Splitting Hares: When climate increases predation on a keystone species

Northern New Hampshire, January 2016.  I was doubtful that I was going to find lynx tracks.  As a Master’s student, I had spent most weekends doing field work in northern New Hampshire and never found lynx tracks.  However, that was three years ago and I have since learned that distribution patterns can change considerably within that timeframe.  I drove my truck around the mountain and slowed down as I passed the area where I used to count carnivore tracks.  Turning the corner, I spotted a set of tracks crossing the road.  I immediately thought they looked cat-like.  The warm sun had begun to degrade the snow but I could see the telltale characteristics of a lynx.  The imprints were large and round with toe and heel pads obscured, and the gait was a classic cat-like walking stride.  Canada lynx have relatively large feet compared to their body size, with hair covering most of their pads that provides an advantage in the cold and snowy conditions of northern New England.  Not only was I gazing at lynx tracks, but I soon figured out the adjacent tracks were that of a bobcat – a species that I only detected 3 times since 2010!

Lynx and bobcat walking stride for stride. Photo: Alexej Siren

As you might imagine I was out of my truck pretty quickly.  My research partner had never tracked lynx so she was excited too.  We set out on a tracking adventure and within an afternoon followed the lynx to a day bed, found a scat, and discovered some of the best snowshoe hare habitat that I’ve seen all year.  One of the most interesting and notable finds, though, was that the bobcat and lynx tracks were side by side and overlapping for the entire backtrack.  I had seen this once before but it was very rare to find the two species together.  We couldn’t quite tell if they were together or if one was lagging behind, but we found what looked to be a bed where the two species interacted.  Lynx and bobcats are competitors but they can also breed and hybridize.  This may be seen as a detriment to lynx because bobcats are a better competitor and hybridization may lead to decreased fitness.  However, there is evidence that hybridization may also increase the adaptive capacity (i.e. ability to withstand and absorb change) of lynx to adapt to a changing climate.  A changing climate with less snowfall may increase overlap and interactions between these similar species.

Snowshoe hare Photo credit: Mills et al., 2013, PNAS

Let me frame this story within the context of competition for food.  Many of the mid-sized carnivores in the northeastern United States rely on snowshoe hares during the winter months.  Since hares are snow-adapted species, with higher survival in deep and powdery snow that persists the length of time their coat-color remains white, then a changing climate with shorter winters with shallow and crusty snow spells trouble.  Recent research from SE CSC Fellow Marketa Zimova, indicates that hares have lower survival when their coat-color is mismatched against the background conditions (e.g., white coat-color without snow).  Also, the hardness of the snow surface has been linked with higher predation rates for lynx and coyotes.  Given the predicted increase in snow crusting due to warmer winters, these conditions may further compromise hare populations.  But there may be hope for hares and other boreal forest species in the northeastern US.  Montane forest contains extensive spruce-fir habitat – hares preferred cover – and deep snow that is predicted to persist into the future. 

Investigating snowshoe hare survival and risk to predation is the basis for my PhD research at the Northeast Climate Science Center.  I have been studying boreal forest species for over a decade and am particularly interested in understanding how snowpack mediates community interactions at the distributional edge of the boreal forest.  I am using a combination of camera-traps and snow track surveys to evaluate how carnivore distribution shifts in relation to seasonal and annual changes in snowpack.  I am primarily focused on lynx and martens, but also studying competing carnivores (i.e., bobcats, fisher, red fox, and coyotes) because their presence on the landscape likely influences the viability of boreal forest species.  I am also studying snowshoe hares to compare demographic parameters (e.g., density and survival) along elevational gradients.  Understanding the patterns that drive current population dynamics will be critical for disentangling how these species might respond to climate change and if there will be refugia in the mountains as winter slowly creeps northwards. 

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Alexej is a NE CSC graduate fellow and wildlife ecologist studying boreal and subboreal species. For more information on Alexej’s work check out his recent article in the Wildlife Journal on snowshoe hare research in New Hampshire and Vermont.