Imagine returning to a favorite hiking trail 15 years after your first visit and discovering that many of the plants and animals that once lived there are gone. While these species may still exist elsewhere, these disappearances – known as local extinctions – are among the clearest signs that climate change is already transforming ecosystems and threatening species across the globe.
University of Arizona researchers compared local extinctions from recent climate change among more than 5,100 plant and animal species from around the world, including hundreds of species of moths and beetles, hundreds of fishes and birds, many mammals, frogs, salamanders, and lizards, and almost 3,000 species of plants.
In the study published in Nature Climate Change, the researchers found that 49% of temperate species experienced local extinction at the hottest parts of their ranges, compared with only 33% of tropical species.
The research drew on repeated biodiversity surveys from nearly 40,000 sites worldwide, allowing them to compare historical records with resurveys conducted years or decades later, making it the largest analysis of climate-driven local extinctions conducted to date.
"For decades, scientists generally believed that temperate species were less vulnerable to climate change," said Gopal Murali, the lead author of the paper and former postdoctoral scholar at the University of Arizona. "We were surprised by our results, which showed that was not the case."
These results were consistent across many different groups of organisms, including insects, vertebrates, plants, and marine and freshwater species.
"I actually published a study of 976 species in 2016 using the same type of data that showed the exact opposite pattern, with more local extinction among tropical species," said John Wiens, senior author of the paper and a professor in the University of Arizona Department of Ecology and Evolutionary Biology in the College of Science . "That's part of why we were so surprised."
To understand the unexpected pattern, the researchers analyzed multiple climate-related factors, including long-term warming trends, rainfall changes, drought conditions and heat waves across global sites. They also excluded sites that may have been affected by non-climatic stressors, such as deforestation.
The researchers found one main explanation for the pattern: temperate regions are warming faster than tropical regions.
"The world has changed since 2016," Wiens said. "There's been more heating in the temperate zone, especially at higher latitudes, and it's possible that the pattern has simply flipped in recent decades, and that helps explain the reversal in findings. For animals, we did not find that tropical extinctions were less common than we thought before. Instead, we found that temperate extinctions had outpaced tropical extinctions."
The researchers found that the maximum increase in temperature over a 25-year period was approximately 3.3 degrees Fahrenheit in tropical regions. In temperate regions, the maximum increase was about 6 degrees Fahrenheit – nearly twice as much.
The team also examined how species responded to warming in each region. For decades, scientists believed that tropical species would be especially vulnerable to climate change due to their physiology. Because tropical species evolved under relatively stable temperatures year-round, they were thought to have less tolerance for temperature changes than temperate species, which experience greater seasonal temperature variation.
"While faster warming in temperate regions appears to be the primary driver of local extinctions, we also found that temperate species are at least as sensitive to rising temperatures as tropical species," Murali said.
The observed local extinctions do not necessarily mean that the entire species went extinct, but they do show that the populations cannot survive the changing environmental conditions. Similar losses across a species' range can lead to extinction of the whole species.
"People often think that a species will simply move into cooler areas as the climate warms, but we found that more than 70% of the species were not doing so," said Wiens. "Essentially, the life and death of the majority of species may be determined by these local extinctions and whether local populations can survive in place or not."
For some species, moving to cooler habitat may not even be possible. Animals may be unable to cross highways, cities, or other developed landscapes, while fish and other aquatic species are often confined to specific lakes and rivers. On mountains, species can continue moving uphill as temperatures rise, but many may simply run out of mountain.
On some mountains, such as Mount Lemmon near Tucson, people can see the dead trunks of tree species that used to thrive at lower elevations that now only occur at higher elevations, said Wiens.
The study also revealed important differences in the patterns of extinction within tropical and temperate species. In tropical regions, climate-related local extinctions were concentrated at the warmest parts of each species' range. In temperate regions, however, populations often disappeared in many locations throughout the species' ranges.
"In the past, we have been laser-focused on the warmest areas," said Wiens. "But it turns out that nowhere is really safe for populations of many temperate species."
Across all species included in the study, researchers found that 45% had gone locally extinct at the warmest part of the region where they were previously found. For many groups, that number exceeded 50%, including insects, terrestrial vertebrates, and marine species.
The findings could have important implications for conservation planning. For years, tropical species were widely considered to face the greatest risks from climate change. While the new study does not suggest that tropical species are safe, it does indicate that temperate species may be in greater danger than previously recognized.
The researchers emphasized that the study is not based on projections of future impacts. Instead, it documents real biological changes that have already occurred.
"People often think climate change is something that will affect species in the future," said Murali. "But for both tropical and temperate species, we're already seeing the effects. The patterns we documented show that biodiversity is already changing in ways we are still working to understand."
