Year: 2013
Abstract:
The North American beaver (Castor canadensis) is a large semi-aquatic rodent that has played a central role in shaping the Canadian boreal landscape, and colonial Canadian history. Exploitation of North American beaver populations to supply the European hat industry spurred the westward expansion of European explorers and traders into the continental interior. With intensive unregulated harvest, beavers virtually disappeared across much of their range; though populations are recovering, the species is only about 10% as abundant as it was before the fur trade took its toll. As a result, much of the recent ecological history of the Canadian boreal forest has occurred in the absence of this keystone ecosystem engineer, and the ecological state that we perceive as natural is in many regions quite different than it was a century ago. Beavers, while playing an important role in structuring streams and wetlands by altering vegetation communities and water flow patterns, may also affect human structures. In the mineable oil sands region of northeastern Alberta, much of the landscape will be impacted by mining. Mine sites will have to be reclaimed, and those reclaimed sites will consist of engineered landforms (including water bodies and waterways); the long-term hydrological and ecological function of those sites may be vulnerable to beaver activity. In an effort to determine if approaches exist that could manage the risk of beavers colonizing and negatively impacting reclaimed sites, we performed an extensive literature search and analysis. Our objective was to examine characteristics of beaver ecology that might potentially impact reclamation plans, and to identify possible methods to mitigate those impacts. We also include information on traditional use, historical abundance, and current abundance in the mineable oil sands region to provide important historical and ecological context. Although beavers inhabit a range of aquatic habitats, the focus of our review is on watercourses that could be dammed by beavers. Of the aquatic habitats which will be constructed during reclamation, these systems are probably the most vulnerable to impacts from beaver activity. Note, however, that inlet and outflow streams from lakes may be vulnerable to beaver activity, which could impact the performance of constructed lakes in a variety of ways. Beavers alter stream form and function, create wetlands, and change vegetation patterns. The most important predictor of beaver occurrence is stream gradient, with low gradients being associated with higher beaver activity. Stream depth and width, soil drainage, and stream substrate are also important. Although beavers may also respond to vegetation factors, such as tree or shrub species and density, hydrological factors are more important predictors of beaver occupancy of a site. The primary forage preferred by beavers includes deciduous tree and shrub species. Aspen (Populous tremuloides) is the species most preferred by beaver, and is a common component of reclamation plantings and natural recolonization of reclamation sites in the oil sands region. Beavers are central-place foragers, meaning foraging is concentrated around a central home base. They typically harvest deciduous trees and shrubs up to 60 m or more from the water, but most harvest occurs less than 30 to 40 m from the water’s edge. Predation (and predation risk) restricts the size of beavers’ foraging areas, and may also regulate their population size. Management of wolf populations to limit predation on caribou in northeastern Alberta may have significant indirect effects on beaver abundance and distribution by releasing them from predation pressure. The boreal forest ecosystem of Canada evolved over millennia with the beaver as a keystone species altering hydrological systems, creating vast areas of wetlands and beaver meadows, changing vegetation communities and modifying geomorphological processes. Reclamation of functional ecosystems in the region must therefore integrate beavers and their engineered structures. The most ecologically- and cost-effective approach is to design reclaimed areas with the objective of including beaver, but directing beaver activity to areas away from vulnerable reclamation structures. Ecological function requires the presence of beaver on the post-reclamation landscape, and the species is important to First Nations peoples and other trappers in the area. Although beaver abundance can be expected to increase in the area after reclamation, their activities will result in the replacement of existing vegetation with species of lower nutritional quality to beaver (conifer trees). This is expected to result in a beaver population decline and then stabilization over time. With beavers an integral component of the functional landscape, it is important to create “beaver exclusion zones” to ensure that the impact of the species is diverted to areas where beaver activity does not damage reclamation structures. There are very few existing studies of beaver impacts to reclaimed areas. Incorporating ecologically-based strategies for keeping beaver density low in sensitive areas at the outset of a reclamation project, and then monitoring the effectiveness of that strategy, is the best advice that can be derived from our analysis of the existing literature. Beavers could be discouraged from settling at a site by creating streams with steep gradients (>10%) that are wide and deep enough to ensure substantial water flows, are armoured with rock or cobble bottoms, and are bordered by coniferous tree species and/or grass and sedge species. Trees should be planted at high density to prevent growth of shrubs and deciduous trees in the understory, as these are preferred by beaver. Deciduous vegetation should not be planted during reclamation near sites where beavers are to be excluded, and it may be necessary to remove existing deciduous trees and shrubs and replace them with conifers, grasses and sedges in these areas. Although planting specific types of vegetation may be used to discourage beavers from settling a certain area in the short term, natural succession could eventually result in other vegetation communities attractive to beavers. Therefore, unless long-term vegetation management is envisioned, reclamation plans should not rely on using vegetation to dissuade beaver activity in sensitive areas alone, though this approach may be used in combination with other methods, especially in the few decades immediately following reclamation. Note that the goal is to plan for a maintenance-free environment in which ongoing beaver control is unnecessary, and the use of multiple strategies in tandem to guide beaver activity is more likely to achieve this goal. More active, maintenance-intensive techniques could be used to limit the damage caused by beaver dams to sensitive areas. These techniques include lethal (e.g., kill trapping or shooting) and nonlethal (e.g., relocation) methods to reduce population density. However, these methods require constant effort, and can be expensive. Another approach is to manipulate water flow through existing beaver dams using pipe drainage systems; this allows the beaver dam to stay in place, while reducing the risk that it will trap enough water to be dangerous if the dam should fail. Again, however, these drainage systems require long-term maintenance. One approach may be more sustainable in the long term and require less maintenance: minimize or maximize water flow through engineered channels, as beavers are less likely to use very low-flow and very high-flow watercourses. Note that beavers may still affect these channels, especially when population densities are high or other habitat is unavailable; however, the probability of beavers affecting very low-flow or high-flow channels is lower than for watercourses with more moderate flows. Creating several dispersed low-flow channels may make an area less desirable to beavers compared to a single moderate flow channel. Similarly, multiple low- to moderate-flow channels could be created, with some having characteristics that attract beavers (“decoys”) and others that do not (“exclusions”), allowing water flow to continue through some channels even in the presence of beavers. “Pre-dam” fences can be installed on decoy streams to create a structure to encourage beavers to occupy a site where damage is not a concern. Discharge could be controlled by regulating water flow through exclusion streams that are not dammed, or by installing flow devices though dams on decoy streams. A similar approach might be used on culverts that allow streams to flow beneath roadways; flow devices could be used proactively at these sites, and/or oversized culverts could be installed to allow maintenance of the natural width of the stream channel and reduce the noise of running water, which attracts beaver activity. Although many different landforms on the reclaimed landscape may be vulnerable to beaver activity, a few are considered critical areas where beaver impacts must be controlled, including the outlets of lakes, side-hill drainage systems, and constructed peatlands. Beaver activity at the outlet of constructed lakes could cause instability in containment structures, negatively affect littoral and riparian zones around the lake, and increase the probability of catastrophic outburst flooding. Damming of side-hill drainage systems could cause stream avulsion and routing of water flow into a new pathway not engineered for a stream, causing increased erosion. Flooding of constructed peatlands could convert them to open-water systems, thereby subverting their intended ecological function. These critical areas should be protected from beaver activities, while other areas should be designed to accommodate this important species. In practice, several different approaches – tailored to specific situations and landforms – will be necessary to develop and implement plans that accommodate beavers as a part of the post-reclamation landscape. As so few data exist to inform effective reclamation in the presence of beavers, all of the methods we suggest carry an unknown degree of risk. This risk can be decreased in the future by adapting methods based on observed effectiveness. We recommend implementing a research and adaptive management program on the influence of beavers on reclamation within the context of oil sands reclamation in northeast Alberta. Lack of existing information, particularly in northeast Alberta, illustrates the need to implement research that documents the positive and negative influence of beavers on reclamation sites and tests alternative methods to prevent negative and support positive influences. Otherwise reclamation strategies will be ad-hoc and tenuous, with a mixed success rate. A research and monitoring program would ideally contribute to a standardized strategic approach to mitigating negative beaver influences on reclamation of watercourses in the oil sands region. Beavers are, to a certain extent, unpredictable. No single approach will guarantee that a site will be unaffected by beaver activity. We suggest that multiple management approaches be simultaneously implemented at sites that are particularly vulnerable or critical for the functioning of the reclaimed landscape (e.g., outlet streams from constructed lakes). It is impossible to predict all eventualities, as the character of the reclaimed landscape will change over time due to successional processes, fire, global climate change, and resource extraction. The information we provide is the best available based on limited current knowledge, and provides the best chance for minimizing risk while accommodating this keystone species. Ultimately, the presence of beavers on reclaimed oil sands leases will increase biodiversity, enhance ecosystem goods and services, and assist in developing ecosystems that are consistent with natural systems in the boreal region.
