Nonpredatory Interspecific Interactions among Plants and Animals in Freshwater Communities - Freshwater Ecology: Concepts and Environmental Applications - page 427

Strong Interactors
Keystone species are those that have a disproportionately large impact
on their community or ecosystem relative to their abundance (Power et al.,
1996). An alternative form of the same idea is that some species are strong
interactors. The term keystone species has been used loosely in the eco-
logical literature, so claims that a species is a keystone species should be
viewed with caution. Identifying keystone species requires knowledge of
the particular system of study and the organisms found in it.
An example of a potential keystone species is the detritivorous fish
Prohilodus mariae found in the Orinoco river basin in South America. The
fish removes sediments from the stream bottom through its normal feed-
ing activities (Flecker, 1996). These activities lower algal biomass and in-
vertebrate mass, though some species of invertebrates and algae are stim-
ulated. This detritivore is an example of a strong interactor that has direct
and indirect influences at several levels of the food web.
Bluegill sunfish (Lepomis macrochirus) may structure communities in
small lakes and ponds throughout the eastern United States (Smith et al.,
1999). Frog tadpoles (except the bullfrog, Rana catesbeiana ), newts, and
predatory aquatic insects are much less abundant when bluegill are pres-
ent. Since bluegill are common in many small ponds, their presence could
alter community structure. Power et al. (1996) provide additional exam-
ples of possible keystone species in freshwater systems that include beaver
(Castor canadensis) and their effect on streams, predatory fish in river and
stream food webs, piscivorous fish in the trophic cascade of lakes, preda-
tory salamanders in temporary ponds that prey on anuran tadpoles, and
planktivorous fish that have strong impacts on zooplankton communities.
Finally, a wevil that attacks Eurasian water millfoil (Myriophyllum spica-
tum) could be a keystone species because it stops the millfoil from domi-
nating (Creed, 2000).
SUMMARY
1. Competition has been well documented for species from a variety of
taxonomic groups found in freshwaters.
2. Mutualism, amensalism, and commensalism have been studied much
less than predation or competition, but they may be important at
times. Facilitation by plants in harsh environments is an example of
positive interactions that should not be ignored in wetland
communities.
3. Disturbance is an important ecological process. Perhaps the best
documented form of disturbance in aquatic ecosystems is river
flooding. This is a natural part of the ecosystem, and river restoration
projects are beginning to consider this aspect of aquatic ecosystem
dynamics.
4. Succession occurs both seasonally and over longer time frames in
most aquatic habitats. The best examples of seasonal succession are
from the series of events that occur in the epilimnion of stratified
temperate lakes. Succession after reservoir construction, when a shift