Abundance and genetic structure of two black bear populations prior to highway construction in eastern North Carolina

A 19.3-km segment of U.S. Highway 64 in Washington County, North Carolina is being re-routed and upgraded to a 4-lane highway in what is considered to be crucial black bear (Ursus americanus) habitat. To lessen the impacts on bears and other wildlife, 3 passageways are currently under construction in this segment of highway. In 2000, I began research on a project to determine the effects of the 4-lane highway on black bear ecology and to determine whether wildlife passageways can mitigate impacts of such highways. Research was divided into 2 phases: before highway construction (phase I; June 2000-June 2001) and after the anticipated completion of the highway (phase II; 2005). In addition to the treatment area (area of the proposed highway) a control area was established. I collected baseline information during phase I by estimating population abundance and density on the 2 study areas with DNA from hair samples collected from barbed-wire enclosures. Microsatellite analysis of the DNA was used to identify individual bears for mark-recapture estimates. I examined 3 different groups of closed population estimation models (modified Lincoln-Petersen, Bailey's triple catch, and multiple mark-recapture models). A multiple mark-recapture model with varying capture probabilities provided the best fit to the data and produced the most precise population estimates. Population estimates for the treatment and control areas were 85 (1.20 bears/km2) and 165 (1.78 bears/km2), respectively. Differences in density may be related to differences in hunting pressure; however, both densities were high because of large tracts of forests interspersed with agricultural food crops. Additionally, I used the microsatellite data to compare genetic relatedness, population structure, and gene flow between the treatment and control areas, between areas north and south of the proposed highway within the treatment area, and between areas north and south of a fictitious highway within the control area. The genetic distance between the north and south populations within the treatment area was greater than the genetic distance between the treatment and control areas and between areas north and south of the fictitious highway within the control area. Unrooted neighbor-joining trees constructed from the genetic distance analyses suggested some subpopulation structure. The FST values (0.029, 0.040, and 0.006 for the 3 scales, respectively), which measure the reduction in heterozygosity of subpopulations relative to the entire population, indicated that the amount of population structure was low, but slightly greater within the treatment area than between the 2 study areas or within the control area. Gene flow rates also supported those results, with fewer individuals traveling north and south of the proposed highway (3.3 migrants per generation) than between the 2 study areas (4.6 migrants per generation) or between the north and south populations within the control area (7.6 migrants per generation). I examined isolation-by-distance with Mantel tests to determine relationships between genetic distance and geographic distance based on sample locations. Male bears showed less isolation-by-distance (r = 0.06) than female bears (r = 0.26), probably because of larger home range sizes, which may also explain why the overall population structure was low. Thus, because of the relatively small scale of the study, the unrooted trees likely depict family structure rather than population structure. Finally, I used a geographic information system (GIS) to map allelic diversity throughout the 2 study areas. Allelic diversity was positively correlated with forest density (r = 0.29), indicating that the size and, possibly, the connectivity of habitat patches plays a role in gene exchange, even on a small scale. The density and genetic structure patterns I observed for black bears on the 2 study areas before highway construction will be crucial to understanding the potential impacts of the highway on black bear ecology. However, it may be some time before potential genetic impacts can be detected once highway construction is completed.