Amphibians, such as this emerald glass frog in Panama, are seeing high levels of population declines, according to a new study.
CNN  — 

The global loss of wildlife is “significantly more alarming” than previously thought, according to a new study that found almost half the planet’s species are experiencing rapid population declines.

Humans have already wiped out huge numbers of species and pushed many more to the brink – with some scientists saying we are entering a “sixth mass extinction” event, this time driven by humans.

The main factor is the destruction of wild landscapes to make way for farms, towns, cities and roads, but climate change is also an important driver of species decline and is predicted to have an increasingly worse impact as the world warms.

The study’s authors analyzed more than 70,000 species across the globe – spanning mammals, birds, reptiles, amphibians, fish and insects – to determine whether their populations have been growing, shrinking or remaining steady over time.

- Source: CNN " data-fave-thumbnails="{"big": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/210524110318-video-thumbnail-kenya-1.jpg?q=x_0,y_0,h_1080,w_1919,c_fill/h_540,w_960" }, "small": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/210524110318-video-thumbnail-kenya-1.jpg?q=x_0,y_0,h_1080,w_1919,c_fill/h_540,w_960" } }" data-vr-video="false" data-show-html="" data-byline-html="
" data-timestamp-html="" data-check-event-based-preview="" data-is-vertical-video-embed="false" data-network-id="" data-publish-date="2021-05-24T11:27:55Z" data-video-section="travel" data-canonical-url="https://www.cnn.com/videos/travel/2021/05/24/kenya-wildlife-census-larry-madowo-pkg-intl-ldn-vpx.cnn" data-branding-key="call-to-earth" data-video-slug="kenya-wildlife-census-larry-madowo-pkg-intl-ldn-vpx" data-first-publish-slug="kenya-wildlife-census-larry-madowo-pkg-intl-ldn-vpx" data-video-tags="africa,animal poaching and trafficking,animals,animals and society,business and industry sectors,business, economy and trade,censuses,climate change,continents and regions,coronavirus,destinations and attractions,diseases and disorders,eastern africa,energy and environment,energy and utilities,environment and natural resources,government and public administration,government bodies and offices,government departments and authorities,health and medical,infectious diseases,kenya,life forms,microscopic life,national parks and monuments,parks (green spaces),points of interest,population and demographics,society,tourism,travel and tourism,viruses,wildlife" data-details="">
CNN's Larry Madowo reports from Amboseli National Park in Kenya.
See how this national park in Africa is counting wildlife
03:05 - Source: CNN

They found 48% of these species are declining in population size, with fewer than 3% seeing increases, according to the study published Monday in the journal Biological Reviews.

Co-author Daniel Pincheira-Donoso, from the School of Biological Sciences at Queen’s University Belfast, said their findings are a “drastic alert.”

“Other studies, based on considerably smaller numbers of species, have shown that the ongoing ‘extinction crisis’ is more severe than generally appreciated,” he told CNN. “Our findings provide a stark confirmation on a global scale.”

The study provides a “clearer picture” about the extent of the global erosion of biodiversity, he added.

- Source: CNN " data-fave-thumbnails="{"big": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/230330154404-whal-thumbnail.jpg?c=16x9&q=h_540,w_960,c_fill" }, "small": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/230330154404-whal-thumbnail.jpg?c=16x9&q=h_540,w_960,c_fill" } }" data-vr-video="false" data-show-html="" data-byline-html="
Mark Tutton
" data-timestamp-html="" data-check-event-based-preview="" data-is-vertical-video-embed="false" data-network-id="" data-publish-date="2023-03-30T15:48:42Z" data-video-section="tv" data-canonical-url="https://www.cnn.com/videos/tv/2023/03/30/michael-fishbach-whales-c2e-spc-intl.cnn" data-branding-key="call-to-earth" data-video-slug="Michael Fishbach whales c2e spc intl" data-first-publish-slug="Michael Fishbach whales c2e spc intl" data-video-tags="" data-details="">
whal thumbnail
Why these whale conservationists are investigating the 'poop loop'
02:51 - Source: CNN

For decades, the extinction crisis has been defined by “conservation categories” – labels that the International Union for Conservation of Nature assigns to each species they assess at a given moment in time, Pincheira-Donoso said.

Based on that method, the IUCN’s Red List of Threatened Species classifies about 28% of species as under threat of extinction.

“What our study shows is not whether species are currently classed as threatened or not, but instead, whether their population sizes are becoming rapidly and progressively smaller or not,” Pincheira-Donoso said. Downward trends in population over time are a precursor to extinctions.

According to this assessment, 33% of the species currently classed as “non-threatened” on the IUCN Red List are in fact declining towards extinction.

- Source: CNN " data-fave-thumbnails="{"big": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/230522112000-monarch-butterfly-file-072122.jpg?c=16x9&q=h_540,w_960,c_fill" }, "small": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/230522112000-monarch-butterfly-file-072122.jpg?c=16x9&q=h_540,w_960,c_fill" } }" data-vr-video="" data-show-html="" data-byline-html="
" data-timestamp-html="" data-check-event-based-preview="" data-is-vertical-video-embed="" data-network-id="" data-publish-date="2020-10-02T01:30:23Z" data-video-section="world" data-canonical-url="https://www.cnn.com/videos/world/2020/09/27/knepp-farm-rewilding-spc-intl.cnn" data-branding-key="call-to-earth" data-video-slug="knepp-farm-rewilding-spc-intl" data-first-publish-slug="knepp-farm-rewilding-spc-intl" data-video-tags="biodiversity,continents and regions,england,environment and natural resources,europe,northern europe,united kingdom,agriculture,agriculture, forestry, and commercial fishing,animal farming and livestock,business and industry sectors,business, economy and trade" data-details="">
A Monarch butterfly, which is now placed in the endangered category of the International Union for the Conservation of Nature's Red List of Threatened Species, perches at the Royal Botanical Gardens in Burlington, Ontario, Canada July 21, 2022.
The British farm where the animals rule
03:33 - Source: CNN

Mammals, birds and insects are all seeing species declines, but amphibians have been particularly badly affected overall, the report found, and are facing a multitude of threats, including disease and climate change.

It was better news for fish and reptiles, with more species appearing to have stable, rather than declining, populations.

Geographically, declines tend to be concentrated in the tropics, the report found. One reason for that is that “animals in the tropics are more sensitive to rapid changes in their environmental temperatures,” Pincheira-Donoso said.

“While we agree with the sentiments and concerns about the declines in species that this paper expresses (as it is fundamentally an analysis of IUCN Red List data), we think that the results over-inflate the situation,” said Craig Hilton-Taylor, head of the IUCN Red List.

Using population data across a wide range of animal groups, including those where data is lacking, is a less robust measure than the IUCN’s Red List criteria “which look at the trends of species over much longer time frames,” he told CNN.

But Brendan Godley, a professor of conservation science at the University of Exeter who was not involved in the study, said the research offers novel insights into population trends.

“This is an extremely impactful study, spanning the globe and all vertebrate groups and insects,” Godley told CNN.

- Source: CNN " data-fave-thumbnails="{"big": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/230519172423-cte-caterpillar-video-card.jpg?c=16x9&q=h_540,w_960,c_fill" }, "small": { "uri": "https://media.cnn.com/api/v1/images/stellar/prod/230519172423-cte-caterpillar-video-card.jpg?c=16x9&q=h_540,w_960,c_fill" } }" data-vr-video="false" data-show-html="" data-byline-html="
Mark Tutton
" data-timestamp-html="" data-check-event-based-preview="" data-is-vertical-video-embed="false" data-network-id="" data-publish-date="2023-05-19T16:30:47Z" data-video-section="tv" data-canonical-url="https://www.cnn.com/videos/tv/2023/05/19/call-to-earth-insect-conservation-mexico-spc.cnn" data-branding-key="call-to-earth" data-video-slug="call to earth insect conservation mexico spc" data-first-publish-slug="call to earth insect conservation mexico spc" data-video-tags="animals,continents and regions,environment and natural resources,latin america,life forms,mexico,mexico city,north america,the americas,wildlife" data-details="">
CTE caterpillar video card
Why Mexico City needs more bugs
03:46 - Source: CNN

“By painstakingly combining population trajectories, rather than more limited Red List Assessments, it underlines how much pressure wildlife is under from human influence, and how this is global and across the animal groups,” he said.

There are positive stories of animals being brought back from the brink of extinction, he said, including great whales and sea turtles.

But, Godley added, “we should all be very alarmed about these results.”

“Without thriving populations, species, habitats and ecosystems, we cannot persist,” he said.