Multiple ecological pathways to extinction in mammals

Abstract

As human population and resource demands continue to grow, biodiversity conservation has never been more critical. About one-quarter of all mammals are in danger of extinction, and more than half of all mammal populations are in decline. A major priority for conservation science is to understand the ecological traits that predict extinction risk and the interactions among those predictors that make certain species more vulnerable than others. Here, using a new database of nearly 4,500 mammal species, we use decision-tree models to quantify the multiple interacting factors associated with extinction risk. We show that the correlates of extinction risk vary widely across mammals and that there are unique pathways to extinction for species with different lifestyles and combinations of traits. We find that risk is relative and that all kinds of mammals, across all body sizes, can be at risk depending on their specific ecologies. Our results increase the understanding of extinction processes, generate simple rules of thumb that identify species at greatest risk, and highlight the potential of decision-tree analyses to inform conservation efforts.

Fig. 1.

Relative importance, in rank order, of ecological predictors of mammalian extinction risk. Importance was measured by the drop in classification accuracy after predictor removal in a random forest of 500 trees. Differences in importance between predictors were quantified with pair-wise, 2-tailed z-tests, α = 0.05. The top 8 predictors (geographic range–activity period) were statistically indistinguishable, except that activity period was significantly less than geographic range; all 8 were significantly more important than landmass, trophic group, and sociality, and all other variables were significantly more important than sociality.

Fig. 2.

Decision tree showing extinction risk based on ecological traits (body mass, geographic range size, mass-specific production rate, population density, group size, home range, activity period, type of landmass, habitat mode, sociality, trophic group). Branches in the smaller, optimal tree (see Fig. S1) are shown in bold, but all depicted branches are statistically meaningful (χ² test, P ≤ 0.001). The probability of being threatened is indicated at each node; species with higher risk are at the right of each branch point, and those with lower risk are at the left. Labeled nodes (A–C) are referenced in main text.

Fig. 3.

Bivariate plots of the top 5 continuous ecological predictors as a function of body mass. Within each plot (A–E), the 4 categories of extinction risk cluster consistently. Note that home range (B), population density (C), and mass-specific production rate (E) correlate strongly with body size. The upper cloud of points in E represents placental mammals; the lower cluster represents marsupials.

Fig. 4.

(A) Body size and extinction risk. Proportion of species predicted to be threatened in each body size class (0.25 log g). (B) Mammalian body mass distribution showing the 4 risk categories based on the random forest model.