Environmental Engineering Reference
In-Depth Information
20.1
INTRODUCTION
ing standard
interventions
(Suding
et al
. 2004 ). In
section 20.2 we review the many ways in which inva-
sive plant species can potentially change ecosystem
composition, structure and functioning, looking at
underlying mechanisms and processes of alien plant
invasions.
It has been acknowledged that clearing of invasive
species alone is often not suffi cient for re-establishing
native communities; some form of restoration is
increasingly seen as vital when dealing with alien inva-
sions (Esler
et al
. 2008). However, restoration efforts
are challenged by numerous obstacles caused by the
alien species such as altered ecosystem properties and
ecosystem functions. Consequently, restoration efforts
often have unexpected outcomes or even unforeseen
negative consequences. In section 20.3 we summarize
the most important challenges that restoration ecolo-
gists have to face when dealing with alien invasions
and give examples of successes and failures.
To successfully plan and prioritize restoration efforts
after alien invasion, a fundamental step is to determine
the
degree of ecosystem degradation
. To identify the
degree of ecosystem degradation resulting from inva-
sion is a major challenge for researchers and manag-
ers. In section 20.4 we evaluate how different types of
impact can infl uence the
resilience
of ecosystems,
using the concept of ecosystem
thresholds
to deter-
mine ecosystem degradation. In the context of ecosys-
tem restoration, resilience is defi ned as the ability of
an ecosystem to recover from
disturbance
without
human intervention (Westman 1978). The concept of
ecosystem thresholds
has been identifi ed as a useful tool
for determining the degree of ecosystem degradation
(Groffman
et al
. 2006 ). Briske
et al
. (2006) described a
threshold as the point at which the dominance of the
negative (regulating) feedbacks that maintain ecosys-
tem resilience is replaced by the dominance of positive
(supportive) feedbacks that lead to losses in resilience.
Invasive species may dominate an ecosystem to the
extent that structural and functional changes to biotic
and abiotic processes occur in concert, resulting in
turn in supporting feedbacks that further promote the
spread of the alien species.
To demonstrate the implications of the ecosystem
threshold approach for management and restoration,
we present case studies of
Acacia
and
Pinus
invasions
in South African
fynbos
. Australian species of
Acacia
(known in Australia as wattles) are particularly suita-
ble as study species for the ecosystem resilience concept
as they have huge invasive potential and have been well
The idea that biological invasions are one of the major
causes of ecosystem degradation is currently under
debate. Many authors consider biological invasions
to be important drivers of ecosystem degradation,
whereas others suggest that they are, in many cases,
just ' passengers ' - capitalizing on other anthropogenic
disturbances (MacDougall & Turkington 2005 ; Gasso
et al
. 2009). While some authors consider invasive
alien species to be a substantial threat to biodiversity
and ecosystem stability, and hold them responsible for
the decline in native species richness and the local
extinction of certain species (Richardson
et al
. 1989 ;
Wilcove
et al
. 1998 ; Simberloff
et al
. 2011 ), others
point out that net species diversity, in most cases,
increases following biological invasions, at least over
short time scales (Sax & Gaines 2003).
Consequently, opinions about approaches for man-
aging
invasive alien species
differ; some authors call
for the acceptance of invasive alien species in some
situations, for example where they provide economic
incentives or where they are integrated into native eco-
system networks (Davis
et al
. 2011 ; Vince 2011 ). Some
studies even call for the use of alien species to restore
areas that have been heavily degraded, since alien
species grow faster and can provide a canopy under
which slower-growing native species can (re-)establish
(Ewel & Putz 2004). On the other hand, invasion biolo-
gists argue that focused management is necessary
because invasive species can cause severe impacts on
ecosystems including species extinctions and major
economic losses (Simberloff
et al
. 2011). We argue that
impacts of biological invasions differ from situation to
situation depending on the ecosystem that is invaded
and the particular invader. In our chapter we focus
on those biological invasions that have undeniably
negative impacts on ecosystems, and exclusively on
impacts of invasive plant species. We seek to link the
topic of invasion ecology and the practice of ecological
restoration.
Biological invasions are increasing in importance
worldwide as drivers of ecosystem
degradation
and
transformation. Invasive species have many impacts:
some operate at the level of populations or communi-
ties (Hejda
et al
. 2009), while others interfere at higher
levels, disrupting fundamental processes that affect
entire ecosystems (Parker
et al
. 1999 ). Alterations to
ecosystems can lead to a shift to an
alternative stable
state
which may not be amenable to restoration, utiliz-