If you let a beaver build, mammals will come

Reference: Nummi, P., Liao, W., Huet, O., Scarpulla, E., and Sundell, J. (2019). The beaver facilitates species richness and abundance of terrestrial and semi-aquatic mammals. Global Ecol Conserv, 20: e00701. http://doi.org/10.1016/j.gecco.2019.e00701

Image of a North American beaver (Castor canadensis). Source: Photograph by Steve from Washington, DC, USA.

The beaver: a symbol of Canada that has a special place in each maple leaf heart of ours.  As a child growing up on a farm in Saskatchewan, Canada, the beaver wasn’t held in high regard in my household. They built dams in the most inconvenient places for my father. Their dams caused flooding to his cropland and their incessant noshing on trees made for precarious situations around power lines. It wasn’t until I was much older and studying ecology in university that I started to change my views towards this adorable, yet frustrating rodent that so many Canadians either LOVE or HATE.

Beavers are ecosystem engineers, natural builders that are compelled to change the environment around them. Does it sound like another animal? (throat clearing) Perhaps humans share this quality? Two stubborn and controlling species each battling to construct an environment that suits their needs best. The conflict between humans and beavers is so engaging that the Canadian Broadcasting Corporation (CBC) did a whole episode on this very issue!

A Petite History Lesson
American beaver (Castor canadensis) distribution map. Dark green: native region; Light green: introduced regions.

Early in the 20th century, beavers were on the verge of extinction due to over-harvesting during the fur trade. A beaver pelt was a hot commodity back in olden times. A century later, activists and conservationists were advocating for its protection. By the 1950s, beavers had returned to their original range. Today, the beaver population has made a partial comeback with an estimated number of 30 million in North America. Unfortunately, another species called the Eurasion beaver has not bounced back to the same degree. This has led to the introduction of the North American species into parts of Europe. Interestingly, all of us (even your old grandpap) have never seen a world with a prevalent beaver population. How did the natural environment look when 60 million beavers were in existence?

Much More Than A Pile Of Sticks
Beaver lodge in Ontario, Canada.

We are getting a glimpse at the circle of influence a beaver can have and we are learning to harness that innate compulsion of theirs to build. Scientific investigations into understanding beaver biology, have enlightened us to their benefits. Beaver dams assist in the creation of wetlands, which offers advantages such as slowing the flow of water to prevent flash flooding after heavy rainfalls, or filtering pollutants from the water naturally before it flows downstream. There is also scientific evidence that wetlands produced through beaver lodges/dams have increased the diversity and abundance of plants, insects, birds, fish and amphibian species to the area, many of which are defined as endangered or threatened.

Casual field observations seemed to support that beaver activities also attracted mammals to an area, but there was no concrete scientific evidence that proved that this was in fact true. A team of researchers in Finland decided to use their scientific toolbox to test this hypothesis: do beavers facilitate an increase in the diversity of mammal species in an area?

Scientist Eyes: They Are Watching You
Beaver dam. Source: Photograph by Lars Falkdalen Lindahl.

Nummi and his colleagues chose to conduct their experiment in the boreal forest of southern Finland where a watershed containing the Evo Lakes exists. The landscape in the area has been very stable for over two decades apart from the changes generated by North American beavers that were introduced into the region. The scientists chose 10 beaver-modified sites and 10 control sites within the watershed. The beaver modified sites were characterized as a wetland formed following the construction of a beaver dam in an outlet of a river or lake. The control sites were defined as a lake within the same drainage system as the beaver sites, but NOT altered by beaver activity.

Next, they went about setting up cameras! Each site had a least one stationary camera that was present at all times and another 10 cameras that rotated between sites every 2 weeks. The cameras were active for 24 hours of every day for a total of 8 months! That is a lot of pictures to go through! In addition, the scientists tracked the animals during the winter months. One to four days after the most recent snowfall, scientists would put on their boots and trek to each site to see if there were any visitors. Each site was visited 5 times over this period.

The Evidence

The data from both the cameras and snow tracking showed that the overall number of mammal species visiting the beaver modified sites was greater than the control sites. The cameras detected the presence of several terrestrial species, but particularly noteworthy was the greater numbers of moose, red fox and raccoon dogs that frequented the beaver modified sites. In addition, several semi-aquatic animals such as otters, weasels and pine martens tracks were found in significantly higher abundance around beaver modified sites than at control sites.

The animals listed above are particularly drawn to beaver modified sites for a number of reasons. Moose eat plants; the felled trees and lush aquatic plants at the beaver pond provide an accessible food source for them year-round. Red foxes are drawn to ponds to prey on the abundance of frogs present. Otters benefit from the wealth of dinner options available around beaver ponds as well as using the beaver’s lodges or dens for shelter and breeding. Moreover, beavers create ice holes around their lodges which allows otters to gain access to the water in winter and continue their foraging activities.

A lesson learned

A frustratingly compulsive builder like a beaver needs to be appreciated for the role they play. We need to take the time to observe, understand, and guide them in the right direction, so they can do what comes naturally to them. A family of beavers don’t necessarily realize how they influence the world, but the things they create have far reaching effects that support the existence of others. What can I say? I think I am a fan now.

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Anita Masse

Anita is currently a research manager/administrator for the University of Saskatchewan (Canada) branch of the EcoToxChip project. In 2016, she graduated with a MSc in Aquatic Ecotoxicology focusing on the reproductive and developmental effects of elevated dietary selenium on amphibians. She looks forward to imparting a "bite" of scientific knowledge that will empower readers to engage in discussions that can inspire change.

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