Determining impacts of habitat modification on diversity of tropical forest fauna: the importance of spatial scale

1. There is an urgent need to understand the impacts of anthropogenic habitat disturbance on biodiversity in tropical forests, but no consensus has yet emerged. We reviewed the literature for the most frequently studied taxon (birds, 37 studies) and found that increased and decreased diversity in response to disturbance (selective logging and shifting agriculture) were reported with approximately equal frequency. 2. The spatial scale at which studies were carried out significantly affected the reported response to disturbance: studies where disturbed and undisturbed habitats were sampled at large spatial scales were more likely to report increased diversity following disturbance, whereas studies that sampled habitats at small spatial scales were more likely to report decreased diversity. These results were not a consequence of sampling method: we divided the studies into those using capture methods and those using observation methods and the same result was obtained when the analysis was restricted to only those studies using observation methods. 3. Previously, we have shown that reported impacts of disturbance on Lepidoptera are also affected by the spatial scale of study. We reviewed the Lepidoptera literature published since then and showed that all 12 new studies conformed to the predicted pattern. 4. While sampling scale significantly affected the reported responses of both birds and Lepidoptera, there were opposite effects of scale in the two taxa: large-scale bird studies and small-scale Lepidoptera studies were more likely to report increased diversity following disturbance. Bird studies were generally carried out at larger spatial scales than those of Lepidoptera and these opposite impacts of scale were probably due to a non-linear effect of habitat disturbance on habitat heterogeneity at different spatial scales. 5. Synthesis and applications. The rapid loss and degradation of tropical forests means that an understanding of the general patterns of responses of species to habitat disturbance is urgently needed. However, there has been little discussion of the most appropriate methods to ensure comparability of results between studies. Data presented here indicate that the spatial scale of sampling chosen in studies has a marked effect on the results obtained, and future studies need to account for this by examining explicitly the effects of disturbance at different spatial scales. The effect of spatial scale differs between taxa, and this may explain why the search for indicator taxa of disturbance effects has so far proved elusive.