Biology Reference
In-Depth Information
5.4.2 Traits of Invasive Alien Species
In reference habitat, invasive species may be functionally similar to the local native
species (cf. Dietz and Edwards 2006; Kueffer 2006a). In the case of plant invaders,
shade-tolerance is an example of a trait that facilitates invasion into undisturbed
forests (Daehler et al. 2004); this trait is also common in native forest species. Some
invasive plants are also well adapted to low levels of unused soil resources prevail-
ing in undisturbed habitats (compare Kueffer 2006a; Funk and Vitousek 2007;
Schumacher 2007). Invasive woody plants invading dry inselberg habitats have
been shown to be as well adapted as native species to drought stress (Schumacher
et al. 2008). Such traits are expected to be uncommon among invasive plants associ-
ated with anthropogenic habitat.
However, alien plants invading reference habitat may also profit from having one
or more novel traits not (substantially) present in the native community, for instance
the ability to fix atmospheric nitrogen (Vitousek et al. 1987) or release allelopathic root
exudates (Callaway and Maron 2006). New functional groups such as large predators
(Fritts and Rodda 1998; Courchamp et al. 2003), large herbivores (Courchamp et al.
2003; Culliney 2006), new groups of invertebrates, e.g. earthworms (Hendrix and
Bohlen 2002), ants (Holway et al. 2002), or new diseases (Benning et al. 2002) can
rapidly spread into reference habitat. The nature of the trait novelty may be subtle. For
instance, allelopathy may be a common mechanism among native species but the
native species may not be adapted to the particular allelopathic substances released by
an alien species (Callaway and Maron 2006). Or, native species may be adapted to
respond to predators but not to the specific predatory strategies of an alien invader
(Schlaepfer et al. 2005). The exploitation of the lack of certain traits or functional
groups in an invaded region because of biogeographic barriers is probably relevant in
all four habitat types, but in reference habitat these species are perhaps of most concern
because of their permeating impacts on native ecosystems.
5.4.3 Impacts of Invasive Alien Species
As discussed in the previous paragraph, invaders of reference habitat may have a
combination of traits similar to the native biota and novel traits. It can be expected
that species that lack the latter may have minor ecological impacts. For instance,
shade-tolerant tropical invaders in the Seychelles have leaf litter properties and
decomposition rates similar to native species and consequently have only minor
impacts on soil fertility (Kueffer et al. 2008). Species with novel traits, in con-
trast, can have substantial impacts (Vitousek 1990) even on undisturbed habitat.
Introduced diseases threaten rare bird species (Benning et al. 2002), and preda-
tors with novel behaviors substantially reduce population sizes of native prey
species (Fritts and Rodda 1998). Invasive ants and earthworms can markedly
affect soil biota and processes (Hendrix and Bohlen 2002; Holway et al. 2002).
