Extinction risk is most acute for the world’s largest and smallest vertebrates

Abstract

Extinction risk in vertebrates has been linked to large body size, but this putative relationship has only been explored for select taxa, with variable results. Using a newly assembled and taxonomically expansive database, we analyzed the relationships between extinction risk and body mass (27,647 species) and between extinction risk and range size (21,294 species) for vertebrates across six main classes. We found that the probability of being threatened was positively and significantly related to body mass for birds, cartilaginous fishes, and mammals. Bimodal relationships were evident for amphibians, reptiles, and bony fishes. Most importantly, a bimodal relationship was found across all vertebrates such that extinction risk changes around a body mass breakpoint of 0.035 kg, indicating that the lightest and heaviest vertebrates have elevated extinction risk. We also found range size to be an important predictor of the probability of being threatened, with strong negative relationships across nearly all taxa. A review of the drivers of extinction risk revealed that the heaviest vertebrates are most threatened by direct killing by humans. By contrast, the lightest vertebrates are most threatened by habitat loss and modification stemming especially from pollution, agricultural cropping, and logging. Our results offer insight into halting the ongoing wave of vertebrate extinctions by revealing the vulnerability of large and small taxa, and identifying size-specific threats. Moreover, they indicate that, without intervention, anthropogenic activities will soon precipitate a double truncation of the size distribution of the world's vertebrates, fundamentally reordering the structure of life on our planet.

Multi‐dimensional biodiversity hotspots and the future of taxonomic, ecological and phylogenetic diversity: A case study of North American rodents

“…Although certain small rodent clades are at risk of being lost entirely to extinction, imperilled species are distributed throughout the rodent phylogeny ( Figure 6b). These findings provide further evidence that not all species are at equal risk of extinction (Ohlemüller et al, 2008;Ripple et al, 2017). Although most ecological categories experience some degree of loss, the greatest impacts are felt by medium-sized rodents, rodents at higher trophic levels and rodents occupying higher elevation habitats in the tropics (Figure 6d).…”

Rapid warming is associated with population decline among terrestrial birds and mammals globally

“…Additionally, to account for the effect of species traits we explore the relationship between population growth rates and body mass, which is a correlate of many species traits (Brook et al., ; Hilbers et al., ). Recent research has shown there is a significant relationship between vertebrate body mass and extinction risk, such that heavier species of birds and mammals are likely to be more at risk of extinction (Ripple et al., ). We therefore hypothesized that larger bodied birds and mammals are more likely to have declining populations.…”

Reply to Kalinkat et al.: Smallest terrestrial vertebrates are highly imperiled

“…(1) discuss the biodiversity crisis in their reply to our article "Extinction risk is most acute for the world's largest and smallest vertebrates" (2). We agree with Kalinkat et al…”

“…However, small vertebrates that use terrestrial habitats exclusively or in combination with freshwater habitats also have a greatly elevated risk of extinction (figure S3 in ref. 2). Indeed, our data show that 59% (76 of 128) of terrestrial vertebrate species with body masses ≤ 0.001 kg are threatened compared with 16% (1,965 of 12,015) for all terrestrial vertebrate species.…”

“…(1) that small freshwater species tend to have elevated extinction risk, an issue that we highlight in our paper (2). Specifically, 41% (36 of 87) of freshwater vertebrate species with body masses ≤ 0.001 kg are listed as threatened compared with 29% (1,578 of 5,428) threatened for all freshwater vertebrate species.…”