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Beavers (Castor canadensis) facilitate early access by Canada geese (Branta canadensis) to nesting habitat and areas of open water in Canada's boreal wetlands
Mammalian Biology 78 (2013) 73–77
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Beavers (Castor canadensis) facilitate early access by Canada geese (Branta canadensis) to nesting habitat and areas of open water in Canada’s boreal wetlands Chantal K. Bromley, Glynnis A. Hood ∗ Department of Science, Augustana Faculty, University of Alberta, 4901-46 Avenue, Camrose, Alberta, Canada T4V 2R3
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Article history: Received 10 November 2011 Accepted 28 February 2012 Keywords: Beaver Castor canadensis Nesting habitat Open water Waterfowl
a b s t r a c t Several studies demonstrate how beavers influence waterfowl habitat availability, ultimately improving waterfowl breeding success; however, no current research links beavers to early season nesting activities of Canada geese (Branta canadensis) in northern climates. We examined how beavers facilitate early access to open water for geese at Miquelon Lake Provincial Park (MLPP), Canada. We surveyed 32 active and 39 inactive beaver ponds to examine whether beavers facilitate early access to open water. Open water occurred 10.7 days earlier at active beaver ponds (mean ice-off day = 87.54, s = 13.88) than inactive ponds (mean ice-off day = 98.19, s = 9.07), especially adjacent to main lodge entrances and winter food caches. Snowpack was on average 5.9 cm shallower at active ponds. Prior to availability of open water, Canada geese exhibited intraspecific territoriality over beaver lodges as nest sites and once water was present, preferred island lodges over bank lodges. These findings support other studies that examined island nesting as protection from terrestrial predators and highlight the importance of beavers in creating open water areas earlier in the season. © 2012 Deutsche Gesellschaft für Säugetierkunde. Published by Elsevier GmbH. All rights reserved.
Prior to colonization of North America by Europeans, beavers (Castor canadensis) shaped the ecology of aquatic systems through the creation and modification of wetlands over extensive areas of the continent (Naiman et al., 1988; Johnston, 1994). By the end of the 1800s, North American beavers were overhunted and their population levels consequently declined in many areas (Naiman et al., 1988; Johnston, 1994), including areas of boreal Canada and the United States (Larson and Gunson, 1983; Hood and Bayley, 2008a), a trend similar to that of European beaver (Castor fiber) (Nolet and Rosell, 1998). Despite this dramatic decline, several reintroduction programs have now established viable beaver populations throughout North America and Europe (Jenkins and Busher, 1979; Nolet and Rosell, 1998; Rosell et al., 2005; Hood and Bayley, 2008a). With their return, activities such as building dams, constructing lodges, and digging channels resulted in physical changes in pond structure and water dynamics (Naiman et al., 1986; Johnston and Naiman, 1987; Westbrook et al., 2006; Hood and Bayley, 2008a). These changes have in turn influenced the biodiversity and overall health of wetland ecosystems (e.g., Nummi, 1989; Brown et al., 1996; France, 1997; Stevens et al., 2007). Beavers build dams to increase the extent of their aquatic habitats, which allows them to avoid terrestrial predators and access
∗ Corresponding author. Tel.: +1 780 679 1556; fax: +1 780 679 1590. E-mail address: [email protected] (G.A. Hood).
additional forage areas (Fryxell and Doucet, 1991; Hood and Bayley, 2008b). They often create large impoundments that result in a mix of shallow and deep water habitat. Flooding can also completely surround bank lodges to form island lodges. Beavers can increase the depth of water in existing ponds (Johnston, 1994), and consequently their ponds are often resilient to drought (Hood and Bayley, 2008a). When ponds are abandoned as a result of the colony dying off or relocating, the dams eventually break down, which causes water to drain out and creates drier habitat (Johnston, 1994; Martell et al., 2006). The active maintenance of these wetland areas by beavers sustains open water areas and their associated vegetation zones and biodiversity (Nummi and Hahtola, 2008; Hood and Bayley, 2008a). In many areas, activities by beavers have been shown to influence plant and animal community composition and diversity and to drive nutrient cycling and decomposition dynamics (Naiman et al., 1988; McKinstry et al., 2001). Several researchers have determined that waterfowl prefer ponds with active beaver lodges over inactive beaver ponds (Renouf, 1972; Rempel et al., 1997). Previous studies found reproductive success for waterfowl relies on water availability (Poiani and Johnston, 1991), food resources (Ewaschuk and Boag, 1972), vegetation for nest concealment (Poiani and Johnston, 1991) and protection from predators (Harvey, 1971; Inglis, 1977; Raveling and Lumsden, 1977; Jobin and Picman, 1997). The active management of water levels by beaver (Hood and Bayley, 2008a) could facilitate access to all of these habitat requirements.
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Longcore et al. (2006) and Nummi and Hahtola (2008) reported increased macroinvertebrate abundance in beaver-created wetlands, which was positively associated with increased brood densities of waterfowl (e.g., Eurasian teal, Anas crecca). Beard (1953) determined during his study at the Seney National Wildlife Refuge in Michigan that ducks preferentially breed in marshes and small ponds that were largely created from past or present beaver activity. Waterfowl might gain a selective advantage by nesting on active beaver ponds because they would have early access to food resources in spring, as seen with early-nesting mallards (Anas platyrhynchos; Dzus and Clark, 1998). Elmberg et al. (2005) also noted Eurasian teal that nested earlier had greater reproductive success than those that nested later in the season. Lastly, beaver ponds might provide more stable nesting sites relative to water levels (Hood and Bayley, 2008a) and vegetation cover. The withinpond location of nesting sites on island beaver lodges could also be important. Although many studies have linked the physical alterations in beaver ponds to improved habitat for fish (Snodgrass and Meffe, 1998; Schlosser and Kallemyn, 2000), aquatic macroinvertebrates (France, 1997), herptiles (Russell et al., 1998; Stevens et al., 2007), and for waterfowl (Renouf, 1972; Diefenbach and Owen, 1989; Brown et al., 1996; McKinstry et al., 2001), fewer studies link beavers to waterfowl nesting habitat (Longcore et al., 2006; Nummi and Hahtola, 2008). The influence of beavers on ice phenology, surface snowpack on ponds, and under-ice temperature effects is rarely, if ever, studied. Yet these factors suggest a possible mechanism for early access by waterfowl to nesting habitat and areas of open water in northern climates where water bodies remain frozen into the early spring. Examination of the role of beavers as a “keystone species” (Paine, 1966, 1969) from a seasonal perspective, specifically by studying their influence on the nesting patterns of waterfowl, could add to our understanding of beaver and waterfowl interactions. This knowledge has important implications during times of drought, when increasingly warmer spring and summer temperatures could decrease water availability in the nesting season. For some species, such as Canada geese (Branta canadensis), nesting can occur as early as late March to late April in northern habitats (Leblanc, 1989), despite many ponds still being covered with ice. Canada geese are common “island nesters” in many areas (Brenner, 1960; Lokemoen and Woodward, 1992) and island beaver lodges might provide similar benefits in terms of predator avoidance. Hood et al. (2007) established that beavers in Elk Island National Park, Canada rarely occupied bank dens and were almost entirely found inhabiting island lodges. Most inactive lodges were bank lodges that, although equivalent to island lodges structurally, were still in direct contact with the shoreline and more sensitive to fluctuations in the adjacent shallow water zone. Establishing a nest on an island beaver lodge could benefit waterfowl such as Canada geese, because terrestrial predation would be avoided. Our study focused on the role of beavers relative to breeding activities of Canada geese in northern nesting areas. Specifically, our goal was to determine whether the presence or absence of active beaver lodges could facilitate early melting of the ice from ponds. Theoretically, these ponds would then provide earlier spring access to open water and food resources for geese than inactive ponds. To test these relationships, we hypothesized that: (1) pond ice and snow would melt earlier at active beaver sites than at inactive sites; (2) geese would nest on active beaver lodges more often than on inactive ones; and (3) island beaver lodges would have more goose nesting activity than bank beaver lodges. We examined active and inactive beaver ponds in Miquelon Lake Provincial Park (MLPP; 8.36 km2 ) located in the County of Camrose, 40 km southeast of Edmonton in east-central Alberta, Canada. The
park is located in the southern edge of the Cooking Lake Moraine, an area best described as “knob and kettle” morainal terrain (Hood et al., 2007). MLPP is within the dry mixed-wood sub-region of the Boreal Forest Natural Region (Achuff, 1994). The area is characterized by trembling aspen forests (Populus tremuloides), with a shrub understory dominated by rose (Rosa woodsii and R. acicularis), beaked hazel (Corylus cornuta), and serviceberry (Amelanchier alnifolia), with small isolated ponds and wetlands within depressional “kettles”. The ponds in MLPP lack resident fish populations. Initially, we determined locations of ponds and the potential locations of beaver lodges within the main 8 km2 of the park using 2003 aerial photographs of MLPP and a Geographic Information Systems (GIS, ArcMap 9.2 by ESRITM ) from which we created a georectified air photo mosaic. This image was later supplemented with a 2007 orthophotograph with a resolution of 0.5 m on the ground. Through ground visits to each mapped pond in January and February 2008, we were able to assess beaver lodge occupancy at 71 ponds while also marking all lodge locations using a Garmin 60Cx Geographic Position System (GPS, ±4 m accuracy). Following methods in Hood and Bayley (2008a), beaver lodges were classified as “active” if there was evidence of a food cache and/or an obvious air vent rimmed with frost at the top of the lodge. We also noted other characteristics of each beaver lodge such as the size of the lodge, whether the lodge was attached to the shore (bank) or surrounded by water (island), and the presence of other animal activity on the lodge. All lodge locations were then transferred to the GIS. Field surveys were conducted following the University of Alberta Animal Use Protocol requirements. We used a chi-square goodness of fit test to assess whether there was a significant difference between lodge status (active and inactive) and lodge type (island and bank). Results for all analyses in the study were considered significant if P < 0.05. To control for the effect that snow depth might have on ice development and degradation, we conducted snow depth surveys at a randomly selected sub-sample of 15 active and 15 inactive beaver ponds. From the centre of the pond, we measured snow depths, in each cardinal direction (N, E, S, W) along a 25-m long transect. Depths were measured at 5-m intervals, resulting in a total of 20 measurements per pond. Because the exact centre of all ponds was not always apparent due to shoreline configurations, we used the centroid function within the GIS to determine the centre of the pond. These points were then transferred into a GPS for field use. Snow depth measurements were recorded for all 30 ponds within the same day with the help of several volunteers. To avoid pseudoreplication, we used a linear mixed-model using a nested design, with lodge nested within status (active or inactive), to compare snow depths at active and inactive beaver ponds. To establish the date that open water was first present in the pond, we visited the 32 active ponds and 39 inactive ponds (n = 71) weekly from 20 January to 1 March 2008. These ponds represented all of the ponds in the main area of the park. The date was recorded as days since 1 January 2008 (Julian date). Once open water first appeared anywhere on the pond, the pond was monitored 1–3 times per week to record location (e.g., adjacent to lodge, shoreline, pond centre), area (m2 ) and percent of open water for each pond. Area and percent of open water were visually estimated, based on pond area determined using the 2007 orthophoto in the GIS. Although we attempted to record the area (m2 ) and percent of open water for the entire extent of each pond, the dynamic nature of freeze-thaw cycles and the difficulty in obtaining a true aerial representation of the overall pond surface from the ground resulted in unreliable measurements for these data. These same freeze-thaw cycles influenced the end of our study on 13 April. We decided to sample 48 ponds sites intensively within the 6-week late winter period (1 March to 13 April), to confirm the sustained presence of open water near the active and inactive beaver lodges.
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A total of 25 active ponds and 23 inactive opened up within the study period and allowed tracking of ice break-up. We used a two-tailed Student’s independent t-test with pond status (active/inactive as grouping variable) to determine whether ponds with active beaver lodges had open water sooner than those without beavers (measured as date of first open water). We also used a two-way ANOVA to determine if there was an interaction effect between lodge type (bank or island) and pond status (active or inactive) relative to the date that open water was first present. We began tracking the arrival of Canada geese after 1 March 2008 on the subset of 48 ponds used for the intensive open water surveys. Canada geese do not normally appear in this area until this time (Leblanc, 1989). While visiting the ponds during these surveys (1 March to 13 April 2008), we also noted the date and time of Canada goose arrival and nesting activity (e.g., nesting, mating displays, pair-bonding). Nesting geese were observed from a distance to avoid disturbing the birds. If the birds showed signs of being disturbed by our presence, we retreated until they resumed their previous behaviour. Regardless of nesting activity, the presence and abundance of geese and all other waterfowl using the lodges were recorded. Nesting activity was defined as the presence of an active and attended nest. If a nest was present, we confirmed the pond status as either active or inactive for beaver occupancy and noted the location of the nest (island lodge, bank lodge, elsewhere), as well as the number of geese that nested on the beaver lodge. Breeding behaviors prior to nest construction (e.g., displaying territorial behaviors) were recorded earlier in the study when other waterfowl were present, but not necessarily nesting at a pond. To assess whether there was a significant difference in nesting activity between pond status (active and inactive) and lodge type (island and bank), we used a chi-square goodness of fit test. Within the 8 km2 of MLPP surveyed, 71 beaver lodges were identified. A total of 39 inactive beaver lodges and 32 active beaver lodges were recorded, a density of 3.9 active lodges per km2 . We determined that all ponds were either currently or historically occupied by beavers, as seen by the presence of cut trees or evidence of previous lodges. There were more island beaver lodges (n = 54) than bank lodges (n = 17) in the study area (X2 = 23.68, df = 1, P < 0.001). This result was consistent, regardless of beaver activity. Of the active lodges, 25 were island lodges and 7 were bank lodges, while of the inactive lodges, 29 were island lodges and 10 were bank lodges (X2 = 0.14, df = 1, P = 0.71). There was no difference in the number of bank lodges (X2 = 0.866, df = 1, P = 0.351) or island lodges, regardless of beaver occupancy (X2 = 0.357, df = 1, P = 0.55). We found that snow depths at active ponds were on average, more than 5.9 cm shallower (mean = 24.92 cm, s = 9.58 cm) than at inactive beaver ponds (mean = 30.90 cm, s = 8.22; F1,31 = 57.73, P < 0.001). Surrounding riparian vegetation (e.g., P. tremuloides forest with a Typha latifolia or sedge emergent zone) was similar at all pond locations. The first signs of snow melting near lodges were observed on 17 February 2008. Throughout February, melting ice and snow were also noted by the food cache (e.g., a dark depression in the snow and sometimes frozen slush). On 1 March 2008, permanent open water was first observed, adjacent to an active beaver lodge. Of the ponds surveyed between 1 March and 13 April 2008, 61.3% (19 out of 31) of the active lodges had some open water near them compared to 17.5% (7 out of 40) of the inactive lodges. The date that open water first appeared was, on average, 10.7 days earlier (∼21 March 2008) at ponds with active beaver lodges than ponds without active beaver lodges (t38 = −2.7476, P = 0.0091, Fig. 1). In almost all cases, open water was present immediately adjacent to the main lodge entrance and over the food cache, and was not a pond-wide occurrence. Despite a weak interaction effect between inactive ponds and island lodges, lodge type (bank or island) did
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Fig. 1. Julian days to open water in active and inactive beaver ponds (n = 48) in Miquelon Lake Provincial Park between 20 January and 13 April 2008.
not influence the date that open water was first present on a pond (F1,36 = 1.0889, P = 0.304). On 21 March 2008, Canada geese were first observed standing on a beaver lodge within our study area. Nest construction was not observed directly; instead nesting activity was inferred from behavior. Soon after their arrival, we noticed some Canada geese becoming territorial towards other Canada geese at many of the island lodges and geese sitting on what appeared to be nests on several lodges. Between 21 March and 13 April, 21.9% more active beaver lodges had geese present than inactive lodges (X2 = 6.4056, df = 1, P = 0.011, n = 48; Fig. 2). Of the lodges surveyed, there was a weak trend towards island lodges being more likely to have displaying geese (X2 = 3.01446, df = 1, P = 0.08, n = 48; Fig. 2). In almost all cases the number of geese present consisted of a single nesting pair. The seasonal dynamics of northern ecosystems play an important role in the success of migratory species such as the Canada goose. Access to open water habitats, adequate food resources and suitable nest locations are critical for waterfowl to successfully rear at least one set of offspring prior to fall migration (Raveling, 1981; Murphy and Boag, 1989; Bromley and Jarvis, 1993). Earlier nesting waterfowl also appear to have better breeding success than late nesters (Dzus and Clark, 1998; Elmberg et al., 2005). Access to open water earlier in the season might allow for earlier nesting times, earlier access to aquatic vegetation and invertebrates, and longer maturation times for waterfowl prior to fall migration. Beavers, as a key biotic factor in many boreal wetlands (Martell et al., 2006; Stevens et al., 2007; Hood and Bayley, 2008a), are one species that could facilitate this success, based on our findings. The trend of open water being present sooner at active beaver ponds than at inactive beaver ponds suggests that beavers might facilitate early access to open water for geese and perhaps other waterfowl. Most often, open water was first present at the food cache, which is immediately adjacent to the main entrance to the lodge and often deeper than other locations within the pond. Within a beaver lodge, water is a few inches below the living platform (Jenkins and Busher, 1979). It is possible that higher and less variable temperatures within the lodge than outside the lodge (Stephenson, 1969) could cause the water within the lodge to be warmer than the water outside the lodge. Beavers normally leave their lodge regularly throughout the winter to access their food cache. The mixing of the warmer water within the lodge with the colder water outside of the lodge when the beavers move beneath the ice could facilitate early melting of the ice above the food cache.
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Fig. 2. Percent of active beaver lodges (island and bank) and inactive beaver lodges (island and bank) with breeding Canada geese (Branta canadensis) present of the 48 lodges surveyed in Miquelon Lake Provincial Park between 21 March and 13 April 2008.
In addition, the dark colour of the lodge when free of snow reduces the albedo effect and could accelerate melting of the adjacent snow and ice (Jeffries and Morris, 2007). The fact that snow depths were shallower on ponds occupied by beavers could be a result of later ice formation due to water temperatures and movements of beavers. However, further research is needed to determine why deeper snow was present at inactive beaver ponds than at active beaver ponds. Specifically, assessment for differences in snow density, pond freeze up dates, and pond temperatures would be required, preferably incorporating aerial survey methods. Beaver activities might also explain the presence of open water in the early and later winter months. During a previous study, one of the authors (Hood) observed beavers regularly slapping their tails adjacent to the forming ice in Elk Island National Park, in what appeared to be an attempt to slow the advance of the ice edge towards the lodge. In another case, a colleague in the same park observed beavers pulling at the edge of the ice to break it, thereby keeping an area close to the lodge and food cache ice-free for a longer period of time. This same behavior was observed in the Cooking Lake-Blackfoot Provincial Recreation Area and in MLPP during the early spring months of 2007 and 2008 by another colleague. Delayed and early open water could influence aquatic habitat and pond productivity. Water chemistry would be affected by beavers keeping water open longer in the fall and facilitating melting and break-up in the late winter/early spring. When water is exposed to air, dissolved oxygen and light penetration levels are much higher than when the ice finally forms on the pond surface (Dodds, 2002). Such changes have several implications relative to photosynthetic activity and reduction of winter kill in aquatic organisms (Dodds, 2002), which might increase productivity in a pond. As with Leblanc (1989) we found that Canada geese nest from late March to into April. We also determined that nesting sites were by open water and preferably on an island (Duebbert, 1982; Lokemoen and Woodward, 1992) or in our case, island beaver lodges. The presence of more breeding pairs of Canada geese at active beaver ponds than inactive beaver ponds suggests that beaver activity during the winter could provide early resources for waterfowl in the spring (see Dzus and Clark, 1998). As with Renouf (1972) and Rempel et al. (1997), we determined that waterfowl prefer active over inactive beaver ponds, which suggests that the presence of beavers might play a key role in the choice of nesting locations. This finding confirmed our second hypothesis that waterfowl would choose ponds with active beaver lodges. Waterfowl likely gain an advantage by nesting on active beaver ponds because they have a better nesting location throughout the later spring and summer (e.g., higher water levels and increased vegetative brood cover; Renouf, 1972). Between incubation and fledging (a 68–80
day period) adult Canada geese and their young are restricted to their nest site and the adjacent pond area, where they feed on a variety of aquatic and terrestrial plants (Alsop, 2002). The higher occurrence of Canada geese on island beaver lodges rather than on bank lodges supports our third hypothesis that waterfowl would prefer island lodges as nesting sites. The preference for island nesting grounds is thought to have evolved in waterfowl as a means to avoid terrestrial predators such as coyotes (Ewaschuk and Boag, 1972; Duebbert, 1982; Lokemoen and Woodward, 1992). Although the presence of open water habitat might facilitate access for semi-aquatic predators such as the American mink (Neovison vison) or river otters (Lontra canadensis), neither species has been detected in MLPP. Several species of ducks and Canada geese select island nest sites (Duebbert, 1982; Lokemoen and Woodward, 1992). The higher occurrence of geese observed on island beaver lodges could also be due to the higher number of island lodges within MLPP than bank lodges. Although beavers play an important role in the spatial extent of wetlands (Naiman et al., 1988; Hood and Bayley, 2008a), we determined that beavers could also aid in the temporal aspects of wildlife ecology. Beavers increase the surface area and depth of water, establish extended riparian zones, and enhance biodiversity (Brown et al., 1996; Hood and Bayley, 2008a). They also create and enhance habitat for waterfowl breeding and brood rearing and for resting during migration (Brown et al., 1996). In our study, we determined that beavers could play a further role in Canada goose habitat selection by providing early access to open water. Such access might play an important role in selection of nesting grounds by Canada geese and ultimately influence their breeding success, as found with mallards (Dzus and Clark, 1998) and Eurasian teal (Elmberg et al., 2005). Acknowledgements We thank many volunteers who assisted with this project, especially under cold conditions and long days. To collect these data, 20 volunteers contributed a total of 140 h to the study. We also appreciate the insight and comments provided by 3 anonymous reviewers. Their input has improved the manuscript and presentation of our research. Finally, we acknowledge the support of Alberta Tourism, Parks and Recreation for their logistical support of this project. References Achuff, P.L., 1994. Natural Regions, Subregions and Natural History Themes of Alberta – A Classification for Protected Areas Management, Revised and Updated December 1994. Alberta Environmental Protection, Edmonton, Alberta, Canada. Alsop III, F.J., 2002. Birds of Canada. Dorling Kindersley Limited, Toronto, Ontario, Canada.
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Short Communication
Beavers (Castor canadensis) facilitate early access by Canada geese (Branta canadensis) to nesting habitat and areas of open water in Canada’s boreal wetlands Chantal K. Bromley, Glynnis A. Hood ∗ Department of Science, Augustana Faculty, University of Alberta, 4901-46 Avenue, Camrose, Alberta, Canada T4V 2R3
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Article history: Received 10 November 2011 Accepted 28 February 2012 Keywords: Beaver Castor canadensis Nesting habitat Open water Waterfowl
a b s t r a c t Several studies demonstrate how beavers influence waterfowl habitat availability, ultimately improving waterfowl breeding success; however, no current research links beavers to early season nesting activities of Canada geese (Branta canadensis) in northern climates. We examined how beavers facilitate early access to open water for geese at Miquelon Lake Provincial Park (MLPP), Canada. We surveyed 32 active and 39 inactive beaver ponds to examine whether beavers facilitate early access to open water. Open water occurred 10.7 days earlier at active beaver ponds (mean ice-off day = 87.54, s = 13.88) than inactive ponds (mean ice-off day = 98.19, s = 9.07), especially adjacent to main lodge entrances and winter food caches. Snowpack was on average 5.9 cm shallower at active ponds. Prior to availability of open water, Canada geese exhibited intraspecific territoriality over beaver lodges as nest sites and once water was present, preferred island lodges over bank lodges. These findings support other studies that examined island nesting as protection from terrestrial predators and highlight the importance of beavers in creating open water areas earlier in the season. © 2012 Deutsche Gesellschaft für Säugetierkunde. Published by Elsevier GmbH. All rights reserved.
Prior to colonization of North America by Europeans, beavers (Castor canadensis) shaped the ecology of aquatic systems through the creation and modification of wetlands over extensive areas of the continent (Naiman et al., 1988; Johnston, 1994). By the end of the 1800s, North American beavers were overhunted and their population levels consequently declined in many areas (Naiman et al., 1988; Johnston, 1994), including areas of boreal Canada and the United States (Larson and Gunson, 1983; Hood and Bayley, 2008a), a trend similar to that of European beaver (Castor fiber) (Nolet and Rosell, 1998). Despite this dramatic decline, several reintroduction programs have now established viable beaver populations throughout North America and Europe (Jenkins and Busher, 1979; Nolet and Rosell, 1998; Rosell et al., 2005; Hood and Bayley, 2008a). With their return, activities such as building dams, constructing lodges, and digging channels resulted in physical changes in pond structure and water dynamics (Naiman et al., 1986; Johnston and Naiman, 1987; Westbrook et al., 2006; Hood and Bayley, 2008a). These changes have in turn influenced the biodiversity and overall health of wetland ecosystems (e.g., Nummi, 1989; Brown et al., 1996; France, 1997; Stevens et al., 2007). Beavers build dams to increase the extent of their aquatic habitats, which allows them to avoid terrestrial predators and access
∗ Corresponding author. Tel.: +1 780 679 1556; fax: +1 780 679 1590. E-mail address: [email protected] (G.A. Hood).
additional forage areas (Fryxell and Doucet, 1991; Hood and Bayley, 2008b). They often create large impoundments that result in a mix of shallow and deep water habitat. Flooding can also completely surround bank lodges to form island lodges. Beavers can increase the depth of water in existing ponds (Johnston, 1994), and consequently their ponds are often resilient to drought (Hood and Bayley, 2008a). When ponds are abandoned as a result of the colony dying off or relocating, the dams eventually break down, which causes water to drain out and creates drier habitat (Johnston, 1994; Martell et al., 2006). The active maintenance of these wetland areas by beavers sustains open water areas and their associated vegetation zones and biodiversity (Nummi and Hahtola, 2008; Hood and Bayley, 2008a). In many areas, activities by beavers have been shown to influence plant and animal community composition and diversity and to drive nutrient cycling and decomposition dynamics (Naiman et al., 1988; McKinstry et al., 2001). Several researchers have determined that waterfowl prefer ponds with active beaver lodges over inactive beaver ponds (Renouf, 1972; Rempel et al., 1997). Previous studies found reproductive success for waterfowl relies on water availability (Poiani and Johnston, 1991), food resources (Ewaschuk and Boag, 1972), vegetation for nest concealment (Poiani and Johnston, 1991) and protection from predators (Harvey, 1971; Inglis, 1977; Raveling and Lumsden, 1977; Jobin and Picman, 1997). The active management of water levels by beaver (Hood and Bayley, 2008a) could facilitate access to all of these habitat requirements.
1616-5047/$ – see front matter © 2012 Deutsche Gesellschaft für Säugetierkunde. Published by Elsevier GmbH. All rights reserved. doi:10.1016/j.mambio.2012.02.009
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Longcore et al. (2006) and Nummi and Hahtola (2008) reported increased macroinvertebrate abundance in beaver-created wetlands, which was positively associated with increased brood densities of waterfowl (e.g., Eurasian teal, Anas crecca). Beard (1953) determined during his study at the Seney National Wildlife Refuge in Michigan that ducks preferentially breed in marshes and small ponds that were largely created from past or present beaver activity. Waterfowl might gain a selective advantage by nesting on active beaver ponds because they would have early access to food resources in spring, as seen with early-nesting mallards (Anas platyrhynchos; Dzus and Clark, 1998). Elmberg et al. (2005) also noted Eurasian teal that nested earlier had greater reproductive success than those that nested later in the season. Lastly, beaver ponds might provide more stable nesting sites relative to water levels (Hood and Bayley, 2008a) and vegetation cover. The withinpond location of nesting sites on island beaver lodges could also be important. Although many studies have linked the physical alterations in beaver ponds to improved habitat for fish (Snodgrass and Meffe, 1998; Schlosser and Kallemyn, 2000), aquatic macroinvertebrates (France, 1997), herptiles (Russell et al., 1998; Stevens et al., 2007), and for waterfowl (Renouf, 1972; Diefenbach and Owen, 1989; Brown et al., 1996; McKinstry et al., 2001), fewer studies link beavers to waterfowl nesting habitat (Longcore et al., 2006; Nummi and Hahtola, 2008). The influence of beavers on ice phenology, surface snowpack on ponds, and under-ice temperature effects is rarely, if ever, studied. Yet these factors suggest a possible mechanism for early access by waterfowl to nesting habitat and areas of open water in northern climates where water bodies remain frozen into the early spring. Examination of the role of beavers as a “keystone species” (Paine, 1966, 1969) from a seasonal perspective, specifically by studying their influence on the nesting patterns of waterfowl, could add to our understanding of beaver and waterfowl interactions. This knowledge has important implications during times of drought, when increasingly warmer spring and summer temperatures could decrease water availability in the nesting season. For some species, such as Canada geese (Branta canadensis), nesting can occur as early as late March to late April in northern habitats (Leblanc, 1989), despite many ponds still being covered with ice. Canada geese are common “island nesters” in many areas (Brenner, 1960; Lokemoen and Woodward, 1992) and island beaver lodges might provide similar benefits in terms of predator avoidance. Hood et al. (2007) established that beavers in Elk Island National Park, Canada rarely occupied bank dens and were almost entirely found inhabiting island lodges. Most inactive lodges were bank lodges that, although equivalent to island lodges structurally, were still in direct contact with the shoreline and more sensitive to fluctuations in the adjacent shallow water zone. Establishing a nest on an island beaver lodge could benefit waterfowl such as Canada geese, because terrestrial predation would be avoided. Our study focused on the role of beavers relative to breeding activities of Canada geese in northern nesting areas. Specifically, our goal was to determine whether the presence or absence of active beaver lodges could facilitate early melting of the ice from ponds. Theoretically, these ponds would then provide earlier spring access to open water and food resources for geese than inactive ponds. To test these relationships, we hypothesized that: (1) pond ice and snow would melt earlier at active beaver sites than at inactive sites; (2) geese would nest on active beaver lodges more often than on inactive ones; and (3) island beaver lodges would have more goose nesting activity than bank beaver lodges. We examined active and inactive beaver ponds in Miquelon Lake Provincial Park (MLPP; 8.36 km2 ) located in the County of Camrose, 40 km southeast of Edmonton in east-central Alberta, Canada. The
park is located in the southern edge of the Cooking Lake Moraine, an area best described as “knob and kettle” morainal terrain (Hood et al., 2007). MLPP is within the dry mixed-wood sub-region of the Boreal Forest Natural Region (Achuff, 1994). The area is characterized by trembling aspen forests (Populus tremuloides), with a shrub understory dominated by rose (Rosa woodsii and R. acicularis), beaked hazel (Corylus cornuta), and serviceberry (Amelanchier alnifolia), with small isolated ponds and wetlands within depressional “kettles”. The ponds in MLPP lack resident fish populations. Initially, we determined locations of ponds and the potential locations of beaver lodges within the main 8 km2 of the park using 2003 aerial photographs of MLPP and a Geographic Information Systems (GIS, ArcMap 9.2 by ESRITM ) from which we created a georectified air photo mosaic. This image was later supplemented with a 2007 orthophotograph with a resolution of 0.5 m on the ground. Through ground visits to each mapped pond in January and February 2008, we were able to assess beaver lodge occupancy at 71 ponds while also marking all lodge locations using a Garmin 60Cx Geographic Position System (GPS, ±4 m accuracy). Following methods in Hood and Bayley (2008a), beaver lodges were classified as “active” if there was evidence of a food cache and/or an obvious air vent rimmed with frost at the top of the lodge. We also noted other characteristics of each beaver lodge such as the size of the lodge, whether the lodge was attached to the shore (bank) or surrounded by water (island), and the presence of other animal activity on the lodge. All lodge locations were then transferred to the GIS. Field surveys were conducted following the University of Alberta Animal Use Protocol requirements. We used a chi-square goodness of fit test to assess whether there was a significant difference between lodge status (active and inactive) and lodge type (island and bank). Results for all analyses in the study were considered significant if P < 0.05. To control for the effect that snow depth might have on ice development and degradation, we conducted snow depth surveys at a randomly selected sub-sample of 15 active and 15 inactive beaver ponds. From the centre of the pond, we measured snow depths, in each cardinal direction (N, E, S, W) along a 25-m long transect. Depths were measured at 5-m intervals, resulting in a total of 20 measurements per pond. Because the exact centre of all ponds was not always apparent due to shoreline configurations, we used the centroid function within the GIS to determine the centre of the pond. These points were then transferred into a GPS for field use. Snow depth measurements were recorded for all 30 ponds within the same day with the help of several volunteers. To avoid pseudoreplication, we used a linear mixed-model using a nested design, with lodge nested within status (active or inactive), to compare snow depths at active and inactive beaver ponds. To establish the date that open water was first present in the pond, we visited the 32 active ponds and 39 inactive ponds (n = 71) weekly from 20 January to 1 March 2008. These ponds represented all of the ponds in the main area of the park. The date was recorded as days since 1 January 2008 (Julian date). Once open water first appeared anywhere on the pond, the pond was monitored 1–3 times per week to record location (e.g., adjacent to lodge, shoreline, pond centre), area (m2 ) and percent of open water for each pond. Area and percent of open water were visually estimated, based on pond area determined using the 2007 orthophoto in the GIS. Although we attempted to record the area (m2 ) and percent of open water for the entire extent of each pond, the dynamic nature of freeze-thaw cycles and the difficulty in obtaining a true aerial representation of the overall pond surface from the ground resulted in unreliable measurements for these data. These same freeze-thaw cycles influenced the end of our study on 13 April. We decided to sample 48 ponds sites intensively within the 6-week late winter period (1 March to 13 April), to confirm the sustained presence of open water near the active and inactive beaver lodges.
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A total of 25 active ponds and 23 inactive opened up within the study period and allowed tracking of ice break-up. We used a two-tailed Student’s independent t-test with pond status (active/inactive as grouping variable) to determine whether ponds with active beaver lodges had open water sooner than those without beavers (measured as date of first open water). We also used a two-way ANOVA to determine if there was an interaction effect between lodge type (bank or island) and pond status (active or inactive) relative to the date that open water was first present. We began tracking the arrival of Canada geese after 1 March 2008 on the subset of 48 ponds used for the intensive open water surveys. Canada geese do not normally appear in this area until this time (Leblanc, 1989). While visiting the ponds during these surveys (1 March to 13 April 2008), we also noted the date and time of Canada goose arrival and nesting activity (e.g., nesting, mating displays, pair-bonding). Nesting geese were observed from a distance to avoid disturbing the birds. If the birds showed signs of being disturbed by our presence, we retreated until they resumed their previous behaviour. Regardless of nesting activity, the presence and abundance of geese and all other waterfowl using the lodges were recorded. Nesting activity was defined as the presence of an active and attended nest. If a nest was present, we confirmed the pond status as either active or inactive for beaver occupancy and noted the location of the nest (island lodge, bank lodge, elsewhere), as well as the number of geese that nested on the beaver lodge. Breeding behaviors prior to nest construction (e.g., displaying territorial behaviors) were recorded earlier in the study when other waterfowl were present, but not necessarily nesting at a pond. To assess whether there was a significant difference in nesting activity between pond status (active and inactive) and lodge type (island and bank), we used a chi-square goodness of fit test. Within the 8 km2 of MLPP surveyed, 71 beaver lodges were identified. A total of 39 inactive beaver lodges and 32 active beaver lodges were recorded, a density of 3.9 active lodges per km2 . We determined that all ponds were either currently or historically occupied by beavers, as seen by the presence of cut trees or evidence of previous lodges. There were more island beaver lodges (n = 54) than bank lodges (n = 17) in the study area (X2 = 23.68, df = 1, P < 0.001). This result was consistent, regardless of beaver activity. Of the active lodges, 25 were island lodges and 7 were bank lodges, while of the inactive lodges, 29 were island lodges and 10 were bank lodges (X2 = 0.14, df = 1, P = 0.71). There was no difference in the number of bank lodges (X2 = 0.866, df = 1, P = 0.351) or island lodges, regardless of beaver occupancy (X2 = 0.357, df = 1, P = 0.55). We found that snow depths at active ponds were on average, more than 5.9 cm shallower (mean = 24.92 cm, s = 9.58 cm) than at inactive beaver ponds (mean = 30.90 cm, s = 8.22; F1,31 = 57.73, P < 0.001). Surrounding riparian vegetation (e.g., P. tremuloides forest with a Typha latifolia or sedge emergent zone) was similar at all pond locations. The first signs of snow melting near lodges were observed on 17 February 2008. Throughout February, melting ice and snow were also noted by the food cache (e.g., a dark depression in the snow and sometimes frozen slush). On 1 March 2008, permanent open water was first observed, adjacent to an active beaver lodge. Of the ponds surveyed between 1 March and 13 April 2008, 61.3% (19 out of 31) of the active lodges had some open water near them compared to 17.5% (7 out of 40) of the inactive lodges. The date that open water first appeared was, on average, 10.7 days earlier (∼21 March 2008) at ponds with active beaver lodges than ponds without active beaver lodges (t38 = −2.7476, P = 0.0091, Fig. 1). In almost all cases, open water was present immediately adjacent to the main lodge entrance and over the food cache, and was not a pond-wide occurrence. Despite a weak interaction effect between inactive ponds and island lodges, lodge type (bank or island) did
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Fig. 1. Julian days to open water in active and inactive beaver ponds (n = 48) in Miquelon Lake Provincial Park between 20 January and 13 April 2008.
not influence the date that open water was first present on a pond (F1,36 = 1.0889, P = 0.304). On 21 March 2008, Canada geese were first observed standing on a beaver lodge within our study area. Nest construction was not observed directly; instead nesting activity was inferred from behavior. Soon after their arrival, we noticed some Canada geese becoming territorial towards other Canada geese at many of the island lodges and geese sitting on what appeared to be nests on several lodges. Between 21 March and 13 April, 21.9% more active beaver lodges had geese present than inactive lodges (X2 = 6.4056, df = 1, P = 0.011, n = 48; Fig. 2). Of the lodges surveyed, there was a weak trend towards island lodges being more likely to have displaying geese (X2 = 3.01446, df = 1, P = 0.08, n = 48; Fig. 2). In almost all cases the number of geese present consisted of a single nesting pair. The seasonal dynamics of northern ecosystems play an important role in the success of migratory species such as the Canada goose. Access to open water habitats, adequate food resources and suitable nest locations are critical for waterfowl to successfully rear at least one set of offspring prior to fall migration (Raveling, 1981; Murphy and Boag, 1989; Bromley and Jarvis, 1993). Earlier nesting waterfowl also appear to have better breeding success than late nesters (Dzus and Clark, 1998; Elmberg et al., 2005). Access to open water earlier in the season might allow for earlier nesting times, earlier access to aquatic vegetation and invertebrates, and longer maturation times for waterfowl prior to fall migration. Beavers, as a key biotic factor in many boreal wetlands (Martell et al., 2006; Stevens et al., 2007; Hood and Bayley, 2008a), are one species that could facilitate this success, based on our findings. The trend of open water being present sooner at active beaver ponds than at inactive beaver ponds suggests that beavers might facilitate early access to open water for geese and perhaps other waterfowl. Most often, open water was first present at the food cache, which is immediately adjacent to the main entrance to the lodge and often deeper than other locations within the pond. Within a beaver lodge, water is a few inches below the living platform (Jenkins and Busher, 1979). It is possible that higher and less variable temperatures within the lodge than outside the lodge (Stephenson, 1969) could cause the water within the lodge to be warmer than the water outside the lodge. Beavers normally leave their lodge regularly throughout the winter to access their food cache. The mixing of the warmer water within the lodge with the colder water outside of the lodge when the beavers move beneath the ice could facilitate early melting of the ice above the food cache.
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Fig. 2. Percent of active beaver lodges (island and bank) and inactive beaver lodges (island and bank) with breeding Canada geese (Branta canadensis) present of the 48 lodges surveyed in Miquelon Lake Provincial Park between 21 March and 13 April 2008.
In addition, the dark colour of the lodge when free of snow reduces the albedo effect and could accelerate melting of the adjacent snow and ice (Jeffries and Morris, 2007). The fact that snow depths were shallower on ponds occupied by beavers could be a result of later ice formation due to water temperatures and movements of beavers. However, further research is needed to determine why deeper snow was present at inactive beaver ponds than at active beaver ponds. Specifically, assessment for differences in snow density, pond freeze up dates, and pond temperatures would be required, preferably incorporating aerial survey methods. Beaver activities might also explain the presence of open water in the early and later winter months. During a previous study, one of the authors (Hood) observed beavers regularly slapping their tails adjacent to the forming ice in Elk Island National Park, in what appeared to be an attempt to slow the advance of the ice edge towards the lodge. In another case, a colleague in the same park observed beavers pulling at the edge of the ice to break it, thereby keeping an area close to the lodge and food cache ice-free for a longer period of time. This same behavior was observed in the Cooking Lake-Blackfoot Provincial Recreation Area and in MLPP during the early spring months of 2007 and 2008 by another colleague. Delayed and early open water could influence aquatic habitat and pond productivity. Water chemistry would be affected by beavers keeping water open longer in the fall and facilitating melting and break-up in the late winter/early spring. When water is exposed to air, dissolved oxygen and light penetration levels are much higher than when the ice finally forms on the pond surface (Dodds, 2002). Such changes have several implications relative to photosynthetic activity and reduction of winter kill in aquatic organisms (Dodds, 2002), which might increase productivity in a pond. As with Leblanc (1989) we found that Canada geese nest from late March to into April. We also determined that nesting sites were by open water and preferably on an island (Duebbert, 1982; Lokemoen and Woodward, 1992) or in our case, island beaver lodges. The presence of more breeding pairs of Canada geese at active beaver ponds than inactive beaver ponds suggests that beaver activity during the winter could provide early resources for waterfowl in the spring (see Dzus and Clark, 1998). As with Renouf (1972) and Rempel et al. (1997), we determined that waterfowl prefer active over inactive beaver ponds, which suggests that the presence of beavers might play a key role in the choice of nesting locations. This finding confirmed our second hypothesis that waterfowl would choose ponds with active beaver lodges. Waterfowl likely gain an advantage by nesting on active beaver ponds because they have a better nesting location throughout the later spring and summer (e.g., higher water levels and increased vegetative brood cover; Renouf, 1972). Between incubation and fledging (a 68–80
day period) adult Canada geese and their young are restricted to their nest site and the adjacent pond area, where they feed on a variety of aquatic and terrestrial plants (Alsop, 2002). The higher occurrence of Canada geese on island beaver lodges rather than on bank lodges supports our third hypothesis that waterfowl would prefer island lodges as nesting sites. The preference for island nesting grounds is thought to have evolved in waterfowl as a means to avoid terrestrial predators such as coyotes (Ewaschuk and Boag, 1972; Duebbert, 1982; Lokemoen and Woodward, 1992). Although the presence of open water habitat might facilitate access for semi-aquatic predators such as the American mink (Neovison vison) or river otters (Lontra canadensis), neither species has been detected in MLPP. Several species of ducks and Canada geese select island nest sites (Duebbert, 1982; Lokemoen and Woodward, 1992). The higher occurrence of geese observed on island beaver lodges could also be due to the higher number of island lodges within MLPP than bank lodges. Although beavers play an important role in the spatial extent of wetlands (Naiman et al., 1988; Hood and Bayley, 2008a), we determined that beavers could also aid in the temporal aspects of wildlife ecology. Beavers increase the surface area and depth of water, establish extended riparian zones, and enhance biodiversity (Brown et al., 1996; Hood and Bayley, 2008a). They also create and enhance habitat for waterfowl breeding and brood rearing and for resting during migration (Brown et al., 1996). In our study, we determined that beavers could play a further role in Canada goose habitat selection by providing early access to open water. Such access might play an important role in selection of nesting grounds by Canada geese and ultimately influence their breeding success, as found with mallards (Dzus and Clark, 1998) and Eurasian teal (Elmberg et al., 2005). Acknowledgements We thank many volunteers who assisted with this project, especially under cold conditions and long days. To collect these data, 20 volunteers contributed a total of 140 h to the study. We also appreciate the insight and comments provided by 3 anonymous reviewers. Their input has improved the manuscript and presentation of our research. Finally, we acknowledge the support of Alberta Tourism, Parks and Recreation for their logistical support of this project. References Achuff, P.L., 1994. Natural Regions, Subregions and Natural History Themes of Alberta – A Classification for Protected Areas Management, Revised and Updated December 1994. Alberta Environmental Protection, Edmonton, Alberta, Canada. Alsop III, F.J., 2002. Birds of Canada. Dorling Kindersley Limited, Toronto, Ontario, Canada.
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