Role of Beaver Reintroduction in Ecological Restoration: Beaver as a Tool for Restoring Wetlands in West Scotland and Internationally
Ecological restoration is a process of conservation management which applies ecological principles to promote the recovery and development of the impoverished or destroyed ecosystems (SER 2004). Although the approach to ecosystem recovery may vary significantly from case to case as determined by its specific goals, living organisms are often instrumental in assisting the process (Perrow & Davy 2008). For instance, Grime’s theory of plant strategies is the key in re-vegetation of the denuded sites: harsh abiotic conditions (e.g. low in nutrients) require stress-tolerant plants, while ruderal vegetation fares best in sites affected by disturbance (Grime 2002). Consequently, various plant species characterised by these life strategies aid in fulfilling the goals of the site restoration – reducing environmental stress or mitigating disturbance respectively (Perrow & Davy 2008).
Moreover, some species can deliver multiple goals at once by changing physical characteristics of their environment; these organisms are known as keystone species as the influence they exert on the ecosystem determines community composition (Brown, Mcmorran, & Price 2011; Greipsson 2011). Beaver is a textbook example of a keystone species, also known as ecosystem engineer; Eurasian beaver Castor fiber L. and North American beaver C. canadensis Kuhl. significantly alter their environment by building dams on small, low gradient streams, which eventually results in the formation of the beaver pond (Naiman, Johnston, & Kelley 1988; Rosell et al. 2005; Biebighauser 2007). These modified physical characteristics differ significantly from the previous stream environment and contribute positively to both biodiversity and ecosystem function (Figure 1). Therefore, it is not surprising that beavers are also considered as potential tools in assisting ecological restoration (Wright, Jones, & Flecker 2002; Rosell et al. 2005; Byers et al. 2006; Biebighauser 2007; Lake, Bond, & Reich 2007; Hood & Bayley 2008; Barrett 2009). Beaver reintroduction with the particular aim of restoration of wetland habitats is in turn an emerging practice, especially as multiple benefits in addition to the standard wetland restoration goals (i.e. water table elevation, flood and erosion control, and pollution reduction) are being delivered.
In places like Scotland where beavers had gone extinct due to anthropogenic activities, beaver reintroduction is considered primarily for the purpose of reinstating the species in its historic range (Conroy & Kitchener 1996). The Scottish Beaver Trial currently being carried out in Knapdale Forest, Argyll in West of Scotland is the first legally approved mammal reintroduction project in the whole of the UK (The Scottish Wildlife Trust 2012). Currently in its third year, project is not intended to serve as ecological restoration of the trial site; however, it is likely that historical extinction of beavers had resulted in the loss of many important wetland habitats (Lindenmayer & Hobbs 2007), thus the success of the trial will be in part determined by the meeting of wetland restoration goals (The Scottish Wildlife Trust 2012).
In this essay I will therefore attempt to examine the use of the beaver as a tool for wetland restoration by looking at the potential advantages and disadvantages of this approach. I will discuss how beavers modify their environment, and how beaver reintroduction can be integrated with the overall habitat management, drawing on some case studies. I will also analyse the on-going Scottish Beaver Trial in more detail, mainly considering the effects the beavers have had on the environment since the trial’s inception in 2009. Given the monitoring results thus far, I will discuss the potential of beavers for wetland restoration in Knapdale and the rest of West Scotland.
Beaver ecology: impacts on biodiversity and ecosystem services
Damming of the streams is first of all beneficial to the beaver itself: ponds created provide food (e.g. aquatic macrophytes), serve as food storage, allow rapid movement across landscape and protect from predators (Collen & Gibson 2001; Biebighauser 2007). However, as shown in Figure 1, beaver activities of dam construction and tree felling does significantly affect the stream and the riparian zone: for example, as dammed stream floods the adjacent woodland, flood-intolerant tree species start to die out and become woody debris, releasing organic carbon into the system while also serving as a habitat and food source for different species (Gurnell 1998; Rosell et al. 2005). Moreover, beaver activities enhance delivery of many ecosystems services (Box 1, Case Study 1): regulation of peak discharge and erosion control results from reduced stream velocity and dissipation of water over larger area, while the pond persistence in times of short water supply (e.g. winter or drought) is a consequence of a raised water table (Gurnell 1998; Salmon and Trout Association 2002; USDA Forest Service 2007). Nutrient and organic matter levels are raised by the increase in debris, microbial activity and trapping of the sediment; this promotes growth of phytoplankton and periphyton, which form the basis of many food webs in the ecosystem (Naiman et al. 1988; Collen & Gibson 2001; Rosell et al. 2005). In addition to flooding, beavers affect the successional processes of the riparian woodland by felling the trees for food or construction (Wright et al. 2002). However, beavers target trees that are capable of resprouting from the stumps, for example, Salix spp. (Connell et al. 2008); thus the impact is greater on the structure of the woodland rather than its composition (Jones et al. 2009).
Ecological impact of beavers on other species of animals is substantial: diversity of aquatic and terrestrial invertebrates increase as a result of the impoundment, and some rare beetle species are only found associated with beaver lodges (Rosell et al. 2005). Dam effects on fish are variable: while sediment trapping positively contributes on water clarity in spawning grounds downstream (Rosell et al. 2005), dams can be an obstacle for fish migration, and their effect on the temperature regime might result in unsuitable conditions to at least some fish species (Salmon and Trout Association 2002; Kemp et al. 2012). Diversity of reptile, bird and mammal species are generally higher in areas inhabited by beavers (Rosell et al. 2005), benefitting from creation of the novel aquatic and terrestrial habitats. While the former results from the changes to the stream environment as discussed above, the latter is a consequence of modification to the woodland structure, e.g. emergence of shrubby understory, which is highly favoured by birds and bats (Rosell et al. 2005; Ciechanowski et al. 2010). Finally, some indirect interactions that benefit other species are mediated by beavers (see Martinsen, Driebe, & Whitham 1998).
On the other hand, not all beaver impacts on the ecosystem are positive. Selective harvesting of particular tree species (primarily willow and alder) might eventually lead to a shift in woodland towards non-targeted species (Moore, Sim, & Iason 2011). Moreover, in some cases, this might fuel the spread of non-native invasive tree species, such as Tamarisk and Russian olive (Lesica & Miles 2004). As for animals, lotic species of aquatic invertebrates, for example, freshwater bivalves and various insect taxa are displaced due to disappearance of stream habitat (Rosell et al. 2005); the same applies to some fish species as mentioned above.
Advantages and disadvantages of beaver as a tool for ecological restoration
Wealth of knowledge exists on the ability of ecosystem engineers to significantly modify their environments, however, in practice eco-engineering is still used relatively rarely as a mean of delivering benefits to the ecosystems (Byers et al. 2006). Table 1 thus presents some advantages and disadvantages for utilising beaver in particular as a tool for ecological restoration; a SWOT analysis, while not meant to be exhaustive, evaluates the approach and provides some understanding of why this might be an attractive solution to restoring wetlands. Most important advantages of the beaver induced ecological restoration over the conventional engineering include its significantly lower costs, greater effectiveness, sustainability (Box 1, Case Study 2) and naturalness, making it more acceptable to the society (Barrett 2009). Nevertheless, there are also weaknesses and threats to this approach, which are very similar to those raised by the opposition to beaver reintroduction (Kitchener & Conroy 1997; USDA Forest Service 2007). However, in our case the most important disadvantage is the uncertainty about the final result as the extent of the changes to the landscape is very hard to predict, also not all of the expected improvements will be realized in different projects (Barrett 2009).
Beavers in West Scotland
Eurasian beaver C. fiber was once widespread in the mainland of Scotland, but by 16th century extensive hunting for pelt had driven species to extinction (see Kitchener & Conroy 1997). In 2009 the Scottish Beaver Trial, the first officially recognised attempt to reintroduce beaver to the country, was started: 15 animals from Norwegian populations were released to the site in Knapdale Forest, Argyll in West of Scotland (The Scottish Wildlife Trust 2012). Currently there are 15 beavers in the area, including animals both from the initial release and born on the site (Jones 2012).
Given the small number of beavers released and plenty of suitable loch habitat and adjacent woodland, dam building was not expected at the very beginning (Moore et al. 2011). However, in 2009 a major dam has been built on the drainage connecting Dubh Loch and Loch Coille-Bharr, increasing the size of the former up to five times (Moore et al. 2011; site visit). Smaller dams were also built within the watershed, however, many of them had to be dismantled to avoid negative effects on aquatic macrophyte communities protected by the local Special Area of Conservation (SAC) (Willby, Casas Mulet, & Perfect 2011). Figure 2 demonstrates the effects that the beavers have had on the trial site thus far; monitoring is on-going and data on some taxa, e.g. insects, such as dragonflies, or birds, is not yet available. From the reports produced so far, it is clear that beavers have modified their environment significantly affecting physical conditions in the Dubh Loch and some of the streams (Kettle-White et al. 2011; Moore et al. 2011). The impact on the adjacent woodlands, while mainly concentrated in the 10 m zone within water’s edge, has been immense: 1) about 21 per cent of tree stems had been felled or gnawed on in the survey sites; 2) smaller diameter trees are preferred, but large trees are also targeted, especially rowan Sorbus aucuparia. About 40 per cent of large rowan specimens had been felled in the survey plots, presenting a potential conflict with Taynish and Knapdale Woods SAC as many important lichen species are found primarily on large old trees (Moore et al. 2011). Overall, however, the conflicts between SAC designation and beaver activities are manageable, as in the case of freshwater loch plant communities (see above). Moreover, given the high degree of resprouting recorded thus far, the prior concerns of overexploitation of the woodlands by beavers seem to be unfounded (Moore et al. 2011); effective control of local deer population is much more important in ensuring the establishment of the new shoots (Jones et al. 2009).
While the system-wide changes following beaver reintroduction are obvious, due to limited data it is too early to tell whether the effect of beavers in the Knapdale Forest has been mainly positive. It is clear, however, that the trial is a great research opportunity not only on beaver but also landscape and ecosystem ecology. Moreover, beavers are attracting tourists to the area, which is beneficial to the local businesses (The Scottish Wildlife Trust 2012). Additionally, as discussed above, beavers have great potential to deliver substantial benefits to biodiversity and ecosystem function, while their reintroduction can be successfully incorporated with habitat restoration goals. At the end of the trial in 2014 these three aspects will be among the likely success criteria to inform the Scottish Government’s decision on the future of beavers in Scotland (The Scottish Wildlife Trust 2012).
Future prospects and Conclusions
The prospect of harnessing the beaver work for ecological restoration in Scotland is unclear due to uncertainty surrounding the future of the beaver reintroduction projects themselves. However, the multiple documented advantages of the approach, in addition to the results of the monitoring of the Scottish Beaver Trial demonstrating benefits to the ecosystem, might encourage the Scottish Government to provide a licence for further reintroductions. In general, beaver-created physical changes to the stream channel and riparian zone are compatible with the ecological and socio-economic aims of wetland restoration: wetlands created by beavers deliver multiple ecosystem services, biodiversity in the area increases, while the landscape changes are visually pleasing to the public, among other positive contributions. As for the ecosystem engineering concept, it appears to be a very effective approach to the ecosystem management and conservation and will undoubtedly become more widely adopted as the amount of research carried out increases.
Box 1. Case studies of use of beaver as a tool of ecological restoration.
Case Study 1. The Volga-Kama National Preserve in Tatarstan, Russia (Busher & Dzieciolowski 1999). Conversion of forests to agricultural land in the area has resulted in significant increase in erosion. Sediment carried from the fields upstream causes filling up of the lakes and fens, while area has become more arid due to drop in groundwater levels. In addition to adoption of more sustainable agricultural practices to prevent soil erosion of the agricultural land, beaver reintroduction is being proposed as a way to mitigate erosion, water level drop and lake sedimentation immediately.
Case Study 2. Wood River Resource Conservation and Development Area, Idaho, USA (Barrett 2009). The stream channels in the area are steep, incised, and highly erodible; the problem is exacerbated further by fast-flowing water due to excessive run-off from deforested and overgrazed areas upstream. In an attempt to restore the stream channels, beavers were reintroduced to the area after comparing the costs and benefits with conventional dams being built on the site. The greatest difference came in terms of the price, where one dam costs up to $20,000 in comparison to $100 paid per reintroduction of a pair of beavers. While beaver dams are less resistant, there are little to no maintenance costs as beavers repair the damage themselves. Sustainability of the practice is further highlighted by the fact that over time beavers produce more beavers, which then can move onto the new streams.
Part of In Class Assessment for Edinburgh University’s 4th Year Ecological Science Field Course, November 2012
All figures and tables my own
Word count: 2,067
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