Environmental Engineering Reference
In-Depth Information
studied. Nine Australian wattles have been classifi ed as
major invaders in South Africa and are widely distrib-
uted in different parts of the country (Nel
et al
. 2004 ).
Another group of highly invasive species in South
Africa is the genus
Pinus
, including the three most
widespread invaders in the fynbos biome: Aleppo pine
(
Pinus halepensis
), cluster pine (
P. pinaster
) from the
Mediterranean region and Monterey pine (
P. radiata
),
introduced from California (Richardson 2000).
Management of invaded ecosystems is an increas-
ingly complex problem worldwide (e.g. Roura-Pascual
et al
. 2009). Motives for the restoration and revised
management of invaded ecosystems include the desire
to deal with (1) loss of
ecological integrity
(e.g.
species loss) (Richardson & van Wilgen 1986 ), (2)
impact on
ecosystem functioning
(e.g. changes in
nutrient cycles) (Yelenik
et al
. 2004), (3) impaired
eco-
system services
(e.g. reduction in river fl ow that leads
to lower water supply for human use) (Le Maitre
et al
.
1996), and (4) loss in agricultural productivity (Born
et al
. 2005). In many contexts, the goal of manage-
ment interventions will be
ecological restoration
(i.e. to return a degraded or impaired ecosystem to
some historic trajectory) (SER 2004). However, in
some cases, other options will have to be considered,
especially when signifi cant biological invasions have
taken place. Three key questions need to be addressed
by managers of invaded ecosystems: fi rstly, is restora-
tion feasible, secondly, is it desirable, and, thirdly, what
are appropriate restoration goals? Answers to these
questions need to feed into a
decision - making framework
.
In the last section of this chapter, we will apply the
concept of ecosystem resilience to describe an approach
for decision making in ecological restoration, focusing
on plant invasions in native ecosystems. We address
questions concerning the feasibility of ecosystem res-
toration (i.e. will removal of the alien species be suffi -
cient to facilitate autogenic recovery of the ecosystem?)
and questions concerning the desirability of ecosystem
restoration (i.e. is restoration desirable where the alien
has become integrated into socio-ecological systems
and provides a valued ecosystem service for local
people?).
ration ecology studies often focus on the consequences
of invasions for populations and communities (i.e.
changes in community composition or changes in
species richness). Underlying processes and mecha-
nisms are much less frequently addressed. This has in
some cases led to unexpected results, or outright failure
of efforts aimed at restoration. As a basis to our frame-
work for the restoration of ecosystems impacted by
alien plants (see below), we will therefore differentiate
between (1) the processes and mechanisms underlying
the invader impacts (e.g. competition or allelopathy),
and (2) the outcomes of invasion on the native ecosys-
tem, called 'native ecosystem response' (e.g. decline of
species richness). Another factor central to plant inva-
sions, and also important in our restoration frame-
work, is
disturbance
. Natural disturbances (e.g. due
to fi res or fl oods) and anthropogenic disturbances (e.g.
due to over-grazing or vegetation removal) provide an
invasion window for alien species to establish (Richard-
son
et al
. 2007) and may trigger processes leading to
ecosystem degradation.
20.2.1 Processes and mechanisms
underlying impacts of invasions
The processes and mechanisms underlying impacts of
alien invasion are numerous and complex, including
the displacement of resident plants, the inhibition of
native plant establishment and the modifi cation of
physical and chemical resources (e.g. water and nutri-
ent availability) and native ecosystem processes (i.e.
changes to fi re regimes and nutrient cycling). Success-
ful alien plants can also make use of existing ecosystem
networks to facilitate their establishment (e.g. local
pollinators and dispersal agents). We will fi rstly review
the different biotic and abiotic processes underlying
impacts of alien plant invasions and secondly refer to
the native ecosystem response (i.e. the impacts on plant
population and community levels).
Competition
Invasive alien plants can replace already established
species in a community, prevent the establishment of
new individuals (Yurkonis
et al
. 2005) or both. Dis-
placement of already established plants can occur
through competition for resources (Werner
et al
.
2010), changes in physical or chemical soil properties
(Yelenik
et al
. 2004), an alteration of soil microbial
20.2 HOW DO INVASIVE ALIEN
PLANTS IMPACT ECOSYSTEMS?
The establishment of a new plant species often changes
the structure and functioning of an ecosystem. Resto-
