Environmental Engineering Reference
In-Depth Information
inclusion of living and nonliving objects in ecosystems has practical as well as intellectual
advantages.
Finally, we note one further property of ecosystems—they are open to the flow of
energy and materials. It might be theoretically possible to define particular examples of
ecosystems that are closed systems, not exchanging energy or materials with their sur-
roundings, but nearly all ecosystems as actually defined have important exchanges of
energy and materials with their surroundings. Indeed, such exchanges are one of the cen-
tral subjects of ecosystem science. We note in particular that most ecosystems depend on
energy inputs from external sources, either as energy from the sun or as organic matter
brought in from neighboring ecosystems.
Now consider briefly what is missing from the definition. We have already noted that
the definition does not specify the time or space scales over which an ecosystem is
defined, or where exactly the boundaries are placed. Ecosystems are not required to be
self-regulating, permanent, stable, or sustainable. They are not required to have any partic-
ular functional properties. For example, they need not be in balance or efficient in the way
that they process materials. Our definition does not require ecosystems to have a purpose.
Although ecosystems change over time, the basic definition does not suggest anything
about the nature or direction of that change. It might seem like a shame not to include
such interesting attributes in a definition of ecosystem (
O'Neill 2001
), and indeed some
ecologists have included such attributes in their definitions, but we think it is neither nec-
essary nor helpful to include them in a basic definition. They may, however, be useful
hypotheses and the subject of fruitful research projects. For instance, we might hypothe-
size that as forest ecosystems recover from disturbances like fire or clear-cutting, they
retain a higher proportion of nutrient inputs. This viewpoint is quite different than saying
that ecosystems are systems that tend to maximize efficiency of use of limiting nutrients.
WHAT ARE THE PROPERTIES
OF ECOSYSTEMS?
All systems have characteristic properties that allow us to describe them and compare
them with other similar systems (
Box 1.1
). How might we describe the properties of
ecosystems?
What Is in an Ecosystem?
We might begin simply by listing the contents of an ecosystem. Plants and animals
occur in most ecosystems. As we will see later in the topic, the number and kinds of plants
and animals can have a strong influence on ecosystem function. Many ecosystems also
contain people. Historically, many ecologists treated humans as being outside of the eco-
system, or deliberately studied ecosystems without people, but it has become increasingly
common to treat people and our institutions as parts of ecosystems (e.g.,
Pickett et al.
2001, 2011
; see Chapter 17). Certainly the structure and function of many modern ecosys-
tems cannot be understood without considering human activities.
