A study examines declines in global terrestrial vertebrate species. Accounts of human-caused biodiversity loss often highlight species extinctions but neglect the extinction of populations within species.
To examine the magnitude of vertebrate population reduction, Gerardo Ceballos and colleagues mapped the ranges of 27,600 birds, amphibians, mammals, and reptiles worldwide at a 10,000 km2 scale. The sample represented nearly half of known terrestrial vertebrate species, and included the 8,851 species that are decreasing in population size and range, according to the International Union for Conservation of Nature (IUCN) as of March 2016.
The authors found that the tropics had the greatest number of decreasing species, whereas temperate regions had similar or higher proportions of decreasing species than tropical regions. Furthermore, the authors found that approximately 30% of all decreasing species are considered common species and of low concern by the IUCN.
Geographic analyses of 177 mammal species found that between approximately 1900 and 2015 all species had lost at least 30% of their geographic ranges and that more than 40% of the species had experienced more than 80% range reduction. According to the authors, the Earth is not only experiencing accelerated human-driven species extinctions but also population declines and extirpations.