Image Credit: Yellowstone National Park, CC BY 2.0, Image Cropped

Of wolves and bears: Seasonal drivers of interference and exploitation competition between apex predators (2021) Tallian et al., Ecological Monographs, https://doi.org/10.1002/ecm.1498

The Crux

I’ve written a lot about our relationship with top predators like bears and wolves on Ecology for the Masses, but their relationship with each other is also capable of having a big impact on their surroundings. When bears live in the same regions as wolves, predation levels are generally higher, but how much higher really depends on how much competition takes place between the two species.

Competition can take two forms out in the wild: interference competition, in which a bear might drive wolves away from a kill they’ve made, and exploitation competition, in which wolves have to search longer because bears have reduced the number of prey species in their area. Since both bears (through hibernation) and their prey species (through fixed mating cycles) vary in their behaviour throughout the year, could the type of competition that wolves face vary throughout the year as well? That’s what today’s authors wanted to find out.

What They Did

The study looked at wolves and bears in Scandinavia across two different time points. The first was early spring, when bears are emerging from hibernation and moose (the main prey for both species) have fairly large year-old calves. The second was summer, when the moose have just given birth. The authors also looked at bears and wolves in Yellowstone National park in the USA, where elk are the main prey source, but prey is generally more varied and includes bison, deer, and bighorn sheep.

Wolves were tagged and followed, and their prey items were examined after a kill was made, with the edible biomass of the prey estimated. The handling time was calculated by figuring out how long the wolves spent hanging around a kill, with most of the remaining period defined as search time. Their GPS locations were used to calculate their territories, and these were combined with brown bear density maps to figure out where bears and wolves co-occurred.

Handling and search time for each pack were then modelled against a number of variables, including brown bear density and time of the year. They also had to account for other variables like ‘hunger’ of the wolves. A larger pack will be hungier after consuming the same biomass as a smaller pack, and hungrier packs may behave in different ways.

Did You Know: Bears, Dogs and Bear-Dogs

Bears and canids (dog relatives) are commonplace in most parts of the world, yet both were preceded by the Amphicyonids, or Bear-Dogs, as they’re often known. These were a group of carnivores that evolved both in North America and Eurasia, having shared a common ancestor back when the two continents were connected by the Bering Land Bridge. When the Bering Land Bridge reconnected, the Amphicyons that had evolved in Eurasia moved back into North America and are thought to have wiped out their North American relatives. Ultimately the Amphicyonids were thought to have been wiped out roughly 9 million years ago by the arrival of more recognisable species, such as big cats, dogs and bears.

What They Found

In Scandivanian spring, bear density had a big impact on wolf handling time, suggesting that bears were driving wolves away from kills (i.e., interference competition). Bears rarely prey on moose just after emerging from hibernation, and in line with this there was no effect of bear density of wolf search time, indicating that exploitation competition doesn’t occur while bears are waking from hibernation.

On the contrary, during summer the presence of bears increased the time wolves spent searching for prey, suggesting that exploitation competition was taking place.

The results were slightly different in Yellowstone National Park in summer, where an increase in bear density increased wolf handling time at smaller kills, and decreased handling time at larger kills. This result is somewhat counterintuitive, since it suggests that bears are more likely to disturb wolves at small kills.

Problems

The comparison between the two time periods in Scandinavia is fascinating, which makes the glaring problem here the lack of a second time period (and of the ability to measure search time) in Yellowstone. The comparison between the two regions is also really interesting, but would have been much more so had the time comparison been present in both regions.

So What

I really enjoy any new studies or information on how top predators interact. They’re such key players in ecosystems, and how they affect each other has the capacity to influence a huge chunk of the northern hemisphere’s biodiversity. The switch from interference competition to exploitation competition is a fascinating piece of behavioural ecology, both on the sides of wolves and bears.

From a more practical standpoint, top predators like bears and wolves are starting to trickle back into their former native ranges. Figuring out what sort of effect they’re going to have on local herbivores is important for conservationists, academics, and even for hunters.

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Dr. Sam Perrin is a freshwater ecologist who completed his PhD at the Norwegian University of Science and Technology who misses the Great Barrier Reef and that turtle he swam with once. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.