The influence of winter severity, predation and senescence on moose habitat use

Temporal variation in translocated Isle Royale wolf diet

Wolves (Canis lupus) can exert top-down pressure and shape ecological communities through the predation of ungulates and beavers (Castor spp.). Therefore, understanding wolf foraging is critical to estimating their ecosystem-level effects. Specifically, if wolves are consumers that optimize tradeoffs between the cost and benefits of prey acquisition, changes in these factors may lead to prey-switching or negative-density dependent selection with potential consequences for community stability. For wolves, factors affecting cost and benefits include prey vulnerability, risk, reward, and availability, which can vary temporally. We described the wolf diet by the frequency of occurrence and percent biomass and characterized the diet using prey remains found in wolf scats on Isle Royale National Park, Michigan, USA, during May-October 2019 and 2020. We used logistic regression to estimate prey consumption over time. We predicted prey with temporal variation in cost (availability and/or vulnerability) such as adult moose (Alces alces), calf moose, and beaver (Castor canadensis) to vary in wolf diets. We analyzed 206 scats and identified 62% of remains as beaver, 26% as moose, and 12% as other species (birds, smaller mammals, and wolves). Adult moose were more likely to occur in wolf scats in May when moose are in poor condition following winter. The occurrence of moose calves peaked during June-mid-July following birth but before calf vulnerability declined as they matured. By contrast, beaver occurrence in wolf scat did not change over time, reflecting the importance of low-handling cost prey items for recently introduced lone or paired wolves. Our results demonstrate that the wolf diet is responsive to temporal changes in prey costs. Temporal fluctuation in diet may influence wolves' ecological role if prey respond to increased predation risk by altering foraging or breeding behavior.

The Role of Wolves in Regulating a Chronic Non-communicable Disease, Osteoarthritis, in Prey Populations

It is widely accepted that predators disproportionately prey on individuals that are old, weak, diseased or injured. By selectively removing individuals with diseases, predators may play an important role in regulating the overall health of prey populations. However, that idea is seldom tested empirically. Here we assess the extent that wolves (Canis lupus) select adult moose (Alces alces) in Isle Royale National Park on the basis of age-class and osteoarthritis, a chronic, non-communicable disease. We also assess how temporal variation in kill rates (on moose by wolves) were associated with the subsequent incidence of osteoarthritis in the moose population over a 33-year period (1975–2007). Wolves showed strong selection for senescent moose and tended to avoid prime-aged adults. However, the presence of severe osteoarthritis, but not mild or moderate osteoarthritis, appeared to increase the vulnerability of prime-aged moose to predation. There was weak evidence to suggest that senescent moose with osteoarthritis maybe more vulnerable to wolves, compared to senescent moose without the disease. The incidence of osteoarthritis declined following years with higher kill rates–which is plausibly due to the selective removal of individuals with osteoarthritis. Together those results suggest that selective predation plays an important role in regulating the health of prey populations. Additionally, because osteoarthritis is influenced by genetic factors, these results highlight how wolf predation may act as a selective force against genes associated with developing severe osteoarthritis as a prime-aged adult. Our findings highlight one benefits of allowing predators to naturally regulate prey populations. The evidence we present for predation’s influence on the health of prey populations is also relevant for policy-related arguments about refraining from intensively hunting wolf populations.

Winter Tick Burdens for Moose Are Positively Associated With Warmer Summers and Higher Predation Rates

Climate change is expected to modify host-parasite interactions which is concerning because parasites are involved in most food-web links, and parasites have important influences on the structure, productivity and stability of communities and ecosystems. However, the impact of climate change on host–parasite interactions and any cascading effects on other ecosystem processes has received relatively little empirical attention. We assessed host-parasite dynamics for moose (Alces alces) and winter ticks (Dermacentor albipictus) in Isle Royale National Park over a 19-year period. Specifically, we monitored annual tick burdens for moose (estimated from hair loss) and assessed how it covaried with several aspects of seasonal climate, and non-climatic factors, such as moose density, predation on hosts by wolves (Canis lupus) and wolf abundance. Summer temperatures explained half the interannual variance in tick burden with tick burden being greater following hotter summers, presumably because warmer temperatures accelerate the development of tick eggs and increase egg survival. That finding is consistent with the general expectation that warmer temperatures may promote higher parasite burdens. However, summer temperatures are warming less rapidly than other seasons across most regions of North America. Therefore, tick burdens seem to be primarily associated with an aspect of climate that is currently exhibiting a lower rate of change. Tick burdens were also positively correlated with predation rate, which could be due to moose exhibiting risk-sensitive habitat selection (in years when predation risk is high) in such a manner as to increases the encounter rate with questing tick larvae in autumn. However, that positive correlation could also arise if high parasite burdens make moose more vulnerable to predators or because of some other density-dependent process (given that predation rate and moose density are highly correlated). Overall, these results provide valuable insights about interrelationships among climate, parasites, host/prey, and predators.

Effects of age on foraging behavior in two closely related albatross species

BACKGROUND

Foraging performance is widely hypothesized to play a key role in shaping age-specific demographic rates in wild populations, yet the underlying behavioral changes are poorly understood. Seabirds are among the longest-lived vertebrates, and demonstrate extensive age-related variation in survival, breeding frequency and success. The breeding season is a particularly critical phase during the annual cycle, but it remains unclear whether differences in experience or physiological condition related to age interact with the changing degree of the central-place constraint in shaping foraging patterns in time and space.

METHODS

Here we analyze tracking data collected over two decades from congeneric black-browed (BBA) and grey-headed (GHA) albatrosses, Thalassarche melanophris and T. chrysostoma, breeding at South Georgia. We compare the foraging trip parameters, at-sea activity (flights and landings) and habitat preferences of individuals aged 10-45 years and contrast these patterns between the incubation and early chick-rearing stages.

RESULTS

Young breeders of both species showed improvements in foraging competency with age, reducing foraging trip duration until age 26. Thereafter, there were signs of foraging senescence; older adults took gradually longer trips, narrowed their habitat preference (foraging within a smaller range of sea surface temperatures) (GHA), made fewer landings and rested on the water for longer (BBA). Some age-specific effects were apparent for each species only in certain breeding stages, highlighting the complex interaction between intrinsic drivers in determining individual foraging strategies.

CONCLUSIONS

Using cross-sectional data, this study highlighted clear age-related patterns in foraging behavior at the population-level for two species of albatrosses. These trends are likely to have important consequences for the population dynamics of these threatened seabirds, as young or old individuals may be more vulnerable to worsening environmental conditions.

Glucocorticoid and triiodothyronine concentrations do not correlate with behavior in vicuñas (Vicugna vicugna)

State-dependent foraging theory posits that animals should make foraging decisions based on energetic condition, where animals with fewer energetic reserves prioritize foraging over other behaviors, including antipredator behaviors. However, few studies have investigated these trade-offs at an individual level in wild, free-ranging animals. We investigated the relationships between internal condition and behavior in a wild mammal, the vicuña (Vicugna vicugna), which makes state-dependent decisions about the use of two habitats with different characteristics that contribute to their internal condition. Using non-invasively collected fecal samples, we measured glucocorticoid metabolites (GCMs) and thyroid hormones (THs) as indicators of combined stress (predation and nutritional), and just nutritional stress, respectively. We video recorded 20-minute behavioral observations and focused on behaviors which often demand a trade-off between energy acquisition and antipredator behaviors-vigilance and foraging. We found differences in expression of these behaviors between the two sites but found no relationships between physiological parameters (GCMs and THs) and behavior (vigilance and foraging) at either site. We suggest that state-dependent foraging may be difficult to observe in large mammals under baseline conditions and that GCMs and THs may be insensitive to small changes in stress stimuli at this scale, and where these wild animals have the entire suite of behavioral responses available to them.

Animal responses to environmental variation: physiological mechanisms in ecological models of performance in deer (Cervidae)

Context Proper assessment of the consequences of environmental variation on animals depends on our ability to predict how they will perform under different circumstances. This requires two kinds of information. We need to know which environmental factors influence animal performance and their mode of action, i.e. whether a given factor acts alone or through interaction with other factors, directly or indirectly, instantaneously or after a delay and so on. This essentially correlative process falls within the domain of ecology. We also need to know what determines the direction, amplitude and limits of animal responses to environmental variation and change. This essentially experimental process falls within the domain of physiology. Physiological mechanisms are frequently poorly integrated within the correlative framework of ecological models. This is evident where programmed responses are attributed to environmental forcing and where the effect of environmental factors is evaluated without reference to the physiological state and regulatory capacity of the animal on which they act. Aims Here we examine ways in which the impacts of external (environmental) stimuli and constraints on performance are moderated by the animals (deer) on which they impinge. Key results The analysis shows (1) how trade-offs in foraging behaviour, illustrated by the timing of activity under the threat of predation, are modulated by integration of short-term metabolic feedback and animal emotions that influence the motivation to feed, (2) how the influence of thermal and nutritional challenges on performance, illustrated by the effect of weather conditions during gestation on the body mass of reindeer (Rangifer tarandus) calves at weaning, depends on the metabolic state of the female at the time the challenge occurs and (3) how annual cycles of growth, appetite and reproduction in seasonal species of deer are governed by innate circannual timers, such that their responses to seasonal changes in food supply are anticipatory and governed by rheostatic systems that adjust homeostatic set- points, rather than being purely reactive. Conclusions Concepts like ‘maintenance’ and ‘energy balance’, which were originally derived from non-seasonal domestic ruminants, are unable to account for annual cycles in metabolic and nutritional status in seasonal deer. Contrasting seasonal phenotypes (fat and anoestrous in summer, lean and oestrous in winter) represent adaptive solutions to the predictable challenges presented by contrasting seasonal environments, not failure of homeostasis in one season and its success in another. Implications The analysis and interpretation of responses to environment in terms of interaction between the external stimuli and the internal systems that govern them offer a more comprehensive, multifaceted understanding of the influence of environmental variation on performance in deer and open lines of ecological enquiry defined by non-intuitive aspects of animal function.

Ageing impacts phenotypic flexibility in an air-acclimated amphibious fish

The ability to tolerate environmental change may decline as fishes age. We tested the hypothesis that ageing influences the scope for phenotypic flexibility in the mangrove rivulus (Kryptolebias marmoratus), an amphibious fish that transitions between two vastly different environments, water and land. We found that older fish (4-6 years old) exhibited marked signs of ageing; older fish were reproductively senescent, had reduced fin regenerative capacity and body condition, and exhibited atrophy of both oxidative and glycolytic muscle fibers relative to younger adult fish (1-2 years old). However, age did not affect routine O₂ consumption. We then acclimated adult fish (1-6 years) to water (control) or air for 10 days to assess the scope for phenotypic flexibility in response to terrestrial exposure. In support of our hypothesis, we found that older air-acclimated fish had a diminished scope for gill remodeling relative to younger fish. We also found that older fish exhibited poorer terrestrial locomotor performance relative to younger adult fish, particularly when acclimated to air. Our results indicate that ageing diminishes skeletal muscle integrity and locomotor performance of amphibious fishes, and may, therefore, impair terrestrial foraging ability, predator avoidance, or dispersal across the terrestrial environment. Remarkably, older fish voluntarily left water to a similar degree as younger fish despite the age-related deterioration of traits important for terrestrial life.

Negative frequency-dependent foraging behaviour in a generalist herbivore (Alces alces) and its stabilizing influence on food web dynamics

Resource selection is widely appreciated to be context-dependent and shaped by both biological and abiotic factors. However, few studies have empirically assessed the extent to which selective foraging behaviour is dynamic and varies in response to environmental conditions for free-ranging animal populations. Here, we assessed the extent that forage selection fluctuated in response to different environmental conditions for a free-ranging herbivore, moose (Alces alces), in Isle Royale National Park, over a 10-year period. More precisely, we assessed how moose selection for coniferous versus deciduous forage in winter varied between geographic regions and in relation to (a) the relative frequency of forage types in the environment (e.g. frequency-dependent foraging behaviour), (b) moose abundance, (c) predation rate (by grey wolves) and (d) snow depth. These factors are potentially important for their influence on the energetics of foraging. We also built a series of food-chain models to assess the influence of dynamic foraging strategies on the stability of food webs. Our analysis indicates that moose exhibited negative frequency dependence, by selectively exploiting rare resources. Frequency-dependent foraging was further mediated by density-dependent processes, which are likely to be predation, moose abundance or some combination of both. In particular, frequency dependence was weaker in years when predation risk was high (i.e. when the ratio of moose to wolves was relatively low). Selection for conifers was also slightly weaker during deep snow years. The food-chain analysis indicates that the type of frequency-dependent foraging strategy exhibited by herbivores had important consequences for the stability of ecological communities. In particular, the dynamic foraging strategy that we observed in the empirical analysis (i.e. negative frequency dependence being mediated by density-dependent processes) was associated with more stable food web dynamics compared to fixed foraging strategies. The results of this study indicated that forage selection is a complex ecological process, varying in response to both biological (predation and moose density) and abiotic factors (snow depth) and over relatively small spatial scales (between regions). This study also provides a useful framework for assessing the influence of other aspects of foraging behaviour on the stability of food web dynamics.

Foraging investment in a long-lived herbivore and vulnerability to coursing and stalking predators

Allocating resources to growth and reproduction requires grazers to invest time in foraging, but foraging promotes dental senescence and constrains expression of proactive antipredator behaviors such as vigilance. We explored the relationship between carnivore prey selection and prey foraging effort using incisors collected from the kills of coursing and stalking carnivores. We predicted that prey investing less effort in foraging would be killed more frequently by coursers, predators that often exploit physical deficiencies. However, such prey could expect delayed dental senescence. We predicted that individuals investing more effort in foraging would be killed more frequently by stalkers, predators that often exploit behavioral vulnerabilities. Further these prey could expect earlier dental senescence. We tested these predictions by comparing variation in age-corrected tooth wear, a proxy of cumulative foraging effort, in adult (3.4-11.9 years) wildebeest killed by coursing and stalking carnivores. Predator type was a strong predictor of age-corrected tooth wear within each gender. We found greater foraging effort and earlier expected dental senescence, equivalent to 2.6 additional years of foraging, in female wildebeest killed by stalkers than in females killed by coursers. However, male wildebeest showed the opposite pattern with the equivalent of 2.4 years of additional tooth wear in males killed by coursers as compared to those killed by stalkers. Sex-specific variation in the effects of foraging effort on vulnerability was unexpected and suggests that behavioral and physical aspects of vulnerability may not be subject to the same selective pressures across genders in multipredator landscapes.

Moose movement rates are altered by wolf presence in two ecosystems

Predators directly impact prey populations through lethal encounters, but understanding nonlethal, indirect effects is also critical because foraging animals often face trade-offs between predator avoidance and energy intake. Quantifying these indirect effects can be difficult even when it is possible to monitor individuals that regularly interact. Our goal was to understand how movement and resource selection of a predator (wolves; Canis lupus) influence the movement behavior of a prey species (moose; Alces alces). We tested whether moose avoided areas with high predicted wolf resource use in two study areas with differing prey compositions, whether avoidance patterns varied seasonally, and whether daily activity budgets of moose and wolves aligned temporally. We deployed GPS collars on both species at two sites in northern Minnesota. We created seasonal resource selection functions (RSF) for wolves and modeled the relationship between moose first-passage time (FPT), a method that discerns alterations in movement rates, and wolf RSF values. Larger FPT values suggest rest/foraging, whereas shorter FPT values indicate travel/fleeing. We found that the movements of moose and wolves peaked at similar times of day in both study areas. Moose FPTs were 45% lower in areas most selected for by wolves relative to those avoided. The relationship between wolf RSF and moose FPT was nonlinear and varied seasonally. Differences in FPT between low and high RSF values were greatest in winter (-82.1%) and spring (-57.6%) in northeastern Minnesota and similar for all seasons in the Voyageurs National Park ecosystem. In northeastern Minnesota, where moose comprise a larger percentage of wolf diet, the relationship between moose FPT and wolf RSF was more pronounced (ave. across seasons: -60.1%) than the Voyageurs National Park ecosystem (-30.4%). These findings highlight the role wolves can play in determining moose behavior, whereby moose spend less time in areas with higher predicted likelihood of wolf resource selection.

Declining home range area predicts reduced late-life survival in two wild ungulate populations

Demographic senescence is increasingly recognised as an important force shaping the dynamics of wild vertebrate populations. However, our understanding of the processes that underpin these declines in survival and fertility in old age remains limited. Evidence for age-related changes in foraging behaviour and habitat use is emerging from wild vertebrate studies, but the extent to which these are driven by within-individual changes, and the consequences for fitness, remain unclear. Using longitudinal census observations collected over four decades from two long-term individual-based studies of unmanaged ungulates, we demonstrate consistent within-individual declines in home range area with age in adult females. In both systems, we found that within-individual decreases in home range area were associated with increased risk of mortality the following year. Our results provide the first evidence from the wild that age-related changes in space use are predictive of adult mortality.

The many faces of fear: a synthesis of the methodological variation in characterizing predation risk

Predators affect prey by killing them directly (lethal effects) and by inducing costly antipredator behaviours in living prey (risk effects). Risk effects can strongly influence prey populations and cascade through trophic systems. A prerequisite for assessing risk effects is characterizing the spatiotemporal variation in predation risk. Risk effects research has experienced rapid growth in the last several decades. However, preliminary assessments of the resultant literature suggest that researchers characterize predation risk using a variety of techniques. The implications of this methodological variation for inference and comparability among studies have not been well recognized or formally synthesized. We couple a literature survey with a hierarchical framework, developed from established theory, to quantify the methodological variation in characterizing risk using carnivore-ungulate systems as a case study. Via this process, we documented 244 metrics of risk from 141 studies falling into at least 13 distinct subcategories within three broader categories. Both empirical and theoretical work suggest risk and its effects on prey constitute a complex, multi-dimensional process with expressions varying by spatiotemporal scale. Our survey suggests this multi-scale complexity is reflected in the literature as a whole but often underappreciated in any given study, which complicates comparability among studies and leads to an overemphasis on documenting the presence of risk effects rather than their mechanisms or scale of influence. We suggest risk metrics be placed in a more concrete conceptual framework to clarify inference surrounding risk effects and their cascading effects throughout ecosystems. We recommend studies (i) take a multi-scale approach to characterizing risk; (ii) explicitly consider 'true' predation risk (probability of predation per unit time); and (iii) use risk metrics that facilitate comparison among studies and the evaluation of multiple competing hypotheses. Addressing the pressing questions in risk effects research, including how, to what extent and on what scale they occur, requires leveraging the advantages of the many methods available to characterize risk while minimizing the confusion caused by variability in their application.

Age-related variation in foraging behaviour in the wandering albatross at South Georgia: no evidence for senescence

Age-related variation in demographic rates is now widely documented in wild vertebrate systems, and has significant consequences for population and evolutionary dynamics. However, the mechanisms underpinning such variation, particularly in later life, are less well understood. Foraging efficiency is a key determinant of fitness, with implications for individual life history trade-offs. A variety of faculties known to decline in old age, such as muscular function and visual acuity, are likely to influence foraging performance. We examine age-related variation in the foraging behaviour of a long-lived, wide-ranging oceanic seabird, the wandering albatross Diomedea exulans. Using miniaturised tracking technologies, we compared foraging trip characteristics of birds breeding at Bird Island, South Georgia. Based on movement and immersion data collected during the incubation phase of a single breeding season, and from extensive tracking data collected in previous years from different stages of the breeding cycle, we found limited evidence for age-related variation in commonly reported trip parameters, and failed to detect signs of senescent decline. Our results contrast with the limited number of past studies that have examined foraging behaviour in later life, since these have documented changes in performance consistent with senescence. This highlights the importance of studies across different wild animal populations to gain a broader perspective on the processes driving variation in ageing rates.

Density-dependent habitat selection and partitioning between two sympatric ungulates

Theory on density-dependent habitat selection predicts that as population density of a species increases, use of higher quality (primary) habitat by individuals declines while use of lower quality (secondary) habitat rises. Habitat partitioning is often considered the primary mechanism for coexistence between similar species, but how this process evolves with changes in population density remains to be empirically tested for free-ranging ungulates. We used resource-selection functions to quantify density effects on landscape-scale habitat selection of two sympatric species of ungulates [moose (Alces alces) and elk (Cervus canadensis manitobensis)] in Riding Mountain National Park, Manitoba, Canada (2000-2011). The density of elk was actively reduced from 1.2 to 0.4 elk km(-2) through increased hunting effort during the period of study, while moose density decreased without additional human influence from 1.6-0.7 moose km(-2). Patterns of habitat selection during winter by both species changed in accordance to expectations from density-dependent habitat-selection theory. At low intraspecific density, moose and elk did not partition habitat, as both species selected strongly for mixed forest (primary habitat providing both food and cover), but did so in different areas segregated across an elevational gradient. As intraspecific density increased, selection for primary habitat by both species decreased, while selection for secondary, lower quality habitat such as agricultural fields (for elk) and built-up areas (for moose) increased. We show that habitat-selection strategies during winter for moose and elk, and subsequent effects on habitat partitioning, depend heavily on the position in state space (density) of both species.

Where wolves kill moose: the influence of prey life history dynamics on the landscape ecology of predation

The landscape ecology of predation is well studied and known to be influenced by habitat heterogeneity. Little attention has been given to how the influence of habitat heterogeneity on the landscape ecology of predation might be modulated by life history dynamics of prey in mammalian systems. We demonstrate how life history dynamics of moose (Alces alces) contribute to landscape patterns in predation by wolves (Canis lupus) in Isle Royale National Park, Lake Superior, USA. We use pattern analysis and kernel density estimates of moose kill sites to demonstrate that moose in senescent condition and moose in prime condition tend to be wolf-killed in different regions of Isle Royale in winter. Predation on senescent moose was clustered in one kill zone in the northeast portion of the island, whereas predation on prime moose was clustered in 13 separate kill zones distributed throughout the full extent of the island. Moreover, the probability of kill occurrence for senescent moose, in comparison to prime moose, increased in high elevation habitat with patches of dense coniferous trees. These differences can be attributed, at least in part, to senescent moose being more vulnerable to predation and making different risk-sensitive habitat decisions than prime moose. Landscape patterns emerging from prey life history dynamics and habitat heterogeneity have been observed in the predation ecology of fish and insects, but this is the first mammalian system for which such observations have been made.