Community Ecology

1) Definition: A community is defined as an assemblage of species living close enough together for potential interaction.

2) Populations may be linked by competition, predation, mutualism and commensalism

• Competition.

• Interspecific competition for resources can occur when resources are in short supply.

• There is potential for competition between any two species that need the same limited resource.

• The competitive exclusion principle: two species with similar needs for same limiting resources cannot coexist in the same place.

• The ecological niche is the sum total of an organism’s use of abiotic/biotic resources in the environment.

• An organism’s niche is its role in the environment.

• The competitive exclusion principle can be restated to say that two species cannot coexist in a community if their niches are identical.

• Resource partitioning is the differentiation of niches that enables two similar species to coexist in a community.

• Character displacement is the tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric
populations of the same two species.

3) Trophic structure is a key factor in community dynamics

• The trophic structure of a community is determined by the feeding relationships between organisms.

• The transfer of food energy from its source in photosynthetic organisms through herbivores and carnivores is called the food chain.

• Charles Elton first pointed out that the length of a food chain is usually four or five links, called trophic levels.

4) Dominant species and keystone species exert strong controls on community structure

• Keystone species exert an important regulating effect on other species
in a community.

5) Ecological succession is the sequence of community changes after a disturbance

• Ecological succession is the transition in species composition over ecological time.

• Primary succession begins in a lifeless area where soil has not yet formed.

• Secondary succession occurs where an existing community has been cleared by some event, but the soil is left intact.

• Soil concentrations of nutrients show changes over time.

6) Community biodiversity measures the number of species and their relative abundance

7) Species richness generally declines along an equatorial-polar gradient

8) Species richness is related to a community’s geographic size

9) Species richness on an island depends on island size and distance from the mainland