Environmental Engineering Reference
In-Depth Information
births and deaths, and migrations and colonizations,
roughly match.
Though knowledge of
feedback
relations is crucial to
understanding the dynamics of ecosystems and their
response to changed conditions, this notion was soon
questioned by people studying real ecosystems, who
found that many ecosystems are never in equilibrium
(Ellis & Swift 1988). Rather, ecosystems are constantly
in fl ux and adapting to changing conditions at various
spatial and temporal scales, including changes in
climate and in external inputs of nutrients (see also
Chapter 21). To understand ecosystems, it is essential
to know how they respond to variations in background
conditions.
cially of phosphorus, a shift or transition takes place,
as conditions become ideal for microalgae. In the pres-
ence of unusually high nutrient levels in the water,
therefore, many shallow lakes rather suddenly switch
from a state with clear water and a highly diverse com-
munity of aquatic plants and invertebrates, to a murky
water state, with a much less diverse community of
pelagic microalgae. Submerged aquatic plants disap-
pear, and in some cases, large populations of toxic
cyanobacteria proliferate. The faunistic community in
this new state is characterized by sediment-feeding
fi sh, which exploit and remobilize the phosphorus that
is stored in dead plant material in the sediment, gener-
ating a feedback loop that solidifi es or 'fi xes ' the
microalgae-dominated state in which the lakes now
occur (see also Chapter 18).
The change in ecosystem state that these shallow
lakes experience in response to increased eutrophica-
tion was unexpected, dramatically fast and hard to
reverse. Many lakes stay in their murky state for years
after anthropogenic nutrient inputs are stopped, and
nutrient levels must return to levels well below those
at which the initial switch to a microalgae-dominated
state occurred before submerged plants are able to
recover. At the basis of this lies a phenomenon whose
existence was predicted by Noy-Meir (1975), who pro-
posed that not just one but two different 'steady states'
may occur in a subtropical or semi-arid region grazing
system.
Indeed, we now know that many ecosystems can
manifest two or more
alternative stable states
, each
characterized by a distinctly different community with
different feedback processes and overall functionality.
Severe or ongoing human pressure on ecosystems can
overwhelm the feedback processes that characterize
one state, leading to a dramatic shift from one type of
community to another. This new state typically has its
own stabilizing feedbacks - in other words, it can
become resilient and such that a quick return to the
earlier steady state is effectively blocked. Only a dra-
matic change in conditions will allow a return to the
previous state in which the previous biotic community
can re-establish itself (see e.g. Suding
et al
. 2004 ).
Theory on the potential of such 'catastrophic
changes' in ecosystems in response to (gradually)
changing conditions dominated ecosystem ecology
during the late 1990s and early 2000s. The resulting
models on alternative stable states, that were obtained
primarily from studies of shallow lakes, were subse-
quently applied to a range of other ecosystems in
6.2.2
Ecosystems in a world of change
Environmental changes have always occurred, as testi-
fi ed by records of variation in mean global temperature
over several thousand years (Intergovernmental Panel
on Climate Change (IPCC) 2007b). However, our very
rapidly growing human
ecological footprint
, over
the last two centuries especially, is causing profound
and sometimes very rapid changes in environmental
conditions on our planet - due for example to indus-
trial, agricultural, and urban greenhouse gas emissions
and the artifi cial enrichment of aquatic ecosystems by
inorganic nutrients, especially N and P, 'leaking' from
intensively fertilized fi elds and choking out possibilities
for life through
eutrophication
. This has led ecolo-
gists to ask the question 'How do ecosystems respond
to these changes?'
The presence of feedback relations within ecosys-
tems is not always an adequate safeguard against
potential effects of rapid and/or profound changes in
environmental conditions. In particular, some systems
change signifi cantly, even violently, in response to
shocks and stress induced by profound or prolonged
human infl uences. A striking example of this is shallow
lakes, as reported by Scheffer
et al
. (1993) . Many
shallow lakes have experienced increased inputs of
nitrogen and phosphorus as a consequence of uncon-
trolled seepage of fi eld and garden fertilizers, and
dumping of phosphate-based detergents. Initially, this
only leads to minor effects, as long as increased densi-
ties of dead and decomposing aquatic plants locked up
the nutrients in the sediment, generating a
feedback
loop
that compensates, in part, for increased nutrient
availability. As nutrient inputs increase further, espe-
