Water Resources Research Act Program

Principal Investigators: Magnus McCaffery, Lisa Eby

Abstract: Beaver (Castor canadensis) play a keystone role on the landscape. Their impoundments create lentic habitat in otherwise lotic systems, leading to fundamental changes in channel geomorphology, hydrology and nutrient cycling. Consequently, beaver promote changes in succession dynamics, increase biotic productivity, and enhance diversity of floral and faunal assemblages1. Increases in water storage capacity through impoundments improve riparian habitat, and potentially augment water supply and late season flows2. These aspects of beaver engineering are of increasing interest to landowners and managers, especially as Montanans continue to deal with drought conditions. In the Pacific Northwest, beaver are actively transplanted into degraded wetlands for habitat restoration3. In Montana, beaver are occasionally being introduced to mitigate water quality (D. Sasse, USFS), increase water storage and riparian habitat quality (C. Riley, USFS) and are naturally moving into restored areas, such as Belmont Creek (J. Christensen, BLM). The utility of beaver as possible agents for watershed restoration is exemplified by current proposals to remedy water shortage problems in the upper Big Hole River watershed in western Montana. Transplantation of beaver into tributary streams of the Big Hole River is one option being considered by the Big Hole Watershed Committee as an alternative to increasing landscape water storage through human dam construction. Promoting beaver on the landscape, either through natural population expansion or active transplantation is a controversial strategy. Aside from human-beaver conflicts such as timber damage, flooding of agricultural and developed lands, and damming of culverts and irrigation systems1, there is the possibility of negative effects on native fish species, such as barrier creation and warming of coldwater streams4. Relatively little is known about the effects of beaver impoundments on stream fish assemblages, and the patterns and mechanisms behind how beaver may influence fish community structure, abundance and distribution is a contested issue in the western U.S. and in Montana in particular. The formation of pool habitat may increase water temperatures, prey availability to fish, and juvenile rearing habitat for species such brook trout (Salvelinus fontinalis)5, as well as providing important winter habitat for many stream fishes including cutthroat trout (Oncorhynchus clarki) and bull trout (Salvelinus confluentus)6. In Rocky Mountain streams, brook trout are an exotic species, and their invasion often results in displacement of native cutthroat trout through age-specific biotic interactions that reduce juvenile cutthroat trout survival7. Thus, understanding both what limits the spread of the distribution of brook trout within a system and what factors influence the outcome of westslope cutthroat and brook trout species interactions is critical for the conservation and management of westslope cutthroat trout in western Montana. Gradual upstream declines in growth rates associated with declining water temperatures may explain the upstream limit for brook trout in some Montanan stream systems8. Any factors that result in demographic consequences such as growth rates, age-0 recruitment, and dispersal can influence the spread of an exotic species9. Furthermore, it has been posited that brook trout, which are more pool adapted and temperature tolerant, may have an advantage in beaver ponds, and can use these habitats as source populations, enabling them to colonize colder sink sections of the stream, thus sustaining invasions across a larger range7. In addition, beaver ponds may alter the outcome of species interactions between westslope cutthroat and brook trout. If beaver ponds provide habitat that preferentially increases abundances of brook trout in a stream, then their impact on westslope cutthroat may be larger. Also, temperature has been implicated in enhancing the ability of brook trout to outcompete westslope cutthroat trout10. Therefore if beaver ponds increase stream temperature, this may give brook trout a greater competitive advantage. Actions to improve water retention must work in synchrony with efforts to curtail brook trout spread and maintain native cutthroat, and based on a scientific understanding of ecological mechanisms operating in the system. This work directly complements ongoing in the Rocky Mountain region, including work by B. Shepard (MFWP, MSU) who is examining correlates of patterns of brook trout invasions the landscape. This project will enhance our knowledge as to how beaver activity influences processes related to species invasion in western Montana.