Whilst we often think about human activities negatively altering the landscape, sometimes these changes can have unexpected benefits for predators. Of course, this is not necessarily good news for prey species! A paper recently published in the Journal of Animal Ecology examined this with regard to linear features and predator-prey dynamics in the forests of Canada. Lead author Dr Craig DeMars, a postdoctoral fellow at the University of Alberta, tells us more about these predator highways.
Human changes to natural landscapes have long been known to impact biodiversity. While such changes include habitat loss and fragmentation, it is becoming increasingly recognized that landscape changes can lead to species endangerment by altering community dynamics, particularly those involving predator and prey. Changes in predator-prey interactions can result from differences in behavioural responses to new environmental features created from landscape alteration. Because such features may not have been experienced in a species’ evolutionary past, species responses and the resultant outcomes on predator-prey dynamics are often unpredictable and context-specific. In some instances, prey may win the “space race” if landscape alteration results in additional safe spaces from predators. For example, in the mountainous regions of southeastern British Columbia, Canada, mule deer (Odocoileus hemionus hemionus) have become increasingly habituated to urban areas, presumably because these areas provide a “human shield” against predators (e.g. wolves [Canis lupus] and cougars [Puma concolor]), which are less tolerant of human activity (Fig. 1). In other situations, predators may be favoured if landscape alteration improves their hunting efficiency or increases their spatial overlap with prey.
In our study, we evaluated how human-created linear features such as roads, pipelines, and natural resource exploration (or seismic) lines impact the dynamics among boreal caribou, an ecotype of woodland caribou federally listed as Threatened under Canada’s Species at Risk Act, and their two main predators, wolves and black bears (Ursus americanus), during the calving season of caribou. Linear features (LFs) are abundant within the boreal forests of western Canada (Fig. 2) and have been linked to population declines of boreal caribou via two hypothesized mechanisms. The first is that LFs enhance the movement rate of wolves, thereby increasing their encounter rate with prey. The second is that LFs facilitate predator movement into caribou habitat. We specifically tested this latter mechanism by (i) assessing the behavioural responses of caribou, wolves and bears to LFs; and (ii) evaluating whether use of LFs by female caribou influenced the survival of their neonatal offspring.
In the western half of their distribution, boreal caribou primarily occur within peatlands (bogs and nutrient-poor fens) and are uniquely adapted to forage on terrestrial lichens within this habitat type. The selection of peatlands by caribou is thought to be an evolutionary strategy for reducing predation risk as it spatially separates caribou from other ungulates (e.g. moose [Alces americanus]) and their generalist predators. Because caribou occur at low densities within peatlands, it is unprofitable for predators to hunt within this habitat type. Indeed, results from our study support this refugia effect as peatlands were one of the lowest ranked habitat types selected by wolves and bears.
Within boreal caribou ranges in our study area, LFs currently occur at relatively high densities (average density = 4 km/km2) with many traversing peatland complexes inhabited by caribou (Fig. 3). Our results indicate that wolves and black bears highly select these features when travelling, including those situated within peatlands. In response, female caribou with calves generally avoided LFs (Fig. 4), but complete avoidance is difficult due to the high LF density within caribou ranges – there are literally few places left for caribou to hide or space away from these predator highways. How maternal females responded to LFs also had demographic effects; specifically, the more females used LFs, the higher the probability their calves would die. By facilitating predator movement into and within peatlands and negatively impacting calf survival, LFs appear to have reduced the efficacy of peatlands as safe spaces for caribou.
Results of our study demonstrate how LFs can contribute to species endangerment by altering the “space race” between predator and prey. For boreal caribou, mitigating LF effects has become a management priority for stabilizing and recovering declining populations (Fig. 5). Because one of these effects is a reduction in refuge quality, we suggest that a high priority be given to limiting or restoring LFs that facilitate predator movement into peatland habitats.
DeMars, C.A. and Boutin, S. (2018) Nowhere to hide: Effects of linear features on predator–prey dynamics in a large mammal system. Journal of Animal Ecology, 87: 274–284. https://doi.org/10.1111/1365-2656.12760