Environmental impacts of beavers
[17] Trumpeter swans (Cygnus buccinator) and Canada geese (Branta canadensis) often depend on beaver lodges as nesting sites.
Removal of some pondside trees by beavers increases the density and height of the grass–forb–shrub layer, which enhances waterfowl nesting cover adjacent to ponds.
[22] Coppicing of waterside willows and cottonwoods by beavers leads to dense shoot production which provides important cover for birds and the insects on which they feed.
[24] As trees are drowned by rising beaver impoundments, they become ideal nesting sites for woodpeckers, which carve cavities that attract many other bird species, including flycatchers (Empidonax spp.
[25] By perennializing streams in arid deserts, beavers can create habitat which increases abundance and diversity of riparian-dependent species.
[28] This study also found that beaver ponds increased smolt salmon production 80 times more than the placement of large woody debris.
[35] Most beaver dams do not pose barriers to trout and salmon migration, although they may be restricted seasonally during periods of low stream flows.
[39] In Oregon coastal streams, beaver dams are ephemeral and almost all wash out in high winter flows only to be rebuilt every summer.
[41] Downstream migration of Atlantic salmon smolts was similarly unaffected by beaver dams, even in periods of low flows.
Enos Mills wrote in 1913, "One dry winter the stream ... ran low and froze to the bottom, and the only trout in it that survived were those in the deep holes of beaver ponds.
In contrast, the most often cited negative impact of beavers on fishes were barriers to migration, although that conclusion was based on scientific data only 22% of the time.
Specifically, slow developing northern red-legged frogs (Rana aurora) and northwestern salamanders (Ambystoma gracile) were found almost exclusively in beaver-dammed locations, suggesting that these amphibians depend on beaver-engineered microhabitats.
[54] Not only have aspen and cottonwood survived ongoing beaver colonization, but a recent study of ten Sierra Nevada streams in the Lake Tahoe basin using aerial multispectral videography has also shown that deciduous, thick herbaceous, and thin herbaceous vegetation are more highly concentrated near beaver dams, whereas coniferous trees are decreased.
[56] In a second study of riparian vegetation based on observations of Bridge Creek over a 17-year period, although portions of the study reach were periodically abandoned by beaver following heavy utilization of streamside vegetation, within a few years, dense stands of woody plants of greater diversity occupied a larger portion of the floodplain.
[59] In the 1930s, the U.S. government put 600 beavers to work alongside the Civilian Conservation Corps in projects to stop soil erosion by streams in Oregon, Washington, Wyoming, and Utah.
[63] In a pilot study in Washington, the Lands Council is reintroducing beavers to the upper Methow River Valley in the eastern Cascades to evaluate its projections that if 10,000 miles (16,000 km) of suitable habitat were repopulated, then 650 trillion gallons of spring runoff would be held back for release in the arid autumn season.
This project was developed in response to a 2003 Washington Department of Ecology proposal to spend as much as $10 billion on construction of several dams on Columbia River tributaries to retain storm-season runoff.
Initially, these were made by felling fir logs, pounding them upright into the stream bed, and weaving a lattice of willow sticks through the posts, which beavers would then expand.
[71] Eric Collier's 1959 book, Three Against the Wilderness, provides an early description of a string of beaver ponds serving as a firebreak, saving the home of his pioneer family from a wildfire in interior British Columbia.
[73] One month after the Sharps Fire burned 65,000 acres (260 km2) in Idaho's Blaine County in 2018, a lone surviving green ribbon of riparian vegetation along Baugh Creek was observed, (see image) illustrating how a string of beaver ponds resists wildfires, creating an "emerald refuge" for wildlife.
[74][75] Lastly, two studies of the Methow River watershed, after the 2014 Carlton Complex Fire burned 256,000 acres (1,040 km2) in north central Washington State, have shown that beaver dams reduced the negative impacts of wildfire on sediment runoff, reduced post-wildfire sediment and nutrient loads, and preserved both plant and macroinvertebrate communities.
Methods of damage prevention includes the placement of a mesh wire fence around the tree trunk, planting trees less palatable to beavers near shorelines, placing under-dam drainage systems to control water levels; and placing traps designed to kill instantly, as Alberta Environment and Parks does not allow the relocation of caught beavers to other areas.
[85] The beaver population at Gatineau Park is monitored by the National Capital Commission in an effort to protect local infrastructure, and maintain public safety.
[80] The city's urban forestry department will occasionally install heavy mesh wire fences around the trunks of trees to prevent them from being damaged by beavers.
[90] The Vancouver Park Board approved a strategy that included plans to promote the growth of the beaver population near the Olympic Village in 2016.
[92] After receiving complaints for beaver-related damages in 2012, the city of Winnipeg has placed mesh wire fence around tree trunks along the shore of the Assiniboine River during the winter; as well as laid down traps designed to kill the beavers.
In March 2009, they hired an exterminator to remove a beaver family using live traps, and accidentally killed the mother when she got caught in a snare and drowned.
[114] In the 1940s, beavers were brought to Tierra del Fuego in southern Chile and Argentina for commercial fur production and introduced near Fagnano Lake.
[115] In contrast, areas with introduced beaver were associated with increased populations of the native catadromous puye fish (Galaxias maculatus).
[118] On balance, most favour their removal because of their landscape-wide modifications to the Fuegian environment and because biologists want to preserve the unique biota of the region.
