Competition between members of different species can be an important force in evolution. In addition to shaping the particular ecology of an area ­ which species are present where ­ it also shapes the evolution of the species around them. For example, if two species of bird compete for the same nest sites but not for the same food, the success or failure of one of the species can have enormous effects on whatever organisms the birds usually eat.
Often, competition between two species can have a bigger effect on one of the species than on the other. Sometimes this effect is the result of direct interference: star barnacles interfere with acorn barnacles by preventing them from sitting on the best rocks. But sometimes this effect is indirect, mediated through something else such as food supply.
