Unravelling the structure of species extinction risk for predictive conservation science

Abstract

Extinction risk varies across species and space owing to the combined and interactive effects of ecology/life history and geography. For predictive conservation science to be effective, large datasets and integrative models that quantify the relative importance of potential factors and separate rapidly changing from relatively static threat drivers are urgently required. Here, we integrate and map in space the relative and joint effects of key correlates of The International Union for Conservation of Nature-assessed extinction risk for 8700 living birds. Extinction risk varies significantly with species' broad-scale environmental niche, geographical range size, and life-history and ecological traits such as body size, developmental mode, primary diet and foraging height. Even at this broad scale, simple quantifications of past human encroachment across species' ranges emerge as key in predicting extinction risk, supporting the use of land-cover change projections for estimating future threat in an integrative setting. A final joint model explains much of the interspecific variation in extinction risk and provides a remarkably strong prediction of its observed global geography. Our approach unravels the species-level structure underlying geographical gradients in extinction risk and offers a means of disentangling static from changing components of current and future threat. This reconciliation of intrinsic and extrinsic, and of past and future extinction risk factors may offer a critical step towards a more continuous, forward-looking assessment of species' threat status based on geographically explicit environmental change projections, potentially advancing global predictive conservation science.

Figure 1.

Proposed and tested structural relationship between predictors of species extinction risk in birds (within-family, 8664 species) with rods representing postulated causal relationships and bubbles variables. Binary extinction risk is the main response, encroachment and range size are the secondary responses in the SEM. Encroachment measures the proportion of range size modified by humans. Life history, ecological niche (eco-niche), niche breadth and environmental niche (env-niche) are latent variables (blue) which each have several hypothesized correlates (black). Selected correlates influence bird extinction risk, population decline or sensitivity to habitat modification. Thickness of all non-grey rods increases with their relative importance. Oval labels give significant standardized partial regression coefficient. Faded bubbles and rods indicate originally hypothesized but insignificant (p > 0.05) indicators and relationships, which were excluded from the final SEM. See electronic supplementary material, table S1 for abbreviations and details.

Figure 2.

Key correlates of extinction risk as identified from the main, within-family SEM model. Cross-species results are illustrated. These violin plots combine box plot and density trace (smoothed histogram of data) and embedded boxes indicate first and third quartiles, white circle the median, and horizontal lines 10th and 90th percentiles. Life history and environmental niche are latent variables whose effects are mostly driven by four predictors (electronic supplementary material, figure S1): species occupying low mean AET and high mean seasonality regions (i.e. high environmental-niche scores) are more threatened, and larger and precocial birds (i.e. high life-history scores) are more extinction prone.

Figure 3.

Observed and predicted extinction risk across (a) 8664 species and (b) ca 13 000, 110 × 110 km assemblages worldwide. Predictions are based on the within-family mixed-effect model (GLMM). For details on (a), see figure 2. In (b), the dashed line indicates a 1 : 1 relationship. A different linear regression fit is illustrated for quartiles of assemblage richness. Prediction success increases with increasing richness: 10–90 species: b = 0.17, n = 3276; 91–150 species: b = 0.54, n = 3403; 151–240 species: b = 0.71, n = 3169 and 241–905 species: b = 0.83, n = 3308. Assemblages with less than 10 species are excluded. Richness per assemblage: dark blue solid line, 10–90; light blue solid line, 91–150; yellow solid line, 151–240; red solid line, 241–905.

Figure 4.

The geography of extinction risk based on global occurrence of species across ca 13 000 110 × 110 km equal-area assemblages. Geographical patterns of (a) observed, (b) predicted percentage of threatened species and (c) observed–predicted in units standard deviation per assemblage. The predictions in (b) are from the species-based extinction risk probabilities of the within-family GLMM. In (c), areas in purple and green shades indicate under-prediction and over-prediction of extinction risk, respectively. Assemblages with less than 10 species are excluded.