The classic example is Paine's work in the rocky intertidal of the Pacific Northwest. He removed the sea star Pisaster from an experimental plot and the number of prey species declined from 15 to 8. Removal of the keystone predator allowed barnacles and mussels to crowd out other species. When Pisaster is present it keeps the populations of barnacles and mussels low enough to allow other species to survive. Character Displacement Species in living in the same area are said to be sympatric .
Species living apart are allopatric .
Closely related sympatric species may diverge in morphology or behavior over time via evolution. This phenomenon is known as character displacement . Some ecologists suggest that character displacement is due to past competition between the species. Figure 22-25 illustrates hypothetical character displacement. Figure 22-26 illustrates a possible example of character displacement among some of Darwin's finches.