Final answer:
The competitive exclusion principle states that two species cannot occupy the same niche in a habitat if they are competing for the same resources. This principle leads to the extinction of one species or specialization in both species. An example of this principle is the competition between two protozoan species, Paramecium aurelia and Paramecium caudatum, where the former outcompetes the latter for food.
Step-by-step explanation:
Resources are often limited within a habitat and multiple species may compete to obtain them. All species have an ecological niche in the ecosystem, which describes how they acquire the resources they need and how they interact with other species in the community. The competitive exclusion principle states that two species cannot occupy the same niche in a habitat. In other words, different species cannot coexist in a community if they are competing for all the same resources. An example of this principle is shown in Figure 45.24, with 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 test tube (habitat), P. aurelia outcompetes P. caudatum for food, leading to the latter's eventual extinction.