Multiscale habitat relationships of snowshoe hares (Lepus americanus) in the mixed conifer landscape of the Northern Rockies,USA: Cross‐scale effects of horizontal cover with implications for forest management

Snowshoe hares (Lepus americanus) are an ecologically important herbivore because they modify vegetation through browsing and serve as a prey resource for multiple predators. We implemented a multiscale approach to characterize habitat relationships for snowshoe hares across the mixed conifer landscape of the northern Rocky Mountains, USA. Our objectives were to (1) assess the relationship between horizontal cover and snowshoe hares, (2) estimate how forest metrics vary across the gradient of snowshoe hare use and horizontal cover, and (3) model and map snowshoe hare occupancy and intensity of use. Results indicated that both occupancy and intensity of use by snowshoe hares increased with horizontal cover and that the effect became stronger as intensity of use increased. This underscores the importance of dense horizontal cover to achieve high use, and likely density, of snowshoe hares. Forest structure in areas with high snowshoe hare use and horizontal cover was characterized as multistoried with dense canopy cover and medium‐sized trees (e.g., 12.7–24.4 cm). The abundance of lodgepole pine (Pinus contorta) was associated with snowshoe hare use within a mixed conifer context, and the only species to increase in abundance with horizontal cover was Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa). Our landscape‐level modeling produced similar patterns in that we observed a positive effect of lodgepole pine and horizontal cover on both occupancy and use by snowshoe hares, but we also observed a positive yet parabolic effect of snow depth on snowshoe hare occupancy. This work is among the first to characterize the multiscale habitat relationships of snowshoe hares across a mixed conifer landscape as well as to map their occupancy and intensity of use. Moreover, our results provide stand‐ and landscape‐level insights that directly relate to management agencies, which aids in conservation efforts of snowshoe hares and their associated predators.     

Dynamics of herbivores and resources on a landscape with interspersed resources and refuges

A tradeoff between energy gain from foraging and safety from predation in refuges is a common situation for many herbivores that are vulnerable to predation while foraging. This tradeoff affects the population dynamics of the plant–herbivore–predator interaction. A new functional response is derived based on the Holling type 2 functional response and the assumption that the herbivore can forage at a rate that maximizes its fitness. The predation rate on the herbivore is assumed to be proportional to the product of the time that the herbivore spends foraging and a risk factor that reflects the habitat complexity; where greater complexity means greater interspersion of high food quality habitat and refuge habitat, which increases the amount of the edge zone between refuge and foraging areas, making foraging safer. The snowshoe hare is chosen as an example to demonstrate the resulting dynamics of an herbivore that has been intensely studied and that undergoes well-known cycling. Two models are studied in which the optimal foraging by hares is assumed, a vegetation–hare–generalist predator model and a vegetation–hare–specialist predator model. In both cases, the results suggest that the cycling of the snowshoe hare population will be greatly moderated by optimal foraging in a habitat consisting of interspersed high quality foraging habitat and refuge habitat. However, there are also large differences in the dynamics produced by the two models as a function of predation pressure.     

Snowshoe hare demography during a cyclic population low

1. Snowshoe hare ( Lepus americanus Erxleben) populations were studied in south-west Yukon during the low phase of the 10-year population cycle. Food availability and predator abundance were manipulated in a factorial design to determine the importance of each factor in hare dynamics during this phase.
2. Food was abundant during the low phase, and snowshoe hares were not food limited.
3. Survival of hares was higher than at any other phase of the cycle, and predators were scarce, but >75% of hare deaths resulted from predation.
4. Food addition resulted in higher hare densities and better body condition than on control sites. There were no observable effects of food addition on population rate of increase, recruitment, survival or age structure.
5. Mammalian predator reduction resulted in higher hare densities, higher survival, better body condition and an older age structure. Relative to control populations, recruitment was lower and population rates of increase similar.
6. The joint manipulation of food addition + predator reduction had greater positive effects on hare density and body condition than either single factor manipulation. Survival was better than on control sites, and the age structure was older than on control sites. Population rates of increase were similar, but recruitment was higher on the control areas.
7. We conclude that snowshoe hare dynamics at the low of the cycle are dominated by the interaction of food and predation. Risk of predation also had indirect effects on snowshoe hare age structure and body condition.     

UV-B-induced plant stress as a possible cause of ten-year hare cycles

Predation has been assumed to be a necessary factor in the ten-year population cycle of the snowshoe hare (Lepus americanus) and Canadian lynx (Lynx canadensis). The UV-B-induced plant stress hypothesis, in contrast, predicts that hare performance, especially reproduction, is negatively related to sunspot numbers, because production of UV-B-protective phenolics in food plants in periods of low sunspot activity, when the ozone layer is thin, increases the availability of amino acids and reduces the amount of phenolics that protect against herbivores. In accordance with the UV-B-induced plant stress hypothesis, and despite the absence of predators that have been assumed to be necessary for hare cycles, mountain hare (Lepus timidus) populations in Norway fluctuate in close synchrony with snowshoe hare populations in Alberta and the Yukon, Canada. When adjusting for the phase of the hare cycle, the natality of snowshoe hare in Alberta 1962–1976 was negatively related to sunspot numbers with a time lag of two years. It is concluded that delayed responses to UV-B-induced changes in plant chemistry during the sunspot cycle is a possible cause of ten-year cycles of hares and other herbivores, for example grouse and forest moths.     

Effects of Precommercial Thinning on Snowshoe Hares in Maine

Abstract: Snowshoe hares (Lepus americanus) are an important prey species and a dominant herbivore across much of their North American range, and researchers have questioned the influences of forestry practices that alter habitat for hares and the potential community-level effects on carnivores. We examined the effects of precommercial thinning (PCT) from 1 to 11 years posttreatment on snowshoe hares. In the commercial forests of northern Maine, USA, we counted and cleared hare pellets twice a year during 2001 and 2002 on >46 km of pellet transects across 30 regenerating conifer stands (17 treated with PCT) previously treated with an aerial application of herbicide. We compared densities of snowshoe hare pellets among 3 development classes with (1 yr after thinning, 6 yr after thinning, and 11 yr after thinning) and without thinning (stands with a similar history of clearcut and herbicide treatment but no thinning). During both years, densities of hares were lower in stands treated with PCT than in similar unthinned stands across the 3 development classes and during both leaf-off and leaf-on seasons (P < 0.001). Within both thinned and unthinned stands, hare density was greatest in stands in the 1-year development class when compared to the 6-year and 11-year development classes, but a statistical difference (P = 0.048) among classes was evident only during leaf-off seasons. Precommercial thinning was associated with densities of snowshoe hares that were approximately half of those in similar unthinned stands up to at least 11 years posttreatment; however, thinned stands may retain densities of hares greater than stands managed using other forest harvesting regimes. Our results apply to core portions of stands with crop trees spaced at 1.8–2.4-m intervals following complete overstory removal and herbicide treatment. We advocate caution when applying our results to other thinning regimes or across broader spatial scales.     

Survival of dispersing versus philopatric juvenile snowshoe hares: do dispersers die?

We used radio-telemetry to monitor the survival of dispersing and philopatric juvenile snowshoe hares ( Lepus americanus ) in southwestern Yukon Territory, Canada, during a cyclic population increase. Neither 28-d survival nor the proportion of hares surviving to breed differed significantly between juvenile hares that dispersed and those that did not, nor was there a significant relationship between dispersal distance and fate (dead or alive). Our results indicate that the overall survival cost associated with natal dispersal is low for snowshoe hares during the early increase of the hare cycle.     

Mass-Balance Analyses of Boreal Forest Population Cycles: Merging Demographic and Ecosystem Approaches

UsingEcopath, a trophic mass-balance modeling framework, we developed six models of a Canadian boreal forest food web centered around snowshoe hares, which have conspicuous 10-year population cycles. Detailed models of four phases of the cycle were parameterized with long-term population data for 12 vertebrate taxa. We also developed five other models that, instead of observed data, used parameter values derived from standard assumptions. Specifically, in the basic model, production was assumed to equal adult mortality, feeding rates were assumed to be allometric, and biomass was assumed to be constant. In the actual production, functional response, and biomass change models, each of these assumed values from the basic model was replaced individually by field data. Finally, constant biomass models included actual production by all species and functional responses of mammalian predators and revealed the proportion of herbivore production used by species at higher trophic levels. By comparing these models, we show that detailed information on densities and demographics was crucial to constructing models that captured dynamic aspects of the food web. These detailed models reinforced an emerging picture of the causes and consequences of the snowshoe hare cycle. The snowshoe hare decline and low phases were coincident with times when per capita production was relatively low and predation pressure high. At these times, ecotrophic efficiencies (EE) suggest there was little production that remained unconsumed by predators. The importance of both production and consumption implies that bottom–up and top–down factors interacted to cause the cycle. EEs of other herbivores (ground squirrels, red squirrels, small mammals, small birds, grouse) were generally low, suggesting weak top–down effects. Predation rates on these “alternative” prey, except ground squirrels, were highest when predators were abundant, not when hares were rare; consequently, any top–down effects reflected predator biomass and were not a function of diet composition or functional responses. Finally, several predators (lynx, coyotes, great-horned owls) showed clear bottom–up regulation, reproducing only when prey exceeded threshold densities. Taken altogether, these results demonstrate that ecosystem models parameterized by population data can describe the dynamics of nonequilibrial systems, but only when detailed information is available for the species modeled. Received 30 November 2000; Accepted 6 September 2001.     

Novel wildlife in the Arctic: the influence of changing riparian ecosystems and shrub habitat expansion on snowshoe hares

Warming during the 20th century has changed the arctic landscape, including aspects of the hydrology, vegetation, permafrost, and glaciers, but effects on wildlife have been difficult to detect. The primary aim of this study is to examine the physical and biological processes contributing to the expanded riparian habitat and range of snowshoe hares (Lepus americanus) in northern Alaska. We explore linkages between components of the riparian ecosystem in Arctic Alaska since the 1960s, including seasonality of stream flow, air temperature, floodplain shrub habitat, and snowshoe hare distributions. Our analyses show that the peak discharge during spring snowmelt has occurred on average 3.4 days per decade earlier over the last 30 years and has contributed to a longer growing season in floodplain ecosystems. We use empirical correlations between cumulative summer warmth and riparian shrub height to reconstruct annual changes in shrub height from the 1960s to the present. The effects of longer and warmer growing seasons are estimated to have stimulated a 78% increase in the height of riparian shrubs. Earlier spring discharge and the estimated increase in riparian shrub height are consistent with observed riparian shrub expansion in the region. Our browsing measurements show that snowshoe hares require a mean riparian shrub height of at least 1.24–1.36 m, a threshold which our hindcasting indicates was met between 1964 and 1989. This generally coincides with observational evidence we present suggesting that snowshoe hares became established in 1977 or 1978. Warming and expanded shrub habitat is the most plausible reason for recent snowshoe hare establishment in Arctic Alaska. The establishment of snowshoe hares and other shrub herbivores in the Arctic in response to increasing shrub habitat is a contrasting terrestrial counterpart to the decline in marine mammals reliant on decreasing sea ice.     

A plant toxin mediated mechanism for the lag in snowshoe hare population recovery following cyclic declines

A necessary condition for a snowshoe hare population to cycle is reduced reproduction after the population declines. But the cause of a cyclic snowshoe hare population's reduced reproduction during the low phase of the cycle, when predator density collapses, is not completely understood. We propose that moderate‐severe browsing by snowshoe hares upon preferred winter‐foods could increase the toxicity of some of the hare's best winter‐foods during the following hare low, with the result being a decline in hare nutrition that could reduce hare reproduction. We used a combination of modeling and experiments to explore this hypothesis. Using the shrub birch Betula glandulosa as the plant of interest, the model predicted that browsing by hares during a hare cycle peak, by increasing the toxicity B. glandulosa twigs during the following hare low, could cause a hare population to cycle. The model's assumptions were verified with assays of dammarane triterpenes in segments of B. glandulosa twigs and captive hare feeding experiments conducted in Alaska during February and March 1986. The model's predictions were tested with estimates of hare density and measurements of B. glandulosa twig growth made at Kluane, Yukon from 1988–2008. The empirical tests supported the model's predictions. Thus, we have concluded that a browsing‐caused increase in twig toxicity that occurs during the hare cycle's low phase could reduce hare reproduction during the low phase of the hare cycle.     

Short‐term response of snowshoe hares to western larch restoration and seasonal needle drop

Old‐growth western larch has been degraded throughout much of its historic range due to extensive timber harvest and fire suppression. We examined the effects of a restoration treatment of western larch on snowshoe hares, a denizen of the boreal forest serving as a focal animal species to indicate the health of the restored ecosystem. We implemented a restoration treatment using “doughnut thinning” to accelerate development of old‐growth attributes in larch stands and simultaneously examined the short‐term effects on snowshoe hare density, survival, and movement. Although typical forest management activities tend to have adverse effects on hares especially in the short term, we found that the restoration treatment did not affect hare density or survival in the short term. In addition, despite significant decreases in cover coinciding with the larch needle drop, we found evidence of year‐round immigration into larch stands by hares suggesting larch stands are suitable year‐round hare habitat. Taken together, our findings suggest that a larch restoration treatment designed to accelerate the development of old‐growth attributes can be implemented so as to have no measurable short‐term detrimental effects on hares.     

Influence of forest structure on the abundance of snowshoe hares in western Wyoming

Snowshoe hares (Lepus americanus) are a primary prey species for Canada lynx (Lynx canadensis) in western North America. Lynx management plans require knowledge of potential prey distribution and abundance in the western United States. Whether even-aged regenerating forests or multi-storied forests contain more snowshoe hares is currently unknown. During 2006–2008, we estimated snowshoe hare density in 3 classes of 30–70-year-old lodgepole pine (Pinus contorta) and 4 classes of late seral multi-storied forest with a spruce (Picea engelmannii)-fir (Abies lasiocarpa) component in the Bridger-Teton National Forest, Wyoming. We recorded physiographic variables and forest structure characteristics to understand how these factors influence abundance of snowshoe hares. In many instances, snowshoe hares were more abundant in late seral multi-storied forests than regenerating even-aged forests. Forest attributes predicting hare abundance were often more prevalent in multi-storied forests. Late seral multi-storied forests with a spruce–fir component and dense horizontal cover, as well as 30–70-year-old lodgepole pine with high stem density, were disproportionately influential in explaining snowshoe hare densities in western Wyoming. In order to promote improved habitat conditions for snowshoe hares in this region, management agencies should consider shifting their focus towards maintaining, enhancing, and promoting multi-storied forests with dense horizontal cover, as well as developing 30–70-year-old lodgepole pine stands with high stem density that structurally mimic multi-storied forests. © 2012 The Wildlife Society.     

Selected microbial agents in snowshoe hares and other vertebrates of Alberta

Serologic surveillance of populations of snowshoe hares and other vertebrate species of north-central Alberta from 1961 to 1969, revealed activity of one bacterial and eight viral agents. The most prevalent agents infecting the snowshoe hare were California encephalitis and Silverwater viruses, while in other vertebrates California encephalitis and Western equine encephalomyelitis viruses were the most common. The role of the snowshoe hare in the natural history of the agents is considered as is the effect of the agent on the hare ten-year cycle of abundance.     

Winter Habitat Selection by Canada Lynx in Maine: Prey Abundance or Accessibility?

Abstract: We related winter habitat selection by Canada lynx (Lynx canadensis), relative abundance of snowshoe hares (Lepus americanus), and understory stem densities to evaluate whether lynx select stands with the greatest snowshoe hare densities or the greatest prey accessibility. Lynx (3 F, 3 M) selected tall (4.4-7.3 m) regenerating clear-cuts (11-26 yr postharvest) and established partially harvested stands (11-21 yr postharvest) and selected against short (3.4-4.3 m) regenerating clear-cuts, recent partially harvested stands (1-10 yr), mature second-growth stands (>40 yr), and roads and their edges (30 m on either side of roads). Lynx selected stands that provided intermediate to high hare density and intermediate cover for hares (i.e., prey access) but exhibited lower relative preference for stand types with highest hare densities where coniferous saplings exceeded 14,000 stems/ha.     

A natural feeding experiment on a declining snowshoe hare population

Summary A snowshoe hare (Lepus americanus) population on a 9-ha area was supplied with extra natural food by chopping down large white spruce (Picea glauca) and aspen (Populus tremuloides) trees throughout 3 winters from 1981 to 1984. Hares fed vigorously on the downed trees, but the phase of decline of the ten-year cycle occurred equally on control and experimental grids from 1981–1983, and we could detect no improvement in survival or reproduction on the food area. Growth rates were improved on the food grid during the first winter of the decline (1981–82). We concluded that food shortage is not necessary for the cyclic decline of snowshoe hares in the southern Yukon.     

Relationship Between Fecal Pellet Counts and Snowshoe Hare Density in Western Wyoming

ABSTRACT Snowshoe hares (Lepus americanus) are an important prey species for Canada lynx (Lynx canadensis) and are considered critical for lynx population persistence. Determination of snowshoe hare distribution and abundance is needed by land management agencies for lynx conservation. An accepted approach for estimating snowshoe hare abundance is the use of fecal-pellet plot counts. Locally derived regression equations are preferred for accurate calibration of pellet counts to snowshoe hare density due to local differences in pellet deposition and decomposition. We used linear regression to examine correlations between snowshoe hare density, as determined by mark–recapture estimates, and pellet plot counts on both uncleared plots and annually cleared plots on the Bridger-Teton National Forest, western Wyoming, USA. We found significant correlations between snowshoe hare density estimates and fecal pellet counts for both uncleared and annually cleared pellet counts; however, the relationship was stronger (higher r) when using pellet counts from annually cleared plots. In addition, we found that adjusting the buffer size by omitting hard habitat edges (not used by hares) around trapping grids improved correlations between snowshoe hare density and fecal pellet counts for both uncleared plots and annually cleared plots. Though precision is sacrificed when using uncleared plots, they may be useful as a coarse index of habitat use by snowshoe hares. Our derived regression equations may be useful to identify important foraging habitat for Canada lynx in western Wyoming. Land managers responsible for conserving snowshoe hare habitat in western Wyoming may use these equations to monitor changes in hare populations among habitats and during prescribed management actions.     

Sinks without borders: snowshoe hare dynamics in a complex landscape

A full understanding of population dynamics of wide-ranging animals should account for the effects that movement and habitat use have on individual contributions to population growth or decline. Quantifying the per-capita, habitat-specific contribution to population growth can clarify the value of different patch types, and help to differentiate population sources from population sinks. Snowshoe hares, Lepus americanus , routinely use various habitat types in the landscapes they inhabit in the contiguous US, where managing forests for high snowshoe hare density is a priority for conservation of Canada lynx, Lynx canadensis . We estimated density and demographic rates via mark–recapture live trapping and radio-telemetry within four forest stand structure (FSS) types at three study areas within heterogeneous managed forests in western Montana. We found support for known fate survival models with time-varying individual covariates representing the proportion of locations in each of the FSS types, with survival rates decreasing as use of open young and open mature FSS types increased. The per-capita contribution to overall population growth increased with use of the dense mature or dense young FSS types and decreased with use of the open young or open mature FSS types, and relatively high levels of immigration appear to be necessary to sustain hares in the open FSS types. Our results support a conceptual model for snowshoe hares in the southern range in which sink habitats (open areas) prevent the buildup of high hare densities. More broadly, we use this system to develop a novel approach to quantify demographic sources and sinks for animals making routine movements through complex fragmented landscapes.     

Herbivores as architects of savannas: inducing and modifying spatial vegetation patterning

In this paper, we address the question whether and through which mechanisms herbivores can induce spatial patterning in savanna vegetation, and how the role of herbivory as a determinant of vegetation patterning changes with herbivore density and the pre-existing pattern of vegetation. We thereto developed a spatially explicit simulation model, including growth of grasses and trees, vertical zonation of browseable biomass, and spatially explicit foraging by grazers and browsers. We show that herbivores can induce vegetation patterning when two key assumptions are fulfilled. First, herbivores have to increase the attractiveness of a site while foraging so that they will revisit this site, e.g. through an increased availability or quality of forage. Second, foraging should be spatially explicit, e.g. when foraging at a site influences vegetation at larger spatial scales or when vegetation at larger spatial scales influences the selection and utilisation of a site. The interaction between these two assumptions proved to be crucial for herbivores to produce spatial vegetation patterns, but then only at low to intermediate herbivore densities. High herbivore densities result in homogenisation of vegetation. Furthermore, our model shows that the pre-existing spatial pattern in vegetation influences the process of vegetation patterning through herbivory. However, this influence decreases when the heterogeneity and dominant scale of the initial vegetation decreases. Hence, the level of adherence of the herbivores to forage in pre-existing patches increases when these pre-existing patches increase in size and when the level of vegetation heterogeneity increases. The findings presented in this paper, and critical experimentation of their ecological validity, will increase our understanding of vegetation patterning in savanna ecosystems, and the role of plant–herbivore interactions therein.     

Food competition between a large ruminant and a small hindgut fermentor: the case of the roe deer and mountain hare

In this study, we demonstrate that the mountain hare and roe deer compete with each other. This was determined using "natural experiments" of populations found in sympatry and allopatry on the islands along the west coast of Norway. We demonstrate that both species occupy the same habitats, share the same food resources and that food availability is limited. Two browsing species as different in size as roe deer and mountain hare might be expected to partition the available vegetation (e.g. woody scrub) in terms of height above ground level. However, from the evidence collected, the feeding-height-separation hypothesis must be rejected as an explanation for ecological separation between roe deer and mountain hares because there was extensive height overlap in resource utilisation by both species and neither species changed its feeding height in response to the presence of the other. Total browse utilisation did not increase when both species were together; rather, species-specific browse utilisation declined when the other species was present. However, the foraging behaviour of each herbivore varied significantly between the allopatric and sympatric sites. When both herbivores were present, the clip diameter of shoots browsed by mountain hares declined to match those selected by roe deer, while roe deer switched from a browse-dominated diet to a diet dominated by winter-green gramineae. The change to smaller-diameter shoots likely resulted in the hare increasing its intake of digestion-inhibiting or toxic secondary metabolites, while the alternative choice of digging through the snow like roe deer to reach the winter-green gramineae is a practice considered energetically too costly for hares. On this basis, we conclude that the enforced switch to a nutritionally inferior diet by mountain hares at the sympatric sites may result in changes to growth rate and body size which consequently impact on mortality and may explain the competitive superiority of the roe deer.     

Demographic differences in diet breadth of Canada lynx during a fluctuation in prey availability

Population dynamics of specialist carnivores are closely linked to prey availability, but the extent of variability in diet breadth of individual carnivores relative to natural variability in the abundance of their primary prey is not well understood. Canada lynx (Lynx canadensis) specialize on snowshoe hares (Lepus americanus) and exhibit cyclic fluctuations in abundance that lag 1–2 years behind those of snowshoe hares. Declining hare densities spur demographic changes in lynx, but it is unclear whether a corresponding increase in diet breadth occurs: (1) broadly across a lynx population; (2) only among individuals who are able to effectively switch to alternative prey; or (3) only among individuals who cannot capture sufficient primary prey. We measured stable isotope ratios of lynx muscle tissue spanning a cyclic increase and decline in hare density (1998–2001) in Fort Providence, NT, Canada. We found that lynx cohorts responded differently to hare population change, with yearling animals having broader diets at low hare densities, while adults and dependent juveniles maintained a constant diet through the initial decline in hare density. This result was consistent irrespective of lynx sex and indicates that yearling lynx likely are forced to adopt a broader diet when primary prey densities decline. Our results imply that select cohorts of specialist carnivores can exhibit high dietary plasticity in response to changes in primary prey abundance, prompting the need to determine whether increased diet breadth in young lynx is a successful strategy for surviving through periods of snowshoe hare scarcity. In this way, cohort‐specific niche expansion could strongly affect the dynamics of organisms exhibiting population cycles.