Abstract
In long-lived mammals, females must pay the costs of gestation and lactation without jeopardizing future survival and reproduction. Insufficient energy and protein intake during summer can have severe consequences for nutritional condition and body mass, which directly impact survival and reproduction. Because behavior is one of the primary mechanisms by which animals cope with constraints on survival and reproduction, individual herbivores should attempt to maximize nutrient intake by consistently selecting the best foraging habitats available to them. Yet, herbivores’ ability to optimize their foraging behavior is constrained by a variety of intrinsic (e.g., memory) and extrinsic (e.g., predation risk) factors. One result of such constraints is that foraging strategies often vary considerably among individuals in a population. The causes and consequences of individual variation have long been the subject of investigation but are often overlooked in wildlife ecology and management. Although previous work has clearly established links between nutrition and individual performance, the mechanisms underpinning relationships among individual behavior, nutrition, and population performance remain elusive. We quantified relationships among the foodscape (i.e., spatiotemporal variation in forage biomass and quality), behavior, and performance (i.e., survival and reproduction) in three populations of bighorn sheep (Ovis canadensis) that spanned a range of environmental conditions, in mid-elevation grasslands and mountainous alpine systems. We first investigated the nutritional underpinnings of variation in life-history strategies exhibited by bighorn sheep. Bighorn sheep are typically considered to be capital breeders that rely on stored energy reserves to finance reproduction. This generality is supported by the observation that in many northern bighorn populations, survival of neonates is insensitive to temporal changes in spring green-up, and capital reserves strongly influence performance. However, bighorn sheep in our study exhibited marked variation in their reliance on capital to finance reproduction. In alpine populations, autumn body fat was positively related to the probabilities of pregnancy and overwinter survival of adult females, but selection for the foodscape had mixed effects on the probability of lamb survival. Strong reliance on stored capital to finance survival and reproduction in alpine environments reflected the capital-breeding life-history strategy most commonly ascribed to bighorn sheep. In the grassland population, however, autumn body fat was unrelated to overwinter survival of adult females or probability of pregnancy, and foodscape selection positively affected lamb survival to 120 days. These results reflect a stronger dependence on current foodscape conditions to finance reproduction in grassland sheep, indicative of a more income-based breeding strategy. Our work demonstrates that tradeoffs between survival and reproduction can vary considerably within species across environmental gradients, and that this variation can lead to markedly different strategies of behavior and resource allocation among managed populations. We next investigated the causes (e.g., prior reproductive success, foodscape heterogeneity) and consequences (e.g., nutritional condition, lamb survival) of intraspecific variation in site fidelity among these same bighorn sheep populations. We used distance-based metrics to quantify site fidelity at three temporal scales (i.e., seasonal, monthly, and biweekly) to better understand the behavioral strategies employed by bighorn sheep to mitigate environmental heterogeneity and optimize foraging opportunities. Site fidelity was relatively strong across populations and temporal scales. Prior reproductive success was not an important predictor of site fidelity by bighorn sheep, and instead, quality and predictability of the forage resources within sheep home ranges drove variation in site fidelity. Despite the consistency of these patterns, however, we found little evidence that site fidelity improved nutritional condition of female sheep, and the effects of site fidelity on lamb survival were mixed. Our results support the notion that environmental conditions shape the strength of site fidelity across temporal scales, but that the effects of site fidelity on individual performance are context dependent. Continuing to parse the complex mechanisms that underpin variation in site fidelity will shed important light on the capacity of large herbivores to rapidly adjust to environmental change.
Finally, we used a Bayesian integrated population model (IPM) coupled with life-stage simulation analysis and population viability analysis to (1) evaluate the relative contributions of different vital rates to population growth rate of bighorn sheep, and (2) predict the potential effectiveness of various management actions for meeting management objectives specific to each population. We observed substantial spatial and temporal variation in vital rates among bighorn sheep populations that had important, context-dependent consequences for population growth rates. Contributions of different vital rates to lambda (the finite rate of population growth) were population-specific and did not follow the expected paradigm for large herbivores. As a result, management actions with the greatest potential for improving population performance differed among populations. Augmentation of adult females may be an effective strategy for increasing abundance when populations are small and demographically stagnant. However, in populations where epizootic pneumonia has resulted in suppressed lamb recruitment, increasing summer lamb survival through test-and-remove will be most likely to produce a population increase. Large or increasing populations where epizootic pneumonia has not affected lamb survival may be subject to nutritional limitations, thus, female harvest could be implemented to reduce density and increase vital rates. Nevertheless, female harvest may need to exceed 10% to stabilize population growth. Our results highlight the importance of considering population-specific dynamics when developing monitoring programs and management plans.