Environmental Engineering Reference
In-Depth Information
aquatic ecosystems. But biotic control is not always primarily about the food web; it may
also be exercised through a variety of nontrophic pathways collectively often called ecologi-
cal engineering (
Box 11.1
).
BOX 11.1
ECOSYSTEM ENGINEERS AND CONTROL
ON ECOSYSTEM FUNCTIONING
Living organisms often control ecosystem
functions such as productivity or nutrient
cycling. How? One way is through energy
and nutrient uptake and waste production—
trophic processes that are universal to all
organisms. Not all species exert significant
control this way; it depends on their bio-
mass, stoichiometry (ratio of elements pres-
ent in their bodies; see Chapter 5), and rates
of uptake and waste production.
This is not the sole means of organismal
control, however. Consider beaver that
clear-cut riparian zones and build dams
forming ponds and wetlands that control
hydrology, sedimentation, and biogeochem-
istry, simultaneously creating, maintaining,
and destroying habitats for many species.
Beaver control does not arise from how
much they eat or the waste they produce
(this is a small fraction of ecosystem energy
and material budgets); it arises because
beaver construction alters the physical
template, changing abiotic resources and
conditions that affect energy and material
flows throughout the ecosystem. Or con-
sider forest trees of which the canopies
attenuate wind and cast shade creating hab-
itat for understory species. This habitat
depends on altered abiotic conditions
caused by the physical properties of tree
structure, not the carbon and nutrients taken
up by the tree, even though these build the
tree structure. Both beaver and forest trees
are examples of physical ecosystem engineers
(
Jones et al. 1994, 1997
); they control energy
and material flows via structurally mediated
changes in resources and conditions.
Beaver are allogenic engineers—they cause
structural change by altering extrinsic living
or nonliving materials (a dam from felled
trees and mud). In contrast, the forest tree
example illustrates autogenic engineering—it
is the physical structure of the tree itself that
causes the abiotic changes. The same tree,
through forming soil macropores that affect
soil drainage via root growth, also illustrates
allogenic engineering. Beaver and trees are
not unique examples of ecosystem engineer-
ing by biota. A great variety of free-living
organisms engineer ecosystems to varying
degrees—from earthworms burrowing in
soils, to coral and oyster reefs, to microbial
crusts controlling runoff on desert soils
(
Figure 11.2
). Humans are allogenic ecosys-
tem engineers par excellence.
Physical ecosystem engineering has four
cause-and-effect relationships (
Figure 11.3
):
1.
An engineering species causes structural
change.
2.
Structural change causes abiotic change
(resources and conditions).
3.
Abiotic change and structural change
(via living space) cause biotic change
(i.e., other organisms and associated
ecological processes).
4.
Abiotic change, biotic change (via other
interactions with the engineer such as
