The Science of Canada’s Symbol, the Beaver

2018BIGCOINSUB-7The beaver, Castor canadensis, is an official symbol of Canada, somehow representing our sovereignty. Each time we pick up a 5 cent-coin, the so-called nickel, which except for special collectors’ editions is about 95% steel and only 2% nickel, we see an illustration of a beaver. But how much do we know about the natural history and ecology of our icon?

Like many humans, beavers are monogamous and mate for life. They also impact both the physical landscape and biological diversity in their habitat.  Their exact impact varies from one site to another, depending on the location, relief and habitat type—again parallel to the non-uniform ecological footprint of our societies.

During dry periods, as much as 30% of  water in certain watersheds could be held in beaver ponds. This can decrease erosion when water flow increases to higher levels. If a beaver dam however collapses, the opposite effect can occur. Flooding was  caused by such an occurrence in Alberta the 1990s and in British Columbia in the summer of 2000.

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A beaver chewing on a cottonwood. This will lead eventually to the tree’s production of shoots rich in protective compounds.

The presence of beavers is important for shaping the littoral communities in certain lakes of the Canadian Shield increasing the population of fish, crayfish, diving beetles, large bugs, tadpoles, newts and leeches. This happens not just from the changing water levels but because dams concentrate nutrients.

They are also engaged in a fascinating coevolutionary relationship with the type of trees they use to build dams. Regrowth of cottonwood trees felled by beavers results in the synthesis of much higher levels of phenolic glycosides. These plant compounds then serve as a defence against other mammalian herbivores and beaver themselves, ensuring the long term survival of the cottonwoods. Another beaver-target, the quaking aspen, also uses a chemical defence against beavers. Younger trees, although easier to take down, are avoided by beavers because juvenile suckers contain higher concentrations of salicin, salicortin, tremulacin, and tremuloidin. Juvenile suckers are asexual shoots produced by trees that have been cut down but which still have living roots.

glycosides
Notice that each of the above compounds consists a simple sugar linked to a phenolic compound by replacement of a hydroxyl group in the sugar molecule—hence their name: phenolic glycosides, which protect trees against herbivores. The compounds’  concentrations was measured by HPLC-analysis after methanol extraction. (Structures from chemspider.com)

In the ecological web of mammals, it’s not surprising to see beavers play a more direct role than the consequences of their influence on plant biochemistry.  The world’s second largest rodent is an important food source for wolves and black bears. Abandoned beaver lodges can provide breeding shelters for bobcats and winter shelters for badges and red foxes.

Sources

Ecological impact of beavers Castor fiber and Castor canadensis
and their ability to modify ecosystems Mammal Rev. 2005, Volume 35, No. 3&4, 248–276

Optimal central-place foraging by beavers:
Tree-size selection in relation to defensive chemicals
of quaking aspen https://www.researchgate.net/publication/30848534_Optimal_central-place_foraging_by_beavers_Tree-size_selection_in_relation_to_defensive_chemicals_of_quaking_aspen

Beaver Behaviour and Biology
http://www.beaversolutions.com/beaver-facts-biology-behavior/

Catastrophic Failure of Beaver Dam At Chusnulida
The importance of beaver lodges in structuring
littoral communities in boreal headwater lakes http://www.nrcresearchpress.com/doi/pdf/10.1139/z97-121

Justice Laws Website http://lois-laws.justice.gc.ca/eng/acts/N-17/page-1.html

Enduring the 5-cent coin http://www.mint.ca

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