Star barnacles and acorn barnacles compete with each other for space on the best rocks. If the star barnacles were not there, acorn barnacles would move down from their positions above the low tide line, and be covered by water all day long. But the two are able to coexist because the acorn barnacles can still survive on the less hospitable rocks. In ecology, such a pattern of coexistence is described by something called the ΓÇÿcompetitive exclusion principleΓÇÖ. According to this idea, if two species are in competition with each other, they will only be able to coexist if at least one of the species uses fewer or worse resources than it would if the other species were not there. If both species must have exactly the same resources, however, then one species will exclude the other. So, if acorn barnacles could not live above the low tide line, they would not be able to coexist with the star barnacles: they would have been excluded from their niche by competition.
