Ecological interactions of river otters and beavers in a boreal ecosystem

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Some ecological interactions of river otters (Lutra canadensis) and beavers (Castor canadensis) were investigated over 4 years as part of a long term study of river otter ecology in a mixedwood boreal ecosystem. Densities of both species were moderately high in the northeastern Alberta study area. Three null hypotheses were investigated: (i) otters do not eat beavers, (ii) otters do not actively manipulate water levels in beaver ponds, and (iii) otters do not parasitize beavers for den sites. Twenty-six otters were captured alive and 13 were telemetered for at least one winter. Thirty-eight beavers were captured alive and 12 were telemetered for at least one winter. The incidence of beaver in the otters' diet was studied by scat analysis and survival of marked beavers in otter winter ranges. A systematic monthly collection of 1,140 otter seats indicated that beavers are a rare food item, occurring in only 0 .4% of the otter seats. Incidental collection of all otter seats with mammalian content (22 containing beaver), and the timing of appearance of 6 beaver carcasses eaten in part by otters, suggested that beavers may most frequently occur in the otters' diet in March and April. In general, predation by otters is not a large source of mortality for beavers. Some of the beavers eaten by otters may have been scavenged. A similar degree of overwinter drop in water levels was documented in all types of beaver ponds irrespective of the extent of beaver repairs the previous fall. Investigations of dams in the spring revealed certain sites of water loss, twice as deep as wide, which were positively associated with the otters' ability to access the particular pond and pass through the dam under the ice during the previous winter. Such sites were not found in dams holding ponds which were never used by otters. Sticks exposed in these passages ha d parallel grooves separated by distances significantly similar to the distances separating the canine teeth in otter skulls. It is concluded that the otters actively dig at beaver dams, thereby manipulating water levels, primarily to insure under-ice access between adjacent ponds. Secondary adaptive values of this behaviour are discussed, as are the potential maladaptive consequences, such as declining fish populations in ponds where beavers do not repair the dams. A review of winter den site use by otters and beavers showed that otters very rarely use active lodges but do use many inactive lodges. Limited data suggest that otter (s) will occasionally parasitize a den used by beavers in the fall when both species have used the same den in the previous open water season and are less flexible in den choice in anticipation of winter ice cover. A commensal symbiosis seems to exist between the two species, with river otters benefitting from the previous activities of beavers in provision of den sites and enhancement of habitat quality for fish by impounding low-order streams. Tests are proposed to further investigate this commensalism. Beavers are rarely exploited by otters either through predation or den parasitism. The possible indirect but harmful effects of otter-induced water drawdown are discussed . Management implications of the observed interactions are discussed, including management and trapping strategies which would promote either beaver or otter populations in the presence of the other species.
Bibliography: p. 191-199.
Reid, D. G. (1984). Ecological interactions of river otters and beavers in a boreal ecosystem (Unpublished master's thesis). University of Calgary, Calgary, AB. doi:10.11575/PRISM/16279