Environmental Engineering Reference
In-Depth Information
of soils affects a terrestrial ecosystem's ability to retain water, nutrients, and other ions,
which in turn affect nearly every aspect of terrestrial ecosystems. As an aquatic example,
the clay content of marine sediments may control long-term rates of organic matter
burial, with profound consequences for global biogeochemistry over long timescales (see
Chapter 6).
Biotic control of ecosystems also is common and varied. Perhaps the best-known
examples involve control through a food web. For example, the trophic cascade
(
Figure 11.1
) is a mechanism by which top predators can control both lower trophic levels
and abiotic properties of ecosystems (e.g., oxygen or nutrient concentrations), especially
Food web
response
Food web
response
Increase
in piscivorous
fish
Decrease
in piscivorous
fish
Piscivorous (fish-eating) fish
(e.g., black bass, pike)
Planktivorous (plankton-eating)
fish (e.g., young shad)
Decrease
Increase
Zooplankton
Increase
Decrease
Phytoplankton
(and water clarity)
Decrease
(Increase in
water clarity)
Increase
(Decrease in
water clarity)
Nutrients
Increase
Decrease
FIGURE 11.1
Biotic control of an aquatic ecosystem from the top of the food web. The trophic cascade
hypothesis proposes that top predators may control the abundance of organisms lower in the food chain. By
reducing the abundance of their prey (planktivorous fish), this in turn may allow an increase in the abundance or
size of organisms among the prey's food source (zooplankton), which can lead to grazing down phytoplankton
populations and increase water clarity and concentrations of dissolved nutrients (Trophic cascade available at:
http://
view;id
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