Predator-Prey Dynamics in Southern Boreal Forests of Washington State

Abstract

Wildlife species, particularly predators with specialized habitat needs and ecological roles, are increasingly vulnerable to range contractions due to human influence, including climate change. This vulnerability to anthropogenic impacts is heightened when behavior-specific habitat requirements are not well understood, hindering informed management decisions. While larger predators often receive more attention and research funding, many smaller mammalian predators (i.e., mesopredators) are also impacted by rapid environmental change. Mesopredator habitat needs remain understudied—most crucially, the habitat required for successful foraging—because observing the foraging behavior of these smaller-sized predators is challenging, particularly in areas and seasons without snow-tracking conditions. Prey-tracking, combined with advanced forensic techniques such as swabbing for predator saliva on remains, can reveal otherwise undetectable mesopredator kill sites. This approach offers insight into habitat use beyond traditional methods, illuminating where a successful predation event—which depends on a complex sequence of events—occurs on the landscape. An important mesopredator whose foraging ecology is less understood compared to larger carnivores is the Canada lynx (Lynx canadensis). This snow-adapted felid is currently listed as Threatened under the US Endangered Species Act (ESA) within the contiguous United States, where its range has contracted significantly. These remaining southern-range lynx populations face a complex web of threats, including habitat loss, fragmentation, and increased competition with generalist predators. Climate change adds further complexity through altered snowpack conditions—potentially affecting the lynx’s competitive edge—and by increasing stand-replacing wildfires, which have jeopardized lynx persistence by eliminating large swaths of remaining usable habitat. Understanding how southern lynx utilize their remaining habitat for successful foraging, especially outside of winter, is crucial for their conservation. However, despite a broad-scale co-occurrence with their preferred prey, the snowshoe hare (Lepus americanus), knowledge gaps remain regarding the specific foraging habitat features that support lynx within these areas. This dissertation explores three dimensions of lynx foraging ecology in the context of a multi-predator southern boreal forest landscape across snow-on and snow-free seasons: where lynx successfully target prey on the landscape, how the habitat at lynx kill sites differentiates them from competing predators, and what factors drive the overall survival of hares—the shared prey species intrinsically linked to lynx persistence. To help address these knowledge gaps, I conducted fieldwork within the Okanogan Lynx Management zone (LMZ) in Washington state, USA, where lynx are listed as state Endangered and identified as a species of greatest conservation need. I trapped, radio-collared, and monitored snowshoe hares, applied forensic techniques to identify predator species at hare kill sites, and measured a variety of habitat data. In my first dissertation chapter, I establish the need to better understand mesopredator ecology, particularly lynx conservation needs, and highlight the utility of the insights into predators such as lynx that prey-tracking studies such as those in my subsequent chapters can offer. In Chapter 2, I examine lynx foraging behavior by discerning the fine-scale habitat features that predict where lynx successfully capture hares within preferred hare habitat areas. Lynx caught hares primarily in cover, when broadly classified by category type, but I found more evidence for lynx targeting hares where landscape features facilitate capture (accessibility hypothesis) instead of in areas with the maximum-cover features associated with the highest hare abundance, such as regenerating forest or stem density (prey abundance hypothesis). Compared to available nearby habitat, lynx kill sites were most strongly predicted by nearby ambush features behind which a lynx might hide and forest structural complexity, with ambush features consistently being the strongest predictor. Horizontal cover within a height of 0.5-1 m also helped explain where lynx caught hares on the landscape to a lesser extent. These findings underscore the importance of fine-scale, microhabitat features in enabling lynx to forage within areas supporting their main prey, thus promoting a crucial interaction that directly improves lynx fitness. In Chapter 3, I examine whether lynx differentiate from competing predators in the habitat features at their kill sites. Lynx targeted hares in a narrower range of habitat features compared to generalist predators, bobcats (Lynx rufus) and coyotes (Canis latrans), whose kill sites were more similar to each other and more heterogeneous overall than those of lynx. Variables that significantly differentiated lynx kill sites from both bobcats and coyotes included forest structural complexity, a high-cover type category, percent horizontal cover within a height of 0.5-1 m, presence of snow, and nearby ambush features. Rather than being distinguished by hunting mode—a hypothesis expecting lynx and bobcats to forage similarly because they are both felids—the three mesopredators aligned with a niche breadth hypothesis (specialists versus generalists). This finding suggests that lynx specialize in where they catch hares compared to these competitors in a southern range population. An important implication of the narrower but overlapping range of habitat features used by lynx is that competition with generalist predators may not be alleviated by niche partitioning. To maintain viable lynx populations in areas with coyotes and bobcats, managers should provide ample hare habitat and promote the habitat features that most strongly differentiate lynx from competitors in where they catch hares. In Chapter 4, I investigate seasonal, habitat, and demographic patterns of snowshoe hare survival. Seasonal survival baselines revealed the lowest hare survival in spring and summer, suggesting a potential role for generalist predators, which are most active during those seasons. Season-specific Cox proportional hazards models supported this pattern, showing that habitat fragmentation (open matrix habitat) negatively impacted hare survival in summer, especially for female hares. Female hares were particularly vulnerable overall in both summer and autumn, likely due to increased energetic demands for reproduction affecting their movement patterns. This reduced survival of female hares when breeding, particularly at fragmented sites in summer when generalist predator pressure is highest, has important implications for the demographics and management of this prey resource for lynx in the face of climate change.