Environmental Engineering Reference
In-Depth Information
economists, and a growing number of ecologists, call
humanity's stock of
natural capital
(i.e. renewable
and nonrenewable resources from 'nature') that assure
the fl ow of
ecosystem goods and services
to society
(Aronson
et al
. 2007a). Thus, many motivations and
justifi cations for ecological restoration exist (see Clewell
& Aronson 2006, and Chapter 2), yet a fi nancial - and
perhaps also a social or political - cost is inevitably
involved. Increasingly, it is obvious - at least to us - that
all societies everywhere should be devoting resources
to this activity to insure and enhance the supply of
ecosystem services as well. However, what may seem
like a clear gain for some, can be perceived as a loss or
waste of resources for others. Trade-offs, negotiation
and, above all, good communication are a
sine qua non
in this realm of human endeavour that require both
ecological and environmental as well as socio-economic
and even political criteria for monitoring and evalua-
tion (Blignaut
et al
. 2007 ; Aronson 2010 ).
Needless to say, points of view in most situations will
differ among
stakeholders
, and they will also change
over time, in any heterogeneous society, and even
among specialized scientists. To illustrate this, let us
consider the concept of steady states and disturbance,
a key notion in all discussions of conservation, man-
agement and restoration of ecosystems.
Figure 1.2
Model of three types of system response to a
disturbance factor: resistance, resilience and disturbance.
State A is the starting or 'initial' steady state, State C is a
new, alternative steady state. As long as the system state
does not pass a 'threshold of irreversibility' (State B), the
system remains stable. Restoration of the ecosystem from
State C towards the starting state A is affected by hysteresis
and by the nature and number of the threshold(s) it has
crossed.
farmer clears a piece of woodland and then cultivates
the land, he or she will naturally consider the change
as a gain. Yet, from the point of view of a bird watcher,
or local authority in charge of nature conservation,
such a transformation may be considered as a loss of
habitat
for birds, or a
degradation
of the woodland
ecosystem at the landscape scale. Similarly, local com-
munities dependent on woodlands for various services
(for example watershed protection and outdoor recrea-
tion) will consider it as negatively impacting their
welfare and well-being. Thus, especially if the farmer
eventually abandons production, for reasons of chang-
ing markets for example, there may be a good argu-
ment that ecological restoration should be attempted,
in order to restore the woodland that once was there.
However, the farmer may instead seek other crops or
land uses that raise income to his or her family or cor-
poration. In Chapter 2, we will return to the concept
of
disturbance
, and the related one of
stability
. Here
we will consider points of view on 'nature' as related
to the qualifi cation of ' disturbance ' .
1.2 VIEWS ON STEADY STATES AND
DISTURBANCE
Disturbance
, though it may sound negative, is basi-
cally a neutral term in science. The term is widely used
in
ecology
, and we will also use it in this topic, but the
neutral term 'transformation' is often a better choice
for indicating a change of a complex system from one
state to another one. What we call a
disturbance
factor
causes a change or transformation in an eco-
system's steady state, in terms of its standing biomass,
productivity or biodiversity, which may be followed by
either
recovery
to the former state (through
resilience
or
resistance
) or a change to another state, following
the crossing of a so-called
threshold of irreversibil-
ity
(see Figure 1.2); then the system is disturbed. In the
latter case, the system may shift to another steady
state, or not; in the ecological literature, this new state
is referred to as an
alternative stable state
.
Depending on your point of view, State C, the alter-
native steady state, can be a gain or improvement, or
else a loss or example of degradation. For example, if a
1.3 VIEWS ON NATURE AND NATURE
CONSERVATION
Just as views on what constitutes a disturbance
differ, the same is true for notions of 'nature', 'nature
