Behavior and Interactions among Microorganisms and Invertebrates - Freshwater Ecology: Concepts and Environmental Applications - page 381

These interactions are incredibly varied, so I provide only a few examples
to illustrate their potential importance.
Macrophytes are often covered by epiphytes, which benefit from living
on the macrophytes. The epiphytes compete for light and nutrients with
the macrophytes and may increase drag and associated detachment and
thus have a negative influence on macrophytes. Such interactions are prob-
ably widespread in aquatic communities.
A hypothetical example is a diatom growing on a cyanobacterial Nos-
toc colony under N-limited conditions. The cyanobacteria can fix N and
may leak some to the diatom in addition to providing substrata for at-
tachment. The diatom competes for other nutrients and light. Thus, the di-
atom receives a benefit and the Nostoc is harmed.
When new habitat is exposed in aquatic systems (e.g., floods expose
new rock surfaces), a sequence of colonization occurs. Often, the earlier
colonists must condition the habitat before the later organisms in the se-
quence can become established. This conditioning may inhibit or ultimately
exclude the initial colonists, but it facilitates the later organisms that can
colonize the site.
COMPETITION
Competition has been suggested to account for many species interac-
tions in aquatic communities and is viewed by many as a key driving force
in evolution. In practice, demonstrating competition directly can be diffi-
cult (Connell, 1983; Schoener, 1983). Some competition must occur be-
cause many aquatic organisms have similar requirements. Thus, we might
expect to find competition for scarce resources if physical conditions re-
main uniform for a sufficiently long time.
The actual time required for competition to become important will be
a function of growth and reproductive rate of the potential competitor as
well as the relative supply rate of the resources. As with all ecological
processes, spatial and temporal scale are important considerations.
There are two forms of competition. Exploitation competition is com-
petition by organisms that are both exploiting the same resource. Direct
negative effects on other organisms are termed interference competition .
The consequences of exploitation competition related to simultaneous con-
sumption of several resources have been explored by Tilman (1982). This
theory helps us consider the possible ramifications of unequal abilities
among organisms to utilize nutrients. Often, a tradeoff exists between the
ability to grow well with low nutrients and the ability to grow well with
high nutrients. If one organism can grow well under low nutrient concen-
trations, and the second can grow well under high nutrient concentrations
(Fig. 18.12), they will be competitively dominant at different nutrient con-
centrations. If their competitive abilities are reversed for a second nutrient,
then they are able to coexist under certain ratios of nutrients because they
are both limited by different nutrients. In the example presented in Fig.
18.12, high Si:P ratio favors species 1, low Si:P ratio favors species 2, and
at intermediate ratios both species may coexist (Fig. 18.13). Data suggest
that such mechanisms of competition can be important in phytoplankton