Characteristics of Dispersal by Sub-adult Beavers in an Illinois Watershed
 

Erin Cleere and Thomas Nelson

Department of Biological Sciences, Eastern Illinois University, Charleston, IL

Introduction

Beavers in North America, Castor canadensis, were trapped almost to extinction throughout much of the 1800’s and early 1900’s. By 1929, the Illinois DNR had not only imposed strict trapping regulations, but also began reintroducing beavers to rebuild local populations. Within 15 years, beaver populations had recovered noticeably. Currently, beavers occupy most of the suitable habitat statewide.
 

Beavers are important economically and ecologically because of their ability to create valuable wetland habitat. Unfortunately, along with the advantages provided by beaver dams and ponds, damage caused by flooding crops and roads, and cutting valuable trees have increased significantly in recent years. Preliminary evidence suggests that damage is often caused by subadults when they attempt to colonize marginal habitat (ditches, farm ponds, urban waterways).
 

Natal dispersal in beavers (to find a mate and establish a territory), generally occurs between the ages of 2 and 3 years old. The paucity of available habitat in central Illinois may cause subadults to delay dispersal. This affects the number of breeding individuals, which impacts the effective population size. However, beavers are monogamous, and, with few exceptions, only the adult male and female breed in the colony. Regardless of available territory, the lack of reproductive opportunities may force subadults to disperse. Understanding the rates, timing, distances moved, and success of dispersal in subadults may provide insights that allow for better management of beavers.

Objectives 

Determine subadult dispersal rates in a saturated riverine habitat.

Examine whether dispersal rates or distances differ between sexes.

Estimate mortality rates for dispersers and non-dispersers.

Methods

Beavers were trapped during the fall and winter of 2002-03 and 2003-04 using snares. Captured animals were anesthetized to facilitate handling and tagging. They were weighed, sexed, tagged, and released after recovery. Age was determined by weight. McTaggart (2001) reported weights as: kits 7-11 kg, yearlings 11-16 kg, 2-year olds 16-19 kg, and adults 19-26 kg. Subadults (yearlings and 2-year olds) were radio-tagged with tail-mounted transmitters. Animals were placed in protective bins and released when fully recovered.
 

Tagged beavers were radio-located >3 times per week during the dispersal season (February – June). From July through December, as dispersal activity decreased, monitoring was reduced to two locations per week.

Tagging Beavers

Results 

Of the 7 tagged beavers who have gone through one full dispersal season, 42.8% dispersed from their natal territory. The majority of dispersal occurred in late-February, which coincided with ice-melt on the rivers. The earliest date of dispersal was January 31st and the latest was March 18th.
 

All 3 dispersers in 2003 were males. No marked females dispersed from their natal home ranges, although one did travel south 15 km during March and April, before coming back to her natal territory. The shortest distance dispersed by any individual was 5 km, whereas the longest was 80 km.
 

Dispersers moved exclusively along the river and its tributaries. To date, I have not seen evidence of any overland movement.

Mortality rates for both dispersers and non-dispersers were low. One disperser was harvested by a nuisance trapper during dispersal season. The other two dispersers, as well as the non-dispersers, were still alive as of December 2003 when the batteries in their transmitters failed.
 

During September-December 2003, we trapped and tagged another 13 beavers with radio-transmitters. Information on the locations of their natal lodges and daily foraging movements during winter have been tabulated. Through March 1, 2004, none of these individuals had dispersed.

Prospectus

Another 13 beavers were tagged this past fall. Their dispersal movements will be radio-tracked, compiled with first year data, and analyzed. I will do vegetation sampling to compare natal lodge characteristics with newly settled territories of dispersers. I will record the proximity and number of neighboring colonies to those that have tagged animals in them.
 

Data on rates and distances of dispersal will help us to better understand the ecology of beavers in a riverine habitat in Illinois. We can then use this information to establish management and conservation goals to work towards mitigating human/beaver conflicts.

Acknowledgements

We wish to thank Bob Bluett (IDNR), Stacy Dunn, Brian Towey, Brian Richardson, Dennis Coulter, and Deb Breitenbach for assistance in the field.  Funding for this study was provided by the Federal Aid to Fish and Wildlife Restoration Program, Project W-135-R.