Foster: Many species of extremely large North American mammals became extinct during the last ice age, which was also the time of the first human migration to North America. These species could not survive the dramatic changes wrought by this human migration. Fisch: Those extinctions were caused by the dramatic shift to a harsher climate. The climate changed so rapidly that the species could not adapt.
The statement 'The huge size of the mammals made it difficult for them to migrate the great distances to milder environments' most strengthens Fisch's argument.