Community Ecology

Big Picture

A community is composed of all the populations of species that inhabit a specific area. Community ecology studies the interactions between such populations of species. Three important interspecific interactions are predation, competition, and symbiosis. The structure of communities also undergoes change due to abiotic and biotic disturbances, resulting in ecological succession.

Key Terms

Predation: A relationship in which a member of one species, the predator, eats or preys on a member of another species, the prey.

Interspecific Competition: Competition between two or more species for the same resources.

Intraspecific Competition: Competition between members of one species for the same resources.

Symbiosis: A close relationship between two species that may benefit one or both species.

Parasitism: A relationship in which one organism (parasite) benefits at the other organism’s (the host’s) expense.

Mutualism: A relationship in which both species benefit.

Commensalism: A relationship in which one species benefits while the other is not affected.

Ecological  Succession: The  sequential  change  over time in the composition of a community.

Primary  Succession: Ecological  succession  that takes  place  in  a  previously  uncolonized  area typically lacking soil.

Secondary  Succession: Ecological  succession that takes place in a previously colonized area that has been disturbed.

Pioneer  Species: The  first  species  to  inhabit  a  previously  uncolonized  or  disturbed  area,  leading  to primary or secondary succession.

Disturbance: An occurrence that changes a community, such  as  a  fire,  drought,  or  animal  movement Disturbances help maintain species diversity.

Keystone Species: A species that plays an important role in determining the structure of a community.

Community Interactions


  • We  usually  think  of  carnivorous  species  like  lions hunting  animals  for  food  as  predation,  but herbivorous  species  eating  plants  is  also  a  type  of predation.
  • Predators and preys tend to keep their population in balance - when there is more prey, there is more food for predators, which allows the predator population to grow. With more predators, however, more prey is eaten, causing the prey population to drop. With less food, predator population also begins to drop.
Image Credit: CK-12 Foundation, CC-BY-NC-SA 3.0
  • Keystone species play an especially important role in the community. A change in the population of keystone species will affect the populations of other species.
  • Example: otters eat sea urchins, which can destroy kelp forests if their population is not kept in check

Keystone species do not have to be predators or at the top of the food chain. Otters, for example, are prey for sea lions and orcas (killer whales).

  • Predation has caused mimicry to evolve  as  an adaptation in some species. Mimicry is when two species resemble each other so that one or both of the species gain an advantage with regard to survival. Two harmful species may resemble each other or a harmless species may imitate a harmful species in order to scare of prey.
Figure: Mimicry in butterflies
Image Credit: Repeating Patterns of Mimicry. Meyer
A, PLoS Biology, Vol. 4/10/2006, e341, CC-BY 2.5


Community Ecology Cont.

Community Interactions (cont.)


Two types of competition:

1. Interspecific competition

  • Example: predators of different species competing for the same prey
  • Can  lead  to  competitive  exclusion:  competition over the same resources can lead to extinction or a  shift/reduction  in  niche  for  the  less  successful competitor. In other words, “complete competitors cannot coexist.”
  • Can  lead  to  specialization,  where  competing species  evolve  different  adaptations,  or resource partitioning,  where  the  existing  resources  are divided spatially (two competing species of fish live at different depths) or temporally (two competing species hunt at different times of the day)

2. Intraspecific competition

  • Example: two male birds competing for the same mate


  • Parasitism:  Has  a  (+/-)  effect,  where  the relationship is positive for one species and negative for the other. The species that benefits is called the parasite. The species that  feels the negative effect is the host. Parasites typically have a negative effect on  the  host’s  health  and  ability  to  reproduce,  at times killing the host.
  • Mutualism:  Has  a  (+/+)  effect  where  the relationship is positive for both species.
  • Commensalism:  Has  a  (+/0)  effect  where  the relationship is positive for one species but does not affect the other.

Ecological Succession

Ecological Succession
Image Credit: LucasMartinFrey, CC-BY 3.0


Community Ecology

Ecological Succession (Cont.)

Communities are constantly changing. Ecological succession is a look at how a community changes over time.

  • Primary succession looks at an area that has never been colonized before. The area usually has nothing but bare rock (no soil).
  • Pioneer species, like bacteria and lichens that can live on rocks, come in first.
  • With time, the rocks wear down and soil forms. Plants can move in once soil is available.
  • Other organisms gradually move in.
  • Secondary succession looks at an area that has been disturbed. The disturbance can be a fire, drought, animal movement, or human action such as farming.
  • Occurs faster than primary succession because soil is already in place.