Many scholars believe this to be the result of the spread of European infectious diseases. Others believe that most of the Monongahela were killed by or assimilated into either the Iroquois or the Algonquian-speaking Lenape tribes during warfare, as the more powerful tribes competed to control area hunting grounds for the fur trade. Still others claim that two severe droughts, one from 1587 to 1589 and another from 1607 to 1612, drove the Monongahela from the region in search of a more habitable area.