Competitive exclusion principle
The competitive exclusion principle states that two species cannot occupy the same niche in a habitat and stably coexist. 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 [link] , 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.
Resource patitioning
This exclusion may be avoided if a population evolves to make use of a different resource, a different area of the habitat, or feeds during a different time of day, called resource partitioning. The two organisms are then said to occupy different niches. These organisms coexist by minimizing direct competition. The anole lizards found on a single island are a good example of resource partitioning [link] because it shows the effects of how natural selection has driven the evolution of different species in order to reduce competition.Symbiosis
Symbiotic relationships, or symbioses (plural), are close interactions between individuals of different species over an extended period of time that impact the abundance and distribution of the associating populations. Symbiosis is a greek word meaning “living together”. Two symbiotic species cannot live independently from one another (they are not free-living). Most scientists accept this definition, but some restrict the term to only those species that are mutualistic, where both individuals benefit from the interaction. In this discussion, the first broader definition will be used.
Commensalism
A commensal relationship occurs when one species benefits (+) from the close, prolonged interaction, while the other neither benefits nor is harmed (0). Birds nesting in trees provide an example of a commensal relationship ( [link] ). The tree is not harmed by the presence of the nest among its branches. The nests are light and produce little strain on the structural integrity of the branch, and most of the leaves, which the tree uses to get energy by photosynthesis, are above the nest so they are unaffected. The bird, on the other hand, benefits greatly. If the bird had to nest in the open, its eggs and young would be vulnerable to predators. Another example of a commensal relationship is the clown fish and the sea anemone. The sea anemone is not harmed by the fish, and the fish gains protection from predators who would be stung upon nearing the sea anemone.