Two protozoan species, Paramecium aurelia and Paramecium caudatum, when grown individually in the laboratory, they both thrive. But when they are placed together in the same culture plate, It leads to the latter's eventual extinction. This phenomenon is called

Gause’s ‘Competitive Exclusion Principle’ states that two closely related species competing for the same resources cannot co-exist indefinitely and the  competitively inferior one will be eliminated eventually. This may be true if resources are limiting, but not otherwise.