Environmental Engineering Reference
In-Depth Information
feedbacks (Callaway
et al
. 2004) or allelopathic effects.
Garcia - Serrano
et al
. (2007), for example, investigated
the replacement of a native
Senecio
species (
S. malacita-
nus
), native in the south-east of the Iberian Peninsula
and Maghreb, by two alien
Senecio
species (
S. inaequi-
dens
and
S. pterophorus
, both from South Africa) in a
greenhouse experiment. They found that the alien
S.
inaequidens
was the most competitive species for all
moisture conditions and that the native
S. malacitanus
was more competitive than the alien only under condi-
tions of water stress.
Establishment limitation of new native individuals
can be triggered if alien propagules occupy germina-
tion niches or through decreased resource availability,
such as for light or nutrients (Davis
et al
. 2000 ), but
also by increased resource availability. A further cause
for reduced establishment is the development of a thick
litter layer by the alien which creates a barrier for
emerging native seeds (Marchante
et al
. 2004 ).
enrichment following alien plant invasions has also
been demonstrated in several ecosystems (e.g. Fisher
et al
. 2006). Invasions may also change other aspects
of ecosystem biogeochemistry. For example, some
plants change soil chemical properties by accumu-
lating certain minerals, such as salt (NaCl), in the
soil, leading to changes in plant density and species
composition, such as those following invasion by ice
plant (
Mesembryanthemum crystallinum
)
(Vivrette
&
Muller 1977 ).
Water u se
Invasive alien species can absorb substantial amounts
of water and other limited resources and thus affect
plant communities by reducing soil water availability
for native species. Furthermore, some invasive alien
plants gain competitive advantage by extracting soil
water at different times or from soil depths not exploited
by native plants. In semi-arid regions of the western
United States, for example, Kulmatiski
et al
. (2006a)
showed that winter-active alien annual grasses inhibit
the establishment of native perennial grasses by early
season and deeper use of water resources. Other studies
report increased
evapotranspiration
rates in invasive
alien compared to native plant species (Zavaleta 2000).
In riparian ecosystems, invasions may have wider
implications: invasive alien trees often use signifi cantly
more water than native riparian species and can there-
fore substantially reduce water fl ows (Dye
et al
. 2001 ).
In South Africa, invasive alien trees from Australia
(mainly
Acacia
and
Eucalyptus
species) have reduced
natural river fl ows by 20% or more (Le Maitre
et al
.
1996 ).
Nutrient a ccumulation
Invasive alien plants with a major impact on ecosys-
tem processes can be called ' transformers ' (
sensu
Rich-
ardson
et al
. 2000). Some transformers alter ecosystem
processes by adding resources that are generally
limited in the native ecosystem, for example by nutri-
ents. Many invasive plants accumulate nitrogen
through increased biomass and net primary produc-
tion (i.e. through production of litter with higher
decomposition rates than native species) (Ehrenfeld
2003 ). Especially in nutrient - poor ecosystems, an
increase in nitrogen can have signifi cant impacts,
ameliorating the conditions for invasive alien plants
(Vinton & Goergen 2006) and preventing the re-
establishment of native plants (Marchante
et al
.
2008). Prominent examples for nitrogen increase in
nutrient-poor habitats are Australian wattle invasions
in South African fynbos, invasion on young volcanic
soils in Hawaii by fi re tree (
Morella faya
), native to
Macaronesia (the Azores, Madeira and the Canary
Islands), and invasion of black locust (
Robinia pseu-
doacacia
), native to south-eastern North America, into
north - eastern American pine - oak ecosystems. Inva-
sion by Norway maple (
Acer platanoides
) and tree of
heaven (
Ailanthus altissima
) in temperate forests of the
north-eastern United States considerably accelerated
nutrient cycling rates and increased soil nutrient
levels, with the magnitude of the impact increasing
with soil fertility (Gomez - Aparicio 2009 ). Phosphorus
Altered fi re r egimes
Invasive alien species may suppress or promote fi re,
thereby driving rapid ecosystem transformation
(Brooks
et al
. 2004). Examples of increases in fi re fre-
quency are common among grass invasions in areas
naturally dominated by woody plants (Rossiter-Rachor
et al
. 2008). Even in fi re - prone ecosystems, plant inva-
sions can have severe impacts if they alter the natural
fi re regime to produce a different selective environment
(Keeley
et al
. 2005 ). Besides altering fi re frequency,
aliens can also alter fi re intensity, especially if they
produce a greater biomass and hence larger fuel loads
than native species (D'Antonio
et al
. 2000 ). Aliens can
also decrease fi re frequency and/or intensity, if they
