Transition to independence by subadult beavers (Castor canadensis) in an unexploited, exponentially growing population

Abstract

We conducted a 4-year study of beavers Castor canadensis to compare the movements, survival and habitat of adults established in existing colonies to juveniles dispersing to new sites in a region with high beaver densities along a suburban-rural gradient. Estimates of annual survival were high for adult and juvenile beavers. Of nine known mortalities, seven (78%) were juveniles. Mortalities occurred during spring-summer, and none during fall-winter. There was a trend toward higher-to-lower survival along the suburban-rural gradient, respectively. Human-induced mortality (e.g. trapping and shooting) was higher in rural areas, whereas nonhuman-induced mortality (e.g. disease, accidents) was higher in suburban areas. Fifteen (14 subadults and one adult) beavers moved from natal colonies to other areas. The average dispersal distance for subadults was 4.5 km (SE = 1.0) along streams or rivers, or 3.5 km (SE = 0.7) straight-line point-to-point. Most dispersal movements were made in spring (April-June). In two cases, individual subadults made return movements from their dispersal sites back to their natal colonies. Dispersal sites tended to be in smaller, shallower wetlands or streams and in areas with higher overstorey canopy closure compared with natal colonies. Woody vegetation usually preferred by beavers for food tended to be less common at dispersal sites than at natal colonies. In regions with high densities of beaver, dispersing juveniles are likely to attempt to colonize lower quality sites. High densities of beavers also lead to more human-beaver conflicts and, in Massachusetts, the pest control management options in place during the past decade have been ineffectual at controlling population levels. Alternately, in regions with no beavers or very low densities and where reintroductions are being attempted, the landscape matrix surrounding release sites should include suitable sites for dispersing young to establish colonies.

Demography of Unexploited Beavers in Southern Illinois

“…In central Illinois, Havens (2006) reported 43% of beaver mortalities were from tularemia. Predation accounted for 13% of the mortalities in our study as compared with the 22% value reported by DeStefano et al (2006). We report survival rates of beavers higher than those presented by other researchers, likely because of the lack of harvest on the UCCA (Table 6).…”

“…Seasonal weather patterns likely affected these trends in survival and explained differences in seasonal survival among regions. For example, in Massachusetts, USA, spring-to-summer survival was lower than fall-towinter survival for an unexploited beaver population (DeStefano et al 2006). During fall and winter, beaver ponds were more likely iced over in Massachusetts, and as a result, beavers stayed in close proximity to their lodge.…”

“…Beaver studies have been limited in their use of research techniques commonly employed for other wildlife species. Unlike many abundant mammalian wildlife species, such as white-tailed deer (Odocoileus virginianus) and raccoons (Procyon lotor), few radiotelemetry studies have been conducted on beavers because, until recently, a practical method of attaching a radiotransmitter to beavers did not exist (Rothmeyer et al 2002, McNew and Woolf 2005, DeStefano et al 2006. Recently, new remote-camera technology has been developed to assess demography of wildlife populations (VerCauteren et al 2002, White et al 2007, Bloomquist and Nielsen 2009; these techniques show much promise for studying colonial, lodge-dwelling species, such as the beaver.…”

Transition from maternal provisioning: crunch time in vertebrate life histories?

“…Although DeStefano et al . (2006, this issue) point out the difficulties in determining stage‐specific mortality for mammals such as the North American beaver, which have very low overall mortality rates, high mortality during transition appears to be common.…”

“…Habitats used by juveniles are often distinct from those used by adults. Increased vulnerability to both predators and adverse physical conditions appear to be important in habitat selection (Hannon & Martin, 2006, this issue); however, some differences between adult and juvenile habitats appear to be the result of competition between adults and juveniles (DeStefano et al ., 2006, this issue), possibly resulting in density dependence within the transition stage. Although such habitat differences are obvious in taxa with complex life cycles and indeterminate growth, they are also present in fledgling songbird species that nest in forest interiors but rear juveniles in early‐successional habitat where reduced predation results in increased juvenile survival (King et al ., 2006, this issue).…”