Island biogeography from regional to local scales: evidence for a spatially scaled echo pattern of fern diversity in the Southeast Asian archipelago

Aim The aim of this study was to investigate whether the equilibrium theory of island biogeography (ETIB) is equally applicable at regional and local spatial scales, and whether the 'echo pattern', the correlation between regional species pool and local diversity by which the diversity of the regional species pool 'trickles down' to local scales, influences local diversity equally across a range of sampling scales. Location Twelve mountain regions on islands having different sizes, degrees of isolation and environmental conditions in Indonesia and the Philippines. Methods We sampled ferns in standardized field plots within a fixed spatial design at six spatial scales (with the grain size varying from plot to island). Using ordinary least squares regression and relative variable importance, we tested for the predictive power of area and isolation as well as of local and regional environmental factors for explaining diversity at the chosen spatial scales. Results Consistent with MacArthur and Wilson's ETIB, we found the strongest correlations of area and isolation with diversity at regional scales. The explanatory power of area increased with increasing spatial scale. The strength of the negative relationships among diversity and isolation decreased with increasing spatial scale. We found evidence for an 'echo pattern' at large to intermediate but not at small spatial scales. Main conclusions Area and isolation are strong predictors of biodiversity at regional scales. The size of the species pool has a scale-dependent influence on diversity at smaller scales but is unimportant at the most local scale, where environmental conditions dominate as predictors of diversity. To make predictions about local diversity on islands it is therefore important to take the scale dependence of the 'echo pattern' into account.