Environmental Engineering Reference
In-Depth Information
groups of organisms have complex interactions that vary over space and
time in the environment, can be mediated by other organisms, and fluctu-
ate between being positive and negative.
Another example of complex, and sometimes indirect, interactions was
shown in an experiment that was performed on competition between tufted
ducks
(Aythya fuligula)
and fish in a small pond formed by quarry activi-
ties in the south of England (Giles, 1994). This shallow pond had a fish
community including bream, roach, perch, and pike and was turbid with
few macrophytes. Most of the fish were removed during a 2-year period,
after which midge larvae, macrophytes, and snails increased (Fig. 20.9).
Fish were reintroduced to enclosures inside the lake, and midge larvae and
snail numbers returned to levels found in the entire lake before fish re-
moval. Predation on ducklings by pike is a direct interaction. Feeding by
ducks, numbers of brooding ducks, and brooding success increased after
fish removal because of their dependence on snails and midge larvae. Indi-
rect effects of fish on the ducks include competition for midge larvae. In-
direct effects of fish include the removal of macrophytes, which provided
habitat for snails, an additional food source for the tufted ducks. These
data are instructive because they compare competition between birds and
fish and document an indirect effect mediated by macrophytes.
A
9
6
3
0
15
B
10
5
0
C
150
100
?
50
0
D
150
Fish
removed
100
50
0
1986
1987
1988
1989
1990
Year
FIGURE 20.9
Interactions of fish and tufted ducks in a small English lake. Fish were re-
moved in 1987 and 1988. Peak densities of chironomid midge larvae (A), snails (B), and
macrophytes (C) and number of duck broods (D) all changed. Fish were added to the enclo-
sures in 1990, and midge and snail densities are reported from inside the enclosures, demon-
strating that fish were able to lower both populations (data from Giles, 1994).
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