When organisms require the same limited resources within an environment, they may have to compete for them. Competition is a net-negative interaction. Even if two competing individuals or populations do not interact directly, the overall fitness of both competitors is lowered as a result of not having full access to the limited resource.

Intraspecific competition, which occurs between individuals of the same species, serves as a natural mechanism for regulating population size. Too much population growth can lead to crowding and diminished resources. Stronger members of the population may outcompete weaker individuals for resources, leading to reduced reproduction or death for the weaker individuals and keeping the population size in check.

Competitive exclusion may occur as a consequence of competition between species, where one is better suited to use a resource and forces out the other, but this is not the only possible outcome when a resource is not abundant. Organisms can also find ways to share limited resources. Competing populations may engage in resource partitioning, dividing the resource in a spatial manner by keeping to non-overlapping territories or using the resource at different times of the day. Alternatively, one population might differentiate its niche so that it no longer has to compete.

Many similar species of anole lizard coexist on islands around the Caribbean Sea, and the anoles of each island avoid competition with each other by residing in slightly different locations within their habitat, an example of spatial resource partitioning. The lizards then prey only upon the insects that enter their preferred territory, effectively dividing up available food sources. This diminishes competition for food and reduces direct conflicts between the different species.

Ultimately, competition provides an evolutionary selection pressure both within and between species when resources are not plentiful, forcing organisms to adapt or risk dying out if they cannot successfully compete.