In the first such global evaluation, Stanford biologists found more than 30 percent of all vertebrates have declining populations. They call for curbs on the basic drivers of these losses.
No bells tolled when the last Catarina pupfish on Earth died. Newspapers didn’t carry the story when the Christmas Island pipistrelle vanished forever.
Two vertebrate species go extinct every year on average, but few people notice, perhaps because the rate seems relatively slow – not a clear and present threat to the natural systems we depend on. This view overlooks trends of extreme decline in animal populations, which tell a more dire story with cascading consequences, according to a new study that provides the first global evaluation of these population trends.
“This is the case of a biological annihilation occurring globally, even if the species these populations belong to are still present somewhere on Earth,” said co-author Rodolfo Dirzo, a professor of biology.
A 2015 study co-authored by Paul Ehrlich, professor emeritus of biology, and colleagues showed that Earth has entered an era of mass extinction unparalleled since the dinosaurs died out 66 million years ago. The spectre of extinction hangs over about 41 percent of all amphibian species and 26 percent of all mammals, according to the International Union for Conservation of Nature (IUCN), which maintains a list of threatened and extinct species – the IUCN Red List of Threatened Species™. This global disaster scene has the fingerprints of habitat loss, overexploitation, invasive organisms, pollution, toxification and climate change.
Stanford’s Paul Ehrlich warns that Earth has entered an era of mass extinction:
The new analysis, published online on 10 July in Proceedings of the National Academy of Sciences, looks beyond species extinctions to provide a clear picture of dwindling populations and ranges. The researchers mapped the ranges of 27,600 species of birds, amphibians, mammals and reptiles – a sample representing nearly half of known terrestrial vertebrate species – and analysed population losses in a sample of 177 well-studied mammal species between 1990 and 2015.
Using range reduction as a proxy for population loss, the study finds more than 30 percent of vertebrate species are declining in population size and range. Of the 177 mammals for which the researchers had detailed data, all have lost 30 percent or more of their geographic ranges and more than 40 percent have lost more than 80 percent of their ranges. Tropical regions have had the greatest number of decreasing species while temperate regions have seen similar or higher proportions of decreasing species. Particularly hard hit have been the mammals of south and southeast Asia, where all the large-bodied species of mammals analysed have lost more than 80 percent of their geographic ranges.
The study’s maps suggest that as much as 50 percent of the number of animal individuals that once shared Earth have disappeared, as have billions of animal populations. This amounts to “a massive erosion of the greatest biological diversity in the history of Earth,” the authors write.
Gerardo Ceballos, lead author, National Autonomous University of Mexico
Why does the loss of populations and biological diversity matter? Aside from being what the scientists call a prelude to species extinction, the losses rob us of crucial ecosystem services such as honeybees’ crop pollination, pest control and wetlands’ water purification. We also lose intricate ecological networks involving animals, plants and microorganisms – leading to less resilient ecosystems and pools of genetic information that may prove vital to species’ survival in a rapidly changing global environment.
“Sadly, our descendants will also have to do without the aesthetic pleasures and sources of imagination provided by our only known living counterparts in the universe,” said Ehrlich.
(Source: Stanford University press release, 10.07.2017; The Guardian, 11.07.2017)