The beaver is a keystone species, increasing biodiversity in its territory through creation of ponds and wetlands. As wetlands are formed and riparian habitats enlarged, aquatic plants colonize newly available watery habitat. Insect, invertebrate, fish, mammal, and bird diversities are also expanded. Effects of beaver recolonization on native and non-native species in streams where they have been historically absent, particularly dryland streams, is not well-researched.
Effects on stream flows and water quality Beaver ponds increase stream flows in seasonally dry streams by storing run-off in the rainy season, which raises groundwater tables via percolation from beaver ponds. In a recent study using 12 serial aerial photo mosaics from 1948 to 2002, the impact of the return of beavers on openwater area in east-central Alberta, Canada, found that the mammals were associated with a 9-fold increase in openwater area. Beavers returned to the area in 1954 after a long absence since their extirpation by the fur trade in the 19th century. During drought years, where beavers were present, 60% more open water was available than those same areas during previous drought periods when beavers were absent. The authors concluded that beavers have a dramatic influence on the creation and maintenance of wetlands even during extreme drought.
From streams in the Maryland coastal plain to Lake Tahoe, beaver ponds have been shown to remove sediment and pollutants, including total suspended solids, total nitrogen, phosphates, carbon, and silicates, thus improving stream water quality. In addition, fecal coliform and streptococci bacteria excreted into streams by grazing cattle are reduced by beaver ponds, where slowing currents lead to settling of the bacteria in bottom sediments.
Following findings that the parasite Giardia lamblia, which causes giardiasis, was putatively carried by beavers, the term "beaver fever" was coined by the American press in the 1970s. Further research has shown that many animals and birds carry this parasite, and the major source of water contamination is by humans. Recent concerns point to domestic animals as a significant vector of giardia, with young calves in dairy herds testing as high as 100% positive for giardia. New Zealand has giardia outbreaks, but no beavers, whereas Norway has plenty of beavers, but had no giardia outbreaks until recently (in a southern part of Norway densely populated by humans but no beaver).
In 2011, a Eurasian beaver pair was introduced to a beaver project site in West Devon, consisting of a large enclosure with a long channel and one pond. Within five years, the pair created a complex wetland with an extensive network of channels, 13 ponds and dams. Survey results showed that the created ponds hold of sediment, which stores of carbon and of nitrogen. Concentrations of carbon and nitrogen were significantly higher in these ponds than farther upstream of this site. These results indicate that the beavers' activity contributes to reducing the effects of soil erosion and pollution in agricultural landscapes.
Effects on animals Bird abundance and diversity
Beavers help waterfowl by creating increased areas of water, and in northerly latitudes, they thaw areas of open water, allowing an earlier nesting season. In a study of Wyoming streams and rivers, watercourses with beavers had 75-fold more ducks than those without.
Trumpeter swans (Cygnus buccinator) and Canada geese (Branta canadensis) often depend on beaver lodges as nesting sites. Canada's small trumpeter swan population was observed not to nest on large lakes, preferring instead to nest on the smaller lakes and ponds associated with beaver activity.
Beavers may benefit birds frequenting their ponds in several additional ways. 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. Both forest gaps where trees had been felled by beavers and a "gradual edge" described as a complex transition from pond to forest with intermixed grasses, forbs, saplings, and shrubs are strongly associated with greater migratory bird species richness and abundance. 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. Widening of the riparian terrace alongside streams is associated with beaver dams and has been shown to increase riparian bird abundance and diversity, an impact that may be especially important in semiarid climates.
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.), tree swallows (Tachycineta bicolor), tits (Paridae spp.), wood ducks (Aix sponsa), goldeneyes (Bucephala spp.), mergansers (Mergus spp.), owls (Tytonidae, Strigidae) and American kestrels (Falco sparverius). Piscivores, including herons (Ardea spp.), grebes (Podicipedidae), cormorants (Phalacrocorax ssp.), American bitterns (Botaurus lentiginosa), great egret (Ardea alba), snowy egret (Egretta thula), mergansers, and belted kingfishers (Megaceryle alcyon), use beaver ponds for fishing. Hooded mergansers (Lophodytes cucullatus), green heron (Butorides virescens), great blue heron (Ardea herodias) and belted kingfisher appeared more frequently in New York wetlands where beaver were active than at sites with no beaver activity.
By perennializing streams in arid deserts, beavers can create habitat which increases abundance and diversity of riparian-dependent species. For example, such as the upper San Pedro River in southeastern Arizona, reintroduced beavers have created a willow and pool habitat which has extended the range of the endangered Southwestern willow flycatcher (Empidonax trailii extimus) with the southernmost verifiable nest recorded in 2005.
Bats Beaver modifications to streams in Poland have been associated with increased bat activity. While overall bat activity was increased, Myotis bat species, particularly Myotis daubentonii, activity may be hampered in locations where beaver ponds allow for increased presence of duckweed.
Trout and salmon
Beaver ponds have been shown to have a beneficial effect on trout and salmon populations. Many authors believe that the decline of salmonid fishes is related to the decline in beaver populations. Research in the Stillaguamish River basin in Washington found that extensive loss of beaver ponds resulted in an 89% reduction in coho salmon (Oncorhynchus kisutch) smolt summer production and an almost equally detrimental 86% reduction in critical winter habitat carrying capacity. This study also found that beaver ponds increased smolt salmon production 80 times more than the placement of large woody debris. Swales and Leving had previously shown on the Coldwater River in British Columbia that off-channel beaver ponds were preferentially populated by coho salmon over other salmonids and provided overwintering protection, protection from high summer snowmelt flows and summer coho rearing habitat. Beaver-impounded tidal pools on the Pacific Northwest's Elwha River delta support three times as many juvenile Chinook salmon (Oncorhynchus tshawytscha) as pools without beaver.
The presence of beaver dams has also been shown to increase either the number of fish, their size, or both, in a study of brook trout (Salvelinus fontinalis), rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) in Sagehen Creek, which flows into the Little Truckee River at an altitude of in the northern Sierra Nevada. These findings are consistent with a study of small streams in Sweden, that found that brown trout were larger in beaver ponds compared with those in riffle sections, and that beaver ponds provide habitat for larger trout in small streams during periods of drought. Similarly, brook trout, coho salmon, and sockeye salmon (Oncorhynchus nerka) were significantly larger in beaver ponds than those in unimpounded stream sections in Colorado and Alaska. In a recent study on a headwater Appalachian stream, brook trout were also larger in beaver ponds.
Most beaver dams do not pose barriers to trout and salmon migration, although they may be restricted seasonally during periods of low stream flows. In a meta-review of studies claiming that beaver dams act as fish passage barriers, Kemp et al. found that 78% of these claims were not supported by any data. In a 2013 study of radiotelemetry-tagged Bonneville cutthroat trout (Oncorhynchus clarki utah) and brook trout (Salvelinus fontinalis) in Utah, both of these fish species crossed beaver dams in both directions, including dams up to high. Rainbow, brown, and brook trout have been shown to cross as many as 14 consecutive beaver dams. Both adults and juveniles of coho salmon, steelhead trout, sea run cutthroat (Oncorhyncus clarki clarki), Dolly Varden trout (Salvelinus malma malma), and sockeye salmon are able to cross beaver dams. In southeast Alaska, coho jumped dams as high as two meters, were found above all beaver dams and had their highest densities in streams with beaver. In Oregon coastal streams, beaver dams are ephemeral and almost all wash out in high winter flows only to be rebuilt every summer. Migration of adult Atlantic salmon (Salmo salar) may be limited by beaver dams, but the presence of juveniles upstream from the dams suggests that the dams are penetrated by parr. Downstream migration of Atlantic salmon smolts was similarly unaffected by beaver dams, even in periods of low flows. Two-year-old Atlantic salmon parr in beaver ponds in eastern Canada showed faster summer growth in length and mass and were in better condition than parr upstream or downstream from the pond.
The importance of winter habitat to salmonids afforded by beaver ponds may be especially important in streams without deep pools or where ice cover makes contact with the bottom of shallow streams. 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." Cutthroat trout and bull trout were noted to overwinter in Montana beaver ponds, brook trout congregated in winter in New Brunswick and Wyoming beaver ponds, and coho salmon in Oregon beaver ponds. In 2011, a meta-analysis of studies of beaver impacts on salmonids found that beaver were a net benefit to salmon and trout populations primarily by improving habitat (building ponds) both for rearing and overwintering and that this conclusion was based over half the time on scientific data. 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. They also found that when beaver dams do present barriers, these are generally short-lived, as the dams are overtopped, blown out, or circumvented by storm surges.