Environmental Engineering Reference
In-Depth Information
primary production doubles when we increase nitrogen inputs than if increased nitrogen
merely enhances the flow of dissolved organic matter from a particular plant species to
the soil by 5%. However, because ecologists may assign different values of importance to
different components and flows, their views about the important controls on an ecosystem
may likewise differ. Furthermore, as we will discuss in more detail later, the answer to the
question of what controls an ecosystem depends on the scales of time and space at which
the question is posed. As a result, it can be tricky to make general statements about the
importance of different controls without specifying the components or flows of interest or
the scales of time and space under consideration.
WHY DO WE CARE ABOUT CONTROLS
ON ECOSYSTEMS?
There are several reasons why the topic of controls occupies a central position in ecosys-
tem science. First, understanding the factors that control an ecosystem is important to deci-
phering how and why different ecosystems work the way they do. Why are two adjacent
forests so different with respect to nitrogen cycling? What determines how many fish are
produced in a series of lakes? Knowing the forces that act on ecosystems allows us to
address interesting questions such as these.
In a more applied sense, understanding how ecosystems are controlled allows us to predict
or evaluate the consequences of various management actions. If we reduce inputs of nitrogen
to Chesapeake Bay by 20%, will we see dramatic improvements in water quality, or just minor
changes? How will forest-watershed ecosystems respond to timber cutting at different
frequencies? The ability to predict consequences can be especially valuable if the proposed
management actions are expensive or take a long time to implement, as often is the case.
Finally, with a good understanding of how ecosystems are controlled, we may be able
to anticipate how they are likely to react to future changes in climate, land use, invasive spe-
cies, and other large-scale changes in ecosystems. This capacity gives us the opportunity to
implement management practices and policies to avoid or mitigate undesirable changes that
may lie ahead. Obviously, the more complete and accurate our understanding of controlling
factors, the better we will be able to recommend particular actions or policies.
HOWARE ECOSYSTEMS CONTROLLED?
Here, we review a few of the more important conclusions that ecosystem scientists have
reached about how ecosystems are controlled.
Both Abiotic and Biotic Control Are Important
Ecosystems may be controlled by abiotic or biotic factors, or interactions among these.
Examples of abiotic controls are numerous and varied. Earlier, we learned that primary
production may be limited—or controlled—by light, nutrients, and water. The texture
