Environmental Engineering Reference
In-Depth Information
Almost all ecosystems contain microbes (bacteria and fungi); although not as conspic-
uous as plants and animals, their activities are vital to ecosystem functioning. Viruses
occur in most ecosystems, and may play important roles as regulators of plant, animal,
and microbial populations. Ecosystems also contain water and air, which are themselves
resources for many organisms and also serve as media in which organisms and nonliv-
ing materials can be transported. Finally, ecosystems contain an enormous variety of
nonliving materials, organic and inorganic, solid and dissolved. These nonliving materi-
als, including such disparate items as dead wood, clay particles, bedrock, oxygen, and
dissolved nutrients, interact with the living biota and exercise strong influences on the
character and functioning of ecosystems. Thus, the total inventory of an ecosystem can
be very long; it might contain thousands or millions of kinds of items, living and
nonliving.
Ecosystems Have Structure
This complexity allows for an essentially infinite number of possible descriptions of
ecosystem structure. Nevertheless, only a few descriptions of ecosystem structure are
commonly used by the scientists who study ecosystems. Often ecosystems are described
by the numbers and kinds of objects that they contain, focusing on key materials or
organisms. Thus, we may describe an ecosystem as having a plant biomass of 300 g/m
2
,
or a deer population of 5/km
2
, or a nitrogen content of 200 kg/ha. Sometimes ecosystem
scientists describe ecosystems by the ratios of key elements such as the nitrogen : phos-
phorus ratio of a lake ecosystem. If we were interested in the role of biological communi-
ties in regulating ecosystem function, we would refer to the biodiversity (especially the
species richness) of the organisms in the ecosystem. We may be interested in the spatial
variation, as well as the mean value, of any such key variables (see Chapter 10). Thus,
we may describe ecosystems as being highly patchy as opposed to relatively homoge-
neous in nitrogen content or biodiversity. Finally, scientists often describe ecosystems by
their size or location (e.g., latitude, altitude, biogeographic realm, or distance from the
coast).
Ecosystems Perform Functions
In the broadest sense, ecosystems consume energy and transform materials. As with
all systems subject to the second law of thermodynamics, some of the useful energy that
comes into ecosystems in forms such as solar radiation, chemical energy (e.g., organic
matter), or mechanical energy (e.g., wind) is degraded to heat and becomes unable to
perform further work. In particular, living organisms need a continual source of energy
to maintain biochemical and physiological integrity, as well as to perform activities such
as swimming, running, and flying. Curiously, although these biological energy transfor-
mations are only a part of the energy transformations that occur in an ecosystem, most
studies of energy flow through ecosystems treat only forms of energy that can be cap-
tured and used by living organisms (i.e., solar radiation and chemical energy), and
ignore such purely abiotic processes as the conversion of kinetic energy to heat by flow-
ing water. Organisms can capture solar energy or chemical energy from inorganic
