Composition and Demographics of Beaver Colonies in Illinois
 

Stan McTaggart and Tom Nelson

Department of Biological Sciences, Eastern Illinois University

BACKGROUND

Beavers (Castor canadensis) are an important wildlife species because of their commercial and recreational value to trappers, their potential as nuisance animals and their role as a keystone species capable of creating wetland habitats for a variety of other wildlife. For these reasons, this species is managed intensively to maintain populations at levels compatible with available habitat and public acceptance. Such management requires accurate information on population densities and trends. Although beavers occur throughout Illinois, neither population estimates nor direct indices of abundance are available. There is a need to develop better methods to estimate and detect changes in the relative abundance of this species on regional and statewide scales.

Biologists at Eastern Illinois University and Southern Illinois University at Carbondale are working in collaboration with the Illinois Department of Natural Resources to develop a framework for monitoring beaver populations by using helicopter surveys. Beaver colonies and signs of activity can be readily observed from the air. But these are useful only if we know the size and composition of a typical colony and the variation among colonies. This study was conducted to investigate the size, composition and reproductive potential of beaver colonies in Illinois.

OBJECTIVES

1. Estimate the size and sex-age composition of typical beaver colonies.
2. Compare the composition of whole colonies versus that of harvested samples taken by trappers.
3. Estimate the reproductive potential of female beavers in Illinois.
4. Compare the efficacy of night-vision censuses versus removal trapping for estimating colony size.

STUDY AREA

Beaver colonies were located by conducting ground searches of suitable habitats in central and southern Illinois during the 1999 and 2000 trapping seasons. Beavers usually occupied bank dens adjacent to lakes, rivers or ditches (Figure 1); lodges were rare. A random sample of thirty colonies was trapped, such that complete removal of all individuals in each colony was accomplished.

Figure 1. Typical Bank Den

METHODS

Beavers were trapped from research colonies using #330 conibear traps until none remained in the lodge. In addition, we solicited skinned carcasses from local trappers. Because the latter were taken from colonies that were not completely trapped out, this sample was not necessarily representative of the whole population. Each beaver was sexed, aged and weighed. Beavers were aged based on the eruption, degree of basal closure and deposition of cementum annuli on the premolars and molars. Reproductive organs from females were removed and examined for ovarian structures (corpora lutea and corpora albicantia), placental scars and fetuses.

To provide a second estimate of the number of beaver in each colony, we conducted censuses at each colony using night-vision binoculars. These counts were conducted for 2.5 hrs following sunset by an observer sitting 10-15 m downwind from the lodge or den. Beaver were counted as they emerged and moved around the food cache and surrounding area. Only beaver that could be identified as unique individuals were counted, providing a conservative estimate of the number present.

RESULTS

Size and Composition of Beaver Colonies.  A total of 239 beaver was trapped, including 169 from research colonies and 70 taken by cooperating trappers. Males comprised 53-55% of the kit, yearling and adult classes. The mean colony size was 5.6 beavers with individual colonies ranging from 2 to 11. Colonies tended to be largest in lacustrine habitats and smallest in ditches. A typical colony consisted of a pair of breeding adults, 1-2 yearling or older offspring, and 2 kits (Figure 2).

Figure 2.  Sex-age composition of a typical colony.

An additional adult was found in 43% of the colonies. However, we found no correlation between the number of adult “helpers” in the colony and the number of kits raised. Individuals in our sample ranged in age from 0.5 to 15.5 years old. The age distribution of trapped-out colonies differed from those taken by cooperating trappers (P=0.04). Trappers harvested relatively more 1.5 and 2.5 year-olds and fewer older beaver (Figure 3), suggesting that the composition of statewide harvests is not representative of the composition of the whole population.

Figure 3. Trapped beaver age classes.

Reproductive Rates of Female Beavers.  No kits and few yearlings ovulated; however, 53% of 2.5 year-olds and 77% of all older females ovulated. Ovulation rates ranged from 2.5 ova/female in yearlings to 4.4 in older females. Relatively few females (N=22) were trapped late enough in the season to carry fetuses, but of those that did, the mean number of fetuses was 3.4 for 2.5 year olds and 4.3 for older females (Table 1). There was a positive correlation between the weight of the female and the number of ova shed (r=0.64, P=0.01). We estimated prenatal mortality to be 13%. Based on the ages of the breeding females in our research colonies and age-specific fetal rates, these colonies should have produced 110 kits at birth. However, during the winter only 47 kits were present, suggesting that mortality rates during the first 6 months of life is ~57%. The reproductive performance of these beavers is consistent with populations occupying saturated habitat.

Table 1. Age-specific reproductive rates of female beavers harvested in Illinois.

Comparison of Night-Vision Censuses with Removal Trapping.  Night-vision censuses consistently under-estimated the actual size of beaver colonies. The mean number of beaver observed was 2.9; the mean number actually present was 5.3. Night-vision censuses accounted for only 55% of the beavers present. However, a strong positive correlation (r=0.80) was found between the number observed and the number present.  Based on censuses and complete removal trapping at 16 colonies the number of beaver present can be estimated with the equation:

No. of beavers in colony = (1.33 X No. of beavers observed) + 0.5

ACKNOWLEDGEMENTS

This study was funded by the Federal Aid in Fish and Wildlife Restoration Project W-135-R, the Illinois Department of Natural Resources and Eastern Illinois University.