Environmental Engineering Reference
In-Depth Information
strong positive feedback loops that affect ecosystem
resilience. We now describe the different ways in which
invasions may impact ecosystem resilience and use this
as a basis for exploring ecosystem degradation thresh-
olds following alien invasion.
several interacting ecosystem components, and identi-
fying when such points are reached is therefore diffi -
cult. Some invaded ecosystems may never reach a
threshold and therefore never change into an alterna-
tive ecosystem state. Others do, and depending on the
type of ecosystem invaded and the invader itself may
even cross several thresholds and eventually change
into an alternative ecosystem state. It is important for
restoration ecologists to determine whether thresholds
have been crossed or an alternative ecosystem state has
been reached. In the next section we introduce the
concept of ecosystem thresholds and conclude by pre-
senting the three-threshold model for plant invasion
to guide decision making on restoration goals and
interventions.
20.4.1
Resilience and biological invasion
Loss of ecosystem
resilience
owing to biological inva-
sions can be triggered by biotic interactions: upon
invading an ecosystem, invasive alien species can cause
a loss of resilience by changing community composi-
tion and structure, leading to changes in ecosystem
functioning
(Richardson
et al
. 2007 ). Depending on
the type of ecosystem invaded and the identity of the
invading species, structural changes can be followed
and/or accompanied by changes in ecosystem func-
tions (e.g. changes in nutrient cycling). The most fre-
quent scenario, however, is that alien invasions change
structural and functional conditions in concert, creat-
ing a switch from negative to positive
feedback loops
.
Positive, or supporting, feedback loops can be triggered
by the competitive superiority of invaders. Reed canary
grass (
Phalaris arundinacea
) causes rapid shifts from
diverse native vegetation to a monotypic stand of
Phalaris
in sedge meadows across temperate North
America. Adams and Galatowitsch (2005) suggest that
vegetative growth of
P. arundinacea
facilitates the domi-
nance of this species and that its invasiveness may be
related to high rates of biomass production during
establishment. Another form of a positive feedback loop
is the creation of nutrient cycling patterns that favour
the alien (Kulmatiski
et al
. 2008). Other invasive plants
alter ecosystem properties to facilitate the establish-
ment of their seedlings by inhibiting growth in other
species. Siemann and Rogers (2003) showed that once
Chinese tallow tree (
Sapium sebiferum
) is established in
grasslands in the southern United States, their own
seedlings are indirectly favoured in competition with
neighbouring herbaceous vegetation through increased
soil fertility and shade. Positive feedback loops can also
be initiated by alterations to microbial processes (Díez
et al
. 2010) or changes in fi re regimes (Rossiter-Rachor
et al
. 2008). The type of intervention required to restore
an invaded ecosystem will depend on whether assisted
recovery is possible (i.e. whether the ecosystem is still
resilient). To determine the degree of ecosystem degra-
dation, it may be useful to apply the concept of ecosys-
tem thresholds.
Threshold
crossings are the result of
20.4.2 The concept of ecosystem
thresholds - a tool for determining
the level of ecosystem degradation
The point at which dominance of the negative (regulat-
ing) feedbacks that maintain ecosystem resilience
switch to a dominance of positive (supporting) feed-
backs that lead to loss of ecosystem resilience has been
described as a ' threshold ' (Briske
et al
. 2006 ). At this
point, the maximum level of ecosystem
resilience
to
degradation
is exceeded (i.e. a threshold is crossed)
and the ecosystem will change to an alternative state.
Once this has happened, ecological restoration may be
very diffi cult, if not impossible, and other, less ambi-
tious goals may be more realistic. Thresholds can be
determined by identifying key disturbances (i.e. distur-
bances caused by the alien) and/or key response
variables (i.e. the response of the native ecosystem to
invasion). Both key disturbance and key response vari-
ables can manifest themselves at the level of popu-
lations, communities or ecosystems. At each level,
we can distinguish between structural thresholds and
functional thresholds. Structural thresholds are
defi ned by changes in relative growth form and species
composition, whereas functional thresholds are defi ned
by alterations of ecological processes. Structural and
functional thresholds can occur through either biotic
or abiotic ecosystem changes or a combination thereof
(Figure 20.1). Once key disturbance (KD) and key
response variables (KRV) have been identifi ed, the
next step is to determine the state of these variables
over time (i.e. to defi ne the trajectory of degradation).
Here it is important to distinguish clearly between the
