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"Competition is a negative interaction that occurs among organisms whenever two or more organisms require the same limited resource. All organisms require resources to grow, reproduce, and survive. For example, animals require food (such as other organisms) and water, whereas plants require soil nutrients (for example, nitrogen), light, and water. Organisms, however, cannot acquire a resource when other organisms consume or defend that resource. Therefore, competitors reduce each other's growth, reproduction, or survival."
http://www.biologyreference.com/Ce-Co/Competition.html#b