Why do beavers build dams? - Glynnis Hood
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The ability of beavers to build structures, such as dams and lodges, flood large areas, and dig extensive networks of channels is typical of ecosystem engineers. By altering aquatic and riparian habitats in dramatic ways, beavers increase the types and diverse structure of habitats that then support other species. Along with changing the physical nature of habitats, flooding and foraging by beavers also change the dynamics among species in these ecosystems. As such, beavers are often called a keystone species. In a stone arch, if the keystone is removed, the entire arch collapses. Similarly, if a keystone species is removed from the ecosystem can collapse as well. The presence of beavers is often associated with an increase in aquatic macroinvertebrates, waterfowl, other semi-aquatic mammals (e.g., river otters and muskrats), and many other plants and animals.
As a long-lived monogamous species, beavers can change landscapes over long periods of time. Typically, a family group (“colony”) will have two adults, two to three juvenile beavers, and two to three young of year (“kits”). Generally, juvenile beavers do not disperse to find their own territories and mates until they are at least two years old. As a rule, the adults are monogamous and mate for life. Having multiple generations in one colony provides more workers for maintaining dams and lodges, and for collecting woody stems for their winter food caches, which are critical for accessing food when the ponds are covered in ice for months on end. Beavers do not hibernate and are still active under the ice throughout the winter, a time when they also mate and later give birth to their kits.
Their ability to change landscapes to suit their ecological needs can bring beavers into conflict with humans. The flooding that follows the building of dams or plugging of culverts can be especially irksome for humans. Often beaver dams and/or the beaver colony are removed, only to be reoccupied by more beavers that then build more dams. If the habitat is suitable, it is only a matter of time before a dispersing beaver will choose it for its new territory. One beaver cannot move into another beaver family’s territory without a fight, and it is often the dispersing beaver who loses and continues its search for a new home.
Fortunately, there are several ways that beavers and humans can coexist. To counter flooding, flow devices, also called pond levellers are used so the associated wetland stays intact, and the human facilities remain undamaged. These devices involve the placement of a piping system through the dam at the same height as the desired water level for the pond. When the water gets too high, it flows through the pipe, only to stop once the pond returns to the desired water level. Keeping beavers on the landscape not only increases access to critical habitats for many species, their activities increase groundwater recharge and the availability of surface water for humans as well. Beavers have made a tremendous comeback from near extinction, and with that comeback our ecosystems and water resources have benefited as well.
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