Two lions fight over a kill. Two oak seedlings race for light in a forest gap. Competition is the struggle between organisms for limited resources. It can be within a species, called intraspecific competition, or between species, called interspecific competition. Resources include food, water, territory, mates, and shelter. When resources are plentiful, competition is weak. When they are scarce, it intensifies, and some individuals or species lose out.

Competition shapes evolution. Individuals with traits that give them an edge survive and reproduce more. Over time, species may diverge to reduce overlap. This is the competitive exclusion principle, articulated by Georgy Gause in the 1930s. Two species cannot occupy the same niche indefinitely. One will outcompete the other, or they will partition resources and coexist. Warblers in a spruce forest feed at different heights, reducing direct competition. That partitioning allows several species to share the same habitat.

Types of competition

• Intraspecific: within a species.
• Interspecific: between species.
• Exploitation: using a resource faster than a rival.
• Interference: actively blocking a rival's access.

Competition is not always violent. It can be quiet and slow, played out over generations through slight differences in efficiency and survival.