Some populations of North American caribou, for example many herds in the barren-ground caribou subspecies and some woodland caribou in Ungava and Labrador, migrate the farthest of any terrestrial mammal, travelling up to a year, and covering . Other North American populations, the boreal woodland caribou for example, are largely sedentary. The European populations are known to have shorter migrations. Island herds such as the subspecies "R. t. pearsoni" and "R. t. platyrhynchus" make local movements. Migrating reindeer can be negatively affected by parasite loads. Severely infected individuals are weak and probably have shortened lifespans, but parasite levels vary between populations. Infections create an effect known as culling: infected migrating animals are less likely to complete the migration.