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AP Bio



Unit 8

8.5 Community Ecology

3 min readmay 31, 2020


Caroline Koffke

A community refers to a group of different populations living in the same area. An example of this would be humans, dogs, and rats living in the city of Chicago. A community has many interactions between the populations of species that live there. These interactions can be positive, negative, or neutral. When studying a community, scientists often measure the species diversity and species composition in that area. This refers to the number of unique species living in the area and what percent of the population is represented by each species. A species with a large number of unique species with equally distributed populations would be considered highly diverse.
The diversity of a community can be measured using Simpson’s Diversity Index. The equation is shown below:
Interactions between populations in a community determine the health of that community, changes over time, and how energy is transferred. These relationships can be defined in a number of ways such as predator/prey, competition, mutualism, commensalism, and parasitism.

Predator and Prey

A predator/prey relationship refers to a relationship in which one organism eats another. The predator in one relationship may be the prey in another relationship. An example of a predator/prey relationship is a snake and a mouse. The snake is the predator and eats the mouse, which is the prey. In another scenario, the snake may become the prey. A hawk would serve as a potential predator for the snake. This relationship is the most common way of showing the transferring of energy in an ecosystem.

Image courtesy of NPS.


Competition refers to relationships in which two species are competing for the same habitat, food, or water. Any time that two organisms share a common food source, competition might result. As an example, hawks and badgers might be in competition for a food source of snakes. Competition can lead to a decrease in one population if another population is more fit to hunt prey. It may also lead to a stabilizing of both populations if both groups are equally fit.

Image courtesy of Giphy.

Mutualism (+/+)

Mutualism refers to a relationship in which both species involved benefit from the relationship. A classic example of this is the Acacia tree and the Acacia ant. The tree produces small leaves that possess large quantities of nutrients for the ants to eat. In return, the ants protect the Acacia tree from some predators and reduce the bacterial population on the leaves of the tree. Because both species are equally benefited from the relationship, this is considered an example of mutualism.

Image courtesy of WikiMedia Commons.

Commensalism (+/0)

Commensalism refers to a relationship in which one species benefits and the other species is neither harmed nor helped. An example of this type of relationship is a barnacle and a whale. A barnacle is a sea creature that bonds to a surface and waits for the water to carry particles of plankton and other food sources across their mouths. Barnacles frequently bind to the thick skin of the whale. The barnacle benefits greatly from this because the whale’s frequent movement allows for an increased amount of plankton and other food particles passing over the barnacle. The whale is neither harmed nor helped by the binding of the barnacle.

Image courtesy of WikiMedia Commons.

Parasitism (+/-)

Parasitism refers to a relationship in which one species benefits and the other species is harmed by the interaction. An example of this type of relationship is a tick and a dog. A tick bites the skin of a dog and uses the blood of the dog for nutrients. The dog is harmed by this interaction. There are a number of other parasites that use a host for nutrients. All of these interactions are harmful to the individual bitten by the parasite, as they are losing crucial nutrients.

Image courtesy of Flickr.

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