Since the 1990s, moose populations have declined dramatically in much of temperate North America, although they remain stable in arctic and subarctic regions. The exact cause of the die-off is not determined, but appears to be a combination of factors, including a change in habitat and heat stress caused by global warming,and the northward migration of warmer-weather parasites to which moose have not developed a natural defense, such as liver flukes, brain worms and winter tick infestations.