Beneath the charismatic creatures we celebrate — the birds, the mammals, the amphibians — lies a quieter foundation: the insects. A new paper in Nature Reviews Biodiversity draws the map we have long lacked, showing that nearly three-quarters of terrestrial and freshwater vertebrates depend on insects not merely for food, but for the symbiotic arrangements that make survival possible. To lose the insects is not to lose a part of nature; it is to begin losing nature itself.
Moths in sloth fur reveal insects' hidden role in vertebrate survival
They're little bundles of energy, protein and water that can easily be fed to chicks.
Mark
So the sloth and the moths—that's a genuine symbiosis, right? Both animals need the other to survive?
Mimi
Exactly. The moths live in the sloth's fur, they feed on algae that grows there because of the nitrogen they deposit. The sloth eats that algae. When the sloth goes to the ground to defecate, the moths lay eggs in the dung. It's a closed loop.
Luke
But how many sloths actually have this relationship? Is this common, or is it one of those rare examples that makes for good storytelling?
Mimi
That's a fair question. The paper uses it as an example of symbiosis, but I don't have numbers on how widespread it is across sloth populations.
Mark
And the birds crushing ants—Cooke says it's probably about parasites, but he admits people are still figuring it out?
Mimi
Right. The formic acid from the ants does reduce parasites, but he's careful to say researchers are still open-minded about what's actually happening.
Luke
So we know ants have formic acid and parasites are a problem, but the exact mechanism—why the bird does this, what benefit it gets—that's still being worked out.
Mimi
Yes. It's an observed behavior with a plausible explanation, not a fully understood one.
Mark
The bigger claim is that 75 percent of vertebrates eat insects. That's enormous.
Mimi
It is. And for 40 percent, insects are the only food. That's the real foundation—insects convert plants into protein that everything else depends on.
Luke
Is that 75 percent figure from the paper, or is it an estimate?
Mimi
It's from the paper. But I'd want to know how they arrived at it—whether it's based on direct observation of all those species or extrapolation from known groups.
Mark
And the European birds—insectivores doing worse than seed-eaters. That's a correlation, not proof of cause?
Mimi
Correct. It suggests insect decline is the issue, but it's not definitive proof.
Luke
Because seed-eaters could be doing better for other reasons—less competition, different habitat pressures.
Mimi
Exactly. The pattern is suggestive, but the causation isn't airtight.
Le Pouls
- Insect populations are declining globally, and the vertebrates built upon them — from mountain pygmy possums to European songbirds — are following them downward.
- The dependency runs deeper than a food chain: moths fertilize sloth fur, ants cleanse birds of parasites, and crickets serve as living proof of a mate's worth.
- Forty percent of vertebrate species eat insects exclusively, and even seed-eating birds cannot raise young without caterpillars to fuel their chicks' growth.
- Insectivorous birds across Europe are now faring measurably worse than seed-eaters, offering a continent-scale signal that insect scarcity is reshaping ecosystems.
- Researchers are calling for a fundamental reframing: insects must be understood not as background noise but as the load-bearing architecture of vertebrate life.
Beneath the charismatic creatures we celebrate — the birds, the mammals, the amphibians — lies a quieter foundation: the insects. A new paper in Nature Reviews Biodiversity draws the map we have long lacked, showing that nearly three-quarters of terrestrial and freshwater vertebrates depend on insects not merely for food, but for the symbiotic arrangements that make survival possible. To lose the insects is not to lose a part of nature; it is to begin losing nature itself.
A brown-throated sloth descends from the canopy once a week to defecate — and in doing so, completes a cycle that keeps it alive. Moths living exclusively in its fur deposit nitrogen into its coat, feeding algae the sloth then consumes. When the sloth reaches the forest floor, the moths lay their eggs in the dung. Neither creature survives without the other. A new paper in Nature Reviews Biodiversity uses this relationship as a window into something far larger: the foundational role insects play in the survival of vertebrates across the entire planet.
The examples accumulate quickly. British blackbirds and woodpeckers sometimes spread their wings and allow ants to swarm their feathers, likely using the formic acid the insects produce to reduce parasites on their skin. Male spotted antbirds court females with live crickets — a gift that communicates both nutrition and the promise of a good breeding season. These are not oddities. They are expressions of a dependency so widespread that nearly three-quarters of all terrestrial and freshwater vertebrates eat insects, and for 40 percent of species, insects are the only food source they have.
Even birds that appear to live on seeds cannot raise their young without insects. Parent birds rely on caterpillars to deliver the protein their chicks need to grow. Insects, as senior ecologist Rob Cooke puts it, are the bridge between plants and the animals that eat them — converting sunlight into protein with an efficiency no other group can match.
The cost of losing that bridge is becoming visible. The decline of bogong moths has tracked the decline of mountain pygmy possums. The extinction of the Rocky Mountain locust almost certainly contributed to the disappearance of the Eskimo curlew. Across Europe, insectivorous birds are declining faster than seed-eaters — a divergence that points unmistakably toward insect scarcity as the cause. Cooke's conclusion is both ecological and moral: without insects, the animals we love cannot survive, and the world becomes irreversibly poorer.
A brown-throated sloth hanging in the canopy hosts an entire ecosystem in its fur—moths that live nowhere else, that deposit nitrogen into the sloth's coat and trigger the growth of algae the sloth then eats. When the sloth descends to the forest floor to defecate, the moths lay their eggs in the dung, completing a cycle so tightly wound that neither animal survives it alone. This is not an outlier. It is one thread in a vast web of dependency that a new paper in Nature Reviews Biodiversity has begun to map: the foundational role insects play in the survival of every vertebrate on Earth.
The examples are everywhere once you know to look. British blackbirds, jays, and woodpeckers will sometimes drop to the ground, spread their wings, and allow ants to swarm across their feathers. The birds crush many of the ants in the process. Rob Cooke, a senior ecologist at the UK Centre for Ecology and Hydrology, explains that the formic acid the ants produce likely reduces parasite loads on the birds' skin—though researchers remain open to other explanations for the behavior. Male spotted antbirds court females by presenting them with live crickets, a gift that signals abundance and readiness to breed. "He's telling her, 'There's loads of crickets, let's make some chicks,'" Cooke says. The cricket is both nutrition and promise.
But the primary service insects provide is simpler and more fundamental: they are food. Nearly three-quarters of all terrestrial and freshwater vertebrates—mammals, birds, reptiles, amphibians, and fish—eat insects. For 40 percent of vertebrate species, insects are the only food source. Even birds that seem to live on seeds cannot raise their young without them. Parent birds need caterpillars to feed their chicks the protein required for growth. "They turn plant material into the protein that all the vertebrates depend on," Cooke says. "They're little bundles of energy, protein and water that can easily be fed to chicks." Insects are the bridge between plants and the animals that eat them—a conversion mechanism so efficient and so essential that vertebrate life as we know it cannot exist without it.
The consequences of losing that bridge are beginning to show. The decline of bogong moths has corresponded with the decline of mountain pygmy possums that depend on them. The Rocky Mountain locust, now extinct, almost certainly took the Eskimo curlew with it. These are the cases where the link is clear enough to trace. But the broader pattern is visible in the data: insectivorous birds across Europe are faring worse than seed-eating birds, a divergence that points to insect scarcity as the culprit. The decline is not uniform, and it is not always easy to prove cause and effect, but the direction is unmistakable.
Cooke frames the challenge as one of perspective. "Insects are foundational, and we need to reframe how we appreciate them and interact with them," he says. "It's about thinking how they also help things that we love, and without them, the world would be a poorer place." The sloth and its moths, the bird and its ants, the antbird and its cricket—these are not curiosities. They are demonstrations of a principle that runs through every ecosystem: vertebrates do not stand alone. They are held up by the insects beneath them, the ones we rarely see and often ignore, the ones that convert the sun's energy into the protein that keeps the world alive.
Citations marquantes
They turn plant material into the protein that all the vertebrates depend on. They're little bundles of energy, protein and water that can easily be fed to chicks.— Rob Cooke, senior ecologist at the UK Centre for Ecology and Hydrology
Insects are foundational, and we need to reframe how we appreciate them and interact with them. It's about thinking how they also help things that we love, and without them, the world would be a poorer place.— Rob Cooke
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