Using bear rub data and spatial capture-recapture models to estimate trend in a brown bear population

www.nature.com/scientificreports OPEN Using bear rub data and spatial capture-recapture models to estimate trend in a brown bear population Katherine C. Kendall 1,2*, Tabitha A. Graves 3, J. Andrew Royle Kevin S. McKelvey 6, John Boulanger7 & John S. Waller8 4 , Amy C. Macleod 5 , Trends in population abundance can be challenging to quantify during range expansion and contraction, when there is spatial variation in trend, or the conservation area is large. We used genetic detection data from natural bear rubbing sites and spatial capture-recapture (SCR) modeling to estimate local density and population growth rates in a grizzly bear population in northwestern Montana, USA. We visited bear rubs to collect hair in 2004, 2009—2012 (3,579—4,802 rubs) and detected 249—355 individual bears each year. We estimated the finite annual population rate of change 2004—2012 was 1.043 (95% CI = 1.017—1.069). Population density shifted from being concentrated in the north in 2004 to a more even distribution across the ecosystem by 2012. Our genetic detection sampling approach coupled with SCR modeling allowed us to estimate spatially variable growth rates of an expanding grizzly bear population and provided insight into how those patterns developed. The ability of SCR to utilize unstructured data and produce spatially explicit maps that indicate where population change is occurring promises to facilitate the monitoring of difficult-to-study species across large spatial areas. Understanding trends in population abundance is fundamental to wildlife management and can be critically important for conservation of at-risk taxa1. Trend can be particularly challenging to quantify when population density is low, the population and its range are expanding or contracting, there is spatial variation in trend within the population, or the conservation area is large or remote; traits common to most forest carnivore populations. Grizzly bears (brown bear; Ursus arctos) epitomize the characteristics that make populations difficult to monitor. They occur at low densities2, are solitary (except while females are accompanied by their young), have large home ranges3, often move long distances in short periods of time4, can exhibit strong behavioural responses to sampling that employs consumable baits and handling, and tend to only survive in remote areas5. Many populations are in decline throughout their Holarctic distribution but some have begun to recover and reoccupy former range in recent years6–10. Estimating bear density can be difficult near such expansion fronts because large differences in densities can occur over short distances and may require fine-scale sampling over large areas11. Bears routinely rub vigorously on trees and other objects, often leaving hair behind. This ubiquitous activity is most likely a form of marking behaviour that functions as chemical signaling to other bears12,13. By exploiting natural rubbing behaviour, it is possible to detect a substantial portion (36%8 to 85%13) of a bear population without the use of bait. Hair samples found at bear rub sites usually have sufficient DNA for genetic determination of species, sex and individual identity13–16. Thus, periodic hair collection from rub objects produces a spatial array of repeated detections for individual bears appropriate for capture-recapture modeling17. When detection is imperfect, capture-recapture modeling provides an optimal approach for estimating population size, trend, and other demographic parameters. 1 U.S. Geological Survey, Northern Rocky Mountain Science Center, West Glacier, Montana, 59936, USA. 2Present address: Ursine Ecological, Columbia Falls, Montana, 59912, USA. 3U.S. Geological Survey, Northern Rocky Mountain Science Center, West Glacier, Montana, 59936, USA. 4U.S. Geological Survey, Patuxent Wildlife Research Center, Laurel, Maryland, 20708, USA. 5Applied Conservation Ecology, University of Alberta, Edmonton, Alberta, T6G 2H1, Canada. 6U.S. Forest Service, Rocky Mountain Research Station, Missoula, MT, 59801, USA. 7Integrated Ecological Research, Nelson, British Columbia, V1L 5T2, Canada. 8U.S. National Park Service, Glacier National Park, West Glacier, Montana, 59936, USA. *email: Scientific Reports | (2019) 9:16804 | https://doi.org/10.1038/s41598-019-52783-5 1 www.nature.com/scientificreports www.nature.com/scientificreports/ The Northern Continental Divide Ecosystem (NCDE) in northwestern Montana, USA, is home to one of five North American grizzly bear populations persisting south of Canada. All were designated threatened in 19755. Bear density in the NCDE in 2004 was highest in Glacier National Park (NP) in the north with density declining to the south and to the edges of occupied range8,18. Along the southern boundary of Glacier NP, the presence of a busy highway (US 2) impacts bear crossings19. Gene flow is reduced across the corridor that contains the western portion of US 2 and the rail line that parallels it8. Population size in the NCDE was estimated to have increased from 765 in 20048 to approximately 1,000 by 200920. Because this trend estimate was based on a small portion (2–4%) of the population, which increases the potential for bias, we sought to increase the proportion sampled and improve insight on population status by detecting individual bears from hair at natural bear rub sites. The purpose of our study was to explore this new approach to monitoring the trajectory of a low-density carnivore population at an ecosystem scale and to estimate variation in local density and growth rates within the population using a spatial capture-recapture modeling framework. Methods Field methods. We sampled the NCDE grizzly bear population in a 33,300-km2 area dominated by the rug- ged and remote terrain of the Rocky Mountains (Fig. 1a). We collected hair at bear rubs sites identified by looking for the presence of hair on trees and other objects while searching along trails, roads, and utility and fence lines. Hair deposition was a result of natural behaviour; no attractant was used to draw bears to survey routes or to encourage rubbing. Rubs were fitted with 3 to 4 40-cm lengths of 4-point barbed wire to facilitate hair deposition8. We monitored the rubs we located in 2004 and 2009–2012 (see Supplementary Information online for additional details). We quantified rub sampling effort as the cumulative number of days between the first collection visit (after the initial visit to clear hair that may have been deposited the previous year) and last collection of the year for each rub sampled per year (Table 1). Sampling began in early June in all years, ended in early September in 2004, and most sampling ended in early October 2009–2012. Genetic analysis. Hair samples were analyzed by a laboratory that specialized in genotyping low quantity and quality DNA (Wildlife Genetics International; http://www.wildlifegenetics.ca). We used the G10J marker to distinguish between grizzl (...truncated)