Last month's article described the unique physical and behavioural adaptations of beaver (Castor canadensis). This article will discuss the essential influence this rodent has on its environment.
An ecosystem engineer is an organism that creates, modifies and maintains habitats. Most organisms alter their physical environments in some way, so the term ecosystem engineer is applied only to certain key species that have a profound and wide-ranging influence, changing the distribution and diversity of flora and fauna in their locality. Scientists describe two types of ecosystem engineers.
Autogenic engineers modify the environment using their own physical structures. Trees and coral are two important examples.
Allogenic engineers transform living or non-living material from one form to another using mechanical or other means. The beaver is second only to humans in this capacity.
The beaver engineers its environment in several ways, most obviously by cutting trees and building dams, but the building of lodges and canals is alos important.
It is the only species besides humans that is capable of cutting down mature trees. Beaver activity changes the forest structure and diversity of tree species. The animal will use a wide range of trees, but when given a choice will cut its preferred foods, particularly poplar and willow. Willows and maples send up shoots from the cut stumps, but poplars and some other tree species do not. Beaver often clear-cut the areas around their ponds. In other cases, mature trees may become replaced by a dense undergrowth of willow or other shoots. Other tree species become scarce and may be replaced by ones that the beaver does not favour, such as ash. Other timber along the watercourse may be killed by flooding.
In one year a family of six can consume 0.4 hectares (1 acre) of poplar trees, and is estimated to fell one metric ton of wood. If the family exhausts its food source, it will move to a new location.
This tree-cutting alters forest succession. If the beaver creates forest openings, sun-loving plants may take hold, converting a mature forest to an early successional stand. Sometimes, however, the beaver hastens forest maturity by selecting willows and poplars, which are early succession trees, allowing the rapid development of understory saplings such as fir and spruce.
Dam-building changes the flow of water through the stream. Beaver require deep slow-moving water for storing food, constructing lodges and moving around safely. This is why they build dams. The still water in a beaver pond attracts species normally associated with lakes rather than streams, while species dependent on fast water die out or move elsewhere. In addition, lodges and dams
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