Demography and Genetic Structure of a Recovering Grizzly Bear Population

Abstract

Grizzly bears (brown bears; Ursus arctos) are imperiled in the southern extent of their range worldwide. The threatened population in northwestern Montana, USA, has been managed for recovery since 1975; yet, no rigorous data were available to monitor program success. We used data from a large noninvasive genetic sampling effort conducted in 2004 and 33 years of physical captures to assess abundance, distribution, and genetic health of this population. We combined data from our 3 sampling methods (hair trap, bear rub, and physical capture) to construct individual bear encounter histories for use in Huggins—Pledger closed mark—recapture models. Our population estimate, Ň = 765 (95% CI = 715–831) was more than double the existing estimate derived from sightings of females with young. Based on our results, the estimated known, human‐caused mortality rate in 2004 was 4.6% (95% CI = 4.2–4.9%), slightly above the 4% considered sustainable; however, the high proportion of female mortalities raises concern. We used location data from telemetry, confirmed sightings, and genetic sampling to estimate occupied habitat. We found that grizzly bears occupied 33,480 km2 in the Northern Continental Divide Ecosystem (NCDE) during 1994–2007, including 10,340 km2 beyond the Recovery Zone. We used factorial correspondence analysis to identify potential barriers to gene flow within this population. Our results suggested that genetic interchange recently increased in areas with low gene flow in the past; however, we also detected evidence of incipient fragmentation across the major transportation corridor in this ecosystem. Our results suggest that the NCDE population is faring better than previously thought, and they highlight the need for a more rigorous monitoring program.

Counts of unique females with cubs in the Apennine brown bear population, 2006–2014

“…In contrast to similar hair-snagging surveys (e.g., Kendall et al 2009), in our bear population cubs are apparently inaccessible to hair-snagging during spring and summer using traditional 50 m-high barbed wire traps (Gervasi et al 2012). Along with the overall lower capture probability of cubs (Kendall et al 2009), this is possibly due to their smaller size compared to other brown bear populations, their particularly restricted movements, and the elusive behavior of their mothers, including their lower attraction to lured traps. As a result, no cubs were detected using hair-snagging in previous surveys (Gervasi et al 2012, although cubs were indeed confirmed in the population in the same years.…”

“…We collected each sample possibly containing guard hairs with bulbs with sterilized surgical forceps, and placed each sample in a paper envelope labeled with a uniquely numbered barcode. We then passed a flame under the barbs to remove any trace of hair to avoid contamination between sessions (Kendall et al 2009). Paper envelopes containing samples where then stored in a dark place within a box with silica gel to avoid DNA degradation.…”

“…As outlined above, in these cases we subsampled the one sample with more guard hairs and bulbs, even though we often collected up to 2-3 additional samples farthest apart from each other to be used as replicated samples in case the other would not yield a reliable genotype. Similarly to other noninvasive genetic surveys that adopted rub-tree sampling (i.e., Kendall et al 2009), we started rub-tree sampling in late May and extended sampling through September. Once activated by installing the barbed wire, we visited rub trees every 12-15 days.…”

“…Genetic analyses were conducted at Wildlife Genetic International (WGI) using quality assurance protocols (Paetkau 2003) that have been shown to ensure accurate individual identification (Kendall et al , 2009). As part of the 2011 survey (Paetkau 2012 we: (a) selected 13 microsatellite markers adequate for individual identification for this bear population, (b) provided empirical evidence of the ideal set of markers to be used in subsequent noninvasive genetic surveys, and (c) provided calibration factors between WGI and the previous Italian lab (ISPRA) as to allow comparison of recent vs. previous genotypes detected in the bear population.…”

A Noninvasive Hair Sampling Technique to Obtain High Quality DNA from Elusive Small Mammals

“…A number of studies have reported using noninvasive sampling techniques to investigate population genetics and demography of wild populations 1 . This approach has proven to be especially useful when dealing with rare or elusive species 2 .…”