Abstract:
In 1954 and 55, I conducted studies on four beaver (Castor canadensis) colonies in Routt County, Colorado to develop a reliable and practical censusing method. In the fall of 1955 these colonies were trapped to extirpation to obtain accurate counts and colony compositions. Photo stations were established and a series of photographs taken approximately every five 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 the dams, ponds and canals remained remarkably similar and their maintenance was carried out by succeeding generations of beaver. The aspen and willow boundaries also remained stable, but conifers slowly encroached. No attempts were ever made to construct food caches for over-wintering purposes at three of the four colony sites. By the fall of 2005 the fourth site had also become “cache-less.” Carrying capacity of these areas is likely limited by severe winter weather at 2,620 to 2,900 meter elevations, aspen depletion, pond-depth reduction and sustained grazing by cattle. This slow transition from an aquatic to a terrestrial system should be viewed as a natural progression in this region.
Key words:
Introduction
The history of beaver habitation in the North Platte River Valley in Routt County Colorado was probably typical of other areas in the state and in the West. Prior to 1850, trappers had worked their way up most of the drainages and by the late 1850’s Colorado’s beaver population had been severely reduced (Rutherford, 1964). By 1900 they were given complete protection (except for nuisance animals) and in 1937 all beaver control was restricted to salaried trappers. Under the Beaver Control Act of 1941, trapping remained under the jurisdiction of state trappers, but one-half of the sale value of the pelts taken from their lands would be given to the landowner. Beaver pelt prices began to decline in 1947 and more efficient law enforcement reduced poaching (Yeager and Hill, 1954). As a result, beaver populations on public lands rebounded. In the early 1950’s, beaver numbers and their subsequent impact on the environment could no longer be ignored and the Colorado Division of Wildlife initiated Federal Aid Projects to study their populations including censusing, food usage and colony abandonment. In 1955, the Colorado Legislature passed a law opening the first public beaver-trapping season since the 1890’s (Rutherford,1964). In 1996, however, Colorado voters approved a constitutional amendment banning the use of leg-hold traps, but allowing live-trapping and hunting of beavers.
In 1954 I started a Federal Aid Project in the North Platte River Valley in Northern Colorado with the goal of developing a census method for beaver. Subsequent to completing the project I periodically visited the study sites for this work approximately every five years and collected photo documentation of changes in the habitat at fixed photo locations (see Figure 1). The photos provide valuable information on the succession of beaver-pond habitats in a high elevation mountain system. All changes in habitat occurred through natural processes other than cattle grazing.
Colorado Division of Wildlife trapping records are recorded by county, not by specific drainages or location, thus there are no documented trapping records for the North Platte drainage.. A retired state trapper, still living in the area, stated that any trapping within the study area over the last 50 years, was limited to “trouble” sites where beaver were blocking irrigation canals or roads (Don Gore, personal communication). Census studies in lower beaver draw (see map of study region) were discontinued at the beginning of the study due to subsequent interference by other trapping and a lack of accurate data taken by the trappers. Beaver have few natural enemies in this area and no evidence of predation was ever documented by the Division of Wildlife and no fires had occurred on the sites over the course of these observations.
Tularemia among beavers in the North Park region, may have influenced beaver populations. In 1957 this disease reached epidemic proportions in Northern Colorado; a specimen was taken in the region, autopsied by the College of Veterinary Medicine at Colorado State University, and received a positive diagnosis. The disease resulted in a drastic reduction of beaver during the 1957 fall population estimate. By 1959, however, beaver populations began to increase to their former levels. (Rutherford, 1964)
Study Area
The study’s location lies within a rectangular tract 4 km long and 1.6 km wide, in the Routt National Forest in North Park, Jackson County, Colorado. The area was drained by the North Fork of the North Platte River and was 10 km south of the Big Creek Lakes at elevations between 2,620 and 2,900 meters. The region is characterized by dense stands of lodge-pole pine (Pinus contorta var. latifolia Engelm.), and Engelmann spruce (Picea engelmanni Parry). Aspen (Populus tremuloides Michx.), alder (Alnus rubra Bong), willows (Salix spp.) and sedges (Carex ssp.) dominated the valley slopes. Aspen is a sub-climax species occurring between 2,150 and 2,750 elevation, is relatively intolerant to continued utilization, and can be easily depleted or destroyed by beavers (Rutherford, 1964 and Beier and Barrett, 1987). Replacement of aspen, even under moderate use, rarely equals the amount utilized, and for this reason aspen cannot be regarded as a “renewable resource” within the life of any single beaver colony (Hall, 1960). Even when their favorite food of aspen becomes difficult to access and constrains construction activities, they can continue to subsist on sedges, willows, water lilies (Nuphar spp.) and other herbaceous plants. Willows can tolerate sustained feeding by beavers in the absence of aspen (Hall, 1960 and Kindschy, 1985).
Free-ranging cattle have a long history of utilizing the stream valleys in the region during the spring, summer and critical fall months. They, along with elk (Cervus elaphus) and mule deer (Odocoileus hemionus), graze the grasses, sedges, young willows and aspen sprouts and effectively limit the growth and ability of aspen to spread. Plant succession in areas devoid of mature aspen stands revert back to a grass-forb state—the original under-story of the aspen stand.
Fall is a critical period for beavers in temperate climates. Activity is high with the repair of dams, lodges and the building of food caches for the coming winter. Canals or channels are often extended to allow safe access to new food sources without getting far from water.
Each beaver colony is a family unit and occupies a discrete territory (Hodgdon and Lancia, 1983). During the summer one colony can maintain and inhabit as many as three different lodges, huts or bank burrows. True bank dens were never encountered in my work, however, of the 22 lodges studied in 1954-55, ten were located on shorelines and were difficult to distinguish from the surrounding terrain. Only the presence of a food cache permitted positive colony identification. Thus, a lodge was not found to be synonymous with a colony. In the fall, however, they build a common food cache and use a single lodge for their winter quarters. A reliable census technique for beavers in mountain terrain may be obtained by counting the number of food caches in a given area in the late fall and multiplying by the average number of animals per winter colony (Hay, 1958).
The use of one large lodge and pond may be essential to beaver survival in the interior mountains, where winter temperatures can reach -50 degrees C. (Colorado Climate Center, 2007). It is unlikely that beaver could survive these intense winters in the Rockies without relatively deep water, a stored supply of food, and the added thermal value inherent in the close proximity to other warm-blooded family members. Gregariousness, therefore, serves as a key protective function to groups of related beavers. The presence of a fresh food cache is clear evidence of a colony. Yearling animals are allowed to remain in the lodge until they reach the age of about two years. They then disperse on their own or will be driven from the colony by the adults (Grasse and Putnam, 1950).
Retzer et al (1955) investigated 365 high-elevation stream sections in Colorado to develop site-suitability classifications for beaver based on physical conditions. They found three features to be important: valley grade, valley width and rock type. About 70 percent of the beaver-occupied stream sections had grades between 1 and 6 percent and no occupation occurred above 15 percent. The quality of beaver habitat is higher as valley gradient decreases and valley width increases. Subsequent studies have since confirmed that stream gradient, valley depth and width, steepness of adjacent slope and water reliability are the most important physical factors affecting colony site longevity. (Slough and Sadlier 1977, Howard and Larson 1985, Beier and Barrett 1987 and Curtis and Jensen 2004).
Low gradients allow beaver to increase their foraging area, while steep topography hinders food transportation (Slough and Sadlier, 1977). Gentle slopes also provide riparian vegetation and woody food plants such as aspen and alder. Such gradients, however, slow the water velocity and increase sediment deposition in the impoundments, thus reducing depth and utility for beaver over-wintering purposes in mountain terrain.
All of the study areas had favorable geological features with no terminal moraines or dikes. Stream gradients varied from 2 percent to 7 percent. Floodplains varied from channel width to 515 feet. The maximum bank gradient was 77 percent and the minimum 2 percent.
The areas studied were a part of the North Platte River’s drainage system and had been transformed into staircases of seepage ponds by generations of beaver. I could find no evidence in this study to support a relationship between the number of beaver impoundments and beaver density as reported in boreal regions by Broschart et al (1989). Impoundments (dams) were not considered a census index because of their obvious dependence upon topographical conditions and proximity of food. I also found no significant statistical relationship between the numbers of beaver and the size of the main lodge. (Hay, 1958)
Methods
These observational studies to evaluate beaver re-population dynamics and changes in their habitat, began in the fall of 1955 and were conducted approximately every 5 years until concluded in the fall of 2005.
All of the photo locations were taken from marked stations in late September. Written and recorded observations of each colony site and its surroundings were also made. To provide reference-point locators, each photo has a white arrow designating the same location (trees or plants) throughout the series.
Results
Four colonies, totaling 32 animals, originally occupied the four sites in this study. (Table No.1) Each had a lodge, a food cache, fresh “scent mounds” and well maintained dams
in the fall of 1955. To obtain an accurate count of each colony and its composition, the colonies were dead-trapped to extirpation by state trappers. The leg-hold traps remained set throughout October, November and into December. In the fall of 2005, none of the original lodges had structurally survived and no food caches were found at the old colony locations.
Table No.1
________________________________________________________________________
Colony Number Composition Number of Animals
1 2 adults, 3 yearlings, 5 kits 10
2 6 adults, 1 yearling 7
3 2 adults, 2 yearlings, 4 kits 8
4 6 adults, 1 kit 7
____________________________________Total_______32______________________
Colony No. 1 (Upper Beaver Draw)
All white arrows point to the same location.
In the fall of 1955 a refurbished dam, lodge and food cache had been constructed. By the fall of 1960 the pond was nearly drained and the lodge was slowly “melting” into the pond area. The small stream continued to pass through the site. Five years later, in 1965, the pond had completely drained and only a remnant of the lodge was left. For the next 20 years water continued to flow through the site which became completely over-grown with sedges, grass, willows and young lodge-pole pines. The dams immediately below this site, however, were routinely repaired by beavers and their ponds remained full. By 1992, (38 years later) beaver activity on this site was again noted and repairs made to the old dam. In 1996, beavers had up-graded the dam, carefully following its original design. In 2001, the dam and pond were still in good repair and by 2005 beaver activity had increased and turned the water turbid again; however, there was still no evidence of a new lodge or a food cache. This site was never re-occupied by an over-wintering colony.
Colony No. 2 (Forester Seep)
In 1955, this huge lodge, over-looking the North Platte River, had a food cache of willow and young aspen and was bounded by a series of deep ponds filled with water. Two large conifers were growing near the lodge. By 1970 one of the trees had fallen into the pond and the other followed by 1975. Portions of these trees remained until 2001. The lodge was constructed of old aspen logs, willows, sedges and mud. Through a series of extensive canals, generations of beaver had depleted nearby aspen stands and their food source was largely that of willows and sedges. This lodge was never reclaimed by a colony and gradually decayed and sank into the pond. Sedges, grasses and willows rebounded around the pond. Throughout the five decades, it is believed that beaver from the adjacent river or up-stream colonies maintained the dam structures, but never attempted to rebuild the lodge or a food cache. The ponds and canals slowly filled with silt and vegetative debris. The conifer on the left (in the 2005 photo) was present as a small tree in the 1955 photo and now towers over the old lodge site now reclaimed by the full pond.
Colony No. 3 (Forester Seep Draw)
In 1955, this was a relatively new, small impoundment with a lodge and an aspen food cache. By 1960, however, with some water still impounded, two small “huts” had been constructed, one on either end of the main dam. These ”improvements” were soon abandoned and by 1965 the dam was breached, and only a trickle of water ran through the heavy overgrowth of sedges and young conifers. This site was ignored by beavers in subsequent years. The main lodge slowly decayed into the bank with one large aspen log visible in all of the photos. The three trees to the left in the photos still remain. This site was never reclaimed during the 50-year period.
Colony No. 4 (Goose Creek)
In 1955, this old lodge was situated on an extensive dam, in a seepage zone that impounded a lake of several acres where pond lilies flourished. The lodge was a part of the dam structure and made of mud, aspen logs, willows and pond-lily rhizomes. The food cache was composed of herbaceous vegetation, willows and small branches of aspen. The site was at the bottom of a long series of beaver impoundments approximately 66 meters from Goose Creek, a tributary of the North Platte River. In 1960, a smaller lodge was added adjacent to the old one, but no food cache was present. By 1965 a food cache was built and the two lodges were combined into one. This single lodge, with a food cache, remained until 1980 when a new lodge was built in the lake just off-shore from the dam and the old lodge. Without repair or use, the old lodge slowly disappeared. A “causeway”, however, using material from the old lodge, was used to connect the dam to the new lodge and its food cache. It was not until 1996 that complete separation of the lodge from the dam took place. In the fall of 2005 the single lodge, remained intact and was kept repaired, but no food cache was observed.
In 1954-55, beavers on this site were using three lodges, in three separate ponds above this main pond during the summer months. Throughout the study period all dams and water levels in this area were also maintained. Note that the sedges in the foreground of the photos remained stable over the years.
Lower Beaver Draw
In the series of photos for Lower Beaver Draw (located below Colony No. 1), the entire length of this stream section, with its narrow valley, was covered by beaver impoundments that formed ”chains” down the streambed, virtually inundating the entire valley bottom. Although the dams were refurbished over the years their number and configuration remained the same. Three of the dams were broken during the 50-year time-period, two of which were promptly repaired, but no food caches were ever found in this stretch of dense dams. The lack of a food cache and previous trapping precluded the inclusion of this area as a study site. The aspen boundary remained stable, but conifers were slowly reclaiming the bottomlands. This section also provided multiple benefits that increased wildlife diversity and species richness. The area was a haven for numerous species of waterfowl. The old dead conifer in the foreground has lost a few limbs over the decades, but has remained quite stable.
Discussion
Habitat Suitability and Re-occupation
All of the four areas were within one-half mile of the Platte River and connected to it by natural waterways or seepages that were filled with beaver ponds. With the removal of the animals in the study areas, the dispersal of yearlings, sub-adults and other transients from the river to the vacated colony sites was expected.
Within the first five years, other beavers had invaded all of the former colony sites and ponds and were maintaining all the dams with fresh mud and willow-cuttings. This rapid entry by other beaver probably began within the first 2-3 years following removal since all pond and dam systems had remained unchanged in 1960. Colony No.3 was abandoned sometime between 1960 and 1965. By keeping the extensive pond and canal systems repaired, beavers could now safely travel throughout the old colony sites and nearby foraging areas.
Despite the region’s suitable geological features e.g., stream and bank gradients, wide floodplains and ample water supply, three of the four sites were never re-occupied by over-wintering colonies. The fourth site (Colony No. 4) on a large lake, was re-occupied and food caches were present 10 years later in 1965 and continued to be found until 2001. By the fall of 2005, however, all four study sites were “cache-less.”
Management Implications
The ponds and canals at the three remaining sites were carefully maintained over the 50-year period following colony extirpation. Eutrophication, however, is slowly taking place, filling the ponds with organic debris and allowing them to become invaded with vegetation. Most ponds were found to be too shallow to sustain beaver activity throughout the winter. Additionally, no fish were observed.
The areas bordering the ponds received sustained grazing by cattle from early spring to late fall. Stem cutting by beaver and intense browsing by livestock or native ungulates can strongly suppress re-growth and may result in declining riparian plants. Summer livestock browsing that congregate along riparian areas can be particularly detrimental to recovery of beaver-cut willows. (Kindschy, 1989)
By 2005, no new food caches were located in any of the colony areas indicating that suitability levels for over-wintering may be receding. Although beaver have continued to maintain the dams and circulate though the old, established pond systems and riparian areas for food, now largely willows and sedges, many of the shallow impoundments are following a natural progression and turning into what has often been called “ beaver meadows.” In his study of abandoned beaver ponds in Colorado, Neff (1957) found that they were quickly invaded by vegetation and within a few years became valuable as grazing lands for livestock and wildlife. These ponds are following that trajectory.
The old adage that “the scale of observation creates the phenomenon” is appropriate to this study. When one observes this high-mountain terrain, with its “beaver valleys and meadows” from a distance, little has changed in past decades. Photo documentation reveals that the dam and pond structures and the aspen and willow boundaries have remained stable. Exceptions were the growth and encroachment of conifers. Looking closely, however, one can now see the absence of healthy beaver habitat and a decline in their population density, available food and depth of pond water.
After five decades, these slow and subtle transitions, photographically documented, offer an “ecological snap shot” illustrating a natural transition of beaver habitat, in some mountainous terrain, from an aquatic environment to a terrestrial system.
Acknowledgements
The original study (1954-55) was sponsored, financed and supervised by the Colorado Cooperative Wildlife Research Unit at Colorado State University and the Colorado Game and Fish Department. Appreciation is extended to William H. Rutherford and Jack D. Remington, former members of the Beaver Investigations Project W-83-R, Federal Aid in Wildlife Restoration for the Colorado Game and Fish Department who reviewed the manuscript. The study was financed by the author.
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