Nonpredatory Interspecific Interactions among Plants and Animals in Freshwater Communities - Freshwater Ecology: Concepts and Environmental Applications - page 413

COMPETITION
Competition (both species have a negative effect on each other) has
been described as a dominant community interaction. It can occur between
organisms from any taxon, but it is more likely to occur among organisms
in the same functional group. I discuss competition among species of plants
and animals in this section. Competition occurs within a species (intraspe-
cific) or between species (interspecific). Use of the word competition refers
to interspecific interactions herein. Competition can manifest itself either
as competition for resources (exploitative) or as interference with a poten-
tial competitor (see Chapter 18).
Competition is often difficult to establish directly in the field, in part
because evolution leads to decreased overlap in resource use, so organisms
that are usually found in the same habitat at the same time rarely special-
ize on the same resource. For example, three-spined sticklebacks ( Gas-
terosteus spp.) in three lakes of British Columbia have recently evolved
from a single marine species. In all these cases, the new pairs of species
found in each lake partition the habitat by feeding on benthic or limnetic
invertebrates. They have evolved mechanisms of reproductive isolation to
ensure that their offspring will continue to use separate habitats. Limnetic
adults prefer to mate with limnetic adults and benthic adults prefer to mate
with benthic adults (Rundle et al., 2000).
Primary producers compete for nutrients and light. Competition be-
tween macrophyte species has been well documented (Gopal and Goel,
1993). Macrophytes can form dense stands that shade all the producers be-
low them (Haslam, 1978). Floating leafed macrophytes, such as water hy-
acinth (Eichhornia) or duckweed (Lemna), can blanket the surface of a
lentic habitat and essentially remove all light. Macrophytes can intercept
90% of the incident light before it reaches a lake bottom, leading to a 65%
decrease in benthic algal production (Lassen et al., 1997). If systems are
sufficiently shallow, emergent primary producers intercept light before it
reaches the water's surface.
Competition for light has been documented for macrophytes inhabit-
ing a stream in North Carolina (Everitt and Burkholder, 1991). In this case,
riparian vegetation and macrophyte communities were characterized in 10
stream segments. The red alga Lemanea australis or the aquatic moss
Fontanalis dominated low-light communities in the winter. High-light sites
were dominated by L. australis and the angiosperm Podostemum cerato-
phyllum . These sites were on the same stream and had similar water ve-
locity and depth, indicating that light availability as influenced by riparian
shading was the major abiotic difference between them. Apparently, com-
petition for light structured the macrophyte communities.
Wetland plant communities can be shaped by competition. Competi-
tion between two species of wetland plants may occur aboveground (for
light) or belowground (for nutrients), and both may be important simul-
taneously (Twolan-Strutt and Keddy, 1996). Competitive rankings of indi-
vidual plants have been demonstrated to remain stable against changes in
nutrients and flooding in some instances (Keddy et al., 1994). However,
wetland plants that occur in disturbed habitats may be released from com-
petition (Keddy, 1989). These studies suggest that competition can deter-
mine which plant species dominate in a particular wetland.