disturbance. This model was proposed as a nonequilibrium explanation of species richness in tropical forests and coral reefs. A second model of succession, initial floristic compo sition, states that nearly all species, including late seral species, are present at the start of succession. This leads to the prediction t hat richness should be highest immediately following disturbance. We t ested these predictions using plant species composition data from two long-term field experiments in North American tallgrass prairie vegeta tion. In contrast to one prediction of the intermediate disturbance hy pothesis, there was a significant monotonic decline in species richnes s with increasing disturbance frequency, with no evidence of an optimu m, in both field experiments. Species composition on an annually burne d site was a subset of that of infrequently burned sites. The average number of species per quadrat and the number of grass, forb, and annua l species were lowest on annually burned sites compared to unburned si tes and sites burned once every 4 yr. The second prediction of the int ermediate disturbance hypothesis, however, was supported. Richness rea ched a maximum at an intermediate time interval since the last disturb ance. This contradicts the prediction from the initial floristic compo sition model of succession. These results also suggest that the two pr edictions of the intermediate disturbance hypothesis are independent a nd unrelated. We propose that this may be explained by uncoupling the effects of disturbance as a single, relatively discrete event from sys tem response to disturbance. From this perspective, disturbance become s an extinction-causing event in these grasslands, whereas recovery fo llowing disturbance is a balance between immigration and extinction.