Beaver Foraging Research Articles

An overview of the current state of the beaver’s environmental activity

A review of scientific publications devoted to the study of the impact of beaver foraging and construction activities on the plant communities of small rivers in different natural zones has been conducted. Based on the analysis of publications devoted to the study of the beaver's environmental activity, its key role in the successional processes of phytocenoses of riverine biotopes in all natural zones within the native and acquired range is noted. The stages of succession transformations initiated by the beaver's vital activity are considered. The degree of succession transformations depends, first of all, on the density of populations and their stages of maturity. The successions are highly intensive, multidirectional and do not reach the final stage, but are concentrated in the floodplains of small rivers and streams, in the coastal strip of other reservoirs. The vital activity of beavers determines the floristic, faunal, and ecologicaltopological diversity of coastal biogeocenoses. An increase in the proportion of groups of aquatic and wetland plant species was noted, and an increase in biological diversity was revealed in the beaver pond backup zone, regardless of zonal factors. However, being strongly transformed, the coastal vegetation remains zonally conditioned, does not become azonal.

Wolves alter the trajectory of forests by shaping the central place foraging behaviour of an ecosystem engineer.

Predators can directly and indirectly alter the foraging behaviour of prey through direct predation and the risk of predation, and in doing so, initiate indirect effects that influence myriad species and ecological processes. We describe how wolves indirectly alter the trajectory of forests by constraining the distance that beavers, a central place forager and prolific ecosystem engineer, forage from water. Specifically, we demonstrate that wolves wait in ambush and kill beavers on longer feeding trails than would be expected based on the spatio-temporal availability of beavers. This pattern is driven by temporal dynamics of beaver foraging: beavers make more foraging trips and spend more time on land per trip on longer feeding trails that extend farther from water. As a result, beavers are more vulnerable on longer feeding trails than shorter ones. Wolf predation appears to be a selective evolutionary pressure propelled by consumptive and non-consumptive mechanisms that constrain the distance from water beavers forage, which in turn limits the area of forest around wetlands, lakes and rivers beavers alter through foraging. Thus, wolves appear intricately linked to boreal forest dynamics by shaping beaver foraging behaviour, a form of natural disturbance that alters the successional and ecological states of forests.

Beaver Foraging Patterns in a Human-Dominated Landscape: Effects on Woody Vegetation and Mammals

Beaver Foraging Patterns in a Human-Dominated Landscape: Effects on Woody Vegetation and Mammals

A non-invasive, DNA-based method for beaver species identification in Finland

For the ability to control an invasive species and to protect an ecologically similar native species it is essential to map the exact distributions of both species. This is difficult if the species are so morphologically similar that their identification in the field is almost impossible. In Finland, the invasive North American beaver Castor canadensis is spreading towards the range of the native Eurasian beaver Castor fiber and at present, these species are partly sympatric. Effective management of these morphologically similar species requires an efficient method for species identification, ideally one that is non-invasive. Non-invasive genetic methods are used in a wide variety of wildlife species, for example in the research of large carnivores. Feces are a good source of DNA for terrestrial animals, but for the semi-aquatic animals like beavers, feces are not the best option. However, environmental DNA (eDNA) has been successfully used to detect species non-invasively in aquatic and terrestrial environments. We developed a non-invasive, eDNA-based method to map the distribution of the beaver species in Finland and to investigate within-species genetic diversity. The eDNA was obtained from the feed remains (wood chips) from beaver forage sites. With the help of Citizen science, wood chip samples were collected from different parts of Finland. We used our eDNA method to identify the ranges of both beaver species. Additionally, the presence of Eurasian beavers in south-east Finland was proven for the first time. Our non-invasive eDNA method is an effective way to accurately identify the ranges of both beaver species and will allow for the control of the invasive North American beaver and conservation of the native Eurasian beaver in Finland.

Beaver dam capacity of Canada\u2019s boreal plain in response to environmental change

Environmental changes are altering the water cycle of Canada’s boreal plain. Beaver dams are well known for increasing water storage and slowing flow through stream networks. For these reasons beavers are increasingly being included in climate change adaptation strategies. But, little work focuses on how environmental changes will affect dam building capacity along stream networks. Here we estimate the capacity of the stream network in Riding Mountain National Park, Manitoba, Canada to support beaver dams under changing environmental conditions using a modelling approach. We show that at capacity, the park’s stream network can support 24,690 beaver dams and hold between 8.2 and 12.8 million m3 of water in beaver ponds. Between 1991 and 2016 the park’s vegetation composition shifted to less preferred beaver forage, which led to a 13% decrease in maximum dam capacity. We also found that dam capacity is sensitive to the size of regularly-occurring floods—doubling the 2-year flood reduces the park’s dam capacity by 21%. The results show that the potential for beaver to offset some expected climatic-induced changes to the boreal water cycle is more complex than previously thought, as there is a feedback wherein dam capacity can be reduced by changing environmental conditions.

Assessment of the effect of beaver foraging activities on the alteration of waterside forests in northern and middle taiga of Russian Karelia

The territory of the Republic of Karelia is latitudinally elongated and traversing two boreal subzones such as the northern and middle taiga. Pine and spruce stands predominate in the northern taiga, while forests in middle taiga are more diverse, represented by secondary stands, and often dominated by deciduous species. These factors define the foraging behaviour of beavers and their role in alteration of riparian forests. Dispersing in the northern taiga, beavers tend to choose waterside areas with a higher proportion of deciduous species. However, such habitats occupy less than 1% of the forested area in this subzone, so the overall effect of beaver foraging on forest stands would be minor. On the other hand, inside beaver colonies, stand alteration is far more pronounced than changes in colonies in the middle taiga, and the effects are the following: 1) waterside forests in the northern taiga lose 2.5 times more deciduous trees than those in the middle taiga (61.4 and 26.3 %, respectively); 2) in waterside stands, aspen is totally removed, the share of conifers is doubled, and the share of birch is reduced (in the middle taiga, the share of birch around beaver colonies slightly increases, and the share of aspen is reduced by a factor of 1.5); 3) beavers in the north of Karelia consume thick birch trees more often than in the south of the region; 4) the regeneration capacity of damaged trees in the north is lower than in the south of Karelia.
 Key words: beavers, foraging activities, waterside forests, northern and middle taiga, tree stands

Modelling Eurasian beaver foraging habitat and dam suitability, for predicting the location and number of dams throughout catchments in Great Britain

Eurasian beaver (Castor fiber) populations are expanding across Europe. Depending on location, beaver dams bring multiple benefits and/or require management. Using nationally available data, we developed: a Beaver Forage Index (BFI), identifying beaver foraging habitat, and a Beaver Dam Capacity (BDC) model, classifying suitability of river reaches for dam construction, to estimate location and number of dams at catchment scales. Models were executed across three catchments, in Great Britain (GB), containing beaver. An area of 6747 km2 was analysed for BFI and 16,739 km of stream for BDC. Field surveys identified 258 km of channel containing beaver activity and 89 dams, providing data to test predictions. Models were evaluated using a categorical binomial Bayesian framework to calculate probability of foraging and dam construction. BFI and BDC models successfully categorised the use of reaches for foraging and damming, with higher scoring reaches being preferred. Highest scoring categories were ca. 31 and 79 times more likely to be used than the lowest for foraging and damming respectively. Zero-inflated negative binomial regression showed that modelled dam capacity was significantly related (p = 0.01) to observed damming and was used to predict numbers of dams that may occur. Estimated densities of dams, averaged across each catchment, ranged from 0.4 to 1.6 dams/km, though local densities may be up to 30 dams/km. These models provide fundamental information describing the distribution of beaver foraging habitat, where dams may be constructed and how many may occur. This supports the development of policy and management concerning the reintroduction and recolonisation of beaver.

Habitat and forage selection by the American beaver (Castor canadensis) on a regulated river in the Chihuahuan Desert

The American beaver (Castor canadensis) has been promoted as a tool for restoring aquatic and riparian ecosystems in Western North America. However, little is known about the ecology of beavers in arid environments. The purpose of this study was to examine selection of habitat and woody forage species during winter along the Rio Grande in southern New Mexico. We compared habitat characteristics at sites where beavers were present and absent and evaluated forage selection of woody plants at presence sites. Beavers occurred at sites that retained water over winter and that had larger pools, higher cover of sedges (Cyperaceae), higher abundance of narrowleaf willow (Salix exigua) stems, and higher diversity of woody stems. For foraging, beavers strongly selected Rio Grande cottonwood (Populus deltoides) and Russian olive (Elaeagnus angustifolia) and used willows slightly more than available proportionately. We concluded that, while cottonwoods might require protection from beaver foraging, beavers were not having a negative impact on willows and did not promote saltcedars via ecological release.

Forage Selection of Native and Nonnative Woody Plants by Beaver in a Rare-Shrub Community in the Appalachian Mountains of North Carolina

Castor canadensis (Beaver) is a selective forager that can modify the species composition and structure of plant communities. However, no studies have examined the use of woody plants by Beaver in temperate forests that contain a dominant nonnative plant. We investigated foraging of woody plants by Beaver in a riparian shrub community that is dominated by both native and nonnative species, including the federally threatened shrub Spiraea virginiana (Virginia Spiraea). We established 48 random, 25-m transects along a 12-km reach of the Cheoah River in the Appalachian Mountains of North Carolina. We sampled woody plants every 5 m using a modified point-centered quarter method to estimate relative abundance and to quantify browsing by Beaver. We used a mixed linear model to determine Beaver forage selection on the 9 most abundant plant species and Virginia Spiraea. We recorded 984 plants of 58 woody species (55 native, 3 nonnative). Beaver browsed 24% of the woody species sampled and 8% of all stems. This finding suggests that the overall effects of browsing in this community were relatively low, likely because of the high gradient and turbulent nature of the Cheoah River. Relative stem abundance and location along the river did not differentially affect local levels of browsing. However, Beaver were selective foragers at both the species and individual-plant level. Of the 9 most abundant species, Carpinus carolinana (Musclewood), Liquidambar styraciflua (Sweetgum), and Alnus serrulata (Tag Alder) were selected most often; Lindera benzoin (Spicebush), Virginia Spiraea, Cornus amomum (Silky Dogwood), and Ligustrum sinense (Chinese Privet) were moderately selected. Least frequently selected species were Rhododendron maximum (Rosebay Rhododendron), Leucothoe fontanesiana (Doghobble), and Xanthorhiza simplicissima (Yellowroot). Browsing appeared to have a positive effect on both the invasive nonnative shrub, Chinese Privet, and the rare Virginia Spiraea by stimulating asexual reproduction and inducing plants to spread through suckering. This study demonstrates the importance of understanding the reproductive strategies of woody plants when gauging the community-wide effects of foraging by Beaver, particularly when an invasive plant species is present.

Evaluation of tree and shrub resources of the Eurasian beaver (Castor fiber L.) and changes in beaver foraging strategy after resources depletion

The results of comparative analysis of tree and shrub resources quantitative characteristics in the active and abandoned beaver habitats within the territory of the Tadenka River basin, Prioksko-Terrasny Nature Reserve, where beavers have been dwelling for more than 60 years, are presented. Using one-way ANOVA analysis with the settlement type (active or abandoned) as a factor, it was shown that long-term foraging by beaver leads to a decrease in the species richness of trees (P = 0.068). Different hypotheses of the impact produced by beaver selective foraging on various forest layers (overstorey and undestorey) are discussed. Factors promoting thinning and changes in the age structure of the overstorey and undestorey are considered. There were observed changes in the species composition, accompanied by an increase in the proportion of low preference and nonfood species. The conclusion is made that, if the foraging zone does not exceed 50 m at the beginning of habitat utilization, it extends up to 165 m in the absence of predators when the habitat is used repeatedly. This observation is confirmed by Fisher’s exact test (P = 0.005), Yates’ chisquared test (P = 0.002), and the likelihood ratio test (P = 0.0002). In the case of beavers recolonizing their abandoned habitats with unrecovered food resources in the Tadenka River basin, a distant zone within 50 to 165 m from the bank becomes the main zone for tree-shrub foraging.

Impacts of Beaver Foraging on the Federally Threatened Virginia Spiraea (Spiraea virginiana) along the Cheoah River, NC

Spiraea virginiana (Virginia Spiraea) is a federally threatened shrub endemic to the southern Blue Ridge and Appalachian Plateau physiographic provinces. Observations along the Cheoah River, where the largest population of S. virginiana occurs in North Carolina, indicate that Castor canadensis (Beaver) feed on S. virginiana. However, the effects of Beaver foraging on this imperiled shrub are unknown. To address this issue, we randomly located 50 belt transects (25 m × 2.5 m) along the center of the scour zone of the Cheoah River and assessed all basal stems (<2.5 cm in diameter) of woody plants for evidence of browsing by Beaver. We recorded a total of 4963 basal stems, consisting of 59 species. Of those stems, 14.7% were browsed by Beaver. Spiraea virginiana accounted for 3.5% of the total basal stems and ranked fourth in the total number of stems browsed with 8.8%. After adjusting for transect effect, S. virginiana ranked second as a preferred forage species, with 32.0% of its stems browsed. This study suggests that overall browse levels of this community are relatively low compared to other communities inhabited by Beaver because of the high-gradient and turbulent nature of the Cheoah River. Our findings provide strong evidence that S. virginiana is an important food to Beaver, and as a consequence, Beaver foraging has the potential to affect this population. Although more information is needed to determine the long-term impacts, we suspect that in the short-term, S. virginiana may benefit from the levels of browsing found in this study. Field observations suggest that Beaver foraging stimulates asexual reproduction of S. virginiana by inducing rhizomatous growth. Beaver also may help S. virginiana with dispersal, as fresh cuttings are often left at feeding sites and then may be transported downstream for rooting.

Proximate weather patterns and spring green‐up phenology effect Eurasian beaver (Castor fiber) body mass and reproductive success: the implications of climate change and topography

Low spring temperatures have been found to benefit mobile herbivores by reducing the rate of spring-flush, whereas high rainfall increases forage availability. Cold winters prove detrimental, by increasing herbivore thermoregulatory burdens. Here we examine the effects of temperature and rainfall variability on a temperate sedentary herbivore, the Eurasian beaver, Castor fiber, in terms of inter-annual variation in mean body weight and per territory offspring production. Data pertain to 198 individuals, over 11 years, using capture-mark-recapture. We use plant growth (tree cores) and fAPAR (a satellite-derived plant productivity index) to examine potential mechanisms through which weather conditions affect the availability and the seasonal phenology of beaver forage. Juvenile body weights were lighter after colder winters, whereas warmer spring temperatures were associated with lighter adult body weights, mediated by enhanced green-up phenology rates. Counter-intuitively, we observed a negative association between rainfall and body weight in juveniles and adults, and also with reproductive success. Alder, Alnus incana, (n = 68) growth rings (principal beaver food in the study area) exhibited a positive relationship with rainfall for trees growing at elevations >2 m above water level, but a negative relationship for trees growing <0.5 m. We deduce that temperature influences beavers at the landscape scale via effects on spring green-up phenology and winter thermoregulation. Rainfall influences beavers at finer spatial scales through topographical interactions with plant growth, where trees near water level, prone to water logging, producing poorer forage in wetter years. Unlike most other herbivores, beavers are an obligate aquatic species that utilize a restricted 'central-place' foraging range, limiting their ability to take advantage of better forage growth further from water during wetter years. With respect to anthropogenic climate change, interactions between weather variables, plant phenology and topography on forage growth are instructive, and consequently warrant examination when developing conservation management strategies for populations of medium to large herbivores.

The Effects of Forestry Site Preparation on Mountain Beaver Demographics and Associated Damage to Tree Seedlings

Abstract Timber resources are a vital part of the economy in the Pacific Northwest, yet reforestation efforts are often hampered by animal damage. Understanding the factors that influence seedling damage can assist managers in implementing appropriate techniques to reduce species-specific damage. I radio-collared and monitored mountain beavers (Aplodontia rufa) from preharvest to seedling planting to determine the impacts of forest management practices on demographics. In addition, I monitored Douglas-fir (Pseudotsuga menziesii) seedlings on 28 plots on two harvest units (Vesta and Schoolhouse) to determine the effects of chemical site preparation on mountain beaver foraging and subsequent seedling damage. Mountain beaver densities differed between silvicultural treatments (F3,39 = 3.58, P &lt; 0.02), although reproductive success did not (χ2 = 0.14, df = 1, P = 0.70). Larger mountain beaver home ranges were documented in the mature timber compared with those following harvest. Home ranges were also larger in the chemical site preparation portion of the unit (mean = 3.37 ± 0.9 ha) than in the nontreated portion of the unit (mean = 0.82 ± 0.17 ha) on Vesta. Herbicide treatment did not promote seedling damage on either unit. Vegetation does not appear to be the only factor influencing mountain beaver movements and seedling damage. Availability of water is very important for mountain beavers and may also influence their foraging choice, as well as contributing to their increased movements in mature timber (reduced groundwater) versus clearcut areas. Understanding the effects of vegetation on population demographics and subsequent seedling damage may allow for initiating a nonlethal management method using alternative forage.

Simulation modeling to understand how selective foraging by beaver can drive the structure and function of a willow community

Beaver–willow ( Castor- Salix) communities are a unique and vital component of healthy wetlands throughout the Holarctic region. Beaver selectively forage willow to provide fresh food, stored winter food, and construction material. The effects of this complex foraging behavior on the structure and function of willow communities is poorly understood. Simulation modeling may help ecologists understand these complex interactions. In this study, a modified version of the SAVANNA ecosystem model was developed to better understand how beaver foraging affects the structure and function of a willow community in a simulated riparian ecosystem in Rocky Mountain National Park, Colorado (RMNP). The model represents willow in terms of plant and stem dynamics and beaver foraging in terms of the quantity and quality of stems cut to meet the energetic and life history requirements of beaver. Given a site where all stems were equally available, the model suggested a simulated beaver family of 2 adults, 2 yearlings, and 2 kits required a minimum of 4 ha of willow (containing about10 stems m −2) to persist in a steady-state condition. Beaver created a willow community where the annual net primary productivity (ANPP) was 2 times higher and plant architecture was more diverse than the willow community without beaver. Beaver foraging created a plant architecture dominated by medium size willow plants, which likely explains how beaver can increase ANPP. Long-term simulations suggested that woody biomass stabilized at similar values even though availability differed greatly at initial condition. Simulations also suggested that willow ANPP increased across a range of beaver densities until beaver became food limited. Thus, selective foraging by beaver increased productivity, decreased biomass, and increased structural heterogeneity in a simulated willow community.

Manipulating Beaver (Castor canadensis) Feeding Responses to Invasive Tamarisk (Tamarix spp.)

To evaluate methods for promoting consumption of tamarisk plants by beavers (Castor canadensis), we determined the feeding responses by captive beavers to diets that contained tannins and sodium chloride (hereafter referred to as tamarisk diet). In two-choice tests, beavers consumed equivalent quantities of tamarisk diet and control diet. Treatment with polyethylene glycol and fructose did not increase beaver preferences for the tamarisk diet. When offered the choice of control diet and casein hydrolysate-treated control diet, beavers strongly avoided the latter, showing feeding deterring activity of casein hydrolysate. However, when tamarisk diet was the alternative to the deterrent treatment, beavers consumed similar quantities of the two diets. Finally, beaver foraging preferences for actual plant cuttings were assessed. Casein hydrolysate application to cuttings of black poplar (Populus nigra) and Scouler's willow (Salix scouleriana) reduced browsing of these highly preferred species and promoted a marked increase in browsing of tamarisk (Tamarix ramosissima). These results suggest that casein hydrolysate treatment of desirable riparian plant species such as Salix and Populus may promote beaver foraging of invasive tamarisk.

The effects of high ungulate densities on foraging choices by beaver (Castor canadensis) in the mixed-wood boreal forest

In some areas of North America previous management policies have created competition between beaver ( Castor canadensis Kuhl, 1820) and ungulates, resulting in dramatic declines in beaver populations. Some authors attribute this decline to competitive exclusion. Generally, the less niche overlap between competitors, the lower potential competition between them. Differences between foraging behaviour of beaver and ungulates suggest that they could not compete to the point of either competitive exploitation or complete exclusion except in restricted habitats. We tested this assumption under two levels of foraging intensity by ungulates by examining the effects of resource competition on beaver forage choices in the context of central place foraging theory. Ungulate densities and foraging intensity within Elk Island National Park (EINP) in Alberta, Canada, were significantly higher than those immediately adjacent to the park, where foraging pressure was lower. Within EINP, forage availability (e.g., stem densities and stem diameters) of many woody plants preferred by beaver, such as Populus L. and Salix L., were depressed by intense foraging by ungulates. Beaver adapted to the effects of high ungulate densities on forage resources by adapting their foraging behaviour. This finding suggested that competitive exploitation, rather than exclusion, exists in EINP. EINP is a productive system that offers an array of forage species, which potentially buffers the effects of competition between ungulates and beaver.

Do beavers promote the invasion of non-native Tamarix in the Grand Canyon riparian zone?

Beavers (Castor canadensis Kuhl) can influence the competitive dynamics of plant species through selective foraging, collection of materials for dam creation, and alteration of hydrologic conditions. In the Grand Canyon National Park, the native Salix gooddingii C.R.Ball (Goodding’s willow) and Salix exigua Nutt. (coyote willow) are a staple food of beavers. Because Salix competes with the invasive Tamarix ramosissima Ledeb., land mangers are concerned that beavers may cause an increase in Tamarix through selective foraging of Salix. A spatial analysis was conducted to assess whether the presence of beavers correlates with the relative abundance of Salix and Tamarix. These methods were designed to detect a system-wide effect of selective beaver foraging in this large study area (367 linear km of riparian habitat). Beavers, Salix, and Tamarix co-occurred at the broadest scales because they occupied similar riparian habitat, particularly geomorphic reaches of low and moderate resistivity. Once the affinity of Salix for particular reach types was accounted for, the presence of Salix was independent of beaver distribution. However, there was a weak positive association between beaver presence and Salix cover. Salix was limited to geomorphic settings with greater sinuosity and distinct terraces, while Tamarix occurred in sinuous and straighter sections of river channel (cliffs, channel margins) where it dominated the woody species composition. After accounting for covariates representing river geomorphology, the proportion of riparian surfaces covered by Tamarix was significantly greater for sites where beavers were present. This indicates that either Tamarix and beavers co-occur in similar habitats, beavers prefer habitats that have high Tamarix cover, or beavers contribute to Tamarix dominance through selective use of its native woody competitors. The hypothesis that beaver herbivory contributes to Tamarix dominance should be considered further through more mechanistic studies of beaver foraging processes and longterm plant community response.

Beaver herbivory on aquatic plants

Herbivores have strong impacts on marine and terrestrial plant communities, but their impact is less well studied in benthic freshwater systems. For example, North American beavers (Castor canadensis) eat both woody and non-woody plants and focus almost exclusively on the latter in summer months, yet their impacts on non-woody plants are generally attributed to ecosystem engineering rather than herbivory. Here, we excluded beavers from areas of two beaver wetlands for over 2 years and demonstrated that beaver herbivory reduced aquatic plant biomass by 60%, plant litter by 75%, and dramatically shifted plant species composition. The perennial forb lizard's tail (Saururus cernuus) comprised less than 5% of plant biomass in areas open to beaver grazing but greater than 50% of plant biomass in beaver exclusions. This shift was likely due to direct herbivory, as beavers preferentially consumed lizard's tail over other plants in a field feeding assay. Beaver herbivory also reduced the abundance of the invasive aquatic plant Myriophyllum aquaticum by nearly 90%, consistent with recent evidence that native generalist herbivores provide biotic resistance against exotic plant invasions. Beaver herbivory also had indirect effects on plant interactions in this community. The palatable plant lizard's tail was 3 times more frequent and 10 times more abundant inside woolgrass (Scirpus cyperinus) tussocks than in spatially paired locations lacking tussocks. When the protective foliage of the woolgrass was removed without exclusion cages, beavers consumed nearly half of the lizard's tail leaves within 2 weeks. In contrast, leaf abundance increased by 73-93% in the treatments retaining woolgrass or protected by a cage. Thus, woolgrass tussocks were as effective as cages at excluding beaver foraging and provided lizard's tail plants an associational refuge from beaver herbivory. These results suggest that beaver herbivory has strong direct and indirect impacts on populations and communities of herbaceous aquatic plants and extends the consequences of beaver activities beyond ecosystem engineering.

Beavers, &lt;em&gt;Castor canadensis&lt;/em&gt;, Feeding on Salmon Carcasses: Opportunistic Use of a Seasonally Superabundant Food Source

We report observations of Beavers (Castor canadensis) foraging and feeding on discarded Chinook Salmon (Oncorhynchus tshawytscha) carcasses within the confines of the Susitna River drainage in southcentral Alaska on three separate occasions between 1999 and 2004. In all three instances, Beavers were observed actively seeking out freshly discarded carcasses or transporting “fresh” salmon carcasses in their mouths. In one instance, Beavers were seen using their dextrous forefeet to “handle” chunks of salmon while hunched over carcasses and in this case we actually witnessed Beavers “chewing” and ingestion was assumed. In the other two instances, Beavers were observed swimming with salmon carcasses in their mouths. Though unique within the framework of Beaver foraging ecology, we suggest this behavior may be a fairly common strategy employed by Beavers in Alaskan streams and rivers to take advantage of a seasonally superabundant source of protein.

Irrigation Increases Verticillium Wilt Incidence in a Susceptible Alfalfa Cultivar.

Verticillium wilt (VW) caused by Verticillium albo-atrum occurs in many irrigated alfalfa (Medicago sativa) fields in southwestern Saskatchewan, Canada, but is very rare in dryland stands. A trial was established in 1994 to determine if incidence of VW is affected by the amount of irrigation water applied. A line-source irrigation system was used to produce an irrigation water gradient on two adapted alfalfa cultivars, Beaver (VW-susceptible) and Barrier (VW-resistant). The irrigation gradient was subdivided into six irrigation level treatments within each cultivar plot. Irrigation plus precipitation from April to August averaged 265, 291, 309, 330, 351, and 353 mm for irrigation levels one to six, respectively, over 3 years. A 1-m-wide strip through the center of each subplot was inoculated with a spore suspension of V. alboatrum before the first harvest in 1995. Plant density (ground cover) and incidence of VW-diseased plants per subplot were recorded and forage yield was assessed twice per year. Beaver had more VW-infected plants in 1995 and 1996 than Barrier, and the number of diseased plants of Beaver increased with irrigation level. Ground cover of Beaver declined in 1997 and this response was correlated to VW-diseased plant counts from 1995 and 1996. There was no significant correlation between VW-diseased plant counts and ground cover for Barrier. Forage yield increased linearly with irrigation level for both cultivars in two harvests during 1995. However, yield of Beaver did not respond to irrigation during cut 2 in 1996. Beaver forage yield declined with irrigation level at cut 1 in 1997 because ground cover had declined by 40% at irrigation level six compared with 22% at irrigation level one. The ground cover of Beaver at irrigation level one was similar to that of Barrier at all irrigation levels. We conclude that VW will affect susceptible alfalfa cultivars on irrigated stands in proportion to the amount of irrigation water applied. Irrigated alfalfa producers should adopt VW-resistant cultivars even when irrigation is limited.