Scared of a ghost: Why Newfoundland caribou migrate despite the disappearance of their historic predator

By Guillaume Bastille-Rousseau 

Migration in animals is one of the most spectacular biological phenomena on the planet. Theory predicts that, to evolve, the benefits of migration (in terms of lifetime reproductive success) should outweigh the costs of moving over such long distances. Once a common behaviour, recent studies have shown that, in many taxa, migratory behavior is disappearing.

Woodland caribou (Rangifer tarandus) are well known for their long-distance migration, a strategic behaviour generally expected to reduce predation. On the island of Newfoundland (located at the eastern point of Canada) around 14 caribou herds migrate to their calving ground annually, despite the absence of their historical predator, grey wolves (Canis lupus) for almost a century. Over the last two decades, caribou in Newfoundland have declined by over 60%, raising questions about the effectiveness of their behavioural strategies.

We sought to quantify the potential benefits of large-scale movement and habitat selection of Newfoundland caribou in regards to vegetation availability and predation risk on calves. We used remote sensing index to quantify vegetation abundance and monitored movements of 66 black bears (Ursus americanus) and 59 coyotes (Canis latrans) using GPS telemetry to create models of predation risk. We then used GPS telemetry locations from 114 female caribou to asses food-predation trade-offs. To isolate the benefits of habitat selection associated specifically with caribou large-scale movements, we developed a mechanistic model of movement mimicking caribou finer-scale movement. Comparing habitat use predicted from this model to the actual habitat use of caribou allowed us to infer what caribou are selecting and avoiding while performing large-scale movement.

Our results showed that migratory movements of caribou were oriented mainly towards habitats with abundant forage and lower risk of bear (and to a lesser extent coyote) encounters. Caribou are therefore able to balance food acquisition against predation risk and continue to benefit from migration in the current environment. In our fast-changing world, it is perhaps comforting to see that, sometimes, an old strategy is still up to today’s challenges.    

Average (± 95% C.I.) exposure to forage biomass, coyote encounter risk and bear encounter risk for female caribou from five herds, Newfoundland. Actual exposure (Use) is compared to availability represented by two scales of movement: (i) Selection of a calving ground (2nd-order)) and (ii) movement within calving-ground (3rd-order)). Availability at each scale was also defined using two approaches; (i) a random model (Random) and (ii) simulated locations based on mechanistic modelling of fine-scale movement (Mechanistic). Overall, selection is inferred when use is higher than availability while avoidance is inferred otherwise.

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