Nutrition, predation, and parasitism as drivers of moose population dynamics in Montana

Abstract Effective wildlife management relies upon understanding a hierarchy of demographic characteristics of wildlife populations, including population growth rate, age structure, component vital rates, and factors influencing those vital rates. Moose ( Alces alces ) occupy a circumboreal range spanning North America and Eurasia, across which their ecology and demography vary substantially. Given uncertainties regarding the status of moose in Montana, USA, and factors limiting their population growth, we studied moose population dynamics during 2013–2023 and focused on 3 study areas. We estimated population growth rates as a function of population age structure and several key vital rates, including adult female survival, adult female fecundity (the product of pregnancy and litter size rates), and calf survival. We also evaluated relationships amongst a suite of environmental conditions (weather metrics of temperature and snow conditions, seasonal forage quality, parasite effects including winter ticks ( Dermacentor albipictus ) and arterial worms ( Elaeophora schneideri ), and relative predator abundances) and vital rates, with the ultimate goals of determining the status of moose populations in Montana and the relative importance among factors affecting them. We studied adult female survival for 615 moose‐years of monitoring across 186 individual moose, fecundity with pregnancy testing across 779 moose‐years and litter size for 491 observed litters post‐parturition, and calf survival using calf‐at‐heel monitoring of 619 pregnancies and 541 calves throughout their first year of life. We used previously reported results to monitor ambient temperature, snow, arterial worms, and winter ticks in these study areas and present new results concerning diet, forage quality, and relative predator abundance in these study areas. All vital rates (adult female survival, fecundity, and calf survival) showed significant effects of maternal age, which, in combination with differences in age distribution among study areas, had important effects on population growth rates. Adult female survival was 0.88–0.92 across study areas, and causes of mortality for adult females were predominantly health‐related (annual cumulative incidence function [CIF] = 0.075) but also included effects of predation (CIF = 0.017) and humans (CIF = 0.014). Arterial worm infection intensities were associated with roughly 1–3% of annual mortality due to health‐related causes. Health‐related mortality was also higher for animals in poor nutritional condition, those who failed to recruit a calf the previous year, and in animal‐years with more snow. Wolves ( Canis lupus ) were responsible for the highest proportion of predation‐caused adult mortality, followed by grizzly bears ( Ursus arctos ) and mountain lions ( Puma concolor ). In addition, relative predator densities within seasonal spatial polygons were predictive of predation‐caused mortality across individual moose. At the population‐level, moose summer diet reflected selection of forage species that offered higher digestible energy (DE), whereas such selectivity in diet was less evident according to variation among plants in DE during winter or in digestible protein during either season. Furthermore, summer DE was the forage quality metric most related ( R 2 = 0.876) to differences in fecundity (calves/female; range = 0.74–0.96) among populations. Calf survival was negatively related to predator densities during the fetal‐neonatal period, to predator densities and prior reproductive effort during the summer, and to snow during the winter. Population growth rates were stable to increasing (1.00, 1.06, 1.10 in Big Hole, Cabinet‐Salish, and Rocky Mountain Front, respectively) among populations, and adult female survival was the vital rate with the highest elasticity ( e = 0.83–0.88) in all populations. Perturbation analyses indicated, in order of importance, effects of nutrition, predation by wolves, parasitism by arterial worms, and predation by other predators each playing roles in driving variation in population dynamics of moose in Montana. Simulated predator reduction management at levels recently prescribed in parts of Montana had negligible predicted effects on moose population dynamics, with a maximum projected increase in population growth rate of 1.1% in the study area with the strongest effects of predation. Our results also suggest a potentially important role of arterial worm parasitism in driving adult mortality and population growth rates in some parts of Montana, but more research is needed to fully understand the mechanisms behind infection‐caused mortality and the dynamics of sublethal impacts on other moose vital rates. Ultimately, our results uphold the importance of nutrition as a driver of moose population dynamics even in the midst of other top‐down forces such as predation and parasitism.