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Beavers: The Boreal Engineers

Mid June 2024

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Featured Species: Can’t Live Without the Dam Beavers  

Beavers that have been entombed in their lodges all winter are out and active. They are busy repairing damages to their dams, and in case of dispersed young, building new ones. Beavers are nocturnal and meticulously maintain their dams, so don’t be surprised if any damage to a dam is repaired by the next morning!

Beavers are good swimmers, but fairly slow and vulnerable on land.  They build dams to raise the water level upstream high enough that it doesn’t freeze in the winter, creating suitable and safe habitat for their lodge and winter food cache, and easier access to stands of Trembling Aspen (a favoured food). The lodge is built so that it has above ground chambers, but only aquatic entrances/exits that are inaccessible to land predators. Their winter food cache is placed under water so that in the winter they never have to go out on land.

Beavers prefer waterways with a firm, mud bottom that aren’t too deep or fast flowing. Trees, sticks, rocks, mud, and grass are used as construction materials.  Trees up to about 40 cm in diameter can be felled, and if needed two Beavers will work together.  Dams are built at narrow points in the waterway where the current is fastest. To begin a dam, Beavers jam in branches with the butt ends facing upstream secured with mud and stones, using the current to spread and securely embed them. Layer upon layer, branches and sticks are weaved in, and stones, roots, and mud are packed in. A pair of beavers needs just a few days to build a basic dam.  Large dams can be as high as 5m, and as wide as 33m (note how these ‘engineers’ curved the dam for greater strength).  Dams are maintained throughout the year, but the most material is added during periods of high water, typically in the spring. 

These Beaver-created wetlands, found across almost all of North America, result in the loss of habitat for some species, and creation of habitat for others. Because of this role, the Beaver is considered a keystone species.  Flooded trees die and attract woodpeckers. Sediments and organic materials accumulate upstream, which increase bottom-dwelling invertebrates that feed on the debris.  By felling trees more sun reaches the area, causing increases in plankton, which in turn increases larger aquatic invertebrates.  Habitat is created for aquatic plants like WatershieldCommon Bladderwort, White Water Lily, and Bullhead Lily

Dragonfly and damselfly larvae, whirligig beetles, and water striders are common residents that are preyed upon by Bullfrogs, Green Frogs, Mink FrogsEastern Kingbirds, and Tree Swallows.  Beaver ponds also attract waterfowl such as Wood Ducks and Black Ducks.  The threatened Blanding’s Turtle may use the lodge and dam to sun on.  Moose feed on the water lilies and new shoots. Downstream habitat is likely important for the very rare Hungerford’s Crawling Water Beetle.

Natural succession occurs (scroll down to graphic) when Beavers leave the area once their food supply, such as Trembling Aspen, has been depleted, or when a colony is killed. The dam breaks, the area drains, and nutrient rich muck that was previously the pond bottom is colonized by sedges and grasses, and turns into a lush meadow for species such as the Savannah Sparrow, and Meadow Jumping Mouse. Eventually, sun-loving Trembling Aspen return, attracting Beavers yet again.

Other Happenings:

  • Loons are incubating their eggs. They build their nest near shorelines and islands on a pile of aquatic vegetation. Usually the same nest site is used year after year. Loons are easily scared off their nests, which can cause egg chill and increase the likelihood of predation. The low-lying nests are also vulnerable to swamping from boaters.  Boaters can help by slowing down near shorelines and avoiding nesting areas.  Loons will also nest on loon nest platforms built to attract loons to lakes without suitable habitat.  If you consistently paddle a particular lake, consider joining the Canadian Lakes Loon Survey to help monitor population health.
  • Male Mallards start moulting. Males only stay with females for about the first 10 days of incubation. They then leave breeding territories, and hide out in vegetation while they moult all their feathers, which renders them flightless. Their new “eclipse moult” resembles the female plumage, not their brilliant breeding plumage.  Males can still be distinguished by their more yellow bill.
  • Meanwhile, moths are trying their best to avoid bats.  To do so, they have a few handy adaptations.  Moths have ears sensitive to bats’ echolocation calls .  If however, they are surprised at close quarters they fold their wings and dive-bomb directly down.  Tiger moths (family Arctiidae) even produce their own ultrasonic sounds that serve to advertise that they are poisonous (many of the larvae, like Monarch Butterflies, feed on milkweed).  These sounds are not emitted until bats are within a meter, and may also serve to startle bats.  Some moths even have sound-absorbing ‘stealth’ scales to reduce their echolocation signature.
  • Giant silk moths are mating. In fact mating is about all they do - the adults don’t even eat! The females attract males by releasing airborne sex pheromones in extremely small quantities. The large, feathery-looking antennae of males are able to detect these chemical molecules as far as 5km away! Luna and Cecropia Moths are two well-known giant silk moths. Others include the Polyphemus, the Promethea, and the Io Moth. They are attracted to bright, white lights particularly near water.   Note the eyespots on many of the wings.  These may serve to startle predators, or make them think that the moth is actually a predator itself.
  • Brook Sticklebacks are spawning and like members of the Sunfish Family it is the male that does most of the work.  The male builds a nest with vegetation, small sticks, and a sticky secretion produced by his kidneys. He entices one or more females to lay eggs in it. The male also fans and defends the eggs and young.

  INVASIVE ALERT – Purple Loosestrife

  • One of the prettiest wetland flowers is also one of the most unwanted!  Nicknamed “beautiful killer”, Purple Loosestrife is an invasive species that has spread throughout wetlands across Canada.  The attractive, tall spikes of purple flowers led to the plant being imported and sold in garden centres.  Unfortunately like many alien species it soon escaped into wetland ecosystems where it outcompetes native plants and provides absolutely no habitat value to wildlife.  The only species that really enjoy munching on the plant are two types of leaf eating beetles known as  Galerucella sp.  These tiny insects have become the soldiers in the battle against this invader but it is important that all of us help stop this invasive species at its source by not planting it in our gardens.  There are many other purple flowers that we can enjoy which won’t harm our sensitive wetlands. 
  • Yellow pollen from Eastern White and Red Pines will dust lakes and flat surfaces for a week or more. The pollen grains  have two air bubbles to make it light so that it can be carried by the wind to facilitate pollination.
  • The summer solstice occurs on June 21st this year.  People have been celebrating this event for thousands of years, so consider planning your own celebration of the Earth’s longest day. If you’re an early riser, sunrises are at their earliest the week before the solstice. Find out why.                                                                                                    
  • Mercury has vanished behind the sun, but Jupiter, Mars and Saturn remain visible in the east to southeast sky before dawn (scroll down).