Environmental Engineering Reference
In-Depth Information
restoration, particularly if the site occurs within a pro-
tected area. Whether or not restoration is feasible will
depend both on the site location in relation to natural
vegetation remnants and on available fi nancial
resources. If restoration is not feasible, it is important
to at least halt further spread of the invasive species.
For landscape-scale restoration, biological control is
thus essential: a gall .fungus on
A. saligna
which was
introduced intentionally as a biological control (bio-
control) agent reduces the life span of
Acacia
trees to
5-7 years (Wood & Morris 2007) and allows the
canopy to remain open, thus promoting survival of
some native fynbos plants. Restoration of long-invaded
sites, characterized by elevated soil nitrogen and lack
of native soil-stored seed banks, is very diffi cult, and to
succeed it has to overcome several barriers.
Invasive pines in fynbos have a very different strat-
egy to the wattles. Their key adaptation for survival,
persistence and rapid invasion in fynbos, and else-
where, is their serotinous habit: winged seeds are pro-
tected from fi res in woody cones which open upon the
death - by - fi re of the parent plants. This mechanism
allows them to germinate rapidly after fi re and disperse
successfully over large distances. Invasive pines have
become highly disruptive, displacing native plant
species, altering fi re regimes and reducing stream fl ow
from invaded catchments (Le Maitre
et al
. 1996 ). Pine
invasions also differ from
Acacia
invasions in how they
degrade invaded ecosystems. Fynbos community
structure and composition are impacted by dense pine
invasion (Richardson & van Wilgen 1986). However,
unlike
Acacia
, pines do not fi x nitrogen and they grow
more slowly. Whereas in established alien
Acacia
stands
canopy closure occurs within 6 months of a fi re, this
can take several years for pines, allowing coexistence
of perennial fynbos species and the opportunity for
native seed bank replenishment. As invasion intensi-
fi es, pines change the abiotic conditions (e.g. they have
a much higher biomass than fynbos and use more
water, shade out fynbos and may cause higher inten-
sity fi res). Pine invasion does not enhance soil nitrogen
levels, so there is no apparent positive feedback loop to
reinforce pine or secondary alien species invasion.
Rather, pine invasions persist through high propagule
pressure and highly dispersible seeds (Richardson &
Higgins 1998 ) (Table 20.2 , Figure 20.5 ). Restoration
of long-invaded sites will have to include reintroduc-
tion of key native functional groups and possibly
microbial groups. Because of the high propagule pres-
sure post - fi re, follow-up control will be essential to
prevent reinvasion. To date biological control is seldom
integrated into restoration projects, either because of
a lack of knowledge or because of legal implications.
However, especially for landscape-scale restoration, the
use of biological control agents would often be essen-
tial in slowing the rate of degradation.
The two case studies presented in this section show
that different invaders impact ecosystems in very dif-
ferent ways, with resultant implications for restora-
tion. In the
Acacia
example, long - standing invasion
results in degradation beyond structural, functional
and positive feedback thresholds, causing multiple bar-
riers to restoration. As funding for restoration projects
is limited, managers must prioritize sites for restora-
tion. The conservation status of the invaded area as
well as its restoration feasibility should be considered.
In section 20.5, we present criteria for restoration fea-
sibility and desirability and conclude with a decision-
making framework for the management and restoration
of invaded ecosystems.
20.5 TOWARDS A DECISION-MAKING
FRAMEWORK FOR THE MANAGEMENT
AND RESTORATION OF INVADED
ECOSYSTEMS
Once the degree of ecosystem degradation has been
assessed and the restoration requirements determined,
decisions can be made on whether restoration is feasi-
ble and affordable and, if so, how the restoration
should be approached. Prior to decision making, it is
important to establish restoration goals. Depending on
the degree of degradation, restoration goals will range
from re-establishing a natural ecosystem state, focus-
ing on biodiversity components, to restoring ecosystem
processes and functions. Our framework for restoration
of invaded ecosystems is based on the three-threshold
model (Figure 20.2).
Restoration of a less degraded ecosystem (in our case
study, an ecosystem recently invaded by wattles or
pines; transition state 1 in Table 20.1) is often feasible
because abiotic conditions are relatively unchanged
and clearance of the alien plants is the only action
required to initiate autogenic recovery towards a
target ecosystem state focusing on
biodiversity
. If the
structural threshold has been crossed (transition state
2 in Table 20.1), autogenic recovery may have to be
facilitated, for example by
reintroducing native
species
.
