Vertebrates on the brink as indicators of biological annihilation and the sixth mass extinction

Abstract

The ongoing sixth mass species extinction is the result of the destruction of component populations leading to eventual extirpation of entire species. Populations and species extinctions have severe implications for society through the degradation of ecosystem services. Here we assess the extinction crisis from a different perspective. We examine 29,400 species of terrestrial vertebrates, and determine which are on the brink of extinction because they have fewer than 1,000 individuals. There are 515 species on the brink (1.7% of the evaluated vertebrates). Around 94% of the populations of 77 mammal and bird species on the brink have been lost in the last century. Assuming all species on the brink have similar trends, more than 237,000 populations of those species have vanished since 1900. We conclude the human-caused sixth mass extinction is likely accelerating for several reasons. First, many of the species that have been driven to the brink will likely become extinct soon. Second, the distribution of those species highly coincides with hundreds of other endangered species, surviving in regions with high human impacts, suggesting ongoing regional biodiversity collapses. Third, close ecological interactions of species on the brink tend to move other species toward annihilation when they disappear-extinction breeds extinctions. Finally, human pressures on the biosphere are growing rapidly, and a recent example is the current coronavirus disease 2019 (Covid-19) pandemic, linked to wildlife trade. Our results reemphasize the extreme urgency of taking much-expanded worldwide actions to save wild species and humanity's crucial life-support systems from this existential threat.

Keywords: conservation; ecosystem services; endangered species; population extinctions; sixth mass extinction.

Fig. 1.

Terrestrial vertebrates on the brink (i.e., with 1,000 or fewer individuals) include species such as (A) Sumatran rhino (Dicerorhinus sumatrensis; image credit: Rhett A. Butler [photographer]), (B) Clarion island wren (Troglodytes tanneri; image credit: Claudio Contreras Koob [photographer]), (C) Española Giant Tortoise (Chelonoidis hoodensis; image credit: G.C.), and (D) Harlequin frog (Atelopus varius; the population size of the species is unknown but it is estimated at less than 1,000; image credit: G.C.).

Fig. 2.

Population size of terrestrial vertebrate species on the brink (i.e., with under 1,000 individuals). Most of these species are especially close to extinction because they consist of fewer than 250 individuals. In most cases, those few individuals are scattered through several small populations.

Fig. 3.

Geographic distribution terrestrial vertebrate species on the brink (i.e., with under 1,000 individuals). The colors in the left bar indicate the number of species in a 100 square km global cell grid.

Fig. 4.

Historic (Top) and current (Bottom) geographic range of 48 mammal and 29 bird terrestrial species on the brink (i.e., with under 1,000 individuals). Note the high concentration of species in tropical regions throughout the world.

Fig. 5.

Current distribution of 515 terrestrial vertebrate species on the brink (i.e., with under 1,000 individuals; Top) and 903 species with under 5,000 individuals (Bottom). Of the 388 species under 5,000 that have populations larger than 1,000 individuals, 84% have overlapping distributions with the species at the brink (i.e., with under 1,000 individuals), indicating high distribution congruence.