Community Ecology, I.

I. The community

Community - all the populations of all the species living in an area.

Community Ecology - the study of how populations interact with one another.

Ecosystem - a community and its abiotic environment.

Ecosystem Ecology - the study of ecosystems.

Typically we think of each species or population in a community as occupying a distinct niche.

Niche - the role of an organism in the structure and function of a community.

Niches are ususally thought of as existing in "n-dimensional hyperspace" because it is impossible to define all aspects of an organisms existence and its interactions with other organisms.

A basic premise is that no two species can occupy the same niche at the same place at the same time. Where overlap occurs, you generally get competition.

II. The interactions of organisms in a community.

The interactions of organisms in a community can be classified into several broad categories:

Interaction Species 1 Species 2

Neutral Ø Ø
Commensalism + Ø
Mutualism + +
Competition - -
Predation + -
Parasitism + -

Most interactions between two species in a community are neutral. Neither species directly affects the other (although the two may interact indirectly through a series of interactions with other species).

Commensalism involves a relationship where one species benefits while the other is neither helped nor harmed.

e.g. - epiphytes on a tree.

Mutualism - both members of a pair of species clearly and directly benefit from the interaction.


Two types of mutualistic relatioships:

  1. Facultative mutualism - both species benefit, but if need be, could live without eachother.
  2. Obligate mutualism - neither species can survive for long without the other.

When different species in a community have some requirement in common, their niches potentially overlap. If resource is unlimited, it is not a problem. If resource is limited, species compete.

Competition is the interaction between organisms using the same resources present in limited supply.

Intraspecific competition - among individuals within a species.
Interspecific competition - between members of different species.

Which would you expect to be more intense?

Two main types of competition:

  1. Exploitation Competition - all individuals have equal access to the resource, but they differ in how efficiently they use it.
    e.g. Gause's work on Paramecium spp (Fig 1)

    Complete competitors cannot coexiost indefinitely (competitive exclusion). Over time, either:

    1. one species goes extinct (Fig 2), or
    2. character displacement occurs - the tendency for two similar species to diverge in areas where they compete to reduce interspecific competition. e.g. Tamar Dyan's work on wild felids.
  2. Interference Competition - certain individuals limit or prevent others from using a resource, and thereby control access to it.
    e.g. wolves, coyotes, and ungulate carcasses.

Predation and Parasitism are ecologically quite similar in that both involve one species benefiting by obtaining food from another species (which is by definition, harmed).

What is the difference?

The dynamics of predator-prey interactions usually result in one of three systems:

  1. No impact. The two species interact, but neither's numbers are directly dependent on the other.
    eg. Red-tailed hawks and chicadees. Occasional killings, but relatively insignificant for the population dynamics of either species.

    compensatory mortality vs. additive mortality.

  2. Stable coexistence. Predation keeps prey population in check. Predators can do this when they reproduce relatively quickly relative to the prey and are capable of killing more when the prey numbers increase.
  3. Oscillations. Cycles of predator and prey abundance caused by (among other things) time lags in the predator's response to changes in prey abundance.
    eg. Canadian lynx and snowshoe hares.

Question: Do the same patterns also show up for parasite-host interactions?

III. The Number of Species in a Community.

What determines it? How stable is a community?

MacArthur and Wilson's island biogeography model is an attempt to answer these questions by framing the question in terms of islands and suggesting that for any size island there is an equilibrium number of species. This number will be stable over time, with new immigrant species balancing those species that have gone extinct.

But what about succession?

Ecological succession - the gradual process by which the species composition of a community changes. Succession can be quite regular and predictable.

Updated April 12, 2005
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