Beaver Colonies Research Articles

Effect of agriculture and presence of American beaver Castor canadensis on winter biodiversity of mammals

AbstractSeveral metrics of biodiversity are used to assess the variability and abundance of multiple species across various ecological levels. Previous research suggests that biodiversity in agricultural areas is diminished due to habitat fragmentation and alterations to natural vegetation. Contrary to the effects of agriculture, the American beaver Castor canadensis has been shown to have a significant contribution to increased biodiversity. Our study focuses on the biodiversity of mammals in wetland areas of the southern mixed‐wood boreal forest natural region of east‐central Alberta, Canada. We compared various measures of biodiversity levels of mammals in the winter months between wetlands on agricultural land and wetlands in Miquelon Lake Provincial Park (MLPP). Similarly, we compared wetlands with active and inactive beaver colonies to determine differences in winter biodiversity of mammals and the amount of water coverage within a pond. We collected data using winter tracking surveys and analyzed the data using geographic information systems (GIS) techniques. We found that winter biodiversity of mammals was higher at the sites within the protected area (MLPP) than at those on agricultural lands. However, our data also suggest that forested areas surrounding agricultural wetlands may play an important role in maintaining biodiversity within agricultural areas. The presence of beavers alone was not a significant factor in relation to winter biodiversity, but the presence of the species was important for maintaining water levels of agricultural wetlands, which in turn plays a role in resource heterogeneity.

What should these rivers look like? Historical range of variability and human impacts in the Colorado Front Range, USA

ABSTRACTHistorical range of variability (HRV) describes the range of temporal and spatial variations in river variables such as flow regime or channel planform prior to intensive human alteration of the ecosystem. In mountainous river networks, HRV is most usefully applied to spatially differentiated geomorphic process domains with distinctive form and process. Using the Colorado Front Range as an example, three examples of how knowledge of HRV can assist river management and restoration are discussed. The examples involve instream wood load and channel morphology, beaver colonies and valley‐bottom form and process, and flow thresholds in regulated rivers. The question of what a river should look like – that is, what range of process and form the river included prior to intensive human alteration – can be addressed by (i) placing the river within a process domain, (ii) establishing correlations between form parameters that can be remotely sensed and reach‐scale process and form, so that the spatial extent, connectivity, and rarity of process domains within a river network or a region can be quickly assessed, (iii) inferring characteristics of the river prior to intensive alteration by documenting characteristics of the least altered reference rivers and by using proxy indicators of pre‐alteration conditions, and (iv) establishing process thresholds that must be exceeded to maintain form (e.g. flow thresholds to mobilize bed sediment). Once this context has been established, resource managers can better evaluate the options for restoring altered riverine form and function. Copyright © 2011 John Wiley & Sons, Ltd.

Succession of Beaver Ponds in Colorado 50 Years After Beaver Removal

ABSTRACT In 1954 and 1955, I conducted studies on 4 beaver (Castor canadensis) colonies in Routt County, Colorado, USA, to develop a reliable and practical census method. In fall of 1955, these colonies were trapped to extirpation to obtain accurate counts and colony compositions. I established photo stations and took a series of photographs approximately every 5 years, in late September, from 1955 to 2005 to document beaver‐population dynamics and beaver‐pond successional changes after complete beaver removal. The number and configuration of dams, ponds, and canals remained remarkably similar, with maintenance conducted by succeeding generations of beaver. Quaking aspen (Populus tremuloides) and autumn willow (Salix serissima) boundaries also remained stable, but conifers slowly encroached. At 3 of 4 colony sites, beaver did not attempt to construct food caches for overwintering. By fall of 2005, the fourth site was also without a cache. Carrying capacity of these areas was likely limited by severe winter weather at 2,620‐m to 2,900‐m elevations, aspen depletion, pond‐depth reduction, and sustained grazing by cattle. Although beaver carefully maintained dams, pond depths were reduced in most cases. This slow transition from an aquatic to a terrestrial system is a natural progression in this region.

Seasonal, diel, and ontogenetic patterns of within-den behavior in beavers ( Castor canadensis)

Despite the ubiquity of denning as a natural history strategy among terrestrial vertebrates, little is known regarding basic patterns of within-den behavior, how such patterns are influenced by demographic and environmental parameters, or how within-den behavioral repertoires relate to activities performed in external environments. Den usage is believed to facilitate increased expression of behaviors that compromise fitness in external environments, though empirical data validating these assumptions are generally lacking. Relative isolation from external light cues within dens has been linked to temporal patterns of den use, yet few studies examine associations between photoperiod and rhythmicity strictly for within-den behavior. Also, for denning species with relatively equivalent parental investment, conclusions regarding sex-specific behavior have been equivocal, and no studies have examined potential segregation of parental activity within dens. We videorecorded 1506 h of within-den activity from 23 beaver ( Castor canadensis) colonies and characterized behavioral patterns based on sex and age over daily and monthly intervals. Within-den time-activity budgets were equivalent among male and female adult beavers, with feeding, sleeping, allogrooming, and individual grooming accounting for more than 95% of all recorded behaviors. Behavioral repertoires within dens exhibited distinct seasonality and were influenced by temporal variation in external conditions associated with food availability, indicating linkages between activities within and outside of dens. Lastly, beaver age classes varied considerably in their associations between diel activity patterns and photoperiod, with adults and kits exhibiting single and multiple sleep–wake cycles, respectively.

Demography of Unexploited Beavers in Southern Illinois

ABSTRACT Beavers (Castor canadensis) are important in ecosystems and to humans. Although beavers are increasingly protected from harvest, relatively few studies of unexploited beaver populations have been reported. Furthermore, few radiotelemetry studies exist for beavers because no practical method of attaching a radiotransmitter to beavers was available until recently. We used radiotelemetry, remote videography, and trapping data to quantify survival, dispersal, and natality of unexploited beavers in southern Illinois, USA, during 2004 to 2006. Beaver colony density was one of the highest reported in the wildlife literature at 3.27 colonies/km2. We monitored 62 beavers for survival; all mortalities (n = 15) occurred during the fall and winter seasons. The pooled annual survival rate for adult and juvenile females was (x̄±SE) 0.76±0.05. Annual survival rates for adult and juvenile males were 0.87±0.04 and 0.55±0.07, respectively. Seasonal survival only differed among sex classes and age classes during the fall. Dispersal rates for juvenile beavers ranged from 0.38±0.13 to 0.59±0.13 and did not vary by sex or age. To quantify natality and recruitment, we captured and euthanized 79 beavers adjacent to our live‐capture area; we found a low pregnancy rate of adult females (36%), and no juveniles were bred. Natality of bred females was 3.6 offspring per adult female, and 0.36 kits were recruited per adult female. Apparent kit survival was 28%. Our research provides information to wildlife managers about beaver demographics for a high‐density population, based on relatively large sample sizes and novel research techniques for the species.

A Remote Videography System for Monitoring Beavers

ABSTRACT We developed a remote videography system for monitoring behavior and demography of beavers (Castor canadensis) inside lodges and bank dens. Videography systems taped 6 beaver colonies for 1,080 hours and recorded 300 hours of beaver activity. Upon viewing videotape, we could characterize 86% of beaver activity into 1 of 12 behavioral categories. Of 26 individually marked beavers, we observed 8 (31%) on videotape and obtained a complete count of kits in 4 of 5 colonies. The remote videography system was generally useful and can provide wildlife biologists with demographic and behavioral information to support population modeling and management programs for beavers.

Distribution and conservation of the Sino-Mongolian beaver Castor fiber birulai in China

The Sino-Mongolian beaver Castor fiber birulai lives in the Ulungur watershed in China and Mongolia, an area little known to the people outside this region. We recorded the number of families, adults, subadults and juveniles at each of three beaver lodges in the Bulgan Beaver Nature Reserve in 2003, 2006 and 2007. Along the whole Ulungur watershed in China in each of the 3 years we conducted surveys for the beaver and estimated the total population based on the area of food caches. We recorded 135, 167 and 145 Sino-Mongolian beaver colonies and estimated a population 472-599, 543-700 and 508-645 beavers in 2003, 2006 and 2007, respectively. From 1989 to 2007 the number of human households in the Bulgan Nature Reserve increased by 112% and the population by 71%. Consequently human activities in the river valley, including collection of wood for fuel, increased. We also surveyed the site of a relocation of C. f. birulai from the Bulgan River to the Ertix River in 1992 but found no sign of living beavers. The population of the Sino-Mongolian beaver in China is small and restricted geographically. Threats, such as habitat modification and deterioration and competition with people for wood, continue. We recommend that the Bulgan Beaver Nature Reserve be expanded by a transfrontier agreement with the appropriate authorities in Mongolia, and that a plan for sustainable wetland management and restoration is required.

Variation in abundance across a species' range predicts climate change responses in the range interior will exceed those at the edge: a case study with North American beaver

AbstractThe absence of information about how abundance varies across species' ranges restricts most modeling and monitoring of climate change responses to the range edge. We examine spatial variation in abundance across the northeastern range of North American beaver (Castor canadensis), evaluate the extent to which climate and nonclimate variables explain this variation, and use a species–climate envelope model that includes spatial variation in abundance to predict beaver abundance responses to projected climate change. The density of beaver colonies across Québec follows a roughly logistic pattern, with high but variable density across the southern portion of the province, a sharp decline in density at about 49°N, and a long tail of low density extending as far as 58°N. Several climate and nonclimate variables were strong predictors of variation in beaver density, but 97% of the variation explained by nonclimate variables could be accounted for by climate variables. Because of the peak and tail density pattern, beaver climate sensitivity (change in density per unit change in climate) was greatest in the interior and lowest at the edge of the range. Combining our best density–climate models with projections from general circulation models (GCM) predicts a relatively modest expansion of the species' northern range limit by 2055, but density increases in the range interior that far exceed those at the range edge. Thus, some of the most dramatic responses to climate change may be occurring in the core of species' ranges, far away from the edge‐of‐the‐range focus of most current modeling and monitoring efforts.

Variation in abundance across a species' range predicts climate change responses in the range interior will exceed those at the edge: a case study with North American beaver

The absence of information about how abundance varies across species' ranges restricts most modeling and monitoring of climate change responses to the range edge. We examine spatial variation in abundance across the northeastern range of North American beaver (Castor canadensis), evaluate the extent to which climate and nonclimate variables explain this variation, and use a species-climate envelope model that includes spatial variation in abundance to predict beaver abundance responses to projected climate change. The density of beaver colonies across Quebec follows a roughly logistic pattern, with high but variable density across the southern portion of the province, a sharp decline in density at about 491N, and a long tail of low density extending as far as 581N. Several climate and nonclimate variables were strong predictors of variation in beaver density, but 97% of the variation explained by nonclimate variables could be accounted for by climate variables. Because of the peak and tail density pattern, beaver climate sensitivity (change in density per unit change in climate) was greatest in the interior and lowest at the edge of the range. Combining our best density-climate models with projections from general circulation models (GCM) predicts a relatively modest expan- sion of the species' northern range limit by 2055, but density increases in the range interior that far exceed those at the range edge. Thus, some of the most dramatic responses to climate change may be occurring in the core of species' ranges, far away from the edge-of-the-range focus of most current modeling and monitoring efforts.

Habitat dynamics of beaver Castor canadensis at two spatial scales

Abstract We studied habitat dynamics of the Canadian beaver Castor canadensis in a boreal forest landscape in southern Finland at two scales: the beaver pond level and the landscape level. To explore the changes in tree species composition due to beaver browsing and flooding, six abandoned beaver ponds were sampled (537 sample plots altogether). For habitat dynamics at the landscape scale, the variation in the flooded area and the number of active beaver colonies were recorded during 18 years (1980–1998). At the pond level, flooding appeared to effect more rapid and more pronounced changes in woody growth production than browsing. Coniferous trees were particularly susceptible to flooding, and deciduous trees were more susceptible to browsing. Deciduous trees dominated during succession following a flooding. At the landscape level, mean occupation time of a colony site was found to be short (2.6 years), and the mean area of beaver impoundments was small (0.14% of the total area). Recolonisation of habitat...

Microsatellite analysis of mating and kinship in beavers (Castor canadensis)

Monogamy is rare among mammals and molecular investigations have revealed that many socially monogamous species participate in extrapair mating. The North American beaver (Castor canadensis) is a socially monogamous species that exhibits classic monogamous behavior, generally living in discrete colonies composed of a mated pair and their offspring. We examined genetic relationships within and among beaver colonies for 2 populations in Illinois to investigate average relatedness within colonies, occurrences of extrapair mating within or between colonies, and the influence of geographic distance on intercolony relatedness. Seven microsatellite loci developed for the beaver were used to estimate relatedness and parentage for 55 beavers in central Illinois and 72 beavers in southern Illinois. Average within-colony relatedness varied widely in both populations, ranging from 0.04 to 0.64 in central Illinois and from 0.16 to 0.41 in southern Illinois. Colonies were composed primarily of 1st- and 2nd-order relatives, but included unrelated individuals. Paternity analysis revealed that 5 (56%) of 9 litters had been sired by ≥2 males. Extrapair mating frequently occurred between members of neighboring colonies in southern Illinois. In contrast to long-held views that beavers are genetically monogamous and colonies are typically lst-order relatives, we documented a wide range of relationships among colony members and multiple paternity in >50% of litters.

Carrion – It's What's for Dinner: Wolves Reduce the Impact of Climate Change

The Big Bad Wolf Humans have viewed wolves as competitors, threats to personal safety, and symbols of evil throughout history. By the early part of the 20th century, gray wolves (Canis lupus) had been eradicated from 42% of their historic range in North America (Laliberte & Ripple, 2004). In Yellowstone National Park, gray wolves were hunted to local extinction by 1926, but were reintroduced in 1995 after a decades-long process involving biologists, politicians, ranchers, and the general public (Table 1). By the end of 2006, the wolf population in the park was at least 136 wolves in 13 packs (Smith et al., 2007). In this activity, high school students use mathematical models to explore how the presence of wolves buffers other carnivores and scavengers from the effects of climate change. By the end of the lesson, students should be able to: * define and give examples of keystone species. * demonstrate, using mathematical models, that ecosystems are more resilient to environmental change when they contain a full complement of species, including top carnivores. * recognize that math is a vital tool in scientific investigations. Background From an ecological perspective, it was important to restore the gray wolf to Yellowstone because it is a keystone species. Keystone species, which are usually top predators, affect their communities or ecosystems in a much larger way than expected based on abundance alone (Steneck, 2005). The presence, abundance, and productivity of a wide array of species in Yellowstone National Park are indirectly affected by interactions of wolves with elk and coyotes (Figure 1). For example, the reintroduction of wolves has facilitated the recovery of beavers in Yellowstone. In the 1800s, human trappers decimated beaver populations. After wolves were removed from the park, elk populations grew and competition for willow--the preferred food and construction material of beavers--became intense (Ripple & Beschta, 2003). Although beavers had been protected from trapping since the early 1920s, and beaver reintroduction efforts were underway, competitive pressure from elk suppressed recovery (Baker et al., 2005). Since wolves were restored to Yellowstone, predation, hunting, and drought have reduced elk populations. But elk also have changed their behavior (Creel et al., 2005): When elk detect wolves in a general area, they move toward conifer forests (where they have good protection from wolves) and away from open areas and streams (where they have less protection from wolves; Figure 2). Because of the combined effect of fewer elk and reduced use of willow habitat, the number of beaver colonies (five to six beavers per colony) on the northern range of Yellowstone increased from one in 1996 to nine in 2003 (D.W. Smith, 2006, personal communication). [FIGURE 1 OMITTED] [FIGURE 2 OMITTED] Winter on the Northern Range In Yellowstone National Park, daytime winter temperatures range from -40[degrees]C to -5[degrees]C and snow depths can exceed 7m at high elevations (NPS, 2006). Every autumn, the northern Yellowstone elk herd migrates from high-elevation summer habitat within the park to milder habitat in the northern range, a 1,530 [km.sup.2]-area that includes a portion of the park and some adjacent public and private land (Singer & Mack, 1999). Even so, winter isn't easy. Elk search for grasses and other herbaceous plants by digging in the snow with their hooves. When the snow is deep or covered by a hard crust, digging becomes more difficult, as does the simple act of moving through the snow (Gese et al., 1996). Furthermore, the plants under the snow are less nutritious than in the summer (Jenkins & Starkey, 2003). In severe winters, elk regularly starve to death (Wisdom & Cook, 2000). They also can suffer the same fate during mild winters if conditions during the previous summer were poor (Vucetich et al. …

Low potential for restraint of anadromous salmonid reproduction by beaver Castor fiber in the Numedalslågen River catchment, Norway

AbstractAfter a long absence, beaver Castor fiber are rapidly returning to Europe. Their dam‐building and tree‐felling behaviour may have consequences for salmon Salmo salar and sea trout Salmo trutta management. In 2003 we investigated the parallel use of stream sections by beaver, sea trout and salmon and determined the potential hindrance that beaver dam‐building presented for reproducing salmon and sea trout along 65 km of the Numedalslågen River and tributaries, a major Norwegian catchment. We also surveyed landowner attitude to having beaver on salmon and sea trout streams. Most salmon spawned in the river and most sea trout in 51 tributaries. Nine of these tributaries also hosted spawning salmon. 15 (29%) of the 51 tributaries with spawning sea trout and six (67%) of the nine with spawning salmon had intermittently been occupied by beaver. Though beaver preferred to colonize the same sections of stream used for spawning, only 15% of the stream length navigable by salmon and sea trout on the 51 tributaries had actually been used by beaver, and only three colonies were occupied autumn 2003 (1 colony/25.0 km). Five dams were functioning during autumn 2003 on the 51 tributaries (1 dam/14.3 km). These potentially hindered sea trout and salmon from reaching 18% and 3%, respectively of their potential spawning habitat, though all dams were low (≤0.5 m). Though the autumn density of occupied beaver colonies along the river (1 colony/2.5 km) was 10.0 times the density on the 51 tributaries, no dams were built on the river. Thus most salmon reproduction in the catchment was unhindered by beaver. Nine of 14 landowners were unequivocally positive about having beaver together with salmon and sea trout. We conclude that the presence of beaver on similar catchments will likely have only an insignificant negative impact on the reproduction of sea trout and salmon. Copyright © 2007 John Wiley & Sons, Ltd.

A GIS-based habitat-suitability model as a tool for the management of beaversCastor fiber

A geographical information system (GIS)-based model, including major parameters of topography, hydrology and vegetation, was developed to assess suitable habitats for beavers in the Federal Province of Salzburg, Austria. BeaversCastor fiber Linnaeus, 1758 are on the verge of their distribution in the study area. At present, the 18 existing colonies show a rather scattered pattern, which suggests that the sub-population has not yet reached its carrying capacity, and a rapid growth phase as seen in other regions is expected. Suitable habitats have been calculated for the regions Alpenvorland, Salzburger Becken, Kalkvoralpen and Kalkhochalpen. Here optimal habitat is covering 429 ha (meeting all requirements of beavers) and suboptimal habitat is covering 187 ha (meeting all requirements except a winter food source). This represents 3.4% optimal and 1.5% sub-optimal habitat of the total available area. There are 13–14 suitable regions large enough to host at least one beaver colony. A total of 31 (23–51) colonies were calculated by the model. Based on the current sub-population of 18 colonies (36–100 animals), an average increase of 72% was expected. In an intensively managed landscape, beaver-human conflict should be anticipated and management policies established. Continuous assessment of suitable, unsuitable and sensitive habitats, extending existing conservation areas and river bank habitats, and pursuing protective measures against individuals causing conflicts are presented for the study areas.

Large woody debris characteristics and contributions to pool formation in forest streams of the Boreal Shield

The inputs, characteristics, and function of large woody debris (LWD) were assessed in 16 mid-order (average channel widths of 3–7 m), moderate-gradient (approx. 1%–5% channel slopes) streams in mixedwood forests of the Boreal Shield in Ontario. Three of the streams were adjacent to clearcuts, with the remainder in areas that have not been logged or recently (>70 years) burned. The average frequency (19.9 pieces·100 m–1) and size (mean diameter 16.7 cm) of LWD in these streams were less than reported in most other regions and forest types. Averaged across sites, input sources were undetermined for about 50% of the LWD owing to fluvial displacement from the points of origin. Natural mortality (24%) and windthrow (15%) were primary input sources of the remaining LWD. Windthrow was highly variable and mostly associated with nearby clear-cut logging. At the three sites near clearcuts, windthrow contributed 34%–62% of LWD in streams. In study reaches where active beaver colonies were observed, beaver-felled trees accounted for up to 47% of LWD inputs. The average frequency of debris dams (2.4 dams·100 m–1) was less than those reported from studies in other areas and was positively correlated with an index of bottom substrate size (r = 0.72). Less than 15% of the pools in these streams were formed or influenced by LWD. Most wood pieces appeared to be ineffective as pool-forming agents because of their relatively small size and instability. In these Boreal Shield forests, it appears that most riparian trees do not live long enough or grow to sufficient size to contribute functional LWD and influence stream morphology or pool formation.

Population-Genetic Structure of Beaver (Castor fiber L., 1758) Communities and Estimation of Effective Reproductive Size Neof an Elementary Population

The absence of panmixia at all hierarchical levels of the European beaver communities down to individual families implies a complex organization of the population-genetic structures of the species, in particular, a large intergroup component of gene diversity in the populations. Testing this assumption by analysis of 39 allozyme loci in the communities of reintroduced beaver from the Vyatka river basin (Kirov oblast) has shown that only the beaver colonies exhibit high intergroup gene diversity (G st = 0.32) whereas this parameter is much lower when estimated among beaver groups from individual Vyatka River tributaries and among localities of one of the tributaries (0.07 and 0.11, respectively). The data suggesting genetic heterogeneity among individual settles within colonies have been obtained. The factors affecting the structure of the beaver communities of the lower hierarchical ranks are considered: the common origin, founder effect, selection, gene drift, assortative mating, and social and behavior features of the species. The conclusion is drawn that the founder effect could be the primary factor of population differentiation only at the time of their formation. The heterogeneity among colonies and among settles is maintained largely by isolation of colonies from one another. The strong interspecific competition for food resources, which is behaviorally implemented in the species at the level of minimal structural units (individual settles) creates a profound and unique population-genetic subdivision of the species. These results substantiate the suggestion that an elementary population (micropopulation) of European beaver is a colony, i.e., a set of related settles of different types. Based on ecological and genetic parameters, the effective reproductive size N e of the minimum beaver population was estimated to be equal to three animals. This extremely low value of effective reproductive population size largely explains the high tolerance of European beaver to inbreeding and striking viability of the species, which from the early 19th century has been for more than hundred years on the brink of survival in the condition which would made any other mammalian species vanish from the Earth.

An extensive study of the foraging ecology of beavers (Castor canadensis) in relation to habitat quality

The objective of this study was to examine the foraging behaviour of the beaver (Castor canadensis Kuhl, 1820) and to explain its selection of terrestrial woody plant species according to central place foraging theory. Limitations in variety of food items in most studies with regard to size and (or) distance from the central place and information on availability of forage choices give a partial view of the subject. In this study, the theory is tested in a natural environment with high variability in food items with regard to these factors. Foraging choices by beavers were inspected by measuring variables on cut and uncut trees of every species encountered within 1 m of trail systems made by 25 beaver colonies in Kouchibouguac National Park in New Brunswick, Canada, thereby quantifying the availability of the different food items. The effect of habitat quality (food availability) on the foraging behaviour of beavers was also tested. The results of this study suggest that with increasing distance from the pond, beavers in high-quality habitats selected fewer, but larger, trees and are more species selective. This selectivity was diminished in habitats of lower quality. The results of this study are consistent with the predictions of the central foraging theory.

Composition and Demographics of Beaver (Castor canadensis) Colonies in Central Illinois

Beavers (Castor canadensis) exhibit wide variations in colony composition and demographics over their broad geographic range, so regional population studies are important for sound management of this species. The objectives of this study were to investigate the: (1) size and sex-age composition of beaver colonies in Illinois, (2) reproductive potential of female beavers in these colonies and (3) efficacy of night-vision surveys versus removal trapping for estimating colony size. We harvested and aged 239 beavers (128 males:111 females) during the 1999–2000 and 2000–2001 trapping seasons. The average colony contained 5.6 beavers. Family groups consisting of a breeding pair and at least 1 offspring composed 86% of these colonies; the other 14% consisted of only a breeding pair. Samples of beavers harvested by commercial trappers were skewed towards yearlings and 2-y olds relative to samples taken from trapped-out colonies. Fetal rates were 3.0, 3.4 and 4.2/female for yearlings, 2-y olds and older adults, respectively. In utero loss was estimated as 13%. Our estimates suggest that over 50% of kits die during the first 6 mo of life. We found no evidence that the presence of older offspring in a colony allowed parents to raise more kits. Night-vision surveys conducted 10 m downwind from the den for 2.5 h after sunset underestimated the size of colonies, accounting for only 55% of the beavers present.

Beaver herbivory and its effect on cottonwood trees: influence of flooding along matched regulated and unregulated rivers

AbstractWe compared beaver (Castor canadensis) foraging patterns on Fremont cottonwood (Populus deltoides subsp. wislizenii) saplings and the probability of saplings being cut on a 10 km reach of the flow‐regulated Green River and a 8.6 km reach of the free‐flowing Yampa River in northwestern Colorado. We measured the abundance and density of cottonwood on each reach and followed the fates of individually marked saplings in three patches of cottonwood on the Yampa River and two patches on the Green River. Two natural floods on the Yampa River and one controlled flood on the Green River between May 1998 and November 1999 allowed us to assess the effect of flooding on beaver herbivory. Independent of beaver herbivory, flow regulation on the Green River has caused a decrease in number of cottonwood patches per kilometre of river, area of patches per kilometre, and average stem density within cottonwood patches. The number of saplings cut per beaver colony was three times lower on the Green River than on the Yampa River but the probability of a sapling being cut by a beaver was still higher on the Green River because of lower sapling density there. Controlled flooding appeared to increase the rate of foraging on the Green River by inundating patches of cottonwood, which enhanced access by beaver. Our results suggest regulation can magnify the impact of beaver on cottonwood through interrelated effects on plant spatial distribution and cottonwood density, with the result that beaver herbivory will need to be considered in plans to enhance cottonwood populations along regulated rivers. Published in 2002 by John Wiley & Sons, Ltd.

Habitat suitability and source–sink dynamics of beavers

Summary Theory suggests that territorial species should share many of the same dynamical characteristics as metapopulations, including asynchronous local dynamics, potential for stochastic extinction of the population when rates of successful dispersal fall below mortality risk, and critical importance of the ratio of suitable to unsuitable habitat for long‐term persistence. These propositions were tested on a population of beavers (Castor canadensisKuhl) in Algonquin Provincial Park, Ontario, which has been continuously monitored over 11 years. Results showed that the total population was considerably less variable than local abundance at 14 beaver colonies, due to asynchrony among local populations. This suggests that local ecological interactions were more important in determining year‐to‐year variation in beaver numbers than broad‐scale environmental processes, such as weather. Of the local colonies, 20% were never abandoned over 11 years, although there was considerable turnover among adults. Offspring production exceeded adult abundance at five source colonies, which did not quite compensate for negative net production at nine sink colonies. These observations were consistent with predictions of spatially structured models of territoriality incorporating local variation in habitat suitability. Mean colony size and probability of recurrence from year‐to‐year were associated with local food availability, indicating that trophic interactions were important in determining local population dynamics. The beaver population in Algonquin declined steadily over the study period, however, suggesting that spatial and demographic processes were insufficient to stabilize abundance over time. This is consistent with predictions of spatially structured models of territoriality in which suitable and unsuitable habitats are interspersed. It is hypothesized that long‐term decline in habitat suitability is induced by acceleration of woody plant succession due to selective foraging by beavers.