Competition (biology)

Competition is an interaction between organisms or species in which the fitness of one is lowered by the presence of another. According to evolutionary theory, this competition within and between species for resources plays a role in natural selection.
Competition occurs when organisms or species need or use the same resources. Common resources are food, water and territory.
Two species in competition often continue long-term in the same environment. This suggests they each have an "edge" which the other does not. In may cases "being there first" is a big edge. For example, plants consume nitrogen by absorbing it into their roots, making nitrogen unavailable to nearby plants. 
Species in competition may directly interfere with each other. An example of this can be seen between the ant "Novomessor cockerelli" and red harvester ants, where the first interferes with the ability of the second to forage. It does this by plugging the entrances to their rival's colonies with small rocks.