Some populations of the North American caribou, for example many herds in the subspecies barren-ground caribou and some woodland caribou in Ungava and Labrador, migrate the farthest of any terrestrial mammal, travelling up to 5,000 km (3,000 mi) a year, and covering 1,000,000 km2 (400,000 sq mi).[2][46] Other North American populations, the woodland caribou (boreal) for example, are largely sedentary.[47] In Europe populations have a shorter migration. 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.[48]