Greater Yellowstone Ecosystem study examines large mammals’ responses to heat

Greater Yellowstone Ecosystem study examines large mammals’ responses to heat 

By Diana Setterberg, MSU News Service

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BOZEMAN – A study of nine species of large mammals in the Greater Yellowstone Ecosystem has revealed that their behavioral responses to summer heat were influenced more by the structure of their environments than by their biological traits or greater temperature increases – a result the Montana State University ecologist who co-authored the study finds encouraging as the region’s climate continues to warm.

Justine Becker, an assistant professor in the Department of Ecology in the College of Letters and Science, is the co-lead author of the study published this month in the journal Ecosphere. Becker and a team of researchers from the University of Wyoming, where Becker was a postdoctoral researcher when the study began, collaborated with the Bureau of Land Management, National Park Service, U.S. Geological Service, the Wyoming Game and Fish Department, and Idaho Department of Fish and Game to synthesize data and conduct the multi-species analysis.

Becker studies the ecological factors that cause behavioral variation in wildlife, particularly in animal movement and habitat use decisions. She said the study was designed to determine how different environmental conditions and inborn traits influenced large mammals’ responses to rapidly increasing temperatures – in this study, variations in maximum daytime temperatures in summer. The researchers included several species and populations in the study to help them better understand ecological patterns, as well as trends in the specific ecosystem they studied. Becker said the study was motivated by the understanding that rising temperatures due to climate change present an increasingly prevalent stressor to animals.

The researchers analyzed GPS location data from populations of herbivorous bison, bighorn sheep, moose, mountain goats, mule deer, pronghorn and elk, as well as two carnivorous species, wolves and cougars. The data were collected between 2001 and 2019 by the collaborating agencies in the Greater Yellowstone Ecosystem.

The research team narrowed its analysis to annual data from mid-June to the end of August, when the ecosystem experiences its warmest temperatures. While seeking to determine animals’ behavioral changes in response to the heat and assess how those responses differed across populations, the researchers also evaluated whether groups that encountered higher temperatures experienced greater degrees of stress, manifested through larger behavioral changes.

“As far as we know, this is the first study of this nature that’s taken data from a lot of different populations of large mammals and looked at their behavioral plasticity at the same time,” Becker said. “We were really interested in seeing whether there was any kind of generality in their responses to increasing temperatures, as well as species-specific responses that might be different from one another.”

As expected, individuals in all the populations changed their behaviors in response to rapidly increasing temperatures by seeking cooler conditions and moving more slowly. But Becker said the team was surprised to find that populations living in homogeneous environments – those with mostly uniform landscape features – altered their behavior more dramatically than those living in environments with more varied landscapes. For example, across all species studied, pronghorn living on the prairie in Wyoming’s Shirley Basin were the most likely to alter their behavior to find shade from trees in response to increasing temperatures. Becker said that may be because animals living in homogeneous environments like the Shirley Basin need to travel farther to find relief from the heat than those living in habitats that, for example, encompass both open meadows and areas of tree cover.

The researchers also evaluated behavioral responses according to the animals’ endogenous traits, such as their sex, physiology and body size. The study did not find any correlation between those traits and the animals’ behavioral responses, but the authors state that further study is needed to compare individual characteristics, such as coat color, with behavioral data.

Becker said the findings were encouraging because they indicate that large mammals are well equipped to adjust their behaviors to cool themselves down. That’s important because they are unable to adapt physically to environmental changes as quickly as smaller animals, due to their longer lifespans and slower generational times.

“We often think of climate change as a huge ecological challenge for most species, and it absolutely is, but, at the same time, behavior is this really amazing way that animals are able to buffer themselves against these changes because they can do it right away,” Becker said. “They don’t need to wait around for the process of evolution to take its course.”

She said that because the study showed a relationship between environmental structure and the degree to which different mammals adjusted their behavior, the information may influence habitat managers to consider large ecosystems as a whole, rather than focus on species-specific management strategies.

“A large part of what management agencies work on is habitat treatment, modification and protection. We’re hopeful that this study gives a general sense of the kinds of environmental features needed to help these animals and what types of habitats are going to be important for them in the future,” Becker said.

University of Wyoming wildlife ecologist Jerod Merkle, who oversaw the study group that included co-lead author Rebecca Thomas-Kuzilik, agreed.

“We saw a large variety of responses during hot days across individuals and across species,” he said. “This result tells us that animals have options, and maintaining a landscape that is a mosaic of these different habitats is key. Further, landscapes need to be permeable, such that the animals can access all these different habitats when the heat waves come.”

Becker emphasized that the team was able to conduct this type of study only because many scientists and agencies shared their data.

“Such syntheses, collaboration and comparative analysis are invaluable to the production of new ecological knowledge,” she said.

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