Environmental Engineering Reference
In-Depth Information
species (Brown
et al
. 2002). By contrast, a study on the
competitive advantage of the invasive alien
Impatiens
glandulifera
in riparian ecosystems in Spain did not
provide evidence that this alien was out-competing
native plants for pollinators (Bartomeus
et al
. 2010 ).
This fi nding was corroborated in a cross-European
analysis on integration of invasive alien species into
native pollinator networks (Vila
et al
. 2009 ). Similarly,
studies on the impact of alien plant invasions on the
pollinator itself have not yielded conclusive results to
date (e.g. Severns & Warren 2008).
Invasive alien plants can also impact soil microbes,
indirectly changing soil processes (e.g. nitrifi cation)
(Ehrenfeld
et al
. 2001). In hardwood forests in north-
ern New Jersey, United States, Kourtev
et al
. (2002)
found that microbial communities differed in structure
(measured by phospholipid fatty acid (PLFA) composi-
tion) and microbial function (described by enzyme
activities and substrate-induced respiration (SIR) pro-
fi les) in soils beneath two alien understorey plant
species, namely Japanese barberry (
Berberis thunbergii
)
and Japanese stilt grass (
Microstegium vimineum
), as
compared to soils beneath native plant species. Inva-
sive alien
Acacia longifolia
in sand dunes in Portugal
had substantial impacts on microbial functional diver-
sity increasing microbial biomass and activity (basal
respiration) as well as nitrifi cation and enzyme activity
(β - glucosaminidase) (Marchante
et al
. 2008 ).
This overview shows that invasive alien plants impact
native ecosystems in a variety of ways and that the out-
comes of invasions vary depending on the type of eco-
system invaded and the alien itself. Plate 20.1 shows a
selection of invasive aliens, with differing impacts in
Mediterranean - type fynbos shrubland ecosystems.
of N
2
- fi xing Australian
Acacia
species in Portuguese
coastal ecosystems, it can take several years before soil
nutrients and processes return to pre-invasion levels.
Elevated soil nutrient levels can cause rapid reinvasion
of the same alien species or favour other weedy species,
thus preventing the re-establishment of native species.
Active restoration including removal of N-enriched
litter can facilitate ecosystem recovery (Marchante
et
al
. 2009). In North American wetlands,
Phalaris arund-
inacea
invasion prevents establishment of native sedge
meadow communities. Control of this alien species
with herbicide led to several setbacks, including rapid
Phalaris
recovery, failure in establishment of intro-
duced native sedge meadow species and an increase in
alien weedy species richness and cover (Healy & Zedler
2010). However, a greenhouse study showed that low-
ering of soil organic nitrogen results in a competitive
advantage of the native
Carex hystericina
over
Phalaris
arundinacea
. This suggests that lowering of soil organic
nitrogen can promote the establishment of native sedge
meadow communities and suppress
Phalaris
invasion
(Perry
et al
. 2004 ).
If invaders have been integrated into native ecosys-
tem networks, restoration efforts can have unforeseen
consequences that exacerbate rather than mitigate the
problem (Hobbs & Richardson 2011). For example,
invasive trees (
Tamarix
spp.) provide habitat for an
endangered bird species, the southwestern willow fl y-
catcher (
Empidonax trailii extimus
), in riparian ecosys-
tems in the arid western United States. Removal of the
invader may lead to further reduction of the already
diminished population size of the bird species (Zavaleta
et al
. 2001), especially because native tree establish-
ment after
Tamarix
removal is low owing to lowered
water tables and saline soils caused by the invader
(Harms & Hiebert 2006). Here, the benefi ts of
Tamarix
trees as bird habitat have to be carefully weighed
against the tree's disadvantages.
20.3 RESTORATION AFTER ALIEN
INVASION - SUCCESS AND FAILURE
Whether restoration after alien invasion is required or
not depends on the degree of degradation (often related
to the duration of invasion). If the native ecosystem
has only undergone (initial) biotic changes, then clear-
ing of the alien species may be enough to initiate eco-
system
recovery
. If ecosystem functions have been
changed, then active restoration after alien clearance
may be required. For example, soil nutrient changes
caused by invasive species can persist long after their
removal, and this persistence may retard the recovery
of native communities for a long period. After clearance
20.4 INCORPORATING THRESHOLDS
IN DECISION MAKING FOR
ECOLOGICAL RESTORATION
Invasions can have severe impacts at population, com-
munity and ecosystem levels, often causing shifts to
alternative ecosystem states and trajectories. Depend-
ing on the type and severity of impact, management of
invasion may have to include active restoration. But
the worst-case scenario is where invaders establish
