Graduate Program

Biological Sciences

Degree Name

Master of Science (MS)

Semester of Degree Completion

2005

Thesis Director

Thomas Nelson

Thesis Committee Member

Eric Bollinger

Thesis Committee Member

Robert Fischer

Abstract

Dispersal is an important mechanism influencing the dynamics, genetics, spatial distribution and size of wildlife populations (Gese and Mech 1991). Natal dispersal may afford individuals breeding opportunities and access to resources, while reducing inbreeding and competition with family members. However, dispersal also can be risky if mortality rates are high or if mates or suitable habitat are unavailable. When dispersal costs are high, subadults may be better off remaining in their natal territory.

Although a number of previous studies have investigated natal dispersal in beavers (Castor canadensis), conclusions regarding the percentage of subadults that disperse, timing of dispersal, evidence of sex-bias, and dispersal distances have differed depending upon the location of the study, characteristics of the research population, and the environments they inhabit. When I initiated this study, few ecological studies on beavers had been conducted in Illinois or the lower Midwest and the patterns and characteristics of beaver dispersal in this region were not well known. Therefore, the objectives of my study were to: (1) the percentage of subadult (yearling and two-year old) beavers that disperse, (2) the timing of dispersal and distances traveled, and (3) difference in dispersal rates or distances between males and females, and (4) mortality rates of dispersers and causes of death. During this 2-year study, I radio-tagged 18 subadult (yearling and 2-year-old) beavers, including 10 males and 8 females. Age was estimated by body weight. Tagged beavers were located at least 3 times each week during the dispersal season from February through May. The overall rate of dispersal during this 2-year study was 33.3%; 42.8% during the first year and 27.3% during the second year.

The majority of dispersers (83.3%) were 2-year-old males; no yearlings dispersed. The median dispersal date was 22 February; the earliest dispersal was initiated on 23 January and the latest was 15 April. The shortest dispersal distance was 2.2 km, whereas the longest was 80 km. The median dispersal distance was 17 km. Mortality rates for both dispersing and non-dispersing beavers were low: 16.7% for dispersers and 8.3% for non-dispersers. All mortalities in both groups were attributed to trappers harvesting for fur or to remove nuisance animals.

My results suggest that a large proportion of subadults delay dispersal until they are at least 3-years-old, and that male subadults are more likely to disperse than females. This may be due to a high population density and saturated habitat on the study area, females delaying breeding until they are older and have higher fecundity, or a combination of these factors. With few natural predators or apparent diseases limiting this population, trapping is the primary mortality factor. In the absence of trapping, beaver populations would likely be limited by social constraints on breeding and the availability of habitat. Trapping probably is not necessary to keep beaver populations in check, but it is an important tool for removing nuisance animals. On my study area, there was a surplus of potential breeders capable of replacing individuals removed by trappers. Trapping creates vacant territories and removes some older breeders allowing younger animals the opportunity to breed. This in turn is likely to reduce the dispersal by subadults into marginal habitat and subsequent nuisance problems.

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