Integrated Carnivore-Ungulate Management: A Case Study in West-Central Montana Gestion Intégrée des Carnivores et des Cervidés: Une Étude de Cas dans le Centre-Ouest du Montana

Abstract

Understanding the effectiveness of harvest regulations to manipulate population abundances is a priority for wildlife managers, and reliable methods are needed to monitor populations. This is particularly true in controversial situations such as integrated carnivore-ungulate management. We used an observational before-after-control-treatment approach to evaluate a case study in west-central Montana, USA, that applied conservative ungulate harvest together with liberalized carnivore harvest to achieve short-term decreases in carnivore abundance and increases in ungulate recruitment. Our study areas included the Bitterroot treatment area and the Clark Fork control area, where mountain lion populations (Felis concolor) were managed for a 30% reduction and for stability, respectively. The goals of the mountain lion harvest were to provide a short-term reduction of mountain lion predation on elk (Cervus canadensis) calves and an increase in elk recruitment, elk population growth rate, and ultimately elk abundance. We estimated mountain lion population abundance in the Bitterroot treatment and Clark Fork control areas before and 4 years after implementation of the 2012 harvest treatment. We developed a multi-strata spatial capture-recapture model that integrated recapture and telemetry data to evaluate mountain lion population responses to harvest changes. Mountain lion abundance declined with increasing harvest in the Bitterroot treatment area from 161 (90% credible interval [CrI] = 104, 233) to 115 (CrI = 69, 173). The proportion of males changed from 0.50 (CrI = 0.33, 0.67) to 0.28 (CrI = 0.17, 0.40), which translated into a decline in the abundance of males, and similar abundances of females (before: males = 80 [CrI = 52, 116], females = 81 [CrI = 52, 117]; after: males = 33 [CrI = 20, 49], females = 82 [CrI = 49, 124]). In the Clark Fork control area, an area twice as large as the Bitterroot treatment area, we found no evidence of changes in overall abundance or proportion of males in the population. The proportion of males changed from 0.42 (CrI = 0.26, 0.58) to 0.39 (CrI = 0.25, 0.54), which translated into similar abundances of males and females (before: males = 24 [CrI = 16, 36], females = 33 [CrI = 21, 39]; after: males = 28 [CrI = 18, 41], females = 44 [CrI = 29, 64]). To evaluate if elk recruitment and population growth rate increased following treatment, we developed an integrated elk population model. We compared recruitment and population growth rate during the 5 years prior to and 5 years following implementation of the mountain lion harvest treatment for 2 elk populations within the Bitterroot treatment area and 2 elk populations within the Clark Fork control area. We found strong evidence that temporal trends differed between the 2 areas. In the Bitterroot treatment area, per capita elk recruitment was stable around an estimated median value of 0.23 (CrI = 0.17, 0.36) in the pre-treatment period (2007–2011), increased immediately after treatment (2013) to 0.42 (CrI = 0.29, 0.56), and then declined to 0.21 (CrI = 0.11, 0.32) in 2017. In contrast, per capita elk recruitment estimates in the Clark Fork control area had similar median values during the pre- (2007–2011: 0.30, CrI = 0.2, 0.35) and post-treatment periods (2013–2017: 0.31, CrI = 0.26, 0.36). These changes in recruitment corresponded to similar changes in elk population growth rate, although population growth rates were also subject to variation due to changing elk harvest. In the Bitterroot treatment area, population growth rates in the pre-treatment period were stable to slightly declining, with an estimated median value of 0.92 (CrI = 0.88, 1.07) in the pre-treatment period (2007–2011). Population growth rate during the post-treatment period increased immediately after treatment (2012: 1.17, CrI = 1.14, 1.20) prior to declining to 1.06 (CrI = 1.04, 1.09) in 2016. In contrast, the median population growth rates were roughly equal in the Clark Fork control area during the pre-treatment period (1.01, CrI = 0.86, 1.09) from 2007 to 2011 and post-treatment period (1.00, CrI = 0.83, 1.15) from 2013 to 2017. Together, these results indicate that the harvest treatment achieved a moderate (i.e., 29%) reduction in mountain lion population abundance within the treatment area that corresponded with short-term increases in elk recruitment and population growth. Elk population demographic responses suggest that the harvest treatment effect was strongest immediately after the mountain lion harvest treatment was implemented and lessened over time as the harvest treatment was reduced. This suggests that the short-term harvest treatment resulted in short-term demographic responses in elk populations, and more sustained harvest treatments would be necessary to achieve longer-term elk population demographic responses. We recommend that wildlife managers seeking to balance carnivore and ungulate population objectives design rigorous carnivore and ungulate population monitoring programs to assess the effects of harvest management programs. Assessing and understanding effects of carnivore harvest management programs will help to set realistic expectations regarding the effects of management programs on carnivore and ungulate populations and allow managers to better design programs to meet desired carnivore and ungulate population objectives.