Earth is home to millions of species that have adapted to environments as diverse as deep ocean seabeds and chilly alpine peaks. But in our era of rapid climate change, driven by human activity, many of our planet’s lifeforms will struggle to survive the increased heat stress that comes from warming global temperatures, provoking concern about looming biodiversity losses.
To anticipate these challenges, many scientists have modeled the effect of “lethal temperatures”—the heat level that will kill any given species—across worldwide ecosystems and ranges. But what if fatal temperatures were not the main driver of biodiversity loss in the era of climate change, but mass sterilization?
A team of researchers led by Steven Parratt, an evolutionary ecologist at the University of Liverpool, has raised this prospect by investigating a worrying and overlooked threshold: thermal fertility limits (TFLs). TFLs refer to the level of heat stress that can render a population infertile, a phenomenon that is known to occur in plants, insects, fish, corals, birds, and mammals, including humans, according to a study published on Monday in Nature Climate Change.
“Lots of excellent work had explored the link between lethal temperatures with the distribution of species around the globe,” said Parratt in an email. “Generally there were promising patterns here, but the existing data suggested we were missing something important—some species just weren’t found in locations where they could survive the high temperatures.”
“Some older work had shown that gametes, sperm in particular, were sensitive to heat waves,” he continued, “so we thought this might be part of the puzzle.”
The researchers set out to probe the possible effects of heat stress on male fertility in fruit flies (Drosophila), insects that are widely used as model organisms across a variety of scientific studies.
Parratt’s team exposed males from 43 Drosophila species to four-hour pulses at temperatures ranging from benign to lethal. The males that survived were then allowed to freely mate with two different groups of females over the course of a week to assess their fertility both immediately after the heat stress, and after a more prolonged period.
The results revealed that 11 of the species experienced an 80 percent loss in fertility immediately after cooler-than-lethal heat exposure, or about 25 percent of the total sampled species. However, that number rose to 44 percent after a week, revealing that “rather than seeing a recovery of fertility over time, the impact of high temperatures on fertility was more pronounced seven days post heat stress,” according to the study.
The team also projected how these limits might affect the future fertility of one of the fruit fly species, the North American Drosophila flavomontana, and discovered that more than half of the areas that are currently cool enough for the insect to breed are likely to become too hot by 2080.
“In this single case, the numbers mean that we would predict the species to lose 48 to 59 percent more habitat when we include the sterilizing effect of high temperature, compared to predicted habitat loss when we only consider the lethal effect of temperature,” Parratt said.
While the new research focuses on fruit flies, the implications of thermal fertility losses extends well beyond insects.
“If our data for Drosophila can be extrapolated to other organisms then male fertility losses at high temperatures may be common, occurring at substantially lower temperatures than lethality,” Parratt and his colleagues note in the study. “The limited data on fertility at extreme temperatures supports this, with high-temperature losses in male fertility observed in diverse organisms including some high-temperature-adapted species.”
“For instance, the zebra finch, a desert-dwelling organism with naturally high body temperature and good thermoregulation, shows substantial damage to sperm at temperatures it regularly experiences in nature,” the researchers added.
Parratt predicted that exotherms (species that rely on their environment to regulate their body temperature) and species with relatively short breeding seasons will be particularly vulnerable to fertility loss from climate change.
“Animals that are geographically constrained to small areas (small lakes, mountain tops or isolated valleys) are probably less able to move away to escape heat waves, and so might be more at risk of heat-induced sterility,” he added.
Although the study’s speculations beyond fruit flies are just that at the moment, it shows that scientists are reckoning with the full scope of possible effects due to climate change.
“It will be important now to broaden the link between TFLs and species distribution beyond fruit fly species to other insects and other groups of organisms,” Parratt said. “This will take a lot of work to find ways of measuring this consistently between, say, an elephant and a snail.”
“We would also like to understand why some species have evolved to stay fertile at higher temperatures than others,” he added. “What genes facilitate this? Can natural selection increase TFLs quickly enough to account for the effects of climate change?”
The study underscores the challenges that wildlife around the world will face as heat levels spiral upwards, and the dire need to account for both thermal fertility limits and lethal temperatures in planning conservation efforts for the most vulnerable populations.
“Our work emphasizes that temperature-driven fertility losses may be a major threat to biodiversity during climate change,” Parratt and his colleagues said in the study. “We urgently need to understand the range of organisms likely to suffer thermal fertility losses in nature, and the traits that predict vulnerability.”
“Exploration of the physiological, genetic, behavioral and ecological mechanisms that underlie thermal fertility limits will now be an important step towards linking temperature-driven sterility with species’ responses to climate change,” the team concluded.
Update: This article has been updated to include comments from lead author Steven Parratt.