Beaver Restoration Guidebook

Gregory Lewallen Janine Castro Technical Editor Ann Sihler Contributors Thanks to the numerous individuals who provided case studies, figures and images, with a special thanks to those who reviewed the document and offered their suggestions and advice. The following is an alphabetical list of contributors to the Beaver Restoration Guidebook. Armichardy, Dan US Forest Service Leary, Ryan The Klamath Tribes Armstrong, Bob www.naturebob.com Lundquist, Kate OAEC Water Institute Beardsley, Mark EcoMetrics, LLC Moore, Kelly Oregon Dept. of Fish and Wildlife Bouwes, Nick Ecological Research O’Brian, Mary Grand Canyon Trust Brennan, Matthew Clean Water Services Ojala-Barbour Reed The Methow Beaver Project Caldwell, Chuck The Beaver Patrol, Juneau, AK Portugal, Elijah Utah State University Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp v Conley, Kimberly US Forest Service Rustay, Michael Snohomish County Public Works Fetherston, Kevin R2 Resource Consultants Slocum, Tom Skagit Conservation District Firor, Susan Terra Graphics Environmental Smith, Karri K.A. Smith Consulting, Inc. Frey, Jennifer K. New Mexico State University Stengle, James B. Certified Wildlife Biologist (retired) Gilgert, Wendell Point Blue Conservation Sci. Stockard, Torre The Methow Beaver Project Hocker, Katherine SE Alaska Sketchbook Vickerman, Sara Defenders of Wildlife Hartman, Heidi Oregon Dept. of State Lands White, Seth Columbia River Inter-Tribal Fish Comm. Hemphill, Nina US Forest Service Willson, Mary, The Beaver Patrol, Juneau, AK Hoehne, Suzanne Biohabitats.com, Ohio River Weber, Nick Ecological Research Contributors to Beaver Restoration Case Studies Armichardy, Dan US Forest Service Helzer, David City of Portland Environ. Services Beardsly, Mark EcoMetrics, LLC Hohle, Janet Idaho Office of Species Conservation Ashmore, Julie Okanogan Highlands Alliance Johnson, Brad Johnson Environ. Consulting Bailey, Scott Jay Tillamook Esturary Partnership Kliegman, David Okanogan Highlands Alliance Bistoff, Sean City of Portland Environ. Services Reid, Scott The Town of Breckenridge Claffey, Mike Claffey Ecological Consulting, Inc. Rich, Lauren Upper Skagit Indian Tribe Doran, Jessica Ecomentrics Smith, Kendra Bonneville Environ. Foundation Firor, Susan Terra Graphics Environ. Engineering Weaver, Matt Five Rivers, Inc. Hassmiller, Bob US Forest Service Wild Cathryn Seventh Generation Institute Funding Provided By: The North Pacific Landscape Conservation Cooperative US Fish and Wildlife Service National Oceanic and Atmospheric Administration US Forest Service Finally, many thanks to all the participants that attended and provided feedback at the Beaver Restoration Workshops held this past year in Oregon, Washington, California and Alaska, and to the invited speakers, Melissa Babik of the Mid-Columbia Fisheries Enhancement Group, Shelly Blair, California Department of Fish and Wildlife, Mark Cookson, US Fish and Wildlife Service, William Meyer, Washington Department of Fish and Wildlife, Thomas Stahl, Oregon Department of Fish and Wildlife and Charlie Corrarino (retired), Oregon Department of Fish and Wildlife. Also, many thanks to Brian Turner, Mary Anne Schmidt and Patrick Edwards of Portland State University, Environmental Professionals Program, for helping to organize those workshops. Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp vi About This Guidebook Michael M. Pollock, Janine Castro and Greg Lewallen Beaver as a Partner in Restoration More and more, restoration practitioners are using beaver to accomplish stream, wetland, and floodplain restoration. This is happening because, by constructing dams that impound water and retain sediment, beaver substantially alter the physical, chemical, and biological characteristics of the surrounding river ecosystem, providing benefits to plants, fish, and wildlife. The possible results are many, inclusive of: higher water tables; reconnected and expanded floodplains; more hyporheic exchange; higher summer base flows; expanded wetlands; improved water quality; greater habitat complexity; more diversity and richness in the populations of plants, birds, fish, amphibians, reptiles, and mammals; and overall increased complexity of the riverine ecosystems. In many cases these effects are the very same outcomes that have been identified for river restoration projects. Thus, by creating new and more complex habitat in degraded systems, beaver dams (and their human-facilitated analogues) have the potential to help restoration practitioners achieve their objectives. Beaver can be our new partner in habitat restoration. Yet even though the potential benefits of restoring beaver populations on the landscape are numerous, so, too, is the potential for beaver/human conflicts. These conflicts can arise from an overlap of preferred habitats by both humans and beavers, misunderstandings of how beavers modify their habitats, and a lack of planning or use of adaptive management on restoration projects. Reviewing the information provided in this guidebook will help interested parties approach beaver-based restoration from a more informed perspective, so that they can manage expectations and increase success. Goals of This Guidebook This guidebook provides a practical synthesis of the best available science for using beaver to improve ecosystem functions. If you are a restoration practitioner, land manager, landowner, restoration funder, project developer, regulator, or other interested cooperator, this guidebook is for you. Our overall goal is to provide an accessible, useful resource for those involved in using beaver to restore streams, floodplains, wetlands, and riparian ecosystems. Although the guidebook summarizes current information about how to use beaver in restoration and conservation, the knowledge base on this subject is rapidly expanding. This means that not all of the information provided has been peer- reviewed in scientific journals; some of it is instead based on the real-life experience of restoration practitioners who are conducting ongoing experiments on using beaver to restore habitat. Thus the guidebook is a compilation of the current best available science, and we expect to update it regularly as the science progresses, readers provide information from their ongoing restoration experiments, or from restoration efforts of which we are currently unaware. See Table 1 for the different types of data presented in this document and the relative ranking we used for assessing scientific credibility. Much of the information presented here is applicable across the beaver’s range, but the guidebook focuses on beaver restoration in the western United States. Much of the interest in beaver restoration is Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp vii occurring in the context of restoring habitat for declining populations of Pacific salmon and trout while simultaneously improving stream flows, particularly in drought-prone regions. Structure and Content The chapters of this guidebook fall into two broad sections; beaver ecology (chapters 1-3) and beaver restoration and management Chapters 4-10. The “Beaver Ecology,” chapters discuss both the general life history characteristics and the effects that beaver dams have on physical and biological processes within river ecosystems. This is includes “Frequently Asked Questions” about beaver (Chapter 2) and beaver “Myth Busters” (Chapter 3), which dispel common myths or misperceptions about beaver, including those that, unfortunately, can influence funding and permitting decisions. Readers already familiar with beaver ecology may opt to skip the first section and move directly to the latter portion of the guidebook, which addresses topics related to beaver restoration and management. Chapters 4 through 8 discuss common emerging techniques for using beaver and beaver dams (both natural and human created or assisted dams) to improve ecosystems; Chapter 9 describes methods for mitigating the unwanted effects of beaver activity; Chapter 10 introduces the Beaver Dam Viability Matrix, which grew out of the Project Screening Risk Matrix—one of several tools generated by the River Restoration Analysis Tool Project (RiverRAT), a broad federal effort to more efficiently and effectively evaluate stream management proposals; and Chapter 11 presents real-life examples of pioneering practitioners who have used beaver restoration tools in the field.These case studies include lessons learned that will help guide future restoration efforts. Future Resources We originally intended to include a chapter on “Beaver Rules and Regulations” as they pertain to restoration in western states, but the process of researching this subject revealed a confusing patchwork of state, federal, tribal, and even local rules governing beaver and beaver dams that varies by land ownership, state and federal agencies, and other factors. Untangling the web of rules and policies into a tractable discussion was beyond the scope of this initial document, but we hope to pursue this topic in the future and appreciate any relevant information that readers want to provide. We have also developed a comprehensive beaver ecology library of more than 1,400 references from scientific journals, “gray” literature, websites, legislation, regulations, and presentations that is available for readers either in Endnote or as a text document. We have copies of many of the articles and are building a library of beaver articles, with particular emphasis on the more obscure references that are difficult to obtain from the Internet. Yet, as comprehensive as this library might sound, many references related to beaver ecology are not yet included, particularly those from the gray literature. We look forward to including additional references as they are provided by readers. Finally, since this is a “living document”, we will be updating regularly, including the addition of other beaver restoration-related products so please check the US Fish and Wildlife website for the latest information: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp We will also be sending out occasional notices when updates to the beaver restoration guide become available or additional tools are produced. It won’t be quite as smooth as the automatic software updates on your phone or computer, but we will do our best. Thank you for your interest. We hope that this guidebook facilitates beaver restoration approaches underpinned by sound scientific Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp viii principles, such that a more comprehensive, evidence-based understanding of beaver ecology, restoration, and management emerges. Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp ix Table 1. Common sources of scientific information (adapted from Washington Administrative Code 365-195-905). Information can be considered scientific if its source has the characteristics in Table 1. Table 1 provides a general indication of the characteristics of valid scientific information typically associated with common sources of scientific information and in general order of reliability. Each source of information (including peer-review articles) needs to be evaluated carefully to ensure it contains the characteristics described below. Characteristics Sources of Scientific Information Peer Review Methods Logical Conclusions, Reasonable Inferences Quantitative Analysis Context References A. Research. Research data collected and analyzed as part of a controlled experiment (or other appropriate methodology) to test a specific hypothesis. X X X X X X B. Monitoring. Monitoring data collected periodically over time to determine a resource trend or evaluate a management program. X X O X X C. Inventory. Inventory data collected from an entire population or population segment. X X O X X D. Survey. Survey data collected from a statistical sample from a population or ecosystem. X X O X X E. Modeling. Mathematical or symbolic simulation or representation of a natural system. Models are generally used to understand and explain occurrences that cannot be observed directly. X X X X X X F. Assessment. Inspection and evaluation of site-specific information by a qualified scientific expert. May or may not involve collection of new data. X X X X G. Synthesis. A comprehensive review and explanation of pertinent literature and other relevant existing knowledge by a qualified scientific expert. X X X X X H. Expert Opinion. Statement of a qualified scientific expert based on his or her best professional judgment and experience in the pertinent scientific discipline. The opinion may or may not be based on site-specific information. X X X X = The characteristic must be present for the information derived to be considered scientifically valid and reliable; O = The presence of the characteristic strengthens the scientific validity and reliability of the information derived but is not essential to ensure scientific validity and reliability. Note: Many sources of information usually do not produce scientific information because they do not exhibit the necessary characteristics for scientific validity and reliability. Information from these sources may provide valuable information that supplements scientific information, but it is not an adequate substitute for scientific information. Nonscientific information should not be used as a substitute for valid and available scientific information. Common sources of nonscientific information include (1) anecdotal information (i.e., one or more observations that are not part of an organized scientific effort, such as "I saw a grizzly bear in that area while I was hiking"), (2) nonexpert opinion (i.e., the opinion of a person who is not a qualified scientific expert in a pertinent scientific discipline, such as "I do not believe there are grizzly bears in that area"), and (3) hearsay (i.e., information repeated from communication with others, such as "At a lecture last week, Dr. Smith said there were no grizzly bears in that area"). Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 1 Section I - Beaver Ecology Photo Credit: Bob Armstrong ( www.naturebob.com) Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 2 Chapter 1—Effects of Beaver Dams on Physical and Biological Processes Greg Lewallen, Michael M. Pollock, Chris Jordan and Janine Castro In most of the temperate Northern Hemisphere, beaver historically altered low-gradient, smallstream ecosystems by constructing millions of dams made primarily of wood. Almost every northern temperate ecosystem that had trees or shrubs growing along streams also once had beaver dams. In Eurasia, evidence of beaver has been found in streams as far south as Iraq and Turkey, in the Arctic, and stretching from Scotland in the west to Kamchatka in the east (Halley and Rosell 2002). In North America, beaver were once found far south into the arid environments of Arizona and northern Mexico along rivers such as the San Pedro, Colorado, and the Rio Grande (Pattie 1833, Leopold 1972) and occupied all biomes north of the border from coast to coast, except for the Arctic, the tip of peninsular Florida, and the dry Great Basin and desert country of Nevada and southern California (Figure 1). Figure 1: Probable historic range of the North American beaver. Adapted from Pollock et al. (2003), as modified by Lanman et al. (2012, 2013) and James et al. (2012) for California, and Layne (1965) for peninsular Florida. Absence of historic beaver evidence in the Great Basin, interior southern California, and southern Florida streams, is not evidence of historic absence of beaver in these regions. Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 3 Historically, beaver dams created streams systems with slow, deep water and floodplain wetlands dominated by emergent vegetation and shrubs. Geomorphology and plant communities of small low-gradient streams were much changed throughout much of the Northern Hemisphere after reduction of beaver populations (Rea 1983, Naiman et al. 1988b). In both Eurasia and North America, beaver populations have generally declined as human populations have increased. In both continents, only small populations survived by the end of the 19th century (Seton 1929, Nolet and Rosell 1998, Halley and Rosell 2002). The primary reasons for the declines were that people trapped beavers either because they were resources for fur or oil or competitors for productive valley bottom lands (MacDonald et al. 1995, Mackie 1997, Halley and Rosell 2002). More recently there has been widespread recognition that beaver dams play a vital role in maintaining and diversifying stream and riparian habitat (Pollock et al. 1994, Gurnell 1998, Collen and Gibson 2000, Burchsted and Daniels 2014). In the past century, land managers throughout the Northern Hemisphere have attempted to reintroduce beaver in areas where they have been extirpated. Today, beaver populations are rebounding throughout North America, with the population estimated to be about 10 million and reoccupying most of its former range (Naiman et al. 1988b). Beaver are found across a wide range of aquatic habitat types, but they do have preferences: • Beaver prefer to build dams on small- to medium-sized, low-gradient streams (<6% slope) that flow through unconfined valleys, and generally populate the lowest gradient (slope < 1-2%) sites first. • Beaver generally avoid constrained valleys with high-gradient streams (reviewed in Pollock et al. 2003) but will colonize this less-preferred habitat if their population densities are high (Müller-Schwarze and Schulte 1999). • Beaver also occupy large rivers but restrict their dam-building to off-channel habitat fed by hyporheic flow, groundwater channels, and tributary channels that flow across the floodplains of the larger river channel (Gurnell 1998, Baker and Hill 2003, Pollock et al. 2003). They also will build seasonal dams across large rivers during low flow conditions. • Beaver also build dams on lakes, wetlands, estuaries and just about any water body where additional water can be retained and thus habitat improved (from a beaver’s perspective) by building a dam. In addition to these physical habitat attributes, beaver make use of streams with developed riparian areas that contain (1) vegetation for food, and (2) potential construction materials to build dams and lodges. Although beavers use a wide variety of trees, shrubs, substrate, and herbaceous vegetation as construction material, for food they prefer species from the genera Populus and Salix (i.e. aspen, cottonwood, and willows). Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 4 Hydrology Increased Water Retention and Base Flows Beaver impoundments change the spatial distribution of water (groundwater, pond, or stream), as well as the timing of its release and residence time in the watershed. Beaver dams impound water in ponds and pools, and these impoundments slow the flow of the stream; this holds the water within the stream reach for longer periods and can increase base flows (reviewed in Pollock et al. 2003). Indeed, some perennial streams transform into intermittent and/or ephemeral streams following the removal of beaver dams (Finley 1937, Wilen et al. 1975). Conversely, reintroduced beaver have transformed some intermittent streams back to perennial streams (Dalke 1947, Pollock et al. 2003), and recolonizing beaver have transformed slightly losing streams to gaining streams ((Majerova et al. 2015). Losing streams are characterized by surface water flowing into the subsurface and not returning to the channel, usually associated with local water tables that are lower in elevation than the stream surface. Gaining streams, conversely, are characterised by high local water tables where subsurface water flows into the stream. Additionally, the ponded water expands the saturated surface area of riparian zones, converting previously upland plant communities into wetland plant communities. Thus, beaver create wetlands. Slower water velocities, lateral spreading, and larger areas of soil saturation contribute to increases in both the surface and subsurface water present in a watershed (Naiman et al. 1986, Syphard and Garcia 2001, Pollock et al. 2003, Cunningham et al. 2006, Westbrook et al. 2006, Hood and Bayley 2008). Storage of water within the stream reach is particularly important for many aquatic species during low-flow periods, when direct hydrologic inputs are limited. When beaver recolonize stream systems, their impoundments increase base flows, as well as recharge and elevate the water table (Pollock et al. 2003). Furthermore, given that climate change is expected to increase drought and reduce snow pack, water storage from beaver impoundments may be an effective tool to help mitigate the associated reductions in water resources (see Rosemond and Anderson 2003, Lawler 2009). Climate change is of particular concern in areas that currently depend on glacial and snow-melt runoff. As water storage in the form of glaciers and snow decreases, surface and groundwater storage behind beaver dams high in watersheds may provide a buffer for base flows (Beechie et al. 2013). Hood and Bayley (2008) studied how temperature, precipitation, and beaver activity influenced the area of open water in east-central Alberta, Canada, over a 54-year span that included many periods of drought. The presence of beaver had a substantial effect on the amount of open water in wetlands within the study area. Hood and Bayley’s results indicate that beaver played a larger role in maintaining open-water areas than did temperature, precipitation, and climate. The authors found that, as sites cycled through beaver occupation and abandonment, beavers caused a nine-fold increase in open-water area compared to the same sites without beaver. Their findings indicate that “beaver could mitigate some of the adverse effects of climate change due to their ability to create and maintain areas of open water.” Hood and Bayley conclude by suggesting that “the removal of beaver from aquatic systems should be Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 5 recognized as a wetland disturbance equivalent to in-filling, groundwater withdrawal, and other commonly cited wetland disturbances.” Decreased Peak Flows Beaver activity within a watershed generally reduces peak flows and spreads flows out over longer time periods. Reducing peak stream flows provides water quality benefits in terms of sediment reduction and also retention of water within the watershed as surface or groundwater. By slowing the stream flow, beaver impoundments reduce erosive energy and increase retention time. During floods, energy is dissipated as the water flows through multiple small channels on the downstream side of the beaver dam (Pollock et al. 2003). Floodplain vegetation alongside and below the dam further dissipates energy as the water works its way back to the stream channel (Li and Shen 1973, Woo and Waddington 1990, Dunaway et al. 1994, Pollock et al. 2003). Beaver impoundments attenuate flood peaks by retaining water behind dams and in the subsurface. Beedle (1991) estimated that a single full beaver pond on a southeastern Alaska island reduced peak flows by more than 5 percent. A series of five large ponds could reduce peak flows of a 2-year event by 14 percent and peak flows of a 50-year event by 4 percent. Also, because ponds are not always at capacity, they can allow for additional storage of flood water. For streams with dozens of dams, further reductions in peak flows and stronger cumulative effects should be expected (Scheffer 1938, Smith 1950, Naiman et al. 1986, Pollock et al. 2003). Expansion of Habitat Area and Complexity Beaver dams can create very large and numerous surface pools and ponds, transforming moving-water habitats to a combination of moving- and slow-water habitats (Naiman et al. 1988b, Martell et al. 2006). This increase in surface and subsurface water leads to an expansion of riparian and wetland habitats along streams (see Johnston and Naiman 1990ab, Pollock et al. 2007, Hood and Bayley 2008). Repeated colonization of sites by beaver followed by abandonment creates habitat complexity, or heterogeneity, within the watershed (Burchsted et al. 2010). After abandonment, open-water wetlands drain and may transform into wet meadow habitats called “beaver meadows” (see the subsection below: Habitat-vegetation). In beaver-modified habitat, the continual creation, modification, and abandonment of wetland patches creates a mosaic of wetlands with a large range of ages and successional stages (Wright et al. 2003). The increased heterogeneity, in turn, increases the diversity of habitat types and plant and animal species, as well as the resiliency of the system to disturbance, specifically flooding (Naiman et al. 1988b) and drought (Hood and Bayley 2008). Surface water area is most dramatically affected directly upstream of beaver dams, where it is collected in ponds and pools. The amount of surface water collected in these low-gradient areas ranges greatly, depending on the size and topography of the catchment, the channel form, and the water regime of the region. Typically the amount of surface water present increases with the number of beaver dams on a stream reach (Johnston and Naiman 1990ab). The ponds and pools formed from beaver dams provide important slow-water habitat for birds, waterfowl, fish, aquatic invertebrates, mammals, and amphibians. By increasing the amount of Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 6 riparian area, beaver ponds typically provide important habitat for both terrestrial and aquatic plants and animals. Increased Wetland Area As ponds and pools fill and become deeper, the impoundments force flow laterally, causing overbank flow onto floodplains and creation of side channels, as water flows around beaver dams (Westbrook et al. 2006). These side channels and distributaries provide benefits such as alternative aquatic passage, dissipation of stream energy, hydrologic reconnection to the floodplain, and increases in the soil saturation area. All of these attributes help to create an intricate network of multi-threaded channels and wetlands. Evidence of surface water and wetland expansion caused by beaver dam construction is plentiful. Many studies have documented creation of and changes in surface water and wetland habitats that have resulted from increases in beaver populations. For example, when studying the effects of climate and beaver activity in Elk Island National Park in Alberta, Canada, Hood and Bayley (2008) estimate that beaver reoccupation of the park caused the total area of open water to increase from 365 hectares (in 1948) to 991 hectares (in 1996). In Acadia National Park in Maine, Cunningham et al. (2006) found that beaver contributed to an 89 percent increase in ponded wetlands from 1944 to 1997, by converting forested wetlands and riparian areas to open water and emergent wetlands and by converting forested upland habitat to forested wetlands and riparian areas. In Virginia, Syphard and Garcia (2001) found that, from 1953 to 1994, beaver activity in the Chickahominy River watershed accounted for only 1 percent of wetland gain, but the animal’s activities accounted for 23 percent of the change in wetland types. In a region of northern Minnesota, Johnston and Naiman (1990a) found that the number of beaver ponds increased from 71 to 835 between 1940 and 1986 as beaver reoccupied the area. Increased Groundwater Recharge Beaver dams can play a critical role in replenishing alluvial aquifers by trapping and storing water, redirecting surface water onto adjacent floodplains, and forcing water into the streambed and banks. Overbank flooding is generally thought to be the main hydrologic mechanism for replenishing groundwater in riparian areas (Workman and Serrano 1999, Girard et al. 2003, Westbrook et al. 2006). Subsurface flow patterns may also be affected by beaver impoundments. In two separate studies located in Rocky Mountain National Park (in Colorado) and in Central Oregon, Westbrook et al. (2006) and Lowry and Beschta (1994), respectively, observed groundwater flow moving laterally around the dams (i.e., perpendicular to the river) into floodplain soils, then downstream, and eventually back in toward the river channel. This “looping” pattern of groundwater flow does not always take place; its occurrence depends on topographic relief and beaver dam height, which affect the hydraulic gradient between river and riparian area (Westbrook et al. 2006). Groundwater flow may also be affected by the location of the beaver dam within the valley and the streams geomorphology. Furthermore, Westbrook et al. (2006) found that, in Rocky Mountain National Park, the main effects of beaver on hydrologic processes occurred downstream of beaver dams rather than being confined to the near-pond Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 7 area. In semi-arid streams, the hydraulic head created by beaver dams can affect subsurface flows by increasing hyporheic interactions within and downstream of beaver dam complexes (Lautz et al. 2006). During summer low-flow months, groundwater drawdown often can negatively affect riparian and floodplain plant communities, especially when rainfall and snowmelt flows have already diminished, as well as the frequency and duration of flooding events. In addition groundwater stored in the soil can be depleted by evapotranspiration. By attenuating the rate of water table drawdown during summer low-flow months, beaver dams can provide a constant supply of water to the riparian area, via surface and subsurface flow paths (Westbrook et al. 2006). This influence on the hydrological processes affects the development of the floodplain and riparian areas by maintaining high local water tables and deeper groundwater levels. Thus, beaver influence floodplain structure and function (Westbrook et al. 2006). In addition to mitigating climate change-related decreases in stream flow, via surface water storage, beaver increase the amount of groundwater storage and aquifer recharge (Pollock et al. 2003, Westbrook et al. 2006). This ultimately may be the most important beaver-related factor in mitigating effects from climate change because groundwater is released more gradually than surface water and has no evaporative losses. In areas where groundwater is being depleted faster than it is being recharged naturally, beaver ponds may help to offset the aquifer depletion, especially when beaver activity is occurring at the reach or watershed scale. Furthermore, increased groundwater storage may help to offset rising stream temperatures associated with the increase in open-water surface area. Cold pockets of water have been found downstream of beaver dams, possibly from the upwelling of groundwater and an increase in hyporheic exchange (Pollock et al. 2007). This is particularly important for aquatic species that require cold water. Water Quality Beaver have the ability to improve the water quality of streams by reducing suspended sediments in the water column, moderating stream temperatures, improving nutrient cycling, and removing and storing contaminants. This section highlights how beaver dams can affect the water quality of streams in ways that often mimic common restoration project goals. Sediment Retention Beaver dams affect channel form by creating ponds that increase the local water depth, reduce flow velocities, and dissipate stream energy. This in turn promotes sediment deposition and channel aggradation upstream of the dams (Naiman et al. 1986, Butler and Malanson 1995, Pollock et al. 2007, Green and Westbrook 2009). The size of a pond (i.e., its surface area) is often the best predictor of the rates and volume of sedimentation (Naiman et al. 1986, Butler and Malanson 1995). By trapping sediment, beaver dams cause substantial changes to channel morphology. In contrast, removing beaver dams can transform intricate, multi-threaded channels to a simplified single channel and increase sediment loads. For example, in a study in the East Kootenay region of British Colombia, Green and Westbrook (2009) found that the removal of beaver and their dams from 1968 to 2004 simplified channel structure and resulted in an Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 8 estimated fivefold increase in mean flow velocity and the release of an additional 848 cubic yards of sediment to downstream areas. If suspended sediment is a water quality concern, beaver colonization may be an effective method for reducing the amount of sediment being conveyed through the system. Beaver dams can influence sediment transport rates in a watershed and act as long-term sinks for both suspended and bedload sediments (Green and Westbrook 2009). Sedimentation rates behind beaver dams vary widely and typically are a function of (1) sediment availability from upstream, and (2) flows capable of liberating and transporting this sediment (Pollock et al. 2014). Aggradation rates range from 1 inch to upwards of 1.6 feet per year, depending on the region and the interrelationships among flow, sediment characteristics, and pond geometry (Devito and Dillon 1993, Butler and Malanson 1995, Pollock et al. 2007). As beaver begin to reoccupy sites, they tend to choose dam locations that will pond large amounts of water (Duncan 1984) and have high sediment trapping capabilities (Ringer 1994). Allred (1980) found that 10 beaver ponds along the South Fork Snake River trapped 63 percent of the suspended sediment during peak flow. On Beaver Creek, Idaho, Reiner (1983) reported that four ponds trapped 78 tons of sediment in a single snowmelt period. Brayton (1984) reports that three years after beaver reintroduction, suspended sediment loads in Currant Creek, Wyoming, dropped by about 90 percent (from 33 tons per day to 3 tons per day). Pollock et al. (2007) found that beaver dams in Bridge Creek, Oregon, collected up to 1.5 feet of sediment behind them during the first year they were in place. This aggradation behind the dams (including dams up to 6 years in age) resulted in an average reduction in slope of 1.3 percent within beaver-modified reaches compared to upstream reaches with no beaver dams. The total amount of sediment that can be stored behind beaver dams can be substantial. For example, 22 ponds in a 620-meter stretch of Mission Creek, Washington, stored 5,847 cubic yards of sediment, for an average of 266 cubic yards per pond (Scheffer 1938). In Quebec, Canada, Naiman et al. (1986) measured retained sediment volumes that ranged from 346 cubic yards to 8,502 cubic yards on second- to fourth-order streams. Butler and Malanson (2005) estimated that modern beaver ponds (i.e., after European settlement) are storing between 9.8 x 10 8 and 5.0 x 109 yd3 of sediment. The sediment retained behind beaver dams can remobilize and become available for transport if dams are intentionally removed, breach as a result of high flows, or are abandoned by beaver (see “How do beavers create their own habitat?” in Frequently Asked Questions). However, when dams breach on small streams, most of the sediment can remain in the pond area (Butler and Malanson 2005). This may be due to lack of erosive flows or because the dam breaches only partially (i.e., there is channel avulsion around the dam), leaving most of the dam in place. As the water table recedes, the remaining nutrient-rich sediment in the abandoned ponds becomes exposed and often is quickly colonized by herbaceous plants or shrubs, forming a beaver meadow (Ives 1942, Johnston and Naiman 1987, Westbrook et al. 2011). Temperature Moderation Land use changes and ecosystem degradation already have caused summer water temperatures in streams and rivers to frequently exceed levels suitable for aquatic life (Kaushal et al. 2010). Climate change models predict that in the near future, water temperatures will Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 9 increase even further. Maximum summer temperatures are often the single most important factor limiting the distribution and presence of numerous fish species in rivers (McRae and Edwards 1994, Wenger et al. 2011). Many salmon habitat restoration efforts in rivers and streams focus on increasing shade by bolstering riparian areas to reduce summer peak temperatures. In many regions, beaver dams have the ability to lower stream temperatures through the creation of riparian and wetland habitat. Vegetation associated with these areas offers shade that helps to lower stream and pond temperatures. A common concern about beaver dams is that they may warm streams by increasing surface water area and reducing the amount of shade (Reid 1952, Knudsen 1962, reviewed in Collen and Gibson 2000). Large ponds general do receive more solar radiation than flowing stream reaches and their surface waters can warm substantially in summer. However, large, deep ponds (greater than six feet deep) usually stratify, with cooler water near the bottom and a thin layer of warm water at the surface, separated by a sharp thermocline. The cool water in the depths of beaver ponds can provide a temperature refuge for fish during the warm parts of the day, and the fish can feed in the more productive surface layers during the night and early morning (Hoffman and Recht 2013). Cooling downstream of dams has been reported. Pollock et al. (2007) found that beaver dams in a stream in eastern Oregon created pockets of cool water downstream, presumably caused by hyporheic upwelling that resulted from the head differential created by the dam. The authors also found that the stream temperatures within the beaver dam complexes were cooler than both upstream and downstream reaches that lacked beaver dams (see also White and Rahel 2008). McRae and Edwards (1994) investigated how beaver dams in northern Wisconsin affected stream temperatures. They found slight warming downstream of beaver dams; however, large ponds tended to dampen temperature fluctuations. They also removed several dams to assess what effect dam removal would have on temperature. Dam removal did not generally reduce temperatures and in some cases actually increased warming rates. McRae and Edwards concluded that the disruptive effects of dam removal on the composition of fish and invertebrate communities may outweigh potential direct thermal benefits. Chesney et al. (2010) found that two beaver dams in the Shasta River in Northern California stabilized temperatures relative to upstream and downstream reaches that lacked beaver dams. Small beaver ponds may not have major temperature effects (Hoffman and Recht 2013). Nutrient Cycling Although beaver are less widespread and ecologically influential today than they were in the past, they continue to have substantial nutrient impacts on drainage networks throughout many areas of North America. As Naiman et al. (1994) states, “beaver feeding strategies and physical alteration of the stream environment affect the hydrologic regime as well as community composition (McDowell and Naiman 1986, Naiman et al. 1988b, Johnston and Naiman 1990a, b). In turn, these changes alter biogeochemical cycling and the accumulation of nutrients and ions in soils, sediments, and water.” Beaver ponds have the ability to trap and retain large amounts of material—woody and herbaceous vegetation and organic and inorganic soil particles—that would otherwise be transported downstream (Naiman et al. 1986, Naiman et al. 1994). This can easily be seen in the thick accumulation of material at the bottom of beaver ponds; sometimes these accumulations Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 10 are up to 3 feet deep. Woody debris on pond floors can be important habitat for fish, amphibians, and aquatic invertebrates. Woody debris reaches the pond floor through several mechanism, including upland surface flows and the active process of beaver cutting down woody material for food and construction material, transporting it to the pond, and depositing it in food caches, dams, and lodges. Debris also can consist of emergent vegetation produced within the pond, or forest vegetation that was drowned out during the original inundation of the forest by beaver. Depending on an individual beaver pond’s age, its ecological maturity, the channel morphology, and other factors related to the maintenance of system properties, the pond can act as both a net sink for soil and woody debris and a source of elements that are transported downstream (Naiman et al. 1994). When upland and in-situ vegetation becomes trapped in beaver ponds, it creates a deep organic sediment layer, generally within the first decade following pond creation (Naiman et al. 1994). Anaerobic conditions within the submerged sediment layers can lock nutrients in the pond sediments until high flows wash them downstream or the site is abandoned and drained, after which a meadow typically forms. When newly exposed sediments return to aerobic conditions, nutrients are released in a form that is available to vegetation, resulting in very productive soil conditions that catalyse rapid plant growth and diverse communities during initial successional stages (Naiman et al. 1994). Contaminants In Europe, beaver ponds have been shown to increase the self-purification capacity of small streams that have been polluted by communal sewage, cattle farms, and agricultural discharge (Balodis 1994). Müller-Schwarze and Sun (2003) used a computer model to estimate the retention time of water flowing through a system with and without beaver dams. The model suggested that water flowing through a 1-square-mile area (2.59 square kilometers) with no dams resides for only 3 to 4 hours, while the same area with a 5-foot-high leaky dam retains water for about 11 days. Non-leaky or tight dams hold water almost twice as long—for about 19 days. Retention times of 6 to 8 days are sufficient to remove excess nutrients and toxins such as nitrogen, phosphorus, and herbicides (e.g., atrazine) from the water column (Muller-Schwarze 2011). Removal processes include deposition, microbial decomposition, uptake by plants, and chemical transformation augmented by filtering. Additionally, beaver ponds can be sinks for fine particulate matter such as clay, which nitrogen and phosphorus can adsorb to. Thus, beaver ponds and associated wetlands created by dams can act as sinks for nutrients and toxins that would otherwise stimulate the growth of algae and other water plants and bacteria downstream. As one example, in the Lake Tahoe basin of California, Muskopf (2007) studied how removing beaver dams from Taylor Creek affected concentrations of phosphorus entering Lake Tahoe. The author reported that the mean total phosphorus concentrations downstream of the dams increased from 70.4 micrograms per liter (μg/l) (before dam removals) to 170.5 μg/l (after dam removals). Geomorphology Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 11 The benefits of trapping and storing sediments behind beaver dams go beyond simply improving the water quality of streams. When beavers build dams on stream reaches, over long time periods the deposition of sediment behind the dams tends to raise the elevation of the streambed (Scheffer 1938, Butler and Malanson 1995, McCullough et al. 2005, Pollock et al. 2007) and increase stream channel complexity by expanding riparian area (Polvi and Wohl 2012). These changes may help prevent channel incision and maintain the hydraulic connection between streams and their floodplains. Channel incision—a widespread phenomenon in stream channels throughout the world—has caused extensive ecosystem degradation and is a common focus of river restoration projects. Incision can result from a number of different factors, including the widespread extirpation of beaver in the nineteenth century—as well as changes in climate, land use, grazing, etc (Naiman et al. 1988b, Pollock et al. 2014). The effects of channel incision include lower stream bed elevations, disconnection of the stream from its floodplain, lower groundwater tables, loss of wetlands, decreased summer low flows, higher stream temperatures, less overall habitat diversity, loss of riparian areas, and population declines in fish and other aquatic organisms (Cluer and Thorne 2014, Pollock et al. 2014). Recovery of incised channels can happen naturally (see Cluer and Thorne 2014), but the process may require very long time scales. Pollock et al. (2014) proposed an expanded view to Cluer and Thorne’s (2014) stream evolution model, suggesting that the inclusion of beaver into incised streams may substantially reduce the recovery time, which typically ranges from decades to centuries (Figure 2). Whether beaver can not only stop the incision process but reverse it, creating a positive feedback loop, depends on the quantity of sediment entering the channel and the channel’s ability to retain this sediment (Pollock et al. 2007) (Table 2). Beechie et al. (2008), studied channel incision on the Walla Walla and Tucannon River basins in eastern Washington and estimated recovery times of 60 to 270 years without beaver and assuming relatively low aggradation rates (approximately 1.2 inches per year). When low densities of beaver dams ( 2 km -1 ) were included in their estimates and an estimated trapping of 224 cubic yards of sediment per year per dam, recovery time was reduced by 20 to 84 years—a decrease of up to 33 percent. Table 2. Aggradation Rates behind Beaver Dams Source Location Aggradation Rate (m/yr) Butler and Malanson (1995) Glacier National Park, MT 0.02-0.28 Meentemeyer and Butler (1999) Glacier National Park, MT ≥ 0.06 Scheffer (1938) Eastern Washington 0.55 McCullough et al. (2005) Nebraska 0.04 Pollock et al. (2007) Bridge Creek, OR 0.075 - 0.47 Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 12 Beaver colonization in incised streams may be difficult because of the relatively deep, strong flow, which can breach or blow out beaver dams, especially during high-flow events (Pollock et al. 2012). For example, along incised reaches of Bridge Creek, Oregon, most beaver dams were extremely short-lived; many lasted less than a year before they were washed out by annual spring floods or summer flash floods (Demmer and Beschta 2008). Pollock et al. (2012) actively assisted beaver in the construction of dams by installing different types of Figure 2: Conceptual model illustrating how beaver dams affect the development of incised streams; (a) beaver attempting to build dams within narrow incision trenches where high stream power often results in blowouts or end cuts that help to widen the incision trench, as illustrated in (b), allowing an inset floodplain to form. The widened incision trench results in lower stream power which enables Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 13 beaver to build wider, more stable dams (c). Because of high sediment loads, the beaver ponds rapidly fill up with sediment and are temporarily abandoned, but the accumulated sediment facilitates the growth of riparian vegetation (d). This process repeats itself until the beaver dams raise the water table sufficient to reconnect the stream to its former floodplain (e). Eventually (f), the stream ecosystem develops a high level of complexity as beaver dams, live vegetation and dead wood slow the flow of water and raise groundwater levels such that multithread channels are formed, often connected to offchannel wetlands such that the entire valley bottom is saturated, as described elsewhere (Sedell and Frogatt 1983, Walter and Merritts 2008). Figure from Pollock et al. 2014. beaver dam analogues (BDAs) and dam support structures (i.e., starter dams, post lines with wicker weaves, and post lines—see ”Beaver Dam Analogues” for more detail). Offering structural supports to possible dam sites, abandoned dams, breached dams, and existing dams increases the chance that these structures will withstand large flow events and remain intact for more than one year. A two-year life span for a beaver dam is critical for beaver colony viability because kits typcially remain with their parents for two years before they disperse from the colony. Once beavers have established themselves on incised reaches, the resulting stable beaver colonies cause the reaches to aggrade, resulting in measurable improvements in riparian and stream habitat conditions (Pollock et al. 2012, Woodruff unpublished data). Reponse of Other Species to Beaver Dams Beaver are a keystone species, meaning that they have a disproportionately large effect on their environment relative to their abundance. Beaver play a critical role in the watersheds of North America by maintaining the structure of the surrounding ecological community. Their presence in watersheds affects not only the types and numbers of many terrestrial and aquatic plant and animal species, but also maintains the change over time of channel form and the hydrology of watersheds. The subsections below highlight certain species that benefit from the habitat created by beaver. Vegetation How vegetation responds to habitat modifications by beaver depends on the type of vegetation and the region, but there are common general trends. Beaver ponds initially affect plants by increasing flooding. Typically, small plants within the footprint of the pond die as a result of the initial inundation, while trees are generally affected within the first year. As large trees and shrubs drown, the canopy opens, allowing more sunlight to reach the pond surface. Increased solar energy facilitates the growth of both emergent and riparian vegetation in the newly enlarged riparian area that has developed as a result of creation of the beaver pond and the expansion of the water surface area. Riparian and emergent vegetation begin to dominate where there used to be upland shrubs and trees. Overbank flooding associated with beaver dams may create surface flows onto floodplains, raising the local water table, which initiates succession toward wetland plant communities. Thus, beaver ponds can create aquatic habitat from many riparian, emergent, and wetland plant communities within and adjacent to ponds (Johnston and Naiman 1990a, Burchsted and Daniels 2014). Increased riparian vegetation density results in the accelerated deposition of fine sediment on the floodplain—a result of greater flow resistance and lower velocities, as vegetation increases Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 14 roughness and pulls energy out of the water flow, reducing erosion and transport power. Thus, beaver dams and beaver-assisted alterations in vegetation work in concert to increase sediment deposition. The diversity and form of growth of riparian vegetation also are driven directly by beaver herbivory (Harrison and Stella 2010). Beavers consume their favorite plant species, leaving riparian areas dominated by non-preferred species, such as ninebark in western North America and red maple in eastern North America. In arid and shrub-steppe environments dominated by stands of willow, beaver herbivory tends to drive willow form from taller stands with less branching to shorter stands with more branching (Baker 2003). Beaver dams create habitat while they are impounding water, but they continue to create habitat even after colonies are abandoned, often in the form of beaver meadows, particularly in more mesic climates (Ives 1942, Burchsted et al. 2010, Polvi and Wohl 2012). A large flow event can cause a dam to be breached, or it may be abandoned after a colony has depleted the resources in the surrounding area. After a breach some of the stored sediment is released downstream (Levine and Meyer 2014), but much of it is retained, depending on the local channel and valley form (Butler and Malanson 2005). As the water table drops in response to dam removal, the exposed substrate is usually colonized by vascular plants, including plants that germinate from the seed bank stored in the sediments (Wright et al. 2002). The resulting newly formed “meadow” usually is devoid of trees (because the former forest was drowned out by the beaver pond or removed by the beaver through herbivory). After a beaver meadow forms, it progresses through successional stages of young and wet to old and moist (Naiman et al. 1994, Wright et al. 2002). The meadow may then persist on the landscape for centuries (Wright et al. 2002). Beaver meadows form distinct patches on a landscape (Johnston and Naiman 1987, Terwilliger and Pastor 1999). The meadows act as “islands” of wetland plant communities whose composition differs from that of adjacent, unmodified riparian zones and upland forest (Wright et al. 2002). The variability in plant species composition and richness of beaver meadows may contribute significantly to landscape-level heterogeneity. Studies of the beaver-meadow complex have occurred almost entirely in mesic environments. Whether long-term beaver meadows form in more xeric regions (e.g. lower elevations in much of the American West) is a research question that should be pursued. Primary Productivity and Aquatic Invertebrates When beaver modify streams, they create excellent habitat for many aquatic insect populations by increasing the input and storage of organic material and sediment (reviewed in Collen and Gibson 2000) and increasing primary productivity. Beaver ponds boost primary productivity both by increasing the availability of organic nutrients (Francis et al. 1985) and by allowing sunlight to reach more water surface for photosynthesis. Primary producers such as periphyton, planktonic algae, and aquatic vascular plants take advantage of the increased solar radiation. This sets the stage for the secondary producers—micro- and macroinvertebrates—who, in turn, take advantage of the increase in detritus—i.e., the woody material, decaying leaves, and decaying in-situ vegetation produced in the pond. These micro- Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 15 and macroinvertebrates form the base of the food web that juvenile salmon and steelhead rely on when rearing and overwintering in beaver ponds. Beaver ponds harbor many lentic benthic invertebrates—i.e., invertebrates that prefer slow- water habitats. Riffle reaches between ponds primarily harbour invertebrates which prefer faster flowing water. In comparison to streams that have no beaver activity, beaver-modified streams influence the community structure of aquatic invertebrates by shifting from primarily lotic taxa to a larger presence of lentic taxa (McDowell and Naiman 1986). Overall, having multiple beaver ponds in an area tends to increase the biodiversity of aquatic insect communities by selecting for both lotic and lentic populations. Fish The pools and ponds created by beaver dams are excellent habitat for many fish species. More than 80 North American fishes have been documented in beaver ponds, with 48 species commonly using them (reviewed in Pollock et al. 2003). Because beaver ponds slow down stream flow and have very large edge-to-surface-area ratios, they provide considerable cover for fish and a productive environment for both vegetation and aquatic invertebrates that fish can use for food resources not found in unimpounded stream habitat (Hanson and Campbell 1963, Keast and Fox 1990, reviewed in Pollock et al. 2003). Additionally, fish expend less energy foraging in the slow, productive waters of beaver ponds and side channels than they do in the faster flowing main channel. This leads to increases in fish abundance and size (i.e., weight and length); fish found in stream reaches that have beaver dams are both larger and more numerous than fish found in streams lacking slow water habitat. (see Gard 1961, Hanson and Campbell 1963, Murphy et al. 1989, Leidholt Bruner et al. 1992, Schlosser 1995, reviewed in Pollock et al. 2003, Sigourney et al. 2006). There has been extensive research on both the positive and negative effects of beaver modifications on fish species. Kemp et al. (2012) thoroughly reviewed the primary literature on this topic, focusing on North America, and completed a meta-analysis. They reported the most commonly cited positive and negative impacts to fish as shown in Table 3. Table 3. Potential Impacts of Beaver Modifications on Fish Species Potential Positive Impacts Potential Negative Impacts • Increased fish productivity/abundance • Increased habitat and habitat heterogeneity (which promotes biodiversity) (Smith and Mather 2013)) • Increased rearing and overwintering habitat • Enhanced growth rates • Providing flow refuge • Improved production of invertebrates • Barriers to fish movement • Siltation of spawning habitat • Low oxygen levels in beaver ponds • Altered temperature regime Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 16 Kemp et al. noted that many of the positive effects cited (51.5 percent) were supported by data, while many more of the negative impacts (71.4 percent) were speculative and not supported by data collected in the field. Furthermore, the most commonly cited negative impact of beaver dams—as barriers to fish movement—was highly speculative, as 78.4 percent of the studies did not support this claim with data. The authors report that 49 North American and European experts consider beaver to have an overall positive impact on fish populations, through their influence on abundance and productivity. Along the Pacific Coast of North America, interest in protecting beaver-modified habitat is growing because of the habitat’s potential to benefit anadromous fish populations. Coho salmon (Oncorhynchus kisutch), for example, use various types of slow-water habitat (e.g., sloughs and perennial and seasonal wetlands, off-channel ponds, small lakes, side channels, alcoves, and backwaters) as juveniles (Solazzi et al. 2000, Bramblett et al. 2002, Pollock et al. 2004, Ebersole et al. 2006, Henning et al. 2006) and adults; they use fast water during adult migration and spawning (Reeves et al. 1989). The activities of beaver can create the type of slow-water habitat used by coho juveniles (Swales et al. 1988, Murphy et al. 1989). During summer, beaver ponds are important rearing grounds for juvenile coho salmon (Leidholt-Bruner et al. 1992). For example, in the Fish Creek Basin of Northwest Oregon, Everest et al. (1986) found that the density of juvenile coho in beaver ponds (i.e., 1.43 per cubic meter) was four times higher than the density in side channels and 48 times higher than that in riffles. Beaver ponds constituted only 2.5% of the habitat at Fish Creek but produced 50.4 percent of the coho salmon smolts in 1986, more than in 1985 (reviewed in Müller-Schwarze 2011). In addition to summer rearing grounds—and possibly more critical to coho populations— is the use of beaver ponds and slow-water habitat as overwintering grounds. For example, Pollock et al. (2004) found that in the Stillaguamish River basin in Washington, the decline in beaver populations and subsequent loss of their dams resulted in a 61 percent reduction of summer coho habitat capacity and an 86 percent reduction in overwintering capacity. The authors conclude that the production bottleneck of coho salmon in this watershed was from a lack of overwintering habitat and that increasing beaver populations could be a simple and effective means of mitigating this loss of productivity. Nickelson et al. (1992) reported that, in coastal Oregon streams, beaver ponds and alcoves supported more juvenile coho salmon (about 1 fish per cubic meter) than did other stream habitats, such as backwater pools, trench pools, glides, riffles, and rapids. Beaver ponds and alcoves represented only about 9 percent of the habitat but accounted for 66 percent of the coho salmon found in the system. Likewise, Bustard and Narver (1975) showed that, on Vancouver Island, the overwintering survival rate for juvenile coho behind beaver dams ranged from 61 percent to 74 percent; this was higher than the average rate for the entire stream systems (i.e., 35 percent). Silloway and Beesley (2011) suggested that coho salmon populations in the Klamath River estuary in California were limited by the availability of juvenile overwintering sites such as coastal wetlands, beaver ponds, and alcove/slough habitats. Many other studies confirm the benefits of slow-water habitat on coho populations along the Pacific Coast (Bell et al. 2001, Brakensiek and Hankin 2007, Ransom 2007, Wallace and Allen 2007, Hillemeier et al. 2009, Chesney et al. 2010, Wallace 2010). Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 17 Most of the research on fish populations using beaver ponds and slow-water habitat along the Pacific Coast has been done on coho salmon; however, other fish species also benefit from this habitat. For example, juvenile Chinook salmon (O. tshawytscha) and steelhead (O. mykiss) in British Columbia and Washington also use off-channel and floodplain habitats for overwintering (Swales et al. 1988, Cunjak 1996). Pollock et al. (unpublished data) found that juvenile steelhead in eastern Oregon had higher densities and survival rates in beaver ponds than did juveniles in similar reaches without dams. Juvenile steelhead in the upper Trinity River of California also preferred side-channels during winter (Macedo 1992). In the Sacramento River system of California, juvenile Chinook show more growth and higher survival in floodplain habitats than do fish in mainstem habitats (Sommer et al. 2001, Sommer et al. 2005). Similarly, Limm and Marchetti (2009) found high growth rates in juvenile Chinook salmon in off-channel ponds of the Sacramento River watershed. Salmon recovery plans along the Pacific Coast have recently identified beaver habitat as important for salmon and steelhead that must be protected to ensure future stocks of this important resource. Amphibians Beaver ponds provide important breeding habitat for some amphibians, including Northwestern salamanders, red-legged frogs, Pacific tree frogs, wood frogs, green frogs, cascades frogs, rough-skinned and red-spotted newts, and Western and American toads. By diversifying the landscape with different sizes and ages of ponds, beaver modified streams can significantly increase the biodiversity of amphibians (reviewed in Müller-Schwarze 2011). Red-spotted newts (Notophthalmus viridescens) readily take advantage of the unique aquatic habitat created by beaver activity and may actually depend on beaver ponds for their survival. Because red-spotted newts respond to aquatic habitats that rapidly shift in time and space, they rapidly colonize new beaver ponds (Gill 1978). The newt’s life history seems well-tuned to the shifting mosaic that typifies beaver-maintained habitat (Müller-Schwarze 2011). The wood frog (Rana sylvatica), which breeds in beaver ponds, thrives in marginal ponds with little inflow or outflow; these habitats are primarily found in areas saturated with beaver populations that are not heavily managed. For example, in the central Adirondack region of the northeastern United States, wood frogs living in beaver ponds had higher survival rates of metamorphosed froglets and produced larger juvenile frogs than did wood frogs living in vernal (i.e., seasonal) pools (Karraker and Gibbs 2009). Stevens et al. (2006) suggested that older beaver ponds (older than 25 years) in boreal streams of west-central Alberta, Canada, supported more breeding wood frogs and had higher rates of juvenile growth and development than younger ponds (less than 10 years old). Canal networks created by beaver in the wetlands of Miquelon Lake in Alberta, Canada, may provide essential movement corridors for emigrating juvenile wood frogs (Anderson et al. 2014). Along the West Coast, the Oregon spotted frog uses perennial wetland habitat—as well as pools, ponds, and small floodplain wetlands associated with permanent bodies of water— throughout its life history (Pearl and Hayes 2004, Cushman and Pearl 2007). Habitat loss has caused this species be become extirpated from possibly 70 to 90 percent of its historical range (Cushman and Pearl 2007). Its last refuge may be beaver-modified systems, which offer relatively favorable conditions for the Oregon spotted frog. Oregon spotted frog eggs survive Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 18 and develop best in warm, shallow water where emergent vegetation already is established (Cushman and Pearl 2007). Beaver ponds’ emergent vegetation and slightly warmer surface water (compared to upstream and downstream reaches) may provide critical habitat for this stage of the Oregon spotted frog life cycle. In addition, beaver dams increase the amount of surface water and retention times within their catchments, and this may reduce egg and hatchling larvae’s susceptibility to desiccation. For more information on beaver ponds and amphibians, see Russell et al. (1999), Skelly and Freidenburg (2000), Quail (2001), Crisafulli et al. (2005) and Stevens et al. (2007).). Reptiles Beaver ponds provide important habitat to some reptiles, turtles being the most common. Painted turtles, western-painted turtles, western pond turtles and snapping turtles use beaver ponds. Other terrestrial reptiles that are found near ponds include snakes and lizards. Older beaver ponds seem to attract more reptiles than younger ponds, again highlighting the importance of the diversified landscape that beavers create over long time frames of occupation (Russell et al. 1999, Metts et al. 2001). Birds The water impounded behind beaver dams provides new habitat for waterfowl and many other bird species. This is not news to any avid bird watcher or waterfowl hunter who chooses to set up their blinds in areas colonized by beaver. Beaver-created wetlands and ponds produce numerous species of aquatic insects, which are essential food for hens and rearing broods of waterfowl. The cover offered by lush riparian vegetation— both tall trees and shrubs and emergent herbaceous vegetation—offers cover from predation by flying raptors and terrestrial hunters. Hens often choose beaver ponds to rear their broods because of the protection that ponds offer from predators and the large supply of protein- and calcium-rich aquatic insects. In addition, the habitat created by beaver dams is a refuge for many migratory birds species, providing rest and refuelling locations along their north-south routes. The dead snags created by beaver through girdling and flooding provide excellent nesting habitat for many birds, and attracts numerous woodpecker species. The list of birds that actively use beaver ponds is long and varies by region. Most studies of beaver-modified habitat and its effect on bird populations have been on the East Coast of the United States. Beaver ponds in New York, for example, host American and hooded mergansers, Canada geese, mallards, pintails, buffleheads, wood ducks, horned and pie-billed grebes, great blue and green herons, kingfishers, woodpeckers, chickadees, tree swallows, eastern bluebirds, red-winged blackbirds, and numerous species of flycatchers and warblers. Surveys of birds at beaver ponds across New York show that active beaver sites support more species of birds than do vacant or potential sites (Lochmiller 1979) and the benefit to avifauna persists for decades after beaver are actively using the site (Alza 2014). In the southeastern U.S. state of Georgia, Lochmiller (1979) found that dead snags flooded or girdled by beaver attracted more than twice as many woodpeckers than did a tree stand without beaver. In Maine, the wetlands created by beaver contained flooded alder-willow thickets, herbaceous vegetation, and large water surfaces, all of which are essential brood-rearing habitat for the Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 19 American black duck (McCall et al. 1996). Protected beaver habitat in south-central Maine supported more mallards, hooded mergansers, and Canada geese than did areas where beaver trapping was allowed. Beaver ponds in Wisconsin attract both waterfowl and other birds, including mallards, black ducks, blue-winged teals, ring-necked ducks, hooded mergansers, shorebirds, swallows, flycatchers, hawks, warblers, sparrows, kingfishers, osprey, and bald eagles (Knudsen 1962). Along the Continental Divide in the Rocky Mountains, birds such as the spotted sandpiper, Wilson’s snipe, Brewer’s blackbird, red-winged blackbirds, mallards, and green-winged teals all rely on beaver ponds (Brown et al. 1996). The beaver’s ability to create wetlands is especially important to waterfowl in the western United States, where riparian and wetland habitats make less than 2 percent of the landscape yet provide habitat for more than 80 percent of wildlife species (Hansen 1995). In addition, such beaver ponds may provide isolated breeding-pair ponds for waterfowl at a crucial time in their annual life cycle. After mating, these ponds offer the necessary protein- and calcium-rich invertebrates that sustain breeding pairs of birds during the egg-laying period. In a study of beaver-modified streams in Wyoming, McKinstry et al. (2001) found that the riparian width in streams with beaver ponds averaged 111 feet, in contrast to 35 feet in streams without beaver. This difference may have affected the waterfowl surveys: a total of 7.5 ducks were found per kilometer of stream in areas with beaver ponds, while similar areas that lacked beaver had only 0.1 duck per kilometer of stream. When McKinstry et al. (2001)reintroduced beaver to 14 streams throughout Wyoming, waterfowl quickly took advantage of the newly created wetlands and improved riparian areas. Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 20 Chapter 2—Frequently Asked Questions about Beaver Greg Lewallen, Janine Castro, Chris Jordan and Michael M. Pollock Where do beaver live? How do they make their dams? Why do they slap their tails? This section answers some of the most common questions people have about beaver biology and ecology. How many species of beaver are there? There are two extant beaver species: the North American beaver (Castor canadensis) and the Eurasian beaver (Castor fiber). Habitat loss and trapping extirpated both species throughout most of their range. The Eurasian beaver, which closely resembles its North American cousin in both appearance and behavior, was extirpated from much of its former range by the beginning of the twentieth century (Halley et al. 2012), and the North American beaver soon followed suit. Estimates of the beaver population in North American before European settlement vary, but it is thought that around 55 million dam-building individuals were present (Pollock et al. 2003); Seton (1929) estimated the total population to be between 60 million and 400 million. Fur trapping, which began in the 1700s to support the European fashion for pelt hats (Bryce 1900), resulted in a massive decline in beaver populations. Today beaver are making a comeback—in Europe, Russia, and North America. Reintroductions of the species began in the United States in the early twentieth century and continue today. Although population numbers have not reached historical levels (current rough estimates put them at only 6 million to 12 million individuals (Naiman et al. 1988b), beaver now occupy almost all of their former range in North America. They have been so successful that their burgeoning populations have migrated into human-occupied territory, sometimes causing localized flooding or loss of vegetation. This has contributed to people’s negative perception of the species as a pest or nuisance animal. Within both species of beaver, individuals manifest two very different but critically important behaviors: Some beavers build dams to impound water and some beavers do not. This has bearing on river restoration projects where habitat modification through dam construction by beavers is intended to produce the effects needed to meet specific goals. It is of critical importance to understand why beavers build dams, so that we can try to predict where and when dam-building activity may occur (see “Why do beavers build dams?” in Frequently Asked Questions). The effects of colonies that do not build dams on river systems are not well understood and not the focus of this document. Here, we highlight how beaver dams affect the landscape and how they can be useful in a wide range of restoration scenarios in North America. In the past, numerous subspecies of North American beaver have been identified, but currently, the Integrated Taxonomic Information System (www.itis.gov) does not recognize Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 21 any subspecies of C. canadensis. For a list of formerly recognized subspecies of C. Canadensis, see Table 1 in Appenix B. What is the beaver’s range? The North American beaver occurs throughout most of Alaska, Canada, the continental United States and in portions of northern Mexico (Figure 1) (Pollock et al. 2003). The beaver’s adaptability and ability to modify its environment to create suitable habitat has allowed it to thrive in a wide range of biomes. Novel evidence is challenging previously held assumptions about the historical range of beavers, pushing its territory to include high elevations in the Sierra Nevada Mountains (Lanman et al. 2012), parts of the California coast (Lanman et al. 2013), tidal wetlands in Washington State (Hood 2012), and peninsular Florida (Layne and Johns 1965). In North America, the only areas where beaver may be absent are the Arctic, the very far north of Canada and parts of Alaska, the dry Great Basin and desert country of Nevada and southern California (Jenkins 1979, Pollock et al. 2003). Otherwise, beavers are found throughout northern boreal forests, south to the deserts of northern Mexico, west to the Aleutian Islands, and all the way to the eastern seaboard. What are important habitat elements for beaver? Numerous studies describe detailed life history characteristics of beavers (Morgan 1868, Bradt 1938, Jenkins and Busher 1979a, Hill 1982a, Allred 1986, Hilfiker 1991, Novak 1999, Baker and Hill 2003, Muller-Schwarze 2011). The single most important feature of beaver habitat is the presence of water. Water is essential to the daily life of beavers and can be in the form of a stream, river, lake, or pond, as long as there is a year-round supply sufficient for access to food resources, protection of lodge and burrow entrances, and general safety from predators (Müller-Schwarze and Sun, 2003). Besides the presence of water, beaver need surrounding riparian areas that can provide food resources (see “What do beaver eat?”), construction materials, and places to build scent mounds (see ”How do beavers communicate?”). Are beaver just big rats? Beavers certainly are big. They are the largest rodent in North America and second largest rodent in the world (after the capybara of South America) (Morgan 1868). Adult beaver typically weigh 35 to 71 pounds and can grow to a total length of 4 feet, including the tail (Jenkins and Buscher 1979, Baker and Hill 2003) (Figure 3). The tail alone is about 1.3 feet long, 6.3 inches wide, and 0.75 inch thick. The size and weight of an individual beaver depends on many variables, including the climate, availability and quality of food, extent and condition of habitat, and latitude. Mid-continent beavers, for example, can reach up to 110 pounds (Bailey and Balley 1927). Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 22 Figure 3: Photo of a North American Beaver from Southeast Alaska. Photograph courtesy of Bob Armstrong (Willson and Armstrong 2009). Beaver have evolved to aquire unique features that make it well adapted for its role as both a keystone species and an ecosystem engineer. Baker and Hill (2003) describe the beaver’s body as being drop shaped, thick and heavily muscled, and supported by a large skeleton that is massive in proportion to other mammals of similar length. The beaver’s strong forelegs are shorter than its hind legs; this results in greater height at the hips than at the shoulders. The large head is supported by a short, thick neck almost continuous with the shoulders. The beaver’s stout and powerful body is perfectly suited to manipulating the surrounding environment by gnawing on hardwoods and carrying branches, rocks, and mud with its forelimbs. The skull and mandible (Figure 4) are enormous and thick so that they can withstand the muscular force involved in chewing hardwoods such as oak and maple (Morgan 1868). The beaver uses its four chisel-like incisors to fell trees, cut branches, and peel bark from stems. These teeth grow continuously. The outer enamel layer appears yellow and is much thicker and denser than the white inner enamel. The chiselled edge is sharpened by grinding the upper and lower teeth together. The remaining molars—eight each in the upper and lower jaws—are used for grinding woody and herbaceous food (Müller-Schwarze and Sun 2003). Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 23 Figure 4: Beaver skull. Beavers have massive skulls that include large incisor teeth (a) with chisel-like cutting edges. The molars (b) are used to crush and grind plant material. The deep groove (c) houses a large muscle for closing the lower mandible. The jaw joint (d) is placed high on the skull, well above the tooth rows, which then can meet in parallel. Figure courtesy of Bob Armstrong (Willson and Armstrong 2009). In adaptations to the beaver’s semiaquatic lifestyle, small round eyes and ears sit atop the head. Beavers can close their nostrils and ears when submerged. They have a special membrane that protects their eyes while underwater, and fur-lined lips that can be closed behind the large incisors (Jenkins and Busher 1979). Beavers also have special adaptations to prevent water from entering their larynx and trachea (Müller-Schwarze and Sun 2003). One of the beaver’s most distinguishing and identifiable features is its broad, flat, scaly tail (Baker and Hill 2003)(Figure 16). This multipurpose appendage is used as (1) a prop when cutting trees and when walking on the hind legs while carrying construction materials with the forelimbs, (2) a rudder during swimming, (3) an alarm by slapping the water surface, (4) a fat reserve for lean winter months, and (5) a heat exchange organ to reduce heat losses from 25 percent in the summer to 2 percent in the winter (Marchand 1996). Beaver have well-developed senses of hearing and smell but relatively weak eyesight (Morgan 1868, Novak, 1999). They are prey animals, so their eyes are widely spaced, to provide a large field of vision (Müller-Schwarze and Sun 2003). They use their acute sense of smell to detect Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 24 predators, select palatable hardwoods, and locate other beaver via mud scent mounds (Müller-Schwarze and Sun 2003). Can beavers walk on land? The beaver’s physiology is a product of its aquatic and terrestrial lifestyles. Beaver spend most of their life in water but may need to move overland when cutting woody vegetation for food or construction materials, or when dispersing from areas to find new territory. Because of its body form, a beaver tends to waddle awkwardly when moving overland, but the animal can gallop if frightened (Jenkins and Busher 1979). Adult beavers can walk on their hind legs, leaving their hand-like, dexterous front feet free to grasp and manipulate food, dig, and groom. The two inside toes of each hind foot have movable, split nails that serve as “combs” for preening the fur to keep it fluffy (Wilsson 1971). In the water, beaver are efficient swimmers who use their large webbed hind feet to propel them through the water. Why was their fur so highly valued? The high value placed on beaver fur pelts during the seventeenth, eighteenth, and nineteenth centuries is what led to the near extirpation of the animal from North America. Beaver pelts were used in winter clothing such as jackets and boots, but the primary use for pelts was in the construction of felted hats. From the 1600s through the 1800s, felt was made from the hairs of the under coat and shaped into a wide range of popular hat styles (Figure 5). Coloration of the pelt varies within and among populations, with reddish, chestnut, nearly black, and yellowish-brown specimens possible even within the same watershed (Baker and Hill 2003). A beaver’s fur consists of long, coarse guard hairs that are about 10 times the diameter of the soft, wavy, short underfur. The guard hairs are longest (2.0 to 2.4 inches) and most dense along the back, but the underfur also attains its greatest length (0.8 to 1.2 inch) on the back and can range from a dark grey to a light chestnut in color (Baker and Hill 2003). The extremely dense underfur keeps the body warm and dry. With approximately 12,000 to 23,000 hairs per square centimeter, beavers have more hair per skin area than the South American nutria (i.e. 8,000 to 3,000 hairs per square centimeter), but less than the river otter, which has 25,000 51,000 (Müller-Schwarze and Sun 2003). Beavers molt during the summer, so fur trappers prefer to harvest beaver pelts between December and March when the animals are considered to be in “prime condition” (Müller-Schwarze and Sun 2003). Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 25 Figure 5: Different styles of hats made from beaver felt. Figure copywritten by batashoemuseum.ca. What do beaver eat? As herbivores, beaver consume a wide variety of plant species. They eat the leaves, twigs, and inner bark of most types of woody plants that grow near the water (Jenkins and Busher 1979). In addition, they eat many different kinds of herbaceous plants, including grasses, sedges, and aquatic species such as water lilies. Their diet appears to change seasonally. During the summer months they primarily consume nutritious herbaceous vegetation (Chabreck 1958, Jenkins 1975). During fall and winter, as deciduous leaves and other aquatic vegetation become scarce or unavailable, they switch to primarily the inner bark (i.e., cambium) of woody shrubs and trees. Their digestive tracts are adjusted to this diet high in plant fiber and, through the help of microbial action, are able to use much of the cellulose they consume (Clarke and Hoover Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 26 1972, Hill 1982b, Buech 1984). Estimates of the amount of woody material a beaver eats per day range from 1.0 to 5.5 pounds (reviewed by Novak 1999). On the East Coast, Brenner (1962) estimated that, during the spring and summer, individual beavers consumed about 12 ounces of herbaceous material per night (beaver are primarily nocturnal). Beavers attempt to optimize their energy returns from herbivory by using a central place foraging strategy and choosing smaller trees and stems (i.e., less than 3.9 inches diameter at breast height [DBH]). In general, they seem to choose small trees over large ones of the same species (Aldous 1938, Stegeman 1954, Hall 1960, Jenkins 1979, 1980, Pinkowski 1983, Belovsky 1984). As distance from the pond increases, a beaver’s choice in tree size seems to decrease (Jenkins 1980). Large trees (i.e., more than 3.9 inches DBH) are sometimes felled, debarked in place with only the smaller branches removed, and taken back to the pond (Jenkins and Busher 1979). This strategy may reduce the risk of predation by limiting the amount of time spent on land (smaller trees take less time to fell) and reduces the amount of energy spent transporting material back to the pond. Large trees may also be debarked around the base and left standing. Barking the base of a tree can result in only small pieces of bark removed, or most of the basal bark gnawed off. Varying the amount of barking may be a strategy to measure the relative nutrient value of different trees, which could explain why preferences for certain species of tree change from year to year (Jenkins 1979). Beavers are able to colonize a large and diverse range of habitats throughout almost the entirety of North America because they can use a great number of woody and herbaceous species for food and construction material. (see Appendix A, see also Henker 2009 for a literature review of what beaver eat). Still, our understanding of beaver carrying capacity within a reach of stream based on food availability is somewhat primitive. Herbaceous vegetation taken by beaver is much harder to quantify than woody species (especially because beaver forage primarily at night), so scientists understand less about the impact that herbaceous species have on beavers’ diet, both at the individual and population levels. Some studies have been done to try to answer this question. For example, Collins (1976), using fecal samples of beavers in Wyoming, found parts of 20 species of forbs and 24 species of graminoids (see also Chabreck 1958, Harper 1968, Jenkins 1975, Novak 1999, Parker et al. 2007). Beaver need a reliable source of food, but they are choosy generalists (Harper 1969), consuming a wide variety of plant species. For example, Harper examined the stomach contents of beavers in Mississippi and found that they consumed 42 species of trees, 36 genera of herbaceous plants, four types of woody vines, and many species of grass (Graminae). Yet they are “choosy” because they prefer certain species over others and will take those first if available. Lists of preferred plant species vary by region, and most studies of beaver herbivory have taken place east of the Rocky Mountains. The focus of these studies is primarily woody species because it is easy to identify and count beaver-chewed stems. But beavers also eat a lot of herbaceous material, including sedges and other emergent vegetation and the tuberous roots of water lilies and cattails. In most places generalities can be made as to the food preference of beaver. Woody species preferred by beaver are aspen and cottonwood (Populus) and willow (Salix). These trees grow fast, sprout rapidly, and have soft wood that is easy to fell and peel (Müller-Schwarze and Sun 2003). If beaver have occupied a site long enough to deplete their preferred food source, they Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 27 will resort to less preferred species. Diets vary depending on what is available. For example, Müller-Schwarze and Sun (2003) report that in New York State, after aspen and willow, beavers’ order of preference changes from birch, black cherry, beech, juneberry, and hornbeam to maples, hawthorn, and hemlock. The least preferred tree species were conifers, such as balsam fir, white pine, Scots pine, red pine, and Norway spruce. More recently, beaver have been known to take exotic species such as Japanese knotweed and kochia (Kochia scoparia) and salt cedar (tamarisk). In freezing climates, what do beaver eat during the winter? Where ponds or streams freeze during winter, beavers build food caches near their lodges or burrows (Jenkins and Busher 1979), which they access by swimming under the ice (Baker and Hill 2003). Because beaver do not hibernate or migrate during cold winter months, a reliable supply of food is necessary. Beavers may use the lower temperatures as a cue to start developing food caches during the fall, before freeze-up. Branches of deciduous species are gathered and embedded in the pond bottom or secured by structures such as large woody debris or boulders situated at the bottom. As the supply of woody vegetation accumulates in the cache, the material that is unsecured to the bottom becomes waterlogged and will sink. Generally, the majority of the cache will be submerged, with only a few sticks above and on the surface of the water where it may freeze in the surface ice. Interestingly, beaver may also initiate construction of the cache by selecting large branches of less palatable species and floating them near the lodge. Then selections of more favoured species are brought and placed under this “raft,” which, over time, becomes water logged and sinks, pushing the cache down to the pond floor. The raft or “cap” often remains close to the surface and becomes locked in the ice, leaving access to the woody vegetation below to be consumed over the winter (Slough 1978). Beavers may also supplement their winter diet of woody vegetation with water lily tubers and rizomes, which can be accessed from beneath the ice of the pond, but this component of the beaver diet is not well understood. Beavers remain in the lodge during most of the winter, emerging periodically to swim under the ice, cut branches from the cache, and take them back to the relative warmth and security of the lodge to eat. Beavers may build multiple food caches in a single colony and not consume the entire cache during the winter (Baker and Hill 2003). Without a large enough cache gathered before freeze-up to feed the entire colony for the duration of the winter, starvation may occur. When surface ice is not very thick, such as in late fall and early spring, beavers may break the ice near the lodge and dam to allow access to food on the shore (Jenkins and Busher 1979). They break the ice with their front paws, stand on it until it breaks, or butt it with head and shoulders (Wilson 2009). In climates where water bodies remain unfrozen, beaver typically do not construct food caches because they can forage year-round. How do males and females differ? Visually, male and female beavers are indistinguishable, and, because their sexual organs are located internally, determining a beaver’s sex in the field can be challenging. There are a number of different ways to ascertain their sex. The easiest field method is to locate the four dorsal mammary glands on females; however, this method is of limited value because the glands are visible only during a brief lactation period in the summer (Müller-Schwarze and Version 1.02. Get the latest version at: http://www.fws.gov/oregonfwo/ToolsForLandowners/RiverScience/Beaver.asp 28 Sun 2003) and non-breeding females do not develop conspicuous teats. DNA markers have been developed, so it is possible to identify gender by collecting hair samples (Goldberg et al. 2011). Two other methods are reliable ways to determine sex but require training and a sedated or constrained animal. These methods are (1) checking the color and consistency of anal gland secretions, and (2) palpating the baculum or penile bone (Schulte et al. 1995) (see the section on beaver sexing in ”Relocating Beaver”). When do beaver begin to reproduce? Beavers of both sexes usually reach sexual maturity and are able to produce their first litter by their second winter, at age 1.5 years (Larson 1967, Henry and Bookhout 1969). Regional variation of age at the first litter has been documented, but generally beaver can reproduce by 1.5 to 3 years of age, although puberty may be reached several months before first breeding (Baker and Hill 2003, Fischer et al. 2010). Adults form relatively long-term pair-bonds. Desertion of a