Environmental Engineering Reference
In-Depth Information
Low degradation/
recent invasion
High degradation/
long invasion
1
2
3
4
positive
feed-
backs
Biotic
threshold
Abiotic
threshold
Biotic-abiotic
feedback
threshold
Figure 20.2
Three - threshold model (modifi ed from Whisenant 2002; King & Hobbs 2006) illustrating the concept of
thresholds which indicate break points between alternative ecosystem states: (1) native ecosystem state: no threshold is
reached (e.g. altered species composition but above-ground vegetation and/or seed banks intact); (2) alternative ecosystem
state: biotic threshold is reached (e.g. altered species composition and structure, seed bank depleted); (3) alternative ecosystem
state: abiotic and biotic thresholds are reached (e.g. altered water and nutrient availability); and (4) alternative ecosystem
state: positive feedbacks trigger threshold to be reached (e.g. changed fi re regime favours the growth of invasive species but
causes extinctions of native species).
small area (90 000 km
2
) wherein approximately 70%
of native plants are endemic.
Alien invasions
pose a
serious threat to the region, especially Australian
wattles in lowland and riparian ecosystems, and Euro-
Mediterranean pines in the mountains. Australian
Acacia
species have huge invasive potential and strong
persistence due to their capacity to produce enormous
amounts of long-lived seeds (Richardson & Kluge
2008). Invasion by these wattles leads to increased
biomass and changed fuel properties in fynbos ecosys-
tems (van Wilgen & Richardson 1985). Furthermore,
their presence can cause alterations in nutrient cycling
due to nitrogen fi xation in concert with increased
litter production and mineralization rates (Yelenik
et al
. 2004 ).
Early in
Acacia saligna
invasion of fynbos, changes
occur to the biotic structural components (e.g. altered
native species composition; Table 20.1, Figure 20.3).
With increasing intensity of invasion, these changes in
biotic structure result in alterations of biotic functions
via the suppression of native species. The dominance
of
Acacia
and the decrease in native species density and
cover reduces ecosystem resilience. Once invasion is
dense, a biotic threshold is passed as
Acacia
excludes
the fynbos species and dominates the soil seed bank.
Although biotic and some abiotic components have
been altered, the system may still be capable of
autogenic recovery after
Acacia
control since soil-stored
seed banks of fynbos species may remain and seedlings
of fynbos species can cope with elevated levels of soil
nitrogen (Musil 1993). Dense alien
A. saligna
stands
alter ecosystem functioning via elevated biomass, litter
production, nitrifi cation and nutrient-cycling rates.
This in turn promotes growth of the alien and other
competitive (native and alien) species, such as grasses,
further boosting their competitive advantage over
fynbos species with each ensuing fi re cycle. An abiotic
threshold is reached after a certain duration of dense
invasion (> 1 and > 3 fi re cycles of
Acacia
in lowland
and mountain fynbos, respectively). These structural
