Environmental Engineering Reference
In-Depth Information
produce lower biomass or less fl ammable fuels than
native species.
and antiherbivore defences of the native plant
Lotus
wrangelianus
.
Impacts at the p lant c ommunity l evel
20.2.2
Native ecosystem response
Impacts at the community level follow a common pro-
gression from initial impacts (e.g. increased biomass,
nitrogen fi xation, species displacement and changes in
abundance) to secondary impacts (e.g. decomposition
processes, and changed seed bank dynamics) to terti-
ary impacts (e.g. interactions among fauna) (Le Maitre
et al
. 2011). The most frequently reported initial impact
of biological invasions at the community level is a
decrease in plant species richness or diversity. Such
impacts appear to be largely species-specifi c, with the
severity of the impact depending almost exclusively on
the identity of the invading species (Hejda
et al
. 2009 ).
Another important infl uencing factor is the type of
native ecosystem undergoing invasion (Gaertner
et al
.
2009). Invasive plants can also negatively affect native
community composition and structure (Lenz
et al
.
2003 ). J รค ger
et al
. (2009) investigated the impact of the
red quinine tree (
Cinchona pubescens
) on the fern-sedge
vegetation of the Galapagos Islands. Invasion of the
formerly treeless wetland community by alien tree
species had signifi cant impacts on stand structure and
environmental conditions and led to decreases in
species diversity and cover.
A secondary impact of invasion on plant commu-
nity composition is the depletion and homogenization
of the native soil
seed bank
. Plant invasions can sig-
nifi cantly alter richness, diversity and composition of
native seed banks, resulting in long-term changes for
the plant community (Gioria & Osborne 2009 ). For
example, in South African fynbos ecosystems, invasion
by the Australian wattle
Acacia saligna
causes signifi -
cant reductions in native soil seed bank density and
richness (Holmes 2002). Tertiary impacts are charac-
terized by interference of invasive species in native eco-
system networks and are described below.
The outcomes of alien invasions on the native ecosys-
tem have been summarized in several reviews (e.g.
Parker
et al
. 1999 ; Levine
et al
. 2003) and can be meas-
ured at both population and community levels. Inva-
sions can also affect mutualistic plant-animal
interactions (Traveset & Richardson 2006 ).
Impacts on p lant p opulations
Invasions can have immediate impacts on the perform-
ance and phenotypic traits of individual plant species.
These impacts can manifest in reduced growth or
reproduction or changes in native species morphology
(Parker
et al
. 1999). Invasions can also have direct
genetic impacts on native species through
hybridization
and/or
introgression
(introgressive hybridization; see
also Chapter 7). Hybridization of invasive alien plants
with native plants can cause a loss in fi tness in the
latter and even result in local extinction (Parker
et al
.
1999 ). Prentis
et al
. (2007) found a decline in the
native
Senecio pinnatifolius
from east coast areas of Aus-
tralia, due to hybridization with the introduced
S.
madagascariensis
. Small native populations on islands
are especially vulnerable to extinction by hybridization
(Levin
et al
. 1996). Introgression describes repeated,
interspecifi c hybridization between a new (to a region)
taxon and a resident taxon, followed by backcrossing
to either or both parental taxa. This infl ux of new
genetic information results in novel phenotypes which
could be more robust and have broader ecological
amplitudes than either parental type. They tend to
occupy disturbed habitats - also called 'hybrid habitats'
(Galatowitsch
et al
. 1999 ).
Furthermore, invasions can have indirect genetic
impacts by altering patterns of natural selection within
native populations. Native species in old invasions may
have become adapted to the altered habitat (e.g. they
develop a resistance to potentially allelopathic chemi-
cals produced by the alien species) (Mealor & Hild
2006). Lau (2008) showed that invasion by an alien
plant and herbivore species in open grasslands
throughout the McLaughlin Reserve in Napa County,
California, United States, altered the strength and
direction of natural selection on the competitive ability
Impacts on p lant - a nimal and
p lant - m icroorganism n etworks
Invasive alien plants can infl uence mutualistic plant-
animal interactions, such as pollination and seed
dispersal, in several different ways (e.g. Traveset &
Richardson 2006). Decrease in pollinator visitation to
native plant species has been demonstrated as a result
of a competitive advantage by an invasive alien plant
