, Volume 10, Issue 4, pp 439-461

The general ecology of beavers (Castor spp.), as related to their influence on stream ecosystems and riparian habitats, and the subsequent effects on fish – a review

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Abstract

The Eurasian and North American beavers aresimilar in their ecological requirements, andrequire water deep enough to cover the entranceto their lodge or burrow. A food cache isoften built next to the lodge or burrow, exceptin some southern areas. On small streams (upto fourth order) dams are frequently built tocreate an impoundment, generally on lowgradient streams, although at high populationdensities dams may be built on steeper gradientstreams. On large rivers or in lakes, simply alodge with its food cache may be built. Thebeaver is a keystone riparian species in thatthe landscape can be considerably altered byits activities and a new ecosystem created. The stream above a dam changes from lotic tolentic conditions. There are hydrological,temperature and chemical changes, depending ontypes of dams and locations. Although theinvertebrates may be fewer per unit area, totalnumber of organisms increases, and diversityincreases as the pond ages. In cool, smallorder streams, the impoundments provide betterhabitat for large trout, possibly creatingangling opportunities. However, at sites wherewater temperatures rise above their optimumpreferenda, salmonids may be replaced by otherspecies, such as cyprinids, catostomids,percids or centrarchids. As the habitat isaltered, interactions amongst co-habiting species may change. For example, brown troutor brook trout (charr) may become dominant overAtlantic salmon. In warm water streams theremay be a shift from faster water dwellers topond dwellers. Larger bodied fish, such ascentrarchids and esocids may displace smallerbodied fish such as cyprinids, providing betterangling. Refugia from high or low water flows,low oxygen or high temperatures, may beprovided in adverse conditions in winter orsummer. However, in some cases dams areobstructions to upstream migration, andsediment may be deposited in former spawningareas. The practicality and benefits ofintroducing or restoring beaver populationswill vary according to location, and should beconsidered in conjunction with a managementplan to control their densities.