(L-R): Lillie Strong and Celeste McEwan are second year Recreation, Fish and Wildlife students at Selkirk College. Photo: Submitted

(L-R): Lillie Strong and Celeste McEwan are second year Recreation, Fish and Wildlife students at Selkirk College. Photo: Submitted

Ecological Comment: The relationship between cottonwoods and beavers in Kootenay ecosystem

Submitted by Lillie Strong and Celeste McEwan

Not only are beavers our national animal, the largest rodent in North America, and skilled builders, they are a keystone species. A keystone species is a living thing that fills an essential role in their ecosystem that no other species can.

Our friendly beavers are ecosystem engineers and one of the only mammals capable of changing a habitat. The trees that beavers fall to construct their dams and lodges are also a source of food for them. Unfortunately, this practice has impacted populations of young cottonwoods negatively in out river (riparian) systems.

The question is: are the beavers harming the diversity of our area by selectively harvesting the trees they favor?

The importance of cottonwoods

Beavers are not the only animals who rely on cottonwoods for their homes, local bird species such as owls and woodpeckers use the trees’ hollows as nesting sites. Cottonwoods in riparian zones create shade which keeps water cool and provides healthy fish habitat. Riparian vegetation keeps riverbanks stable, thus, keeping the water clean.

As the deciduous cottonwoods lose their leaves, the organic matter becomes a vital part of the food chain sustaining small invertebrates and microorganisms which are in turn consumed by larger creatures. Cottonwood ecosystems are also valuable to humans as many enjoy using these areas for recreational activities. Since beavers and cottonwoods each play an integral role in our environment, it is important to find the balance between managing both species.

Cottonwood trees are the largest poplar species in North America. They grow quickly and can reach heights of 30 to 50 meters at maturity. The average lifespan of a cottonwood is 80 to 120 years, although some have even surpassed 400 years old.

We often see cottonwoods growing on riversides as they favour floodplains and damp soils, requiring moisture and many nutrients to grow successfully. Mature cottonwoods are rare in the area due to dam-regulated systems, the waterways are redirected, and streams have been channelled and dyked. Without these changes, much of our riversides would be thriving cottonwood ecosystems. The remaining trees are valuable and therefore need to be protected.

Managing the issue

Prior to 2021, staff at Selkirk College were accustomed to having one beaver lodge on the oxbow near the Castlegar campus. Noticable changes were discovered in spring when it was observed that the majority of the young cottonwood population was decimated.

Students of Selkirk College’s Recreation, Fish and Wildlife program responded to this issue by wrapping select trees with metal caging near the confluence of the Columbia and Kootenay rivers. This protects some young trees while still leaving others untouched for the beavers. Wrapping trees is a popular way to deter beavers from overharvesting. As this issue unfolds staff, students, and local professionals will continue to monitor the outcomes to ensure both the cottonwoods and beavers are thriving to promote a diverse and healthy ecosystem.

Lillie Strong and Celeste McEwan are second year Recreation, Fish and Wildlife students at Selkirk College.

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