Attacked from two fronts: Interactive effects of anthropogenic and biotic disturbances generate complex movement patterns

Anthropogenic and biotic disturbances have the potential to interact, generating cumulative impacts on animal movement or, alternatively, counterbalancing or masking each other. Despite their importance, those interactions have not been investigated thoroughly. Our study aimed to fill this knowledge gap by assessing the combined effects of a human activity—that is, military exercises—and a biotic disturbance—that is, insect harassment—on movement rates of free-ranging semidomesticated reindeer (Rangifer tarandus tarandus). From 2010 to 2012, we analyzed location data from fifty-one Global Positioning System (GPS)-collared female reindeer in the largest European military test range, situated in northern Sweden. In the presence of both military exercises and mosquito harassment, reindeer reacted by increasing their movement rates but not as much as when mosquito harassment occurred alone. Conversely, reindeer reduced their movement rates during military exercises performed with aircraft. Moreover, the effect of military exercises performed with vehicles was evident only when combined with mosquito harassment. These results stress the value of evaluating the effects of the interaction between biotic disturbances and human activities, especially in northern ecosystems, because of the predicted climate warming and the growing interest toward natural resource extraction and other forms of land use.