Competitive exclusion (By Gause): It states that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eventually eliminated.

Gause original experiment with ciliates, which is a classic example of this. Paramecium caudatum and Paramecium aurelia, two closely related ciliate protozoans, when grown in separate cultures exhibited typical sigmoid population growth. When both protozoans were placed in the same culture, P. aurelia alone survived after 16 days because it had a more rapid growth rate and thus out-competed P. caudatum for the limited amount of food.