Competition

Competition is the interaction in which two or more species (or two individuals of the same species) are harmed because they depend on a resource that is in short supply relative to demand -- represented as (−, −). Unlike predation, competition does not require any direct physical contact or consumption between the competing organisms; it is enough that both need the same limiting resource, whether that is sunlight, water, soil nutrients, prey, nesting sites, or physical space.

A foundational idea in the study of competition is Gause's Competitive Exclusion Principle, based on laboratory experiments the Russian ecologist G. F. Gause carried out with two species of the protozoan Paramecium, P. aurelia and P. caudatum. When grown separately in culture, each species thrived; but when the two species were grown together in the same culture vessel, sharing the same finite food supply, P. aurelia consistently out-competed and eventually eliminated P. caudatum from the mixed culture. From this and similar experiments, Gause proposed that two species competing for exactly the same limited resource cannot coexist indefinitely in the same place -- the species that is even marginally more efficient at using the shared resource will, over time, competitively exclude the less efficient one.

Field studies have reinforced this idea. In an often-cited study of two species of barnacle (Chthamalus and Balanus) living on rocky shores, the ecologist Joseph Connell showed experimentally that Chthamalus is capable of living over a much broader vertical zone of the shore than it is normally found in, but is confined to the drier, upper zone in nature specifically because the faster-growing Balanus competitively excludes it from the wetter, lower zone whenever the two are allowed to interact. …