Snowshoe hare cycles are one of the most prominent phenomena in ecology. Experimental studies point to predation as the dominant driving factor, but previous experiments combining food ...supplementation and predator removal produced unexplained multiplicative effects on density. We examined the potential interactive effects of food limitation and predation in causing hare cycles using an individual‐based food‐supplementation experiment over‐winter across three cycle phases that naturally varied in predation risk. Supplementation doubled over‐winter survival with the largest effects occurring in the late increase phase. Although the proximate cause of mortality was predation, supplemented hares significantly decreased foraging time and selected for conifer habitat, potentially reducing their predation risk. Supplemented hares also lost less body mass which resulted in the production of larger leverets. Our results establish a mechanistic link between how foraging time, mass loss and predation risk affect survival and reproduction, potentially driving demographic changes associated with hare cycles.
Snowshoe hare cycles are one of the most notable phenomena in ecology. Through an individual‐based food supplementation experiment, we found strong evidence that both food limitation and predation interactively drive hare cycles through changes in behaviour and condition.
Some mammal species inhabiting high-latitude biomes have evolved a seasonal moulting pattern that improves camouflage via white coats in winter and brown coats in summer. In many high-latitude and ...high-altitude areas, the duration and depth of snow cover has been substantially reduced in the last five decades. This reduction in depth and duration of snow cover may create a mismatch between coat colour and colour of the background environment, and potentially reduce the survival rate of species that depend on crypsis. We used long-term (1977-2020) field data and capture-mark-recapture models to test the hypothesis that whiteness of the coat influences winter apparent survival in a cyclic population of snowshoe hares (
) at Kluane, Yukon, Canada. Whiteness of the snowshoe hare coat in autumn declined during this study, and snowshoe hares with a greater proportion of whiteness in their coats in autumn survived better during winter. However, whiteness of the coat in spring did not affect subsequent summer survival. These results are consistent with the hypothesis that the timing of coat colour change in autumn can reduce overwinter survival. Because declines in cyclic snowshoe hare populations are strongly affected by low winter survival, the timing of coat colour change may adversely affect snowshoe hare population dynamics as climate change continues.
Climate warming is causing asynchronies between animal phenology and environments. Mismatched traits, such as coat color change mismatched with snow, can decrease survival. However, coat change does ...not serve a singular adaptive benefit of camouflage, and alternate coat change functions may confer advantages that supersede mismatch costs. We found that mismatch reduced, rather than increased, autumn mortality risk of snowshoe hares in Yukon by 86.5% when mismatch occurred. We suggest that the increased coat insulation and lower metabolic rates of winter‐acclimatized hares confer energetic advantages to white mismatched hares that reduce their mortality risk. We found that white mismatched hares forage 17–77 min less per day than matched brown hares between 0°C and −10°C, thus lowering their predation risk and increasing survival. We found no effect of mismatch on spring mortality risk, during which mismatch occurred at warmer temperatures, suggesting a potential temperature limit at which the costs of conspicuousness outweigh energetic benefits.
Extensive research confirms that environmental stressors like predation risk can profoundly affect animal condition and physiology. However, there is a lack of experimental research assessing the ...suite of physiological responses to risk that may arise under realistic field conditions, leaving a fragmented picture of risk-related physiological change and potential downstream consequences on individuals. We increased predation risk in free-ranging snowshoe hares (Lepus americanus) during two consecutive summers by simulating natural chases using a model predator and monitored hares intensively via radio-telemetry and physiological assays, including measures designed to assess changes in stress physiology and overall condition. Compared to controls, risk-augmented hares had 25.8% higher free plasma cortisol, 15.9% lower cortisol-binding capacity, a greater neutrophil: lymphocyte skew, and a 10.4% increase in glucose. Despite these changes, intra-annual changes in two distinct condition indices, were unaffected by risk exposure. We infer risk-augmented hares compensated for changes in their stress physiology through either compensatory foraging and/or metabolic changes, which allowed them to have comparable condition to controls. Although differences between controls and risk-augmented hares were consistent each year, both groups had heightened stress measures during the second summer, likely reflecting an increase in natural stressors (i.e., predators) in the environment. We show that increased predation risk in free-ranging animals can profoundly alter stress physiology and that compensatory responses may contribute to limiting effects of such changes on condition. Ultimately, our results also highlight the importance of biologically relevant experimental risk manipulations in the wild as a means of assessing physiological responses to natural stressors.
We report a 26-year detailed record of changes in abundance and demography of the deer mouse (
Peromyscus maniculatus
) in old-growth coniferous forest in southern British Columbia, Canada, from 1995 ...to 2020. We evaluated three separate hypotheses that may help to explain population changes: (H
1
) large cone crops of coniferous trees will correspond to peak
Peromyscus
density; (H
2
) spring reorganization of individuals within populations will lead to early breeding season declines; and (H
3
) competition between
P. maniculatus
and the sympatric red-backed vole (
Clethrionomys gapperi
) and hence densities of the former will be lower when the latter are higher. Populations of deer mice showed five periods of high numbers with intervals of 3 to 8 years between fluctuations. Mean density ranges were low, growing from 2–3/ha in spring to 8–13/ha in autumn. The mean abundance of
P. maniculatus
in high years was directly related to number of successful pregnancies, total number of juvenile recruits, and a high index of early juvenile productivity. Irregular population fluctuations were related to pulsed food supplies in the form of coniferous seeds, thereby supporting H
1
. Numbers declined in 16 of 26 summers, reflecting poor overall survival and low reproduction (only 69% of adult females lactating and 51% having only one litter; virtually no juveniles matured and bred). Thus, H
2
was partly supported for a reorganization event that led to breeding season declines, and the immediate cause was very poor survival of juvenile mice. The annual peak numbers of
C. gapperi
ranged from 1.5 to 16.9 times higher than numbers for
P. maniculatus
and hence provided only weak support for H
3
. Field experiments are needed to test these hypotheses as mechanisms driving deer mouse populations.
Food availability and temporal variation in predation risk are both important determinants of the magnitude of antipredator responses, but their effects have rarely been examined simultaneously, ...particularly in wild prey. Here, we determine how food availability and long-term predation risk affect antipredator responses to acute predation risk by monitoring the foraging response of free-ranging snowshoe hares (Lepus americanus) to an encounter with a Canada lynx (Lynx canadensis) in Yukon, Canada, over four winters (2015–2016 to 2018–2019). We examined how this response was influenced by natural variation in long-term predation risk (2-month mortality rate of hares) while providing some individuals with supplemental food. On average, snowshoe hares reduced foraging time up to 10 h after coming into close proximity (≤75 m) with lynx, and reduced foraging time an average of 15.28 ± 7.08 min per lynx encounter. Hares tended to respond more strongly when the distance to lynx was shorter. More importantly, the magnitude of hares’ antipredator response to a lynx encounter was affected by the interaction between food-supplementation and long-term predation risk. Food-supplemented hares reduced foraging time more than control hares after a lynx encounter under low long-term risk, but decreased the magnitude of the response as long-term risk increased. In contrast, control hares increased the magnitude of their response as long-term risk increased. Our findings show that food availability and long-term predation risk interactively drive the magnitude of reactive antipredator response to acute predation risk. Determining the factors driving the magnitude of antipredator responses would contribute to a better understanding of the indirect effects of predators on prey populations.
Climate change-induced shifts in species phenology differ widely across trophic levels, which may lead to consumer-resource mismatches with cascading population and ecosystem consequences. Here, we ...examined the effects of different rainfall patterns (i.e., timing and amount) on the phenological asynchrony of population of a generalist herbivore and their food sources in semiarid steppe grassland in Inner Mongolia. We conducted a 10-y (2010 to 2019) rainfall manipulation experiment in 12 0.48-ha field enclosures and found that moderate rainfall increases during the early rather than late growing season advanced the timing of peak reproduction and drove marked increases in population size through increasing the biomass of preferred plant species. By contrast, greatly increased rainfall produced no further increases in vole population growth due to the potential negative effect of the flooding of burrows. The increases in vole population size were more coupled with increased reproduction of overwintered voles and increased body mass of young-of-year than with better survival. Our results provide experimental evidence for the fitness consequences of phenological mismatches at the population level and highlight the importance of rainfall timing on the population dynamics of small herbivores in the steppe grassland environment.
To understand why population growth rate is sometimes positive and sometimes negative, ecologists have adopted two main approaches. The most common approach is through the density paradigm by ...plotting population growth rate against population density. The second approach is through the mechanistic paradigm by plotting population growth rate against the relevant ecological processes affecting the population. The density paradigm is applied a posteriori, works sometimes but not always and is remarkably useless in solving management problems or in providing an understanding of why populations change in size. The mechanistic paradigm investigates the factors that supposedly drive density changes and is identical to Caughley's declining population paradigm of conservation biology. The assumption that we can uncover invariant relationships between population growth rate and some other variables is an article of faith. Numerous commercial fishery applications have failed to find the invariant relationships between stock and recruitment that are predicted by the density paradigm. Environmental variation is the rule, and non-equilibrial dynamics should force us to look for the mechanisms of population change. If multiple factors determine changes in population density, there can be no predictability in either of these paradigms and we will become environmental historians rather than scientists with useful generalizations for the population problems of this century. Defining our questions clearly and adopting an experimental approach with crisp alternative hypotheses and adequate controls will be essential to building useful generalizations for solving the practical problems of population management in fisheries, wildlife and conservation.
The population dynamics of snowshoe hares (Lepus americanus) are fundamental to the ecosystem dynamics of Canada’s boreal forest. During the 8- to 11-year population cycle, hare densities can ...fluctuate up to 40-fold. Predators in this system (lynx, coyotes, great-horned owls) affect population numbers not only through direct mortality but also through sublethal effects. The chronic stress hypothesis posits that high predation risk during the decline severely stresses hares, leading to greater stress responses, heightened ability to mobilize cortisol and energy, and a poorer body condition. These effects may result in, or be mediated by, differential gene expression. We used an oligonucleotide microarray designed for a closely-related species, the European rabbit (Oryctolagus cuniculus), to characterize differences in genome-wide hippocampal RNA transcript abundance in wild hares from the Yukon during peak and decline phases of a single cycle. A total of 106 genes were differentially regulated between phases. Array results were validated with quantitative real-time PCR, and mammalian protein sequence similarity was used to infer gene function. In comparison to hares from the peak, decline phase hares showed increased expression of genes involved in metabolic processes and hormone response, and decreased expression of immune response and blood cell formation genes. We found evidence for predation risk effects on the expression of genes whose putative functions correspond with physiological impacts known to be induced by predation risk in snowshoe hares. This study shows, for the first time, a link between changes in demography and alterations in neural RNA transcript abundance in a natural population.
Population ecology is the most mature of the three subdisciplines of ecology partly because it has a solid mathematical foundation and partly because it can address the primary questions of ...distribution and abundance with experimental protocols. Yet there is much left to do to integrate our population knowledge into community and ecosystem ecology to help address the global issues of food security and the conservation of biodiversity. Many different approaches are now being developed to bring about this integration and much more research will be necessary to decide which if any will be most useful in achieving our goals of explaining the changes we see in the distribution and abundance of animals and plants. Food web ecology would appear to be the best approach at present because it uses the detailed information of the population ecology of particular species in combination with data on consumer–resource interactions to apply to the applied problems of biodiversity conservation, food security, pest management and disease prevention. If we can use our understanding of population ecology to address the practical problems of our time in a creative way, we will benefit both the human population and the Earth's biodiversity. Much remains to be done.
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