Environmental Engineering Reference
In-Depth Information
Finally, abiotic and biotic factors often interact to jointly control ecosystem structure and
function. One particularly useful framework that includes the interactions of abiotic and
biotic factors is
Jenny's (1941)
state model of soil formation (
Box 11.2
). Beyond soil forma-
tion, however, there are many examples—across the full range of spatial and temporal
scales—of interacting biotic and abiotic factors. To offer just one example, inputs of water to
some ecosystems are determined by the architecture and morphometry of vegetation (biotic
structure) and how it “captures” fog water from the air (abiotic flux;
Figure 11.4
). This cap-
tured fog water can influence ecosystem functions such as primary production, nutrient
flow, and perhaps even soil development (
Ewing et al. 2009
). So our first lesson about con-
trol is that both abiotic and biotic factors are commonly involved, and that the details of
these controls can be as varied as the abiotic and biotic contents of the world's ecosystems.
BOX 11.2
A BROAD PERSPECTIVE ON ECOSYSTEM CONTROLS:
JENNY'S STATE FACTORS
The soil scientist Hans
Jenny (1941)
developed a well-known framework that
defines the factors that account for much of
the variation in the world's soils—a frame-
work that can be extended easily to describe
controls on variation in terrestrial ecosys-
tems. Building from the work of Russian
soil scientist Vasilii Dokuchaev, he identi-
fied a set of five state factors that control soil
genesis. Jenny's list (often evoked by the
ungainly mnemonic “clorpt”) includes cli-
mate (“cl”), organisms (“o,” that is, the
biota), topography (or relief, “r”), parent
material (“p,” the material—usually geolog-
ical—from which the soil formed), and time
(“t”). Jenny's idea was that specifying
values for these five factors for a site of
interest would provide enough information
to predict what kind of soil would occur on
that site. No other information about the
site would be required, but neither could
any of the five state factors be omitted.
Although this framework was developed
for soils, it is widely used to describe the
factors that control terrestrial ecosystems, at
least over broad scales of space and time.
That is, if we specify values for all five state
factors for a site, we can predict a good
deal about the ecosystem, as well as the
soils, that would occur at the site. For exam-
ple, consider a grassland site that has been
developing on a steep limestone slope that
has not been overrun by glaciers in at least
the last 100,000 years and currently has a
continental climate with hot, variably moist
summers and cold, dry winters. An ecosys-
tem ecologist with experience in a range
of ecosystems would not have too much
difficulty making general estimates about
primary production, the types of grazers
that might be present, and the likelihood
that fire will be a part of the ecosystem's
disturbance regime.
Nevertheless, there are limits to the
applicability of Jenny's state factors for eco-
systems. For example, it does not seem very
useful if we are interested in controls over
ecosystem function at short timescales (i.e.,
less than decades), as we often are. Short-
term variation in variables such as weather
or population dynamics may drive consid-
erable variation in ecosystem function (see
