Two centuries ago, extinctions were rare. Islands were hotspots, losing flightless bird species like the dodo and other animals that were hunted out of existence by European traders and colonists or killed off by introduced rats and cats.

The Industrial Revolution changed that: Extinction rates soared across continents, roughly paralleling the exponential growth in human population, and more recently, a rapidly heating planet. The modern era leveled forests, raised sprawling cities, converted huge amounts of wild land for agriculture, and pierced remote areas with roads. Massive growth kicked in post-World War II as 2.5 billion people in 1950 grew to eight billion in 2022. These trends are expected to continue this century, with wildlife increasingly pushed to the margins.

“The major cause behind this current crisis is habitat destruction,” says Daniel Pincheira-Donoso, senior lecturer of evolutionary biology and macroecology at Queen’s University, Belfast.

He’s a co-author of a new study published in May in Biological Reviews. Pincheira-Donoso and his colleagues examined the state of more than 71,000 animal species, looking for both winners and losers and those with stable populations. They found erosion of species across the tree of life, which they called “one of the most alarming consequences of human impacts on the planet”.

Most previous studies estimated extinction risk based on snapshot-in-time assessment, using categories ranging from non-threatened to critically endangered. The new study instead focused on trajectories: whether species are doing well, with numbers increasing or holding steady – or falling. It evaluated species from all five vertebrate groups – mammals, birds, reptiles, amphibians and fish – as well as insects, using data from the IUCN Red List. It also looked at species within geographic regions.

Declines in seed dispersers, like these hyacinth macaws in the Brazilian rainforest, could also contribute to declines in the plants whose fruits and seeds they feed on. Seed dispersers — including birds, insects, and primates — are especially needed now as climate change requires plant species to shift to cooler climes if they are to survive. Credit: Bernard DUPONT from FRANCE, CC BY-SA 2.0, via Wikimedia Commons.

Evaluated in the standard way, roughly 28% of life on Earth is threatened with extinction, Pincheira-Donoso says. However, examining species from this alternative perspective revealed a vast disparity between declining and recovering populations that was “much worse than we thought – or had been shown by the traditional measure”.

According the new study, 48% of the species evaluated are decreasing in number globally while only 3% are increasing. Less than half, 49%, remain stable. Most population losses are concentrated in the tropics.

Among the most concerning discoveries was that one-third of animals that are considered safe, not yet on the endangered list, are perishing in numbers that threaten their long-term survival. These data provide an early warning for preemptive action by spotlighting species that are going downhill, before it’s too late to act and prevent extinction, Pincheira-Donoso says.

“Once a species is critically endangered, it’s difficult to save,” agrees Colin Chapman, a biologist and biological anthropologist at Canada’s Vancouver Island University, who was not affiliated with the new study. “So we should be a lot more worried about species that are not listed by IUCN that may need special attention.”

The study also highlighted huge knowledge gaps, another serious concern. Scientists remain in the dark on the conservation status of many species, with a gross lack of data, especially from the tropics. “We have maps that show ‘hotspots of ignorance,’” Pincheira-Donoso says, “areas that concentrate species for which no data exists.”

Extinction rates for amphibians are higher than for all other vertebrates combined. A clown tree frog photographed in the Colombian Amazon. Credit: Rhett A Butler/Mongabay.

A synergy of threats

Scientists agree: Earth is in the midst of its sixth great extinction. The last exterminated the dinosaurs 66 million years ago. Long before that, the “Great Dying” wiped out more than 95% of life, ending the Permian period roughly 251 million years ago. Studies of past extinctions show that cataclysms of such magnitude are not caused by a single event, but by a convergence – a synergy of threats, Pincheira-Donoso explains.

Theoretical ecologist Stuart Pimm highlights rapidly changing climate and deforestation as major drivers of biodiversity loss: The United Nations estimates that the world lost 14 million square kilometers of forest over the past three centuries. Rainforests are home to two-thirds of all species on Earth, and “the tropics are getting hammered,” Chapman says.

Brazil poses a stark example, where 542,581 square kilometers of rainforest, an area larger than Spain, was lost between 2001-’20, according to the Amazon Network of Georeferenced Socio-Environmental Information – largely to make way for cattle, soy, corn, sugarcane and other commodity agribusiness, as well as mining.

Together, the combination of habitat loss and warming climate are rapidly shrinking and shifting ranges, fragmenting lands available for wildlife, and forcing populations to either climb to higher altitudes or move toward the poles to escape the heat. While some animals can relocate, others are trapped within isolated, contracting scraps of wildland.

The global distribution of animals with decreasing (top), stable (middle) or increasing (bottom) populations combining data from all taxonomic groups. Numbers of species were counted within each 1° × 1° grid cell covering the globe, using a Behrmann’s equal area projection. Credit: Finn et al. (2023), CC BY 4.0.

There are myriad other threats facing animals: hunting for meat; the often illegal global wildlife trade; climate change-intensified wildfires; deadly conflict between people and wildlife; onslaughts of invasive species; and diseases contracted from people or livestock.

More broadly, scientists point to humanity’s growth and industrial progress, which have put intense destabilising pressures on Earth’s natural operating systems. In an attempt to classify and analyse these global stressors, an international group of researchers has identified nine planetary boundaries, biophysical systems and natural processes (such as the carbon cycle or nitrogen cycle), which have been disrupted and may be approaching the breaking point due to anthropogenic activity.

Although there is controversy over what categories to include in this interdisciplinary assessment, and how to set benchmarks for potential tipping points, at least six out of the nine natural systems identified have already been significantly disrupted, causing: climate change; loss of biosphere integrity, conversion of natural landscapes (especially forest); pollution (including toxic chemicals, plastics, and more); over-nutrification by nitrogen and phosphorus fertilizers; and threats to freshwater quality and availability.

These stressors combine and interact, pushing wildlife closer to the edge. Then, as populations shrink and fragment, they lose genetic diversity and resilience. At that point, a single major disturbance – a wildfire or disease outbreak, for example – can cause a species to flicker into oblivion.

Pimm underscores the urgent need to determine which species are going downhill now and why, but also notes the importance of reporting stability and recovery. With active protection, he says, humpback whales, bald eagles, Steller sea lions and Bengal tigers have resurged. Continued vigilance is needed.

While tigers remain the most endangered big cat, India’s conservation efforts have increased its Bengal tiger population. Credit: Girish.hc2016, CC BY-SA 4.0, via Wikimedia Commons

Hotspots of ignorance

But the devil is in the details. Information gaps are enormous, especially for long-term monitoring and for less charismatic or less critically endangered species. Many species fall into the “unknown” group, Chapman says. “There are just so few data for the tropics, so we don’t really know what’s happening.”

For example, the new study found 54% of insects are declining globally, many of them key pollinators for 75% of the crops that feed us. But just as importantly, the authors note that the true state of insect populations remains a black hole: There are more than one million species currently identified by science (most of which are poorly researched), and possibly 4.5 million to seven million species remain unknown to science. But there are clear indications that a “great insect dying” could be underway on every continent but Antarctica. The biggest insect knowledge gap is in the tropics, the world’s most biodiverse region.

Over recent decades, the extinction rate for amphibians has been higher than that of all vertebrates combined. The disappearance of colorful Harlequin frog species, which prefer Latin America’s forested mountains, is among the most alarming: 70% are critically endangered or extinct.

The newly published study, while it provides a sweeping global overview, out of necessity overlooks much detail and history. Overall, for example, mammals in Madagascar are considered stable, but lemurs remain the most endangered mammal group. Fish in the freshwater heart of Africa, Lake Victoria, are increasing, but since the 1970s, more than half of the 350-plus cichlids that live there (and nowhere else) have blinked out or hover on the brink.

Insects are the most understudied organisms on Earth. They play huge roles in ecosystems as pollinators, decomposers, and as food for other animals, like the Cuban iguana. Credit: Przemek Pietrak, CC BY-SA 3.0, via Wikimedia Commons

Crisis of planetary scale

While extinctions are part of the evolutionary process, Pincheira-Donoso emphasises that “[Earth’s] climate is changing way too fast … and environments are changing more rapidly than animals can adapt.” And “failing to adapt often means extinction”. Current species losses are 1,000 to 10,000 time higher than “background” extinction rates. The broad range of these estimates highlights both the severity of the crisis and the need for more information.

The tropics remain the extinction epicenter, and climate change could continue accelerating declines. Some research suggests that since temperatures remain relatively constant near the equator year-round, animals there may be physiologically more vulnerable to slight changes in temperature compared to those living in temperate zones, where wildlife adapts to wide winter-summer variations.

Regardless of location, ripping out the individual threads of an ecosystem’s living fabric ultimately shreds habitats, potentially triggering collapse. Global declines in seed dispersers – including birds, insects and primates — can cause fruiting plants to become rare, starving the animals that feed on them and sending additional shockwaves through an ecosystem. Likewise, the loss of tigers, wolves and other top predators allows rodents, deer and other species to take over, denuding forests. And coral reefs can’t serve as the ocean’s nurseries if they bleach and die, no longer nurturing the fish that one billion people rely on for food.

Both Chapman and Pimm say the new study offers indicators of where conservationists could step in before species are in dire straits. At the 2022 UN biodiversity conference, nations agreed to protect 30% of the planet’s land and water by 2030. While there is little chance that goal will be reached, prioritising species undergoing serious declines, bringing attention to unstudied regions, and focusing attention on places that harbor a preponderance of soon-to-be-endangered species, could have positive conservation impacts, they say.

Percentage of species per taxonomic group that have decreasing, stable, increasing or unknown/unassessed (NA) population trends. Credit: Image courtesy of Finn et al. (2023) (CC BY 4.0).

“Slowing the rate of biodiversity declines must be a global priority,” the study authors write. Collaborations between governments, law enforcement, nonprofits, and local and Indigenous peoples (the best land stewards) can curb hunting, wildlife trafficking and protect and replant forest. Connecting scraps of forest is critical, Pimm adds. “We know that fragmented landscapes lose species quickly.”

Plunging biodiversity is “a crisis of planetary scale, a crisis of our making,” Pincheira-Donoso concludes. “We need to be alarmed about it.” But he emphasises that we shouldn’t feel powerless: each of us can take steps that make a difference, from lowering use of energy and pesticides to supporting effective conservation organisations.

Pincheira-Donoso’s hope: that the work of scientists “inspires our sense of cooperation and our sense of responsibility. We need to work as a global collective”.

This article was first published on Mongabay.