Bogs as Beaver Habitat in North-Central Minnesota

Abstract

In a 100-km2 area surveyed for beaver activity, one-third of the active colonies were in bogs and two-thirds were in lakes and rivers. From 1979 to 1981, the percent of colonies in bogs increased from 29% to 36%. Of 481 bogs in the study area, 200 (42%) had current or previous history of beaver activity (colony sites and work areas). At 101 bogs (21 %), dam construction in seepage zones created flowages. Most impoundments were built at minerotrophic sites. Moats were used at 99 bogs (21 %), where beavers preferred sedge-moss cover and avoided tall shrub and wooded cover. The results show that bogs are suitable habitat for beavers. INTRODUCTION The suitability of lake and stream habitat for beavers (Castor canadensis Kuhl.) has been well-studied (Retzer et al., 1956; Northcott, 1964, Tufts, 1967). Although beavers frequently colonize lakes and streams that are adjacent to bog areas, few researchers have mentioned that beavers are capable of colonizing bogs that lack open water. Northcott (1964) reported a single instance of a colony in a closed bog, but reference by others to similar phenomena has been anecdotal (Schwintzer and Williams, 1974; Buell and Buell, 1975; Worley, 1981). Marshall and Miguelle (1978) in their review of wildlife on Minnesota peatlands mentioned only that beavers colonize bogs that have existing open water. Although there are few accounts of beavers colonizing bogs, colonies in bogs are locally common in N-central Minnesota, and in some areas they are more prevalent than in lakes or streams. Beavers are well-adapted to aquatic habitats, but their ability to create open water in closed bogs is an unusual situation that may involve specialized behavior. The purpose of this paper is to document the extent of bog use by beavers in a section of Cass Co., Minnesota, and to determine what biological and physical features of bogs are associated with use by beavers. METHODS AND PROCEDURE The 100-km2 study area, located E of Backus in Cass Co., included most of Deerfield Township (T139N R31W) and a few adjacent sections of Hiram, Bull Moose, Powers and Birch Lake townships. The topography is characterized by a rugged terminal moraine belt with ca. 250 small lakes and 500 bogs in depressions between hills. The upland forest is predominantly paper birch (Betula papyrifera) and trembling aspen (Populus trernuloides). I used aerial photographs to detect the 512 bogs larger than 0.12 ha in the study area. Bog areas adjacent to lakes and streams were not included unless the connection was minimal and the bog occupied a separate basin. In the field, 481 bogs were evaluated for the presence of beaver activity and classified according to predominant cover type as sedge-moss, tall shrub or wooded bog. Sedge-moss bogs were characterized by floating mats of sedges (Carex) and/or Spagnum moss and ericaceous shrubs covering most of the basin. Tall shrub bogs were dominated by alder (Alnus rugosa) (the brushy fen of Boelter and Verry, 1977), or by Betula pumila, Larix laricina and Picea mariana -the early seral stages of bog forest on ombrotrophic sites. Wooded bogs were dominated by L. laricina, P mariana and occasionally by trees from later stages. Most bogs in the study area were less than 3 ha and had either monotypic vegetation or predominant cover types that were easily recognizable. Aerial and ground surveys were used to detect both colony sites and work areas at 'Present address: Department of Botany, Louisiana State University, Baton Rouge 70803.