Variation in multiple dimensions of biodiversity along a tropical elevation gradient: Inter-taxon comparisons

Background/Question/Methods

Deepening our understanding of biological diversity requires consideration of taxonomic, functional, and phylogenetic dimensions to characterize the evolutionary and ecological variation represented by groups of species.  Nonetheless, simultaneous considerations of multiple dimensions of biodiversity for multiple taxa are lacking with respect to the same environmental gradient.  To address this knowledge gap, we assessed variation in three dimensions of biodiversity along an extensive elevational gradient (500 m to 3500 m) for bats, rodents, and passerines in a tropical biodiversity hotspot in the Peruvian Andes (Manu).  Taxonomic biodiversity was evaluated as specific, generic, and familial richness; phylogenetic and trait-based functional biodiversity were each estimated using Rao’s quadratic entropy.  For each taxon, we fit linear and quadratic models to quantify the relationship between each dimension and elevation, and compared the form and parameterization of relationships between taxa. To reduce collinearity between linear and quadratic terms the independent variable was centered.  Additionally, we used a Mitchell-Olds Shaw test to determine if a relationship with a significant quadratic component represented modal or saturating responses.  Finally, we used a simulation approach to compare empirical functional biodiversity at each elevation to the mean of those based on communities that were randomly assembled from regional species pools.

Results/Conclusions

Patterns of taxonomic or functional biodiversity did not differ among taxa, whereas, patterns of phylogenetic biodiversity were taxon specific. Species richness, generic richness, and familial richness decreased with increasing elevation for each taxon, and generally did so in a saturating fashion regardless of level in the taxonomic hierarchy.  Nonetheless, the rate of decrease with elevation for specific or generic richness was taxon specific, with rates of taxon loss being greatest for passerines and least for rodents.  In contrast, rates of elevational decrease in familial richness were not significantly different among taxa.  There was no relationship (linear or quadratic) between functional biodiversity and elevation for any taxon.  Moreover, empirical functional biodiversity at a particular elevation was rarely different from that calculated based on randomly generated communities.  Relationships between phylogenetic biodiversity and elevation were complex and taxon specific.  Phylogenetic biodiversity decreased with increasing elevation for bats and passerines, but not for rodents.  The lack of variation in functional diversity along the gradient, combined with taxonomic results, suggests that functional redundancy decreases with elevation.  Concordant decreases in taxonomic and phylogenetic biodiversity for bats and passerines suggest niche conservatism in these taxa.