Leopold wolf following grizzly bear. (Provided by Doug Smith, April 2005, via the University of Montana)
Bears are thieves, among the predators that steal carcasses from wolves.
A new study based partly in Yellowstone National Park sheds light on the interactions of these animals and how they compete for food, such as elk or moose. Aimee Tallian, lead author of the international study, said it’s one more puzzle piece that helps build on the collective understanding of a system people care about and want to understand.
“Competition is rampant in nature,” said Tallian, a scientist with the Norwegian Institute for Nature Research. “It’s rampant in society. It drives everything, really, from the couple blades of grass competing for water and sunlight to apex predators.”
Published in Ecological Monographs of the Ecological Society of America, the study was conducted using data partly from Yellowstone and partly from Scandinavia. Co-author Matthew Metz, who recently earned his doctorate in wildlife biology from the University of Montana, said the results come out of years of researchers tromping after animals in the field and recording data.
“The work is built on all the effort of all the people going out and hiking to these (sites),” said Metz, a research associate with the Yellowstone Wolf Project. “I used to be in the field a lot. And now, I’m almost never in the field. These sorts of analyses are really the byproduct of the work of hundreds of people in the field.”
At a basic level, the study looked at “the predation sequence,” Tallian said, made up of “handling time,” or the time animals spend slogging around with their prey and eating it, and then “search time.” She said the research evaluated the difference for wolves when bears were present, or how bears affected the predatory sequence for wolves.
As it turns out, bears have an effect on how wolves behave, the research found. The researchers also noted the effect is different than the impact bears have on larger solitary cat species, according to UM.
Whereas cats are more likely to give up their prey and start hunting again if a bear shows up, wolves spend more time at a kill site, an increase in “handling time,” and they kill less when a bear pays a visit, Tallian said. She said that data point suggests the wolves are spending more time defending their kills, possibly because they can protect the resource as a group, or they’re moving between carcasses to avoid bears.
“Wolves are sticking around, or they are not giving up that resource so quickly,” Tallian said. “Why is a really great question … For cats, it’s not worth sticking around.”
One takeaway in this study is that just because species live together in the same place doesn’t mean they don’t affect each other, she said. But she said every single piece of information is helpful to better understanding the landscape.
“These pieces of information give us a picture, a better picture and understanding, of how these systems work,” Tallian said. “And these are our systems to protect and care for and manage and argue about. Every single piece gives us a clue about what’s happening.”
Metz does most of his work behind a computer these days, but said he’s been fortunate to spend a lot of time in the field and watch a number of interactions between wolves and bears. In a typical exchange, the wolves have a kill, the bears take it over, the wolves wait or maybe move on.
“It’s kind of like Groundhog Day in that scenario,” Metz said.
Sometimes, into the earlier part of winter, he said large males will even follow wolves to their next kill site. He said researchers track GPS locations from wolf collars and go into a place after a wolf has left in order to not affect their behavior.
A good day in the field might mean observing an interesting wildlife interaction, but it might just be a day when the bugs aren’t as bad as expected, he said. In this case, Metz said the most rewarding part of the work was the chance to combine the data from Yellowstone with data from Scandinavia and look at how the systems might be similar or different.
Tallian said the differences start with the presence of the species in both locations: “Bears and wolves are sympatric, they live together throughout Yellowstone. But in Scandinavia, there are wolf populations that are persistent in areas with bears, and wolf populations that persist without bears.”
In Scandinavia, the study said, wolves that lived with bears in spring spent more time handling prey compared to wolves that didn’t live with them. It said this finding corresponds with previous research that suggested wolf kill intervals were longer in places wolves shared the landscape with bears.
However, the study points to one critical similarity between the landscapes as well: “In both systems, handling time and/or time spent near a kill site increased as the distance of the carcass site to the closest road increased during summer, suggesting that human disturbance probably limits wolf foraging ability, particularly during this period. This is important, as anthropogenic disturbance may increase predation pressure if predators cannot efficiently use their kills, ultimately shaping the role of large carnivores on ecosystem function.”
Both wolves and bears are top predators, Tallian said. As such, both affect their ecosystems, and there’s a long running debate about how they do so.
“There’s no one single piece of research that’s going to crack the case,” she said. “You get there through consistent testing and consistent looking into the different pieces of the spectrum and gaining understanding.”
Plus, she said, the work is rewarding on its face too: “Wolves and bears are interesting. They capture the imagination, and so they’re just a fun species to learn about.”
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