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GAINESVILLE, Fla. — In what researchers say is one of the most direct illustrations of global climate change’s impact on animals, a new study shows that longer summers and milder winters have allowed yellow-bellied marmots to grow larger and increase in numbers.

For more than four decades, researchers have trekked out to the Colorado Rocky Mountains to study the population of marmots — considered an ideal species to observe and measure because the rodents live in groups, don’t stray far from their burrows and are relatively easy to handle, said Madan Oli, a population ecologist in the University of Florida’s Institute of Food and Agricultural Sciences.

“It’s the tremendous wealth of information on these animals that have allowed us to take what is, really, an unprecedented look at how climate change has affected a species,” said Oli, a co-author on the study, a collaboration among six academic institutions in the U.S. and United Kingdom.

Published this week in the journal Nature, the work shows that longer summers have allowed the marmots, which hibernate in colder temperatures, more time to grow before the next hibernation.

The researchers have recorded the body mass, survival and reproduction of the marmots by age group from year to year. On average, a 7-pound adult marmot would have added about half a pound over the last 10 years.

The study, which pulls from 33 years’ worth of data, also shows that until 2000, the marmot population size was relatively stable at about 130. Since that time, however, the population has increased to about 300.

The boost is due primarily to extra time for the marmots to eat and grow. Marmots can lose as much as 40 percent of their body mass during hibernation, so extra body mass means a greater chance of surviving winter and a higher chance of reproduction the following summer.

This advantage is most likely enhanced by increasingly mild winters, said Arpat Ozgul, the study’s lead author and a researcher at Imperial College London. Ozgul began his marmot studies as a postdoctoral researcher with Oli in UF’s department of wildlife ecology and conservation before moving to Imperial College London in 2008.

For the moment, the marmot population and size booms have probably had little effect on the local ecosystem. While this seems to be a happy story for the marmots so far, the trend won’t likely end well — for the marmots or their ecosystem, Ozgul said.

Marmots are cold-weather creatures that have trouble adapting to heat, as do the nutritious and moist plants they eat. Meanwhile, the larger population has already started to attract more predators like coyotes to the area.

“This is an example of a system being slowly pushed out of balance,” Ozgul said. “Continuation of this exceptional long-term research will show us how much further a natural system can be pushed before we start to see negative impacts.”

Along with Imperial College London and the University of Florida, this study was also carried out by researchers at the University of Kansas; University of Sheffield; University of California, Los Angeles; and Stanford University. The work was funded by the National Science Foundation, Wellcome Trust, Natural Environment Research Council, National Institutes of Health and the National Institute on Aging.

Writer: Stu Hutson, 352-273-3569, stu@ufl.edu

Sources: Madan Oli, 352-846-0651, olim@ufl.edu

Arpat Ozgul, Imperial College London, a.ozgul@imperial.ac.uk

Kenneth Armitage, University of Kansas, marmots@ku.edu

Caption:

In this file photo released by the University of Florida, ecologist Arpat Ozgul holds a yellow bellied marmot during research on the animals in the Colorado Rocky Mountains. The research revealed that climate change has contributed to a substantial increase in the size and number of the mountain-dwelling rodents.(University of Florida/IFAS/Arpat Ozgul)

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