An improved understanding of ungulate population dynamics using count data: Insights from western Montana

Understanding the dynamics of ungulate populations is critical given their ecological and economic importance. In particular, the ability to evaluate the evidence for potential drivers of variation in population trajectories is important for informed management. However, the use of age ratio data (e.g., juveniles:adult females) as an index of variation in population dynamics is hindered by a lack of statistical power and difficult interpretation. Here, we show that the use of a population model based on count, classification and harvest data can dramatically improve the understanding of ungulate population dynamics by: 1) providing estimates of vital rates (e.g., per capita recruitment and population growth) that are easier to interpret and more useful to managers than age ratios and 2) increasing the power to assess potential sources of variation in key vital rates. We used a time series of elk (Cervus canadensis) spring count and classification data (2004 to 2016) and fall harvest data from hunting districts in western Montana to construct a population model to estimate vital rates and assess evidence for an association between a series of environmental covariates and indices of predator abundance on per capita recruitment rates of elk calves. Our results suggest that per capita recruitment rates were negatively associated with cold and wet springs, and severe winters, and positively associated with summer precipitation. In contrast, an analysis of the raw age ratio data failed to detect these relationships. Our approach based on a population model provided estimates of the region-wide mean per capita recruitment rate (mean = 0.25, 90% CI = 0.21, 0.29), temporal variation in hunting-district-specific recruitment rates (minimum = 0.09; 90% CI = [0.07, 0.11], maximum = 0.43; 90% CI = [0.38, 0.48]), and annual population growth rates (minimum = 0.83; 90% CI = [0.78, 0.87], maximum = 1.20; 90% CI = [1.11, 1.29]). We recommend using routinely collected population count and classification data and a population modeling approach rather than interpreting estimated age ratios as a substantial improvement in understanding population dynamics.

Reproductive senescence in free-ranging North American elk Cervus elaphus Cervidae

Successful production of calves is necessary for growth of North American elk (Cervus elaphus Linnaeus 1758) populations, but few studies have evaluated age-related effects on both the conception and survival of a calf to weaning in multiple free-ranging populations. Conception and survival of calves to weaning were both affected by maternal age, with old (age 9 and older) females showing reproductive senescence as compared to prime-aged (ages 2–8) females despite achieving similar or greater size and condition. Reproductive senescence in our free-ranging populations ultimately resulted in old females weaning fewer calves (0.42 calves/female) than did prime-aged females (0.64 calves/female). Other factors, especially maternal size, also influenced conception and survival to weaning, and these interacted with age in a consistent manner, i.e. larger females or females in better condition were more likely to conceive and successfully wean calves within each age class. Female age structure receives less consideration in ungulate management than does male age structure, despite demonstrated impacts on population productivity of multiple species because of reproductive senescence. Because of the large proportion of individuals in senesced age classes in elk populations, low productivity in populations may simply reflect female age structure, rather than other frequently hypothesized factors.

Population Dynamics Model to Inform Harvest Management of a Small Elk Herd in Central New Mexico

Crop depredation by wildlife is a frequent concern for natural resource managers and mitigation of this issue is often an important task for wildlife agencies. Elk Cervus elaphus and other ungulate species have depredated corn Zea mays at Bosque del Apache National Wildlife Refuge, New Mexico, USA, interfering with the ability of the Refuge to provide sufficient supplemental nutrition to overwintering sandhill cranes Antigone canadensis and geese (Anatidae). We estimated annual adult survival and calf recruitment rates of elk from 2011 to 2013 at Bosque del Apache National Wildlife Refuge. Natural adult survival (excludes human-related mortalities) was high (mean = 98.3%; 95% CI = 95.0–100.0%). Calf recruitment was lower than in some populations, and ranged from 13.0 to 36.7 calves : 100 cows at time of recruitment (March and April) with a mean of 21.9 (SD =12.9). Using this information, we constructed a harvest management model to determine annual harvest quotas required to stabilize the growth of the elk herd on the Refuge. The female segment of the herd is growing at an annual rate of 9.0% (95% CI = −1.1–24.1%). To stabilize the growth rate of the female elk population, 8.0% (95% CI = −1.1–19.4%) of the cows would need to be harvested annually. We estimated an adult elk abundance of 40.0 (SE = 4.57; 95% CI = 33.8–52.6) in 2012 and 61.1 (SE = 7.21; 95% CI = 49.9–78.8) in 2013. Our harvest management model provides Refuge staff, who ultimately intend to improve corn yield, with valuable information needed to stabilize the elk herd. Further, our approach outlines a simple, easily implemented modeling technique that can be used for the management of other ungulate herds.

Population Demographics and Dynamics of Colonizing Elk in a Desert Grassland–Scrubland

Elk Cervus elaphus are rare in Southwestern desert grassland and desert-scrub habitats, and these habitats are anecdotally considered unsuitable for elk. We studied a colonizing herd in a Southwestern desert grassland–scrubland in northwestern New Mexico to determine the condition and population dynamics of elk in this habitat type. We radiotracked ≤ 24 adult (1.5 y and older) cows and ≤ 8 calves annually, 2003–2006; the sample of radiocollared cows in this study was very close to a census of the entire population for 2004–2006 based on repeated surveys and monitoring. Mean body fat of lactating cows in autumn was 10.6–13.2% in 2003–2004, indicating that lactating elk were acquiring high moderate–low good nutrition, but dropped to 5.6% in 2005 and 6.8% in 2006, indicating poor nutrition for these years of low precipitation. We found adult female survival of 0.94–1.00 and calf survival of 0.38–1.00. Calf survival was related to maternal size, maternal condition, and cumulative annual precipitation through parturition; all calf mortality occurred at or near parturition. Pregnancy rates averaged 0.33 for yearling cows and 0.86 for ≥ 2.5-y-olds, 2003–2006, and pregnancy was positively related to body mass for ≥ 2.5-y-old adults. Lactation rates of ≥ 2.5-y-olds averaged 0.59, 2003–2005, but dropped to 0.30 in 2006, due to poor maternal condition and low precipitation. Since colonization, elk have increased from about 20 to ≥ 53 individuals, a mean rate of increase of 18%/y prior to 2006, but decreasing to ≤ 4% in 2006. The high sustained rate of increase (with few exceptions) indicates that desert grassland–scrubland habitats are suitable for elk and merit consideration in elk management plans of Southwestern agencies.