Understanding global patterns of mammalian functional and phylogenetic diversity

Abstract

Documenting and exploring the patterns of diversity of life on Earth has always been a central theme in biology. Species richness despite being the most commonly used measure of diversity in macroecological studies suffers from not considering the evolutionary and ecological differences among species. Phylogenetic diversity (PD) and functional diversity (FD) have been proposed as alternative measures to overcome this limitation. Although species richness, PD and FD are closely related, their relationships have never been investigated on a global scale. Comparing PD and FD with species richness corroborated the general assumptions of surrogacy of the different diversity measures. However, the analysis of the residual variance suggested that the mismatches between the diversity measures are influenced by environmental conditions. PD increased relative to species richness with increasing mean annual temperature, whereas FD decreased with decreasing seasonality relative to PD. We also show that the tropical areas are characterized by a FD deficit, a phenomenon, that suggests that in tropical areas more species can be packed into the ecological space. We discuss potential mechanisms that could have resulted in the gradient of spatial mismatch observed in the different biodiversity measures and draw parallels to local scale studies. We conclude that the use of multiple diversity measures on a global scale can help to elucidate the relative importance of historical and ecological processes shaping the present gradients in mammalian diversity.

Figure 1.

How phylogenetic diversity (PD) and functional diversity (FD) together could inform about spatial and biophylogeographic patterns. The relationship between PD and FD is, although yet unknown, drawn as a nonlinear relationship, since studies of trait evolution suggest that for many life-history traits the tempo of trait evolution slows down after an initial boost in divergence. Therefore, we assumed that the differences in functional divergence will not follow phylogenetic divergence linearly. Overdispersion of FD characterizes communities in which species are distributed farther apart in functional niche space than expected by their PD. FD deficit in contrast describes the situation where species in a community share more functional traits than expected. And finally, evolutionary constraint describes the assumed general condition for communities where the FD of communities follows the phylogenetic divergence of the species composing the community.

Figure 2.

Relationship between (a) PD and richness and (b) FD and richness as well as (c) the correlation between PD and FD colour coded for different continents. Generalized additive models were used (with latitude and longitude of each grid cell used as smooth factors) to account for biases in model estimates due to spatial autocorrelation. Smooth term: estimated degrees of freedom in (a) = 28.17, F = 171.7, p < 0.0001; in (b) = 28.72, F = 278.2, p < 0.0001; in (c) = 28.8, F = 344.3, p < 0.0001.)

Figure 3.

Deviance (residuals) of observed FD compared to the global linear relationship between PD and FD (FD = 0.003·PD; see figure 2c). Blue areas depict areas of lower than expected FD (according to the PD present in the area) whereas red areas are areas with more FD than expected from this linear relationship.