Linking global turnover of species and environments

Abstract

Patterns of species turnover are central to the geography of biodiversity and resulting challenges for conservation, but at broad scales remain relatively little understood. Here, we take a first spatially-explicitly and global perspective to link the spatial turnover of species and environments. We compare how major groups of vertebrate ectotherms (amphibians) and endotherms (birds) respond to spatial environmental gradients. We find that high levels of species turnover occur regardless of environmental turnover rates, but environmental turnover provides a lower bound for species turnover. This lower bound increases more steeply with environmental turnover in tropical realms. While bird and amphibian turnover rates are correlated, the rate of amphibian turnover is four times steeper than bird rates. This is the same factor by which average geographic ranges of birds are larger than those of amphibians. Narrow-ranged birds exhibit rapid rates of species turnover similar to those for amphibians, while wide-ranged birds largely drive the aggregate patterns of avian turnover. We confirm a strong influence of the environment on species turnover that is mediated by range sizes and regional history. In contrast to geographic patterns of species richness, we find that the turnover in one group (amphibians) is a much better predictor for the turnover in another (birds) than is environment. This result confirms the role of amphibian sensitivity to environmental conditions for patterns of turnover and supports their value as a surrogate group. This spatially-explicit analysis of environmental turnover provides understanding for conservation planning in changing environments.

Fig. 1.

Turnover of environment and species with spatial distance (km) for an example location in central Africa (depicted in Fig. 2). Environmental distance is the absolute difference in the environment principal component between locations (1,000 km slope ± 95% CI = 1.6 × 10⁻³ ± 1.7 × 10⁻⁴, F_[1,87] = 320, P < 1.0 × 10⁻¹⁵, r² = 0.78). Species turnover for birds and amphibians is measured as the natural log of Jaccard Similarity in species composition between sites. Distance decay in similarity occurs more rapidly for amphibians (1,000 km slope ± 95% CI = −2.80 × 10⁻³ ± 2.9 × 10⁻⁴, F_[1,102] = 360.7, P < 1.0 × 10⁻¹⁵, r² = 0.78) than for birds (1,000 km slope ± 95% CI = −1.44 × 10⁻³ ± 1.0 × 10⁻⁴, F_[1,92] = 764.4, P < 1.0 × 10⁻¹⁵, r² = 0.89), and the slope of the decay varies with the spatial distances examined (500 km, 1,000 km, and 2,000 km). These slopes formed the basis for the maps of Fig. 2.

Fig. 2.

Spatial patterns of rates of environmental turnover (A) correlate to those of rates of species turnover for birds (B) and amphibians (C). The maps depict slopes (20 quantiles, red: steeper slope) of the relationships between environmental distance or species similarity (ln Jaccard similarity) and spatial distance (km). The example location in central Africa (Fig. 1) is depicted along with 500-km, 1,000-km, and 2,000-km radius circles indicating the sampling distances.

Fig. 3.

The relationships between environmental and avian or amphibian turnover and between amphibian and avian turnover (depicting a 1:1 relationship) across locations worldwide and for three distance windows. Quantile regressions (10%) are depicted for the relationships with environmental turnover, while a linear least-squares regression is depicted for the relationship between avian and amphibian turnover (slopes in Table 1).

Fig. 4.

Turnover patterns vary for birds divided into four range size quartiles with the most narrowly distributed birds exhibiting the fastest rates of turnover (Q1). Data are divided into the 20 quantiles mapped in Fig. 2, with red indicating faster turnover for the focal distance of 1,000 km. White areas are those with an insufficient number of bird species for analysis.

Fig. 5.

The increase in avian and amphibian turnover with increasing environmental turnover is steeper in tropical realms than in temperate realms. The 10% quantiles of species turnover depict the lower bounds on this relationship for a focal distance of 1000 km.