Biology Reference
In-Depth Information
5.3.2 Functional Type of Invasive Alien Species
Invasive species associated with anthropogenic habitat are characterized by traits
typical of agricultural weeds (Baker 1974). They are resource-demanding, fast-
growing, and fast-reproducing ruderal or early successional species. They usually
have high seed output, and an efficient seed dispersal mechanism (particularly wind
or birds). Invasive plants with high qualitative defense (toxins such as alkaloids)
against generalist herbivores but low quantitative defense (e.g. lignins, tannins)
may particularly profit from the high nutrient levels and presence of mainly gener-
alist herbivores in anthropogenic habitat (compare Müller-Schärer et al. 2004;
Blumenthal 2006). Invasive species characteristic for anthropogenic habitat repre-
sent a paradigmatic type of invader that may be called a weedy invader. Species
with weedy traits that profit from resource-rich and highly disturbed anthropogenic
habitat have been important in the development of invasion biology (Baker 1974;
Rejmanek 1996; Davis et al. 2000). Within anthropogenic habitat, traits of invasive
species may differ between agricultural and urban land. For instance, there are
often more annuals in weed communities on arable land and more biennial and
perennials in ruderal vegetation around settlements (Lososova et al. 2006). Further,
weeds on arable land may be associated with particular crops and their management
systems (Pyšek et al. 2005).
5.3.3 Impacts
From a nature conservation perspective, anthropogenic habitat is of major concern
as potential pathways into natural areas, and as reservoirs, where alien species may
build up propagule pressure and adapt to local conditions (Pysek and Jarosík 2005;
Dietz and Edwards 2006; Didham et al. 2007). After evolutionary change (cf. Dietz
and Edwards 2006) or due to source-sink dynamics (cf. Didham et al. 2007), some
species associated with anthropogenic habitat may be able to enter relatively undis-
turbed, reference habitats (see below). With continuing expansion of human land
use, this scenario will require increasing attention.
The impacts of alien species on biodiversity within anthropogenic habitat may
also be relevant. Farmers may in the future be more commonly forced to actively
coproduce biodiversity values on their land, and similarly biodiversity is considered
a product of sustainable forestry. In areas with a long human land-use history, many
native ruderal species depend on anthropogenic habitat, and such areas may be
threatened by the invasion of alien species. However, it seems that in these regions
the land-use history has also selected for native species that are able to coexist with
alien ruderals, and invasive alien species seem to be only infrequently able to
threaten native ruderal biodiversity (Klingenstein and Diwani 2005; Maskell et al.
2006). In fact, alien species generally increase the species diversity in such habitats
(Maskell et al. 2006). Alien weed diversity may not be considered a biodiversity
