Predator-prey relationships between Rocky Mountain elk and black bears in Northern New Mexico
Quintana, Nicole Tatman
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We conducted a 4-year study (2009–2012) evaluating the role of predation and nutrition in limiting the productivity of an elk (Cervus elaphus) population in northern New Mexico. In the years leading up to the initiation of the study, we observed low (<25:100) calf:cow ratios, suggesting calf recruitment was lower than desired. We sought to identify the reason for low recruitment by assessing the role of predation and nutrition in the population. We captured and fixed ear-tag radio transmitters to 245 elk calves (126M, 119F) to determine cause specific mortality. We quantified summer calf survival using Program MARK and annual survival using Cox Proportional Hazards models. During the second half of our study, we implemented spring black bear (Ursus americanus) harvest that included supplemental take by New Mexico Department of Game and Fish personnel and evaluated the response in calf survival. Across all years of our study we quantified how risk of mortality varied for juvenile elk both spatially and temporally by comparing the landscape surrounding black bear and mountain lion predation sites to sites where elk calves were captured. Simultaneously, we captured 9 black bears in 2011 and 2012 and equipped them with Global Positioning System (GPS) collars to quantify patterns of landscape use. Global positioning collars deployed on black bears obtained multiple locations per day and we evaluated bear habitat use in relation to presence of elk calves on the landscape. We also assessed the nutritional condition of adult female elk by quantifying herd-wide percent ingesta-free body fat (IFBF) and pregnancy rates. To achieve this, sport hunters harvested adult female elk from autumn through winter 2009–2012. We estimated autumn ingesta-free body fat (IFBF) using the Kistner subset score when possible (n = 1,130) or the kidney fat mass method (n = 284) when the Kistner score was not possible. We developed a set of models to explain IFBF of females through autumn and winter. The primary cause of death for calves across all years was black bear predation (57 of 140 non-anthropogenic mortalities). Predation was the primary cause of death for juveniles during their first 3 weeks of life, resulting in 84 of 92 non-anthropogenic mortalities. During this time, black bears were the primary predator (n = 49), but coyotes (Canis latrans, n = 26) and mountain lions (Puma concolor, n = 4) were also predators. Black bear and mountain lion predation sites had higher percent canopy cover (64%, 95% CI=0.531–0.741) than capture sites (19%, 95% CI=0.152–0.220). For every 1% increase in canopy cover, a site was approximately 2 times more likely to be a black bear or mountain lion predation site than a capture site. We suspect that increased predation pressure in the forest edge environment likely influenced selective pressure on maternal elk to choose more open habitats postpartum because they were less risky for juvenile elk early in life. Annual calf survival was greater when spring black bear harvest was moderate to heavy (0.44–0.47) compared to low (0.33–0.35). For every additional bear harvested in spring, radio-tagged elk calves were 2.6% more likely to survive the summer. Though black bear predation is typically considered an additive form of mortality, when we only considered Ursid predation patterns we observed this predation on calves to be dependent on the size of the calf (P = 0.0403, P = 0.00251). This size-dependent predation suggests that ursid predation on elk calves may not have been entirely additive during our study. Black bears fixed with GPS-collars used a variety of vegetation types, demonstrating the generalist nature of black bears in New Mexico. Despite having a small sample size of GPS-collared bears during the calving season (n = 4), we found that black bears tended to have smaller home ranges that overlapped the calving area to a greater extent during the calving season compared to other seasons. This suggests that it is possible that black bears are keying in on elk calves during the calving period. Adult female elk were harvested by sport hunters from October through March 2009– 2012 (n = 1,808). Across years and age classes 82% (SE = 1%) of females were pregnant. Pregnancy rate was greatest for prime aged (2–14 years) females (88%, SE = 1%) and lower for young (<2 years, 11%, SE = 4%) and senescent (>14 years, 47%, SE = 5%) females (χ2 = 267.3, P < 0.001). Our herd-wide estimate of autumn IFBF was 11.41% (SE = 0.19) but varied by age class, pregnancy status, and lactation status. Prime-aged females that were pregnant had greater autumn IFBF (12.51%, SE = 0.51%) than females that were not pregnant (9.95%, SE = 0.21; F1,725 = 88.09; P < 0.001). Ingesta-free body fat decreased as winter progressed (F1,1408 = 58.37; P<0.001), with body fat being an average of 1.29% lower during winter than autumn, but this also depended upon age class and lactation status. We found that IFBF was best explained by a model incorporating both environmental (winter severity and harvest unit) and biological (pregnancy status, lactation status, herd-wide calf survival, and age) covariates. The range of variables deemed significant underscores the importance of considering multiple factors that may influence a large herbivore population and IFBF of adult females in particular. Simple models (those with a single predictor variable) performed worse than models that were more complex, suggesting that IFBF is influenced by a combination of environmental and biological factors. Low calf recruitment despite adequate condition and nutrition of adult females in the study area suggested that substantial black bear predation was limiting population productivity. Despite observing black bear predation that may have been partially compensatory, when spring bear harvest was heavy calves were 1.5 (95% CI = 0.97–2.32) times more likely to die compared to when black bears were heavily harvested (P = 0.068). Results from our study suggest that productivity could be increased by implementing a spring black bear harvest strategy, targeting hunting or removal efforts near calving areas. However, we were unable to sustain higher spring black bear harvest with hunter effort alone. We recommend that a combination of analysis of IFBF on hunter harvested adult female elk and an assessment of cause-specific neonatal survival can be used to assess the limiting nature of predation and nutrition in many settings.