Predation risk for hoary marmots in Washington’s North Cascades Mountains

Declines in populations of small mammals associated with high elevations, e.g., marmots (Marmota spp.) and pikas (Ochotona spp.), have been attributed to both direct and indirect effects of environmental changes caused by humans. For example, populations of Olympic marmots (M. olympus) and Vancouver Island marmots (M. vancouverensis) have declined in response to increased predator access to high-elevation marmot habitats. In the North Cascades National Park Service Complex (NOCA), observed mean abundance of hoary marmots (M. caligata) declined by 74% from 2007 to 2016. Although these declines have been linked to harsh winter conditions, the role of predation and its association with decreasing snowpack has yet to be explored in this system. We noninvasively examined these predator–prey dynamics by coupling behavioral surveys of hoary marmots with indices of predation risk. We conducted 145 vigilance samples and 39 flight initiation distance trials to measure Hoary Marmot antipredator behavior. We derived a risk index for each predator by weighting its probability of use estimates (the probability of a predator–prey encounter) with its observed proportion of Hoary Marmot predation (probability of prey death given an encounter). We used genetic metabarcoding of vertebrate DNA from 413 predator scat samples to quantify predation on hoary marmots. We surveyed 9,930 trap-nights using 130 remote cameras to model predator probability of use near Hoary Marmot colonies. From camera traps and scat DNA, we detected 10 predator species in the study area, and we detected hoary marmots in the scats of 5 of these species. The proportion of observed Hoary Marmot predation was highest for coyotes (Canis latrans) at 50%. To our knowledge, we also report the first record of hoary marmots being consumed by Pacific martens (Martes caurina), which were also significant predators (proportion of observed predation = 32%). We expected predators with low-elevation habits to use sites with lesser snow persistence; this prediction was supported for coyotes but not for other typically low-elevation predators. We found estimated risk experienced by hoary marmots was highest from Pacific martens and lowest from Canada Lynx (Lynx canadensis). Contrary to our predictions and despite hoary marmots allocating >50% of their time to vigilance, neither estimates of predator risk nor human presence explained variation in marmot antipredator behavior. Based on the results from both our dietary and probability of use analyses, we expect coyotes to have an increasing effect on NOCA’s Hoary Marmot population as the climate warms and Coyote range continues to expand. Thus, our work highlights the need to better understand the effect of climate-driven shifts in predator–prey ecology in high-elevation systems.