The residents of Tecolutilla, Mexico, knew the heatwave was bad when they heard the thuds. One by one, the town’s howler monkeys, overcome with dehydration and exhaustion, were falling from the trees like apples, their limp bodies smacking the ground as temperatures sizzled past 43C (110F) in spring last year.
Those that survived were given ice and intravenous drips by rescuers. At least 83 of the primates were found dead in the state of Tabasco, though local veterinarians estimated hundreds throughout the region probably perished.
Episodes such as this are unfolding across the world as the climate crisis delivers harsher and more frequent heatwaves. Flying foxes have tumbled from trees in Australia; billions of barnacles have baked in tide pools in Canada; male beetles have been virtually sterilised by soaring temperatures.
Beyond these local die-offs, ecologists are only just beginning to grasp the full threat that extreme heat poses to the world’s wildlife populations, and how quickly it can drive species towards extinction.
Maximilian Kotz, a climate scientist at the Potsdam Institute for Climate Impact Research, says: “As human emissions shift the temperature distribution upwards, this manifests as a strong increase in the number of very hot days – not just an increase in average temperatures.”
While the steady drumbeat of climbing average temperatures has long been expected to push species out of their preferred habitats and make food scarce, these episodes of blistering heat constitute a unique threat to wildlife, scientists say.
Heatwaves are reaping such long-term population crashes in some regions that experts say the climate emergency has become a driver of biodiversity loss on a par with – or worse than – deforestation and habitat loss.
Recently, Kotz and his colleagues dug through decades of global monitoring data on more than 3,000 bird populations and daily weather records to tease out whether heatwaves had contributed to observed declines in some parts of the world.
After controlling for other factors, they found bird populations in temperate, boreal and tundra zones did not seem to suffer much from scorching heat.
But for those in the tropics, the findings were sobering: extreme heat had slashed tropical bird populations by 25% to 38% over the past 70 years. Tropical songbirds, they found, were hit hardest.
The losses in the tropics have been so profound, in part, because species there are already living near their limit of heat tolerance, Kotz says. At the same time, birds in tropical regions are experiencing dangerously hot days about 10 times more often than they did in the past.
“Heatwave frequency has just increased that much more quickly in the tropics,” Kotz says.
Exactly how heatwaves drive population declines is not clear from their study, published in Nature Ecology & Evolution. But Kotz says it is likely to be a combination of birds dying during heatwaves, heat stress in nests and eggs, and effects on the birds’ food, such as insects, which similarly struggle to survive soaring temperatures.
Their research suggests that across the tropics changes to the climate have played a greater role in driving bird numbers down than direct human activities, such as logging, mining or farming.
“For many of these species that are in protected areas in pristine habitats, there can still be really substantial effects from intensifying heat extremes through our greenhouse gas emissions,” Kotz says.
When a heat dome descended on the Pacific north-west in the summer of 2021, unleashing air temperatures above 46C (116F), scientists walking the beaches along the strait of Georgia, off Vancouver Island, were horrified to find tide pools containing thousands of lifeless molluscs and barnacles. Attached to rocks, the creatures were unable to escape to cooler waters.
Billions of animals dead
At the time, Prof Christopher Harley, a marine biologist at the University of British Columbia, estimated the heatwave had killed 1 billion animals along the coast.
“I was nervous making that estimate publicly because it was a bit back of the envelope,” he says, but now adds that they underestimated the toll. “We now have better numbers for that: we estimate 10 billion barnacles and 3 billion mussels [died] just in the strait of Georgia.”
Harley says that while deaths in the billions may sound like an extinction-level event, abundant and fast-reproducing tidal species have proved surprisingly resilient.
“The barnacles came back really fast. It took a few more years for the mussels to start creeping back in,” he says. “But if you went out to the shore of Vancouver right now, you wouldn’t immediately think, ‘There was a disaster here four years ago’.”
Those animals that managed to survive the heatwave may also be slightly better adapted to extreme temperatures, meaning their offspring could carry that same tolerance.
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Other coastal species that thrive at the ebb and flow of the ocean, such as starfish, may have crawled into cooler waters when the heatwave took hold, Harley says. But among those that perished, it could take decades to recover the more slow-reproducing species.
Harley’s preliminary data also examines the outcomes for insect populations. Honeybees appeared to have fared well, he says, but “aphids had a really tough go”.
It fits with global research that finds temperature extremes can cause a mix of reactions among insect species, from outbreaks to breakdowns.
In a 2020 research review paper in Global Change Biology, scientists noted that many of the world’s insects were unable to regulate their body temperature during heatwaves because they mimicked ambient temperatures. Extreme heat can therefore push species beyond their limits.
Yet heatwaves can also trigger a temporary outbreak: a dramatic increase in a population that quickly burns through local food sources and ultimately causes a crash. This, the authors noted, could lead to local insect extinctions.
How heat changes animal bodies
Epidemiologists have clearly established how heatwaves affect humans, but there has been scant research exploring the physiological consequences for non-human mammals.
PJ Jacobs, an evolutionary biologist at the University of Pretoria in South Africa, is trying to close that gap. His recent research has focused on small mammals, which are especially vulnerable to heatwaves because of their larger surface area to volume ratio, which means they heat up more easily.
In a study published last December, Jacobs examined the effects of heatwaves on small African rodents – the mesic four-striped field mouse and Namaqua rock mouse.
He simulated a heatwave in a lab to test how the fertility of the male mice would be affected. Both experienced a serious drop in testosterone levels and other fertility measures.
This, Jacobs says, suggests that intensifying heatwaves could jeopardise the ability of small animals to reproduce, potentially shrinking populations if they cannot access cooler refuges, such as shady spots or grassy shelters. That could have a serious effect further up the food chain.
Heatwaves also affect animals at a behavioural level that poses limits to survival.
“If there’s a massive heatwave happening, animals are not just going to go out and be active when temperatures are so extreme,” Jacobs says. “They have to rest in the shade which means they can’t go and do other things related to their fitness – reproduction and feeding.”
At the same time, he adds, “If you hit those severe temperature extremes, you’re going to get those effects at the physiological and molecular level. That’s going to cause disorientation and dehydration. And you just collapse – you faint.”
Even when the lab mice sought water to cool down, “the brain was the one tissue that was still affected”, says Jacobs. “The brain is obviously the control box of the body. If the brain doesn’t function properly, you’re going to have monkeys falling out of trees.”
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