Divergent population trends following the cessation of legal grizzly bear hunting in southwestern British Columbia, Canada

Abstract

We conducted DNA capture-recapture monitoring of grizzly bears (Ursus arctos) from 5 to 17 years after hunting was stopped in two adjacent but genetically distinct populations in southwestern British Columbia, Canada. We used spatial capture-recapture and non-spatial Pradel robust design modelling to estimate population density, trends, and the demographic components of population change for each population. The larger population had 21.5 bears/1000 km 2 and was growing (λ Pradel = 1.02 ± 0.02 SE; λ secr = 1.01 ± 4.6 × 10 −5 SE) following the cessation of hunting. The adjacent smaller population had 6.3 bears/1000 km 2 and was likely declining (λ Pradel = 0.95 ± 0.03 SE; λ secr = 0.98 ± 0.02 SE). Estimates of apparent survival and apparent recruitment indicated that lower recruitment was the dominant factor limiting population growth in the smaller population. Factors limiting reproductive rates and population density could include poor habitat quality, particularly the abundance of high-energy foods, genetic Allee effects due to a long period of population isolation, or demographic effects affecting infanticide rates. The cessation of hunting was insufficient to promote population recovery for the low density, isolated population. Our research highlights the importance of considering mortality thresholds in addition to small population effects and habitat quality when recovering large carnivore populations.

Vital rates of two small populations of brown bears in Canada and range‐wide relationship between population size and trend

“…The McGillvary Mountains and North Stein‐Nahatlatch populations are located in the Coast Mountains of British Columbia, Canada (Figure 2). The McGillvary Mountains population has approximately 45 resident bears, and one‐quarter of its geographic perimeter was connected to other bear populations to the north, while the North Stein‐Nahatlatch population has fewer than 25 individuals and appeared genetically isolated (Apps et al., 2014; McLellan et al., 2019). The western portion of the study area is a temperate rain forest dominated by western red cedar ( Thuja plicata ) and western hemlock ( Tsuga heterophylla ).…”

“…We estimated survival (S) for adult and subadult female bears using the staggered entry design for the Kaplan–Meier estimator (Pollock et al., 1989). We considered population (S~Pop) and age class (S~Age) as categorical covariates in candidate models along with the null model (S~1) because we were interested in establishing whether the previously identified differences in population trend (McLellan et al., 2019) were indicative of differences in population‐specific survival and because survival often differs among age classes (Mace et al., 2012; McLellan, 2015; Schwartz et al., 2006). We used months as our monitoring interval from April to October when most bears were active (McLellan & McLellan, 2015) and amalgamated November through March into one monitoring interval because monthly mortality would not be distinguishable when bears were hibernating.…”

“…Specifically, the larger, higher density McGillvary Mountains population, with some genetic and demographic connectivity to other large populations, was estimated to increase at 2% each year while the adjacent, small (<25 bears) and F I G U R E 1 Mother brown bear and offspring in a waterhole. These bears are part of the growing McGillvary Mountains population in southwest British Columbia, Canada long-isolated (Apps et al, 2014;McLellan et al, 2019) North Stein-Nahatlatch population declined by approximately 5% each year. We then compare our population growth estimates to those published for other small (<300 individuals) brown bear populations across their distribution using a literature review.…”

“…Here, we use data collected from telemetered brown bears to estimate causes of mortality, age‐specific survival probabilities, reproductive rates (age of primiparity, litter size, and interbirth interval), and population growth in two adjacent but genetically separate populations at the southern edge of brown bear distribution along the Coast Mountains of British Columbia, Canada. Previous research using genetic‐based mark–recapture monitoring of these populations from 2005 to 2017 identified divergent population trends following the cessation of legal hunting (i.e., human‐caused mortality) in 2000 (McLellan et al., 2019). Specifically, the larger, higher density McGillvary Mountains population, with some genetic and demographic connectivity to other large populations, was estimated to increase at 2% each year while the adjacent, small (<25 bears) and long‐isolated (Apps et al., 2014; McLellan et al., 2019) North Stein‐Nahatlatch population declined by approximately 5% each year.…”

“…Previous research using genetic‐based mark–recapture monitoring of these populations from 2005 to 2017 identified divergent population trends following the cessation of legal hunting (i.e., human‐caused mortality) in 2000 (McLellan et al., 2019). Specifically, the larger, higher density McGillvary Mountains population, with some genetic and demographic connectivity to other large populations, was estimated to increase at 2% each year while the adjacent, small (<25 bears) and long‐isolated (Apps et al., 2014; McLellan et al., 2019) North Stein‐Nahatlatch population declined by approximately 5% each year. We then compare our population growth estimates to those published for other small (<300 individuals) brown bear populations across their distribution using a literature review.…”