Are Wolves Wiping Out Beaver Populations?

 

Fake anti-wolf statements such as "They have killed nearly every deer in this area. They need shooting!" are on most Face Book groups looking at wolves. One of the latest6 lies is that wolves are wiping out beavers.

The Voyageur Wolf Project responded to this:

"Are wolves decreasing the beaver population in our area via predation? An interesting question and one we have continuously examined since starting our research. We thought folks might enjoy learning about the research and science that answers that question!

"The first thing to note is that beaver populations are pretty resilient to any form of lethal removal, whether that is human-caused or via predation. For instance, studies examining how human hunting/trapping affects beaver populations have found that somewhere around 25-33% of beaver populations can be killed before populations decline.

"In other words, a large proportion of the beaver population can be killed without decreasing the population year over year. This is, in part, why trappers can sustainably trap beavers in the same areas year after year without decreasing the population. This happens in portions of the Greater Voyageurs Ecosystem (our study area).

"The second thing to note is that the beaver population in the GVE has remained at a high-density for 30-40+ years, despite a sustained high-density wolf population for a similar period. Of course, the beaver population has fluctuated but there is no indication such fluctuations are driven by wolf predation.

"This past fall we mapped out and counted all active beaver lodges in the GVE. The average beaver density across the GVE was roughly 1 beaver lodge per square kilometer (that is high-density!). The typical beaver lodge in our area has ~5.3 beavers in it.

"And this is where wolves come in.

"First to estimate the impact of wolf predation, we need to know how many beavers wolves kill. Based on our work, the most beavers we have documented a wolf killing during the open water season (~April to October) was 42. Only one other wolf has come close to that with 38 beavers.

"Most wolves kill way fewer than that with a typical wolf killing somewhere between 8-15 beavers per year. But for the sake of argument and illustration, let's assume the high end of that range: that the typical wolf kills 15 beaver per year.

"A typical wolf territory in our area is about 100 square kilometers, which means that there are roughly 530 beavers (100 beaver lodges x 5.3 beavers per lodge) per wolf territory on average.

"An average pack is about 4.5 wolves in our areaso if each wolf kills 15 beavers, then the typical pack removes 67.5 beavers per year, which equates to 13% of the beaver population. This is well below the 25-33% removal rate needed to induce a decrease in the beaver population.

"Even if each wolf killed 20 beavers per year on average, wolves would still only remove about 17% of the population, well below amount needed to decrease populations. And the average wolf is not killing 20 beavers per year in our area.

"Supporting our data from the wolf side of things, was a study from the beaver side of things.

"A few years back, a study examining the survival rate of beavers in Voyageurs National Park determined that beaver mortality rates were 0.22-0.24 (i.e., 22-24% of beavers die each year from myriad sources of mortality including predation, disease, starvation, etc).

"And again, that overall level of beaver mortality does not appear sufficient to induce beaver population declines which is why the beaver population has continued to remain at high and stable densities in the area.

"In sum, wolves simply do not kill enough beavers each year to induce population declines. And our area has some of the highest-sustained wolf populations documented. In other words, if there was any place wolves could/should impact beavers, one would anticipate it would be our area.

"Now, one could argue that wolves would have a greater impact on beavers in areas with less dense beaver populations. That certainly seems plausible, however, the number of beavers wolves kill each year is very likely a result of the beaver density in an area.

"In other words, as beaver populations become denser, wolves likely kill more beavers and vice versa (ecologists call this the “functional response” of a predator).

"So if there are fewer beavers, wolves very likely kill fewer of them and thus the percent of the beaver population killed by wolves could very well be the same as in our area or even potentially lower.

"Some evidence exists, per a study in 2020, that wolves likely are not influencing beaver population size in other areas of Minnesota with less beavers than our area. That study looked at population trends from 1975-2002 of 15 beaver populations across northern Minnesota.

"The study found that wolves did not appear to influence beaver population growth rates or population change. Rather myriad other factors influenced beaver population change including beaver dispersal patterns, territoriality, winter temperature, and precipitation.

"Ok, we could continue on as this is a pretty interesting topic but we will end it there for today because otherwise, this post is going to get even longer than it currently is!

"Sources used/referenced for this post:

Gable et al. 2023. Wolf-beaver dynamics in the Greater Voyageurs Ecosystem in the book The Wolf: Culture, Nature Heritage.

Gable et al. 2018. The forgotten prey of an iconic predator: a review of interactions between gray wolves and beavers. Mammal Review.

Johnson-Bice et al. 2020. Ecological forecasts reveal limitations of common model selection methods: predicting changes in beaver colony densities. Ecological Applications.

Smith et al. 2016. Do transmitters affect survival and body condition of American beavers? Wildlife Biology.

Gable and Windels. 2018. Kill rates and predation rate of wolves on beavers. Journal of Wildlife Management.

Parker et al. 2002. Rapid rebound in colony numbers of an over-hunted population of Eurasian beaver. Wildlife Biology.

Novak. 1987. Beaver. A chapter in Wild Furbearer Management and Conservation in North America.

Rosell and Campbell-Palmer. 2022. Beaver: ecology, behavior, conservation, and management."