I. Common use of scarce resources leads to competition

A. Competition = an interaction between two species using the same resource, such as food, living space, light, nutrients, water, nesting sites

B. Niche = the functional role of a particular species in an ecosystem

• niche, which is an entire pattern of living, is not the same as habitat, which is the location where an organism lives
• niche is often described as how an organism affects energy flow within an ecosystem

C. A species niche can vary in size

• fundamental niche = the entire range of conditions an organism can potentially occupy
• realized niche = the part of the fundamental niche that a species actually occupies

D. Resources are often partitioned between species, so that realized niches are smaller than fundamental niches

• MacArthur study of Cape May warblers describes 5 species of insect-eating birds living together but in different parts of the same spruce trees, thus reducing competition
• Connell study of barnacles describes 2 species of crustaceans living in different parts of the same intertidal habitat, as the result of competition

II. Competition without division of resources leads to extinction

A. Competitive exclusion = the elimination of one species when two species are in direct competition for the same resources

• Gause study of Paramecium shows that 2 species competing for the same bacterial food in a test tube always leads to the extinction of one of the two

B. Coexistence between competitors is possible when resources are divided

• Gause study of Paramecium shows that 2 species competing for the different foods allowed the coexistence of both species

C. Predation can lessen competition

• Paine study of intertidal regions showed that the removal of sea stars, a top predator, led to a decrease in the number of other species from 15 to 8, the 7 eliminated species driven to extinction by increased competion
• therefore, by reducing competition among prey, predation can increase biodiversity, the variety of species living in a community

D. Biodiversity increases ecosystem productivity and stability

• Tilman study of grasslands showed that ecoystem productivity increased with increasing biodiversity
• Tilman also showed that grasslands with greater biodiversity were able to recover from drought more quickly