Academic journal article The American Midland Naturalist

Dispersal and Survival of Juvenile Beavers (Castor Canadensis) in Southern Illinois

Academic journal article The American Midland Naturalist

Dispersal and Survival of Juvenile Beavers (Castor Canadensis) in Southern Illinois

Article excerpt

ABSTRACT.-

We used radiotelemetry to estimate natal dispersal patterns and survival rates of 13 yearling and 19 subadult beavers (Castor canadensis) at two geomorphologically different sites in southern Illinois. Overall, we observed a 55% dispersal rate for yearlings and a 73% dispersal rate for subadults. Normally, juveniles (yearlings + subadults) initiated dispersal around 16 February (range = 28 Jan.-20 Mar.) and settled around 18 April, with juveniles remaining transient from late January through late June. Nine subadult beavers dispersed significandy earlier at one site (x = 25 Nov., range = 31 Oct.-16 Dec.), presumably due to intracolony strife brought on by management-induced autumn flooding of wetlands for waterfowl. Dispersal occurred earlier than documented in northern regions. Beavers dispersed farther from natal colonies with free-flowing water access (x = 5.9 km) than those landlocked (x = 1.7 km). Males moved more frequently and traveled greater distances per individual move than did females, but overall dispersal distances were similar between sexes. Dispersal distances of beavers with access to free-flowing waterways generally were similar to other studies. Survival during dispersal was greater for beavers emigrating from lodges on the land-locked, less densely populated site (0.80), than for those dispersing in an area of higher population densities (0.43), but did not differ between dispersers and non-dispersers at either site.

INTRODUCTION

Dispersal and survival patterns have profound effects on population demography (Lidicker, 1975; Sun et al, 2000). Dispersal affects composition, size and spacing of populations which, in turn, affects productivity of that species (Lidicker, 1975). Further, dispersal provides the primary means by which most mammals occupy new areas or areas where populations have been reduced. Beaver dispersal can potentially affect species management as well. Van Deelen (1991) noted colony-based indices were insufficient to estimate total beaver populations because they often ignored dispersing or transient individuals. These transients have the potential to significantly bias estimates garnered from methods historically used to survey beaver abundance.

Previous studies of juvenile beaver dispersal and survival have been conducted in the northern portions of the species range (Leege, 1968; Jackson, 1990; Van Deelen, 1991); few have been conducted in the Midwest (McTaggart and Nelson, 2003) and southern portion (Davis, 1984; Weaver, 1986) of their North American range. Information gathered regarding dispersal and survival in boreal systems probably is not applicable to the southern region because of differences in habitat and climate as well as differences in predatory species and harvest pressure. Further, few studies in the Midwest (none in Illinois) have used radiotelemetry to determine the fates of dispersing juvenile beavers. This study was designed to address dispersal patterns and survival rates of beavers in southern Illinois. The objectives of our study were to estimate distances and relative timing of juvenile beaver dispersal, test whether dispersal patterns differed between sexes and landscapes and estimate survival and cause-specific mortality rates during the dispersal period.

MATERIALSAND METHODS

Study sites.-Research was conducted at 2 locations in southern Illinois, Amax Delta (AD) reclaimed surface mine and Union County Conservation Area (UCCA) between August 2001 and July 2003. The 543-ha AD site was located in the Southern Till Plain Division and UCCA in the Lower Mississippi River Bottomlands Division (Luman et al., 1996). Intermittent surface mine reservoirs and farm ponds create a pot-hole landscape of suitable beaver habitat within a predominantly agricultural area surrounding the AD site. Dominant vegetation surrounding the waters of the mine was composed of reedgrass (Phragmites australis), buttonbush (Cephalanthus occidentalis) and maples (Acer sp. …

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