Testing Rapoport's Rule for Species-Rich Genera of the Malay Archipelago
The Malay Archipelago is home to habitats with some of the highest plant species richness on Earth, but how these patterns vary long environmental gradients are still not well understood. Elevation gradients are considered to show patterns of diversity that are similar to latitudinal gradients, but the extent to which this is the case has not been systematically tested in Southeast Asia, nor have competing hypotheses to explain these patterns been tested. We examined the applicability of Rapoport's rule (the observation that species range size increases with latitude or elevation) for the top twenty most species-rich plant genera of this region and tested four hypotheses about species range size, relative to peaks in species richness, along an elevation gradient. First, we summarized the elevation minimum and maximum for more than 18,000 plants species using global herbarium records and grouped information into 100 m elevation bins. We then plotted species richness, compared to range size, along an elevation gradient (0-4000 m), analyzing the overall pattern for all species richness and elevation range size versus elevation was linear for all species, with richness declining, and range size increasing, with elevation. However these overall patterns did not hold for most genera, which varied substantially. Genera with peaks in species richness in lower elevation forest types (tropical rainforest) were generally consistent with Rapoport's rule, with trends that could be explained by either the 'niche packing' or the 'climate variability' hypotheses. Genera with peaks in richness within mid elevation forest types (cloud forest, lower montane forest) displayed a similar range size trend, but with stronger support for temperature variability as an underlying mechanism (with smaller range sizes associated with habitat with more constant temperature conditions). Genera with peaks in richness within high elevation forest types (elfin forest, upper montane forest) did not support Rapoport's rule and displayed little to no change in range size. Results for the hihg elevation forest types did not match any of our prior hyotheses, but may indicated unique ecological mechanisms shaping these communities. In conclusion, trends observed when grouping all species together differs from those on a per genera basis.