Abstract
Insect macroecology and conservation biogeography studies are disproportionately scarce, especially in the Neotropics. Dung beetles are an ideal focal taxon for biodiversity research and conservation. Using distribution and body size data on the ecologically important Phanaeini, the best-known Neotropical dung beetle tribe, we determined elevational patterns of species richness, endemism, body size, and elevational range in Bolivia, specifically testing Bergmann’s and Rapoport’s rule. Richness of all 39 species and of 15 ecoregional endemics showed a hump-shaped pattern peaking at 400 m, but overall declined strongly with elevation up to 4000 m. The relationship between endemic and total species richness appeared to be curvilinear, providing only partial support for the null hypothesis that species-rich areas are more likely to be centers of endemism by chance alone. An elevational increase in the proportion of ecoregional endemics suggests that deterministic factors also appear to influence endemism in the Andes. When controlling for the effect of area using different species-area relationships, the statistically significant richness peak became more pronounced and shifted upslope to 750 m. Larger species did not have higher elevational mid-points, and mean body size decreased significantly with elevation, contradicting Bergmann’s rule. Rapoport’s rule was supported: species with higher elevational mid-points had broader elevational ranges, and mean elevational range increased significantly with elevation. The elevational decrease of phanaeine richness is in accordance with studies that demonstrated the combined influence of temperature and water availability on species diversity, but also is consistent with niche conservatism. For invertebrates, confirmation of Rapoport’s and refutation of Bergmann’s rule appear to be scale-invariant general patterns. Analyses of biogeographic patterns across elevational gradients can provide important insights for identifying conservation priorities. Phanaeines with narrow elevational ranges on isolated low-elevation mountains in eastern Bolivia are at greatest climate-change related extinction risk from range-shift gaps and mountaintop extinctions.
Highlights
After several decades of research elucidating patterns of species richness along elevational gradients (e.g., [1,2,3,4,5,6]), in recent years much emphasis has been placed on developing a comprehensive understanding of these patterns and their underlying causes [7,8,9,10,11,12,13,14,15]
Despite a minor peak at 250–499 m (Fig. 1), the proportion of ecoregional endemics showed an overall increase with elevation (OLS regression: R2 = 0.71, P,0.01), so above 1000 m, in remarkable contrast to endemic species richness (Fig. 1)
An examination of the residual plot of the regression of endemic against total species richness indicated that the distribution of residuals may be non-random, suggesting a curvilinear relationship, the low number of data points (N = 8 elevational zones) and the lack of total richness values between 10 and 23 species precluded a definite appraisal
Summary
After several decades of research elucidating patterns of species richness along elevational gradients (e.g., [1,2,3,4,5,6]), in recent years much emphasis has been placed on developing a comprehensive understanding of these patterns and their underlying causes [7,8,9,10,11,12,13,14,15]. Elevational patterns of other biogeographic parameters such as body size distributions (e.g., [17]) or endemism (e.g., [18,19]) have received considerably less and more recent attention. This is the case for invertebrates, which comprise the vast majority of known biodiversity on Earth. Meiri [24] defined Bergmann’s rule in a broad sense as ‘a tendency of organisms to be smaller at high temperatures and low latitudes and larger at low temperatures and high latitudes’ and argued that it is a pattern that can be studied regardless of mechanism in any taxon and at any taxonomic level, which we follow here. Bergmann’s rule was applied to elevational gradients by Hawkins and DeVries [25] and more explicitly by Brehm and Fiedler [17], predicting that animal body size increases with elevation
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