3.3 The struggle for existence
During their lives, guppies face a variety of environmental hazards which cause mortality. They must find food and, if food supply is limited, some will die through starvation. Heavy rain periodically causes floods which may wash a large part of a population out to sea; occasional droughts cause populations to perish when streams dry out. Like all organisms, guppies are attacked by a rich variety of parasites and diseases. Of most interest to us in this discussion is that guppies are preyed upon by larger, predatory fishes. Of importance to what follows is the fact that, in their natural habitat, some streams contain many predatory fish, others contain few or none. There is thus variation in the level of predation to which wild populations of guppies are subjected.