The Nature and Consequences of Indirect Effects in Ecological Communities

The Nature and Consequences of Indirect Effects in Ecological Communities

J. Timothy Wootton

Annual Review of Ecology and Systematics, Vol. 25:443-466 (Volume publication date November 1994)

PDF Available

Five simple types of indirect effects have been regularly demonstrated in nature: exploitative com­petition, trophic cascades, apparent competition, indirect mutualism, and in­teraction modifications. Detailed experimental investigations of natural com­munities have yielded complicated effects.

Let’s examine those types more closely since all are common in our reef tanks, even if most of them happen out of view, on a microscopic level.

  1. Exploitative Competition
    For example, corals and algae competing with one another for light, space and nutrients.
  2. Trophic Cascade
    For example, overfishing large fish-eating fish would release predatory pressure on smaller zooplankton-eating fish.

    Releasing predatory pressure on smaller zooplankton-eating fish would increase predatory pressure on zooplankton.

    Increasing predatory pressure on zooplankton would probably allow greater amounts of phytoplankton to grow.

  3. Apparent Competition
    For example, two algae that are predated upon by the same snail, but where one algae grows better and ends up sustaining a larger snail population.  The larger snail population could also increase grazing pressure on the algae that doesn’t grow as well.
  4. Indirect Mutualism
    For example, a stable community of unrelated species that utilize similar resources will tend to minimize competition with one another.

    Under harsh conditions, some individual species may even indirectly facilitate positive conditions for other species.

    This allows the community to survive under harsh conditions, but since they are all inter-dependent it also means the communities tends to collapse together once a tipping point is passed and conditions become too harsh.

    Community recovery from this collapsed stage is known to require a much more considerable improvement to environment than would have been required prior to collapse.

  5. Interaction Modification
    For example, introducing a fish predator to a tank that was dominated by a planktivore will cause the planktivorous fish to begin hiding during a large part of the day.  The plankton in a tank will increase in numbers due to the change in behavior.

    This “increase” may mostly be noticeable as uneaten food since there’s usually not a lot of plankton in a reef tank to begin with…and it’s very hard to notice the amount without close observation with a microscope.


The key feature of indirect effects is that they require the presence of intermediary species in order to arise. Thus, they are a property arising in multi-species assemblages.

It feels like Dr. Wootten is basically speaking directly to us in the next passage, below (emphasis mine):

In some instances, indirect effects have also been considered to arise through a change in a physical or chemical variable in the environment as well as through another species (116). For example, fish foraging activity may change the sedimentation regime of a stream, which in turn may influence invertebrates and algae (37, 94, 96). Similarly, a species may affect the availability of inorganic nutrients in the system, which in turn affects the population dynamics of producers (9, 41, 123, 131). The major feature in such examples is that the physical or chemical variable of interest acts in a dynamic manner, much as another species would.

It almost sounds like he’s referring specifically to a dinoflagellate bloom in an aquarium – but obviously he’s speaking in general.


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