Predicting the Fitness Effects of Climate Change on Snowshoe Hares

C CHANGE IS CONSIDERED ONE OF the greatest potential threats to global biodiversity (Thomas et al., 2004). Currently, it is altering the distribution and abundance of numerous species (Parmesan, 2006), causing numerical declines in populations that are unable to adapt (Kausrud et al., 2008) and potentially leading to species extinction (McLaughlin et al., 2002). Predicting the impacts of climate change on species demography, however, remains challenging because of the complexities associated with species’ responses to shifting environments. One such complexity is how climate disruption affects biotic interactions (Heller and Zavaleta, 2009) and in particular, predator-prey dynamics (Post et al., 1999). Predator-prey interactions are characterized by five stages (see Fig 1; adapted from Sih, 2011), and the likelihood of a prey being consumed is determined by the frequency of encounters and the conditional probability of each subsequent stage in the interaction. Therefore, climate change may reduce prey survival either by increasing encounter rates or by increasing the probability of any stage in the interaction. For example, increased snow depth caused wolves (Canis lupus) to hunt in larger packs, improving hunting success, which subsequently tripled kill rates of moose (Alces alces; Post et al., 1999). Furthermore, warming temperatures increase swimming speed of northern pike (Esox lucius) relative to brown trout (Salmo trutta) and increase attack success (Öhlund et al., 2015). Most systems, however, consist of multiple predator species, and climate change may alter the relative importance of different predators to prey mortality rates through differential influences on any stage in the interaction (Fig. 1). This alteration may in turn influence future population dynamics of the prey. However, these complexities remain largely unexplored, because of the difficulties of simultaneously monitoring detailed changes in environmental conditions and cause-specific mortality of the target species. The snowshoe hare (Lepus americanus) is a keystone prey species of the boreal forest (Krebs, 2011) and experiences regular population cycles that occur over 10-year periods (Krebs et al., 2001). Individual behaviour and population dynamics of hares are shaped heavily by predation, which can account for 85% – 100% of mortalities in certain locations and years (Hodges, 2000). Changing winter conditions could be especially relevant for disrupting interactions between hares and their predators. Hares have a lower foot load than other species (Murray and Boutin, 1991), which provides them with an advantage over their predators in deep, soft snow. Changes in winter climate and the subsequent consequences for snow conditions could influence hare survival by altering predator hunting success Predicting the Fitness Effects of Climate Change on Snowshoe Hares