When Anne Charmantier set out to check her great tits—a songbird native to Europe—on June 28, she expected to find healthy, spry chicks.
As she slowly opened the doors to the wooden nest boxes—a trick to study these birds—the quiet at the nest disturbed her. Peering in, she encountered a grim scene: All chicks lay dead in their nests. An evolutionary ecologist at the Center of National Scientific Research in France, Charmantier has studied great tits for 15 years—long enough to know that this was not normal.
The culprit was a heat wave that had swept through Europe in late June. In Montpelier, where she checked the nest boxes, temperatures exceeded 110 degrees Fahrenheit, a record by more than 10 degrees.
“The heat was so much over anything we've ever experienced,” Charmantier says. “It was creepy.”
Though this is just one anecdote, scientists predict that extreme heat waves will become more common with climate change, carrying huge consequences for the survival of some populations.
The question plaguing scientists is this: Is climate change happening too fast for animals to save themselves—and their future offspring—by adapting quickly?
“Climate change is one of the key threats to biodiversity and to human society in the coming century,” says Thomas Reed, an evolutionary ecologist at University College Cork. “Ultimately, populations must evolve” to survive.
Early to breed
Scientists like Charmantier and Reed are studying how animals across the world will respond to a new environment shaped by climate change. Warming air and water temperatures, rising sea levels, increasing storm intensity, and disappearing ice create a drastically different world for species that evolved to live in specific conditions.
Multiple studies have looked at how individual species will fare in the face of climate change, but a study published in Nature Communications in July performed a meta-analysis, pulling all the data together to try to provide a broader conclusion.
Viktoriia Radchuk, lead author of the study, scanned over 10,000 abstracts from published studies to search for bits of data that she could include in the analysis.
Researchers focused on research from taxa beyond plants—amphibians, mammals, birds, reptiles, and insects—allowing researchers to establish a trend of warming temperatures in those study locations.
A warmer world unleashes cascading effects in a terrestrial environment: Temperatures stay warmer at night, trees shoot out leafy-green tendrils, and insects start their frantic mating dance. The abundance of new, yellow-green leaves and budding flowers supplies a smorgasbord for hungry caterpillars. These small caterpillars make their way into the begging mouths of baby birds, delivered by active parents.
Spring comes earlier with climate change, with consequences for animals. Radchuk’s team found a strong relationship between timing of key life cycle events, like breeding, and the warming pattern: Animals generally shift their breeding earlier to match the new timing.
On average, the window of time when birds lay their eggs has gotten earlier by almost two weeks over half a century. Since many small songbirds can raise their young in roughly one month, two weeks is a big shift in their timing.
“If [birds] don't adjust, then the chicks will arrive way after the caterpillars are gone. And so, they starve,” Charmantier says.
Knowing that warming occurred in the studies and that animals were able to breed earlier, the team then asked if animals can evolve with the changing climate. The pool of studies with those detailed data dropped to 13 species—and almost all were birds.
Researchers were interested in understanding cases where they could “witness evolution in action…that happens at the speed of a few generations,” Charmantier says.
Evolution can happen very fast—over a few years—or it can be a slower process. In insects that breed much more quickly, evolution can occur faster, whereas longer-lived birds and mammals usually see these processes occur at a slower pace.
Natural selection is just one force that drives evolution. For natural selection to occur, explained Charmantier, some individuals with a particular ability—like starting breeding earlier—need to be favored. If breeding earlier means parents are able to raise more offspring—and this is linked to a gene—that means natural selection is at work to breed earlier.
The team found evidence of selection for most species, toward breeding even earlier with the warming climate. “But [evolution is] never going to be fast enough if we match this with the climate prediction,” Charmantier says.
Birds would have to shift even earlier to ensure a population can remain stable.
Those animals that can’t match the rate of warming and delay breeding, like the roe deer and Columbian ground squirrels, don’t raise as many young. If an animal can’t adapt, it’s possible a local population would go extinct, said Reed, a coauthor on the Nature Communications study.
Reed admits that the study is a small snapshot to generalize for all populations and animals. Generalists that eat different types of food or inhabit a wide range of habitats might be more resilient to climate change. (Read how a generalist, the raccoon, spread widely across the globe.)
Jeremy Cohen, a postdoctoral fellow at the University of Wisconsin who was not involved in the study, doubts it is a clear case of an evolutionary response. “Birds may be quickly reacting to what’s going on in the weather, rather than evolving these responses across generations,” Cohen says.
Because previous studies have noticed that changes in the timing of migration and breeding are linked to warming temperatures, the changes suggest more of a behavioral response. Scientists would need high-quality genetic data from related individuals in a population to show that genes that tell a bird to breed earlier are actually appearing in the next generation. Cohen noted that changes in morphological characteristics, such as body size, would have been more convincing evidence of an evolutionary response to climate change.
“It’s a really valuable comprehensive assessment,” says Jeffrey Lane, an evolutionary ecologist at University of Saskatchewan, who was not involved with the study. “The bigger advance is that [animals] aren’t keeping pace.”
The data from the Nature Communications study stem mostly from temperate climates in the United States, and Europe, which have the longest continuous data. The tropics and Arctic represent large unknowns. But researchers are working to fill the gaps.
Northward to the Arctic
Researchers that study an animal population intimately for years have accumulated that kind of genetic data to answer if animals are evolving with climate change, and the answer so far isn’t positive.
At a small sandbar island in the Arctic, Mandt’s black guillemots are breeding earlier, trying to keep up with the pace of ice melt. George Divoky has documented change over 47 years in this small seabird living at the northern extent of its range, watching as the sea ice disappears earlier and more quickly each year.
Another striking feature of climate change at this small colony: Polar bears now visit the sandbar and try to eat the small guillemot snacks. Researchers sleep behind an electrified fence because the polar bears have become so common.
In this Arctic species, “they're advancing their laying date in the right direction,” says Drew Sauve, a Ph.D. student at Queen’s University in Ontario. “But it doesn't seem to be enough.”
Sauve was curious if there was a genetic link—do birds that breed earlier have more breeding success and pass that down to their offspring? Researchers know all the birds on the island—down to the family tree—allowing them to test for selection of earlier breeding.
Reported in Functional Ecology in July, researchers found very little genetic effect. The earlier breeding seemed more related to birds following the environmental cues.
“There's probably not much of an evolutionary response going on, nor much of a potential to do so,” Sauve says.
It’s possible that these birds experienced strong selection in the past to breed during this period and are now limited in how much they can shift the timing of their breeding.
Climate change and extreme events
How animals adapt to warming temperatures is just one piece of the puzzle. The impact of extreme events on animals, like France’s heat wave, needs to be further studied.
An added factor in the extreme heat wave event that killed the great tit chicks, Charmantier says, is the “heat island effect,” where cities tend to be hotter than surrounding areas. In a nearby oak forest in Southern France, where temperatures remained cooler, fewer chicks died.
These extreme heat events could have a huge impact on selection. But some offspring need to survive during an extreme event for those beneficial genes to be passed into the population. If an extreme heat wave kills everyone, natural selection can’t occur.
We do know that these “extreme climatic events are creating a new selection pressure,” Charmantier says. “We hope that this will trigger new evolution, but we know that any evolution will be too slow to catch up with climate change.”