Environmental Engineering Reference
In-Depth Information
Structure
Function
altered soil nutrient availability (KD)
altered water availability (KD)
increased fire intensity/frequency (KD)
altered nutrient cycling rates (KD)
Abiotic
increased invader biomass (KD)
high invader seed production (KD)
dominance of alien seedlings (KD)
altered community structure (KRV)
high native seed mortality (KRV)
depleted native seed bank (KRV)
declines of native species richness (KRV)
decrease of native functional groups (KRV)
suppression of native species (KRV)
enhanced growth of other alien species (KD)
increased competition for resources e.g. light
and water (KD)
allelopathy (KD)
altered microbial systems
Biotic
Figure 20.1
2 × 2 matrix showing structural and functional thresholds described by biotic and abiotic ecosystem changes
and responses. KD, key disturbance; KRV, key response variable.
alteration processes that start immediately after the
establishment of the alien (e.g. litter input and water
use), and the response of the native ecosystem to these
processes, which may occur only after a lag phase.
Figure 20.2 shows the
three - threshold model
for plant
invasions which can be a useful tool to determine the
level of ecosystem degradation. The model illustrates
the trajectory of invasion with three thresholds being
crossed in the course of increasing ecosystem degrada-
tion. We use ecosystem response (ER) (i.e. outcomes or
consequences of invasion on a native ecosystem) as the
means of determining the level of ecosystem degrada-
tion. The fi rst threshold is reached by a change of
structural biotic interactions within the invaded com-
munity, and is followed by a second structural thresh-
old, after a longer duration of invasion. This second
threshold is harder to reverse and can be caused by
amplifi ed biotic interactions, abiotic changes or a com-
bination thereof (Suding & Hobbs 2009; see also
Chapter 21). At this point, most of the ecosystem's
measurable traits (e.g. nutrient load) are the same as
before, but the species composition has changed. In a
later stage of invasion, a third threshold (positive feed-
back threshold) may be crossed. At this stage, changes
in structural components result in a change of func-
tional components (e.g. competitive interactions);
these changes in functional components initiate posi-
tive feedback loops (i.e. the alien alters the ecosystem
to its own favour but to the disadvantage of native
species, thereby increasing the rate of invasion).
In practice, measuring changes in ecosystem func-
tion over time or over a range of sites that have experi-
enced different levels of invasion allows us to defi ne
levels of ecosystem functioning that represent thresh-
olds. To identify whether an ecosystem has crossed one
or more thresholds, it is necessary to evaluate ecosys-
tem response (i.e. the location of the ecosystem on the
Y-axis), which can also be interpreted by determining
the degree of invasion (i.e. the location on the X-axis)
(King & Hobbs 2006 ).
The identifi cation of variables that represent thresh-
olds can be illustrated with the example of the well-
studied case of
Lantana camara
L. in New South Wales
(Australia). Invasion by this vertebrate-dispersed South
American shrub reduced native species richness
(key response variable (KRV)) signifi cantly in heavily
invaded sites (above a level of 75% cover). Below 75%
invader cover, native species richness remained stable.
The authors therefore suggest that 75% cover of
L.
camara
can be defi ned as the threshold beyond which
native species richness declines (Gooden
et al
. 2009 ).
20.4.3 Case studies from South Africa:
Acacia
and
Pinus
invasions in fynbos
ecosystems
Fynbos shrublands are the dominant vegetation of the
Cape Floristic Region, a global
biodiversity
hotspot.
More than 9000 plant species occur in a relatively
