Environmental and historical constraints on global patterns of amphibian richness

Abstract

Our knowledge of the broad-scale ecology of vertebrate ectotherms remains very limited. Despite ongoing declines and sensitivity to environmental change, amphibian distributions are particularly poorly understood. We present a global analysis of contemporary environmental and historical constraints on amphibian richness, the first for an ectotherm clade at this scale. Amphibians are presumed to experience environmental constraints distinct from those of better studied endothermic taxa due to their stringent water requirements and the temperature dependence of their energetic costs and performance. Single environmental predictors set upper bounds on, but do not exclusively determine, amphibian richness. Accounting for differing regional histories of speciation and extinction helps resolve triangular or scattered relationships between core environmental predictors and amphibian richness, as the relationships' intercepts or slopes can vary regionally. While the magnitude of richness is strongly determined by regional history, within-region patterns are consistently jointly driven by water and temperature. This confirms that ecophysiological constraints extend to the broad scale. This coupling suggests that shifts in climatic regimes will probably have dramatic consequences for amphibians. Our results illustrate how the environmental and historical explanations of species richness gradients can be reconciled and how the perspectives are complements for understanding broad-scale patterns of diversity.

Figure 1

Global amphibian species richness. Richness was compiled within equal area quadrats equivalent to 0.5° in size by overlaying species' distribution maps (Global Amphibian Assessment).

Figure 2

Amphibian species richness is constrained by multiple environmental variables. Bivariate plots of environmental effects on richness across 40 315 equal area quadrats equivalent to 0.5° size covering the world except islands showing 10, 50 and 90% quantile regressions. Quantile regressions (90% quantile mean(95% CI)) suggest the constraints imposed by each variable: (a) mean net primary productivity for the lowest three months (NPP_min) (log(SR)=−0.78(0.02)+1.12(0.09) log(NPP_min), F_[1,40312]=60 906, p<1×10⁻¹⁵); (b) annual actual evapotranspiration (AET) (log(SR)=−0.94(0.05)+0.88(0.02)log(AET), F_[1,40312]=9211, p<1×10⁻¹⁵); (c) annual precipitation (log(SR)=−0.59(0.04)+0.73(0.01)log(pre), F_[1,40312]=12 646, p<1x10⁻¹⁵) and (d) mean annual temperature (log(SR)=−55.5(0.47)+23.18(0.29)log(temp), F_[1,40312]=54 815, p<1×10⁻¹⁵). Darker regions indicate a higher density of observations.

Figure 3

Regional species pools influence environmental constraints on amphibian richness. The relationships between three-month mean minimum net primary productivity (NPP_min) and richness is relatively consistent between the six biogeographic realms (mean(95% CI)): Nearctic (log(SR)=−0.84(0.01)+0.92(0.01)log(NPP_min), F_[1,6309]=12 700, p<1×10⁻¹⁵, r²=0.67); Palearctic (log(SR)=−0.80(0.01)+0.84(0.00)log(NPP_min), F_[1,16523]=35 470, p<1×10⁻¹⁵, r²=0.68); Indomalay (log(SR)=−0.25(0.03)+0.73(0.01)log(NPP_min), F_[1,2200]=3404, p<1×10⁻¹⁵, r²=0.61); Neotropics (log(SR)=−1.84(0.02)+1.51(0.01)log(NPP_min), F_[1,6115]=18 620, p<1×10⁻¹⁵, r²=0.75); Afrotropics (log(SR)=−0.55(0.01)+0.91(0.01)log(NPP_min), F_[1,6715]=16 640, p<1×10⁻¹⁵, r²=0.71); and Australasia (log(SR)=−1.03(0.04)+1.10(0.02)log(NPP_min), F_[1,2240]=2107, p<1×10⁻¹⁵, r²=0.46). Darker regions indicate a higher density of observations.