The Canadian elk has traditionally been limited in its geographic range by the presence of elderberries, its primary source of food, which only grow up until a certain point in the tundra. A recent rise in temperatures, however, has seen a spread in the growth of elderberries northwards into the tundra. Therefore, the overall range of the Canadian elk can be expected to increase.
Which of the following, if true, best casts doubt on the argument?
【选项】The grizzly bear, the Canadian elk`s primary predator, has also started moving north into the tundra.