Environmental Engineering Reference
In-Depth Information
compounds (photosynthesis and chemosynthesis, respectively), store energy, obtain
energy from other organisms (e.g., predation), or convert energy into heat (respiration).
Patterns of energy flow through ecosystems can be of direct interest to humans who har-
vest wild populations, and can tell ecosystem scientists a good deal about how different
ecosystems function.
Ecosystems also transform materials in various ways. Materials that come into the eco-
system may be taken up by some part of the ecosystem and accumulate. In some cases,
this accumulation may be temporary so that the ecosystem acts as a sort of capacitor,
releasing the material at a later time. The lag time between atmospheric deposition of sul-
fate onto a terrestrial ecosystem and its export in stream water from that system is an
example. Ecosystems may also be a source of material, releasing their internal stores to
neighboring systems. Weathering of soils and bedrock is a prime example. Finally, and
perhaps most interesting, ecosystems transform materials by changing their chemical and
physical states (see Chapter 5). Nitric acid contained in rainwater falling on a forest soil
may react with the soil and form calcium nitrate in soil water. The nitrate in the solution
may then be taken up by a plant and incorporated into protein in a leaf. At the end of the
growing season, the leaf may fall into a stream where it is eaten by an insect and chopped
into small leafy bits, which then wash out of the ecosystem. The description of chemical
and biological transformations by ecosystems forms the field of biogeochemistry
(
Schlesinger 1997
; see Chapter 5), a major part of modern ecosystem sciences (and this
topic). Many biogeochemical functions are important to humans (e.g., the removal of
nitrate by riparian forests in the Mississippi River basin), as well as essential to under-
standing how different ecosystems work.
Ecosystems often are described by their functions as well as their structures. One of the
most common functional descriptions of ecosystems is whether the system is a source or a
sink of a given material; that is, whether the inputs of that material to the ecosystem are less
or more, respectively, than the outputs of that material from the ecosystem. In the special
case of energy flow through ecosystems, the degree to which an ecosystem is a source or a
sink is described by the P/R (gross photosynthesis to respiration) ratio for the system. At a
steady state, ecosystems with a P/R ratio less than 1 must import chemical energy (usually
organic matter) from neighboring ecosystems and are called heterotrophic; those with a
P/R ratio greater than 1 export chemical energy to neighboring ecosystems and are called
autotrophic. Another useful functional description is the residence time of a given material
in an ecosystem; that is, the average amount of time that a material spends in an ecosystem.
Residence time is calculated by dividing the standing stock of the material in the ecosystem
by its input rate.
Recently, people have begun to formally recognize that ecosystem structures and
functions may have economic value. For instance, ecosystems provide lumber, they
purify water and air, they regulate the prevalence of human diseases, and they provide
pollination for crop plants. These and many other goods and services provided by eco-
systems are now commonly called “ecosystem services”—the benefits that people derive
from ecosystem structures and functions (e.g.,
Millennium Ecosystem Assessment 2005;
Kareiva et al. 2011
). Developing ways to estimate quantitatively the value of ecosystem
services is an important and developing field at
the intersection of ecology and
economics.
