Changes in species diversity following habitat disturbance are dependent on spatial scale: theoretical and empirical evidence

1. Many studies have examined the impacts of tropical habitat disturbance. However, the effects of moderate habitat disturbance on species diversity show little consensus, with both increased and decreased diversity following disturbance being reported with approximately equal frequency. Previous work has shown that the spatial scale of sampling affects the reported changes in diversity following habitat disturbance, and here we present new theoretical and empirical data which explain why this is so. 2. We assume that habitat disturbance reduces the slope of the species-area relationship (SAR), and we show theoretically that this reduction in the slope results in a scale-dependent response of diversity to disturbance. Thus, following moderate habitat disturbance, diversity is reported to increase when measured at small spatial scales but declines when measured at large spatial scales. Our findings suggest that even a very small change in the SAR slope following disturbance corresponds with a scale-dependent response of diversity to disturbance. 3. We analyse new empirical data for tropical butterflies at a range of spatial scales(approximate to 3-80 ha). Our results support our theoretical findings and the notion of scale-dependence in estimates of diversity. We show that this scale-dependence occurs because alpha and beta diversity increase with spatial scale at a significantly faster rate in undisturbed forest compared with disturbed forest. This is due to reduced habitat heterogeneity and reduced spatial autocorrelation of butterfly diversity data following disturbance. 4. Synthesis and applications. There is little consensus in the reported responses of species diversity to moderate tropical habitat disturbance, and the spatial scale at which studies are carried out largely pre-determines the findings. Here we demonstrate, both theoretically and empirically, the mechanisms that produce a scale-dependent response of diversity to habitat disturbance. There is little agreement among researchers about the best methods for sampling tropical species in the field, and our findings highlight the problems of using diversity changes that do not account for the spatial scale of sampling. We conclude that in the future, studies should assess spatial patterns in diversity over a range of spatial scales and should not evaluate changes in diversity at a single spatial scale.