Environmental Engineering Reference
In-Depth Information
Control Can Be “Bottom-Up” or “Top-Down”
Historically, ecologists tended to think of ecosystems as being controlled from below—by
inputs of nutrients, light, or water that set levels of primary production, which in turn control
many other aspects of ecosystem function. Certainly, such “bottom-up” control is important
in ecosystems: primary production in many ecosystems is controlled by inputs of energy or
limiting materials (see Chapter 2), and many parts of ecosystems are controlled by the amount
and quality of that primary production (see Chapter 3).
However, it has become increasingly apparent that not all control is from the bottom-
up. As we have seen, ecosystems can be strongly controlled from the top-down through
the trophic cascade (
Figure 11.1
) or by engineers at any point between the “top” and “bot-
tom” of ecosystems (
Box 11.1
). Furthermore, ecosystems can just as easily be controlled by
losses of key materials, such as nutrients and water, as by inputs. For instance, two forests
receiving the same inputs of water and nutrients, but growing on soils with high and low
abilities to retain nutrients and water, will develop very different structures and functions
(
Ewing 2002
; see Chapter 7). Likewise, the activity of consumers in an ecosystem is tied to
its ability to retain organic matter (see Chapter 3). Thus, although energy may flow from
the bottom to the top of ecosystems, control may be exerted from the top, from the bottom,
or from any point in between.
Control Can Come from Outside or Inside the Ecosystem
All ecosystems are controlled by inputs from outside their boundaries. At the level of
the planet, inputs of energy from the sun drive all of Earth's ecosystems, and variation in
inputs of solar energy and resultant patterns in precipitation across the globe account for
large structural differences in ecosystems (
Figure 11.5a
). At a more local level, we know
that inputs of nutrients from outside the system control the character of ecosystems, for
example, phosphorus loading from land to lake. Likewise, agents of disturbance such
as hurricanes or lightning strikes that come in across ecosystem boundaries can play
important roles in structuring ecosystems (see Chapter 10).
However, ecosystems also are controlled by their own internal structure. It is perhaps
easiest to appreciate this by noting how much ecosystems can change following abrupt
changes in biotic structure such as species invasions (
Figure 11.5b
) or overharvest, but
there are many other examples of how the internal structure of an ecosystem controls how
that ecosystem functions. Factors such as whether a lake is thermally stratified (i.e., water
temperature varies with depth, with the warmest water at the top, and the coldest at the
bottom) or mixes to the bottom, whether a river is clear or turbid with silt, or whether
a forest soil is shallow or deep, clearly will affect primary production, decomposition, and
other ecosystem functions. Thus, ecosystems are jointly controlled by their own internal
structure and exchanges with the environment that surrounds them.
Control Can Follow Various Mathematical Functions
Most of the examples of control mentioned in this topic are simple linear functions,
although some are presented on log-log scales (
Figure 11.6a, b
). Linear functions are
