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.

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.

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

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.


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

Beaver Behaviour and Biology

Catastrophic Failure of Beaver Dam At Chusnulida
The importance of beaver lodges in structuring
littoral communities in boreal headwater lakes

Justice Laws Website

Enduring the 5-cent coin


The Science and Spirituality of Trees

P1090816At the risk of sounding like a nineteenth century Romantic painter or poet, I feel that trees echo a part of the human spirit that otherwise remains silent in our day to day urban lives. Although their leaves exist to provide more surface area than what green branches alone would provide, trees assume a wide variety of hues in variable sunlight. Their matrices of lignin and cellulose form towering trunks spreading an intricate canopy over our view of the sky. From dawn to dusk, or from spring to autumn, the smells they evoke are not constant. With different wind speeds, a spectrum of rustling and swaying sounds are created. The symphony of colors, fragrances and sound waves of trees along with their imposing strength can interact with our memories and inner feelings. Although they are not of the intensity brought on by another human, trees evoke notes within us that no other living thing can replicate.

Existential issues aside, from the point of view of smaller organisms, a tree is in a sense its own forest. Over the ridges of the bark of some species, there are lichen—  algae and fungus, mutually helping each other* survive and serving as an indicator of pollution in cities. All tree-species provide niches and shelter for insects, birds and small mammals. Most trees would not survive if their roots were without the company of fungi known as mychorrhizas that help them absorb nutrients in exchange for carbohydrates.

And how they produce carbohydrates is a scientific wonder. Photosynthesis occurs in chloroplasts, structures which on a microscopic and evolutionary level reveal another partnership. A billion years ago, endosymbiosis, a process by which large cell engulfed smaller ones without killing them, led to the formation of plasmids. These evolved into the modern set of membranes and genetic material serving as the sites of photosynthesis in all plants.

The overall reaction of photosynthesis mocks what is actually occurring in the cells of trees: 6 CO2 + 6 H2O –> 6 O2 + C6H12O6 . Mix the reactants in vitro, and you would get nothing but carbonic acid! How does a tree or any plant manage to come up with a gas that is more often than not at the opposite end of the reaction arrow? How does it generate something sweet that flows through veins know as phloem, veins that the tree itself constructed from the same building blocks that it made with sunlight, with mychorrihizas’s transferred ions and the two official reagents water and carbon dioxide? Essentially through a network of cooperating cycles, chloroplasts absorb light frequencies in order to eject electrons from chlorophylls. These electrons are returned after they are ultimately taken away from water and transferred to molecules that use them to bond carbon dioxide. But the energy of the sun is not only invested in an electricity-like movement but in creating a voltage by temporarily isolating the hydrogen ions that also result when electron-yielding water splits into oxygen.

In the tropics there are at least 40 000 tree species but possibly more than 53 000. Temperate Europe, in contrast,  has only 124. Although forest cover has improved in Europe since the Middle Ages and has continued to do so recently, elsewhere on the planet areas with the most biodiversity have experienced the most loss. Here is a map showing which areas have done well and which haven’t between 1999 and 2012.mapping_world_trees

Landsat 7 data from 1999 through 2012 were obtained from a freely available archive at the United States Geological Survey’s center for Earth Resources Observation and Science (EROS).  More than 650,000 Landsat images were processed to derive the final characterization of forest extent and change.

From the vantage points of economics, carbon footprints and time management, it would make more sense for urban dwellers (now 54% of the planet) to access wooded areas as close to home as possible.

from National Geographic

In the United States, Pittsburgh, formerly known as the Steel City, has steadily reforested its surrounding hillsides, which had been previously cleared for logging and mining. Currently, this new growth forest along with four large parks occupy 42% of the urban area in Pittsburgh. Not only do the trees help filter pollution, avoid soil runoff in the sloped areas, they provide citizens a chance to have their spirits uplifted in an alternative way.

  • Postscript. Many biologists argue that the relationship only helps the fungus and not the algae. And yet some lichens show a three-way symbiosis involving a yeast. See  The previous reference gives no credit to Goward, a naturalist who first realized that a 3rd partner had to be involved in order to explain a mystery:  Bryoria fremontii, is hairlike, often brown and eaten by Northwestern indigenous peoples, but the lichen, Bryoria tortuosa,  is often yellow- green and toxic, with high levels of vulpinic acid. Yet both species had the same alga and fungus. It turned out that the toxic species had a lot more yeast.

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