AP Biology/Community Ecology

Community ecology is the study of the relationship of populations in an area.

a) The competitive exclusion principle, also called Gause's Principle, states that when two species compete for exactly the same resources (thus, they occupy the same niche), one is likely to be more successful. As a result, one species "outcompetes" the other species, and eventually the second species is eliminated. This principle means: no two species can sustain coexistence if they occupy the same niche. Gause mixed two species of Paramecium that competed for the same food. One population grew rapidly and was more successful at finding food. This population thrived.

b) Resource partitioning occurs when two species coexist in spite of apparent competition for the same resources. Close study would reveal that they actually occupy slightly different niches. By pursuing slightly different resources or obtaining resources in slightly different ways, individuals minimize competition and maximize success. Dividing up resources in this manner is called resource partitioning. Five species of warblers coexist in spruce trees by feeding on insects in different regions of the tree and by using different feeding behaviors to obtain insects.

c) As a result of resource partitioning, certain characteristics may enable individuals to obtain resources in their partitions more successfully. Selection of these characteristics reduces competition with individuals in other partitions and leads to a divergence of features. This is called character displacement or niche shift. Two species of finches that live on two different Galapagos Islands have similar beaks, both suited for using the same food supply. On a third island, they coexist, but due to evolution, the beak of each bird species is different. This minimizes competition by enabling each finch to feed on seeds of a different size.

d) Realised niche -- The niche that an organism occupies in the presence of predators and other environmental constraints. When competitors are absent the niche occupied is called fundamental niche. That is, when they do not compete for the same resources. Under experimental conditions, one species of barnacle can live on rocks that are exposed to the full range of tides. The full range, from lowest to highest tide levels, is its fundamental niche. In the natural environment, however, a 2nd species outcompetes the first species, but only at lower tide levels where desiccation is minimal. The first species survives in its realized niche, where the tide is higher.

Predator-Prey Relationships

e) A true predator kills and eats another animal.

f) A parasite spends most of its life living on another organism, obtaining nourishment from host by feeding on its tissues. A parasite lives on a living organism.

g) An insect that lays its eggs on a host (usually an insect or a spider) is called a parasitoid. After the eggs hatch, the larvae obtain nourishment by consuming the host's tissues. The tarantula hawk wasp stings a tarantula, paralyzing it. It then lays its eggs in the live tarantula, which eventually hatch. The newborn babies feed on the tarantula until they begin pupation.

h) A herbivore is a creature that eats plants. There are three types of herbivores. Granivores eat seeds. Grazers eat grass. Browsers eat leaves.

i) Symbiosis applies to two species that live together in close contact. There are three forms of symbiosis.

• 1) Mutualism - benefits both species (+,+)
• 2) Commensalism - benefits one species, does not affect other (+,0)
• 3) Parasitism - benefits one species, hurts the other (+,-)