Interactions include competition, predation and symbiosis.
Interspecific competition is when two species compete for the same limited resources. The competitive exclusion principle (Gause's principle) states that if two species occupy the same niche and resources are limited, one will be outcompeted and eliminated. Gause experimented with different species of paramecium to prove this.
A fundamental niche is the niche that may be occupied by a species. This is often different from the realized niche which is part of the niche the species actually takes up. Competition often prevents species from occupying their entire fundamental niche.
Predation is when one organism eats another. There are a variety of defenses that have developed to avoid predators. One defense is warning coloration or aposematic which is when a poisonous animal becomes brightly colored to warn other organism; cryptic coloration is camouflage. Batesian mimicry is when a harmless species mimics the coloration of a harmful species (e.g. flies with yellow and black coloration). Mullerian mimicry is when organism with the same defense mechanism share the same coloration (e.g. bees and wasp). An example in plants include toxins or spines.
Symbiosis is when two species live in contact with one another. The three examples include parasitism, commensalism and mutualism. In parasitism one organism relies on the other but harms that organism (tapeworm). In commensalism, one species benefits and the other is neither harmed nor helped (egrets on cattle). In mutualism, both species are benefited (lichen, mycorrhizae).
Source: Holtzclaw, Fred, and Theresa Holtzclaw. AP Test Prep Series. San Francisco: Pearson Education Inc., 2013. Print., PACK, P. (2013). AP BIOLOGY. (4TH ED., PP. 73-81). BOSTON: HOUGHTON MIFFLIN HARCOURT., N.d. Photograph. Biology NotebookWeb. 22 Feb 2014.