Species extinction has been underestimated so far

frog

According to the Red List, the Emerald Glass Frog (Espadarana prosoblepon) is not endangered, but its populations are also shrinking. © Roberto García-Roa

The global biodiversity is dwindling - and may have been underestimated so far, as a study now suggests. In it, biologists have studied the evolution of 71,000 vertebrate and insect species worldwide at the population level. Accordingly, there are significant population declines in 48 percent of all species examined - these species are therefore in decline. The losses of terrestrial species are particularly pronounced in tropical regions, but many populations are also shrinking in temperate latitudes. Amphibians, mammals and insects are particularly affected, and the team has determined that their numbers and diversity have each fallen by more than 50 percent.

Whether insects, birds or terrestrial vertebrates: the biodiversity of our planet is dwindling rapidly. According to estimates, around one million species worldwide could now be threatened with extinction. Responsible for this are the destruction of habitats by us humans, climate change, but also targeted hunting. In view of the massive loss of species, scientists are already talking about a sixth mass extinction - the largest since the disappearance of the dinosaurs. Most current conservation surveys are based on the International Union for Conservation of Nature (IUCN) Red Lists. In them, data on the population of various species are compiled and then classified into endangerment classes. According to these lists, around 28 percent of the world's species are currently threatened with extinction.

Population development of 71,000 animal species

But these lists don't capture the true extent of species and population declines, say Catherine Finn of Queen's University Belfast and her colleagues. “You can think of these protection categories as snapshots of a species' endangerment. In contrast, the development of the populations offers a more dynamic picture of the threat over time.” Because such a population development also includes species that are not endangered or officially classified as threatened, but which are rapidly developing towards such a status. "The background is that the process of extinction is initiated by demographic collapses within a species, which then leads to extinction over time," the team explains. "Population trends are therefore a good tool for predicting the future fate of a species." For their study, Finn and her colleagues evaluated the population trends of a total of 71,000 animal species and determined whether their populations were decreasing, increasing or remaining stable. The animal groups studied include all five major vertebrate groups – mammals, birds, reptiles, amphibians and fish – as well as insects.

The result: The scientists found a decline in the population of 48 percent of all animal species examined, the populations of 49 percent are stable and only three percent of the species show increasing population sizes. "This means that almost half of all animal species on Earth for which we have data are declining," says Finn. Given the unprecedented rate at which human activity is destroying and degrading habitats and environments around the world, it is not surprising that the number of populations that are shrinking or collapsing in response far outnumbers the animal species that are adapting fast enough able to adapt to changes. In addition, many animal species are already affected by the decline that were previously not considered threatened with extinction. For the animals currently listed as Least Concern by the IUCN, 33 percent showed a population decline. If this trend continues, at least 2,136 species previously considered to be of less concern could be threatened in the near future.

"On the Brink of an Extinction Crisis"

Amphibians have been hit hardest, with populations declining 63 percent, followed by mammals with 56 percent and birds with 53 percent, the biologists determined. In the case of insects, around 54 percent of the species are affected by population decline, if you only look at the terrestrial species, the figure is even 66 percent. However, according to the research team, there is a very high number of unreported cases in this group because the population development for almost three quarters of the species is not yet known. As already established in earlier surveys, the loss of species is highest in the tropical regions. However, there are also individual stable populations of mammals and birds in this climate zone, as Finn and her team determined. Similar refuges are found for amphibians on the American continent.

Overall, however, the result is alarming - and the situation is far more dramatic than the endangerment categories of the Red Lists reflect: "The rate of global defaunation is continuing at an ever faster rate, despite increasing calls for immediate measures to preserve biodiversity," state the Scientist. "Our findings reinforce the warning that biodiversity is on the cusp of an extinction crisis. This crisis will have significant ecological and ecosystemic consequences, as the population declines and the resulting changes in the communities affect the entire ecology.” In view of the clear signals of the current biodiversity crisis, the time for discussions about this development is over. Action must now be taken to preserve the future integrity of biodiversity and thus of humanity.

Source: Catherine Finn (Queen's University Belfast) et al., Biological Reviews, doi: 10.1111/brv.12974

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