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 intermediate 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 composition, states that nearly all species, including late seral species, are present at the start of succession. This leads to the prediction that richness should be highest immediately following disturbance. We tested these predictions using plant species composition data from two long-term field experiments in North American tallgrass prairie vegetation. In contrast to one prediction of the intermediate disturbance hypothesis, there was a significant monotonic decline in species richness with increasing disturbance frequency, with no evidence of an optimum, in both field experiments. Species composition on an annually burned site was a subset of that of infrequently burned sites. The average number of species per quadrat and the number of grass, forb, and annual species were lowest on annually burned sites compared to unburned sites and sites burned once every 4 yr. The second prediction of the intermediate disturbance hypothesis, however, was supported. Richness reached a maximum at an intermediate time interval since the last disturbance. This contradicts the prediction from the initial floristic composition model of succession. These results also suggest that the two predictions of the intermediate disturbance hypothesis are independent and unrelated. We propose that this may be explained by uncoupling the effects of disturbance as a single, relatively discrete event from system response to disturbance. From this perspective, disturbance becomes an extinction-causing event in these grasslands, whereas recovery following disturbance is a balance between immigration and extinction.