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Scientists close in on explaining confounding predator-prey cycle

It’s a photo often found in biology textbooks: a snowshoe hare runs from a Canada lynx through a snowy Arctic clearing, a classic representation of predator versus prey.

This snapshot in time also represents one of the biggest mysteries in animal ecology.

Scientists have long known that snowshoe hare populations follow a 10-year boom and bust cycle. But what drives this cycle has fascinated ecologists for more than a century.

In a new study, scientists from the University of Florida and other institutions in the United States and Canada analyzed four decades of field data to reveal the mechanics driving these fluctuations.

“These 10-year cycles produce some of the foundational questions of animal ecology,” said Madan Oli, professor of wildlife ecology at UF/IFAS and the study’s lead author.

“For this study, we wanted to know: what’s happening within the hare population that’s driving that pattern?” Oli said. “We found that two factors drive the whole thing: how well the hares survive during the winter and how much they reproduce during the summer. Population crashes were triggered by low winter survival followed by poor summer reproduction. On the other hand, surges in population were triggered by increases in summer reproduction followed by higher winter survival.”

These findings support existing hypotheses that predators propel the boom and bust cycles.

“During the winter, every predator — lynxes, owls, coyotes, foxes — eats snowshoe hares. Experiments by Dr. Krebs and others indicate that predators, not lack of food or other factors, are what primarily kill the hares. Research by Rudy Boonstra, one of the co-authors of this paper, indicates that dealing with all these predators stresses the hares and reduces their ability to reproduce. These indirect predator effects are passed on from mother to offspring epigenetically. The demographic signature we see in the data backs these two hypotheses,” Oli said.

Oli, who specializes in evaluating animal populations from a statistical perspective, analyzed data collected by teams of researchers led by Charles Krebs, a professor at the University of British Columbia whom many ecologists consider the preeminent expert on snowshoe hare cycles.

Since the early 1970s, Krebs and his collaborators have gone to the Kluane National Park and Reserve in Canada’s Yukon Territory several times a year to count and tag snowshoe hares.

“When we’re up there, it can be as cold as minus 22 degrees Fahrenheit. It’s extremely remote, and there are hardly any people in the area, so it’s the perfect place to study an environment unaffected by human activity,” said Krebs, who is now 83 years old and retired, though that hasn’t stopped him from continuing to travel to the Yukon and collect data. He considered trips to the Yukon his “vacation time.”

“To do this work, it’s kind of an insanity,” he said with a laugh. “It’s my interest in the north that keeps bringing me back.”

This study shows the value of long-term ecological research, something that still is not very common, Krebs said. However, as climate change continues to affect places like the Yukon and elsewhere, long-term, collaborative studies will be needed to understand how ecosystems are responding, he said.

Even the 10-year snowshoe hare cycles could shift because of climate change, according to both Oli and Krebs.

“The snowshoe hare’s coat changes from brown to white for the winter. It’s camouflaged in snow. But what happens if there is less snow due to climate change — will predators have an easier time catching hares?” Oli said. “On the other hand, we know warmer temperatures will increase the amount of plant material available to the hares to eat. All these changes could affect the 10-year cycle.”

The new study appears in the journal Ecology.

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