Characteristics, significance, and human dimension of global invasive weeds - Invasive Plants: Ecological and Agricultural Aspects - page 228

subsequently reproduce) outside its natural range. Their movement can be
either within a country or between countries. Biological invasions are impor-
tant components of human-caused global environmental changes. Williamson
and Fitter [20] estimated that only 0.1% of all plant species that are introduced
outside their native ranges by humans become invasive.
Invasive species have been introduced in a variety of ways. The means and
routes by which they are introduced are called invasion “pathways”. People
introduce exotic plants to new areas with intent and by accident by variety of
means. Some species are introduced for use in gardening and landscaping, for
erosion control, forage and other purposes. For instance, in the 1930s, the
Civilian Conservation Corps planted kudzu vine [ Pueraria lobata (Wild.)
Ohwi.], introduced from Japan, throughout the Southeast to help stabilize soil
in erodable areas. The recent intentional introduction of purple loosestrife
( Lythrum salicaria L.) is another example of a successful plant invasion. This
species was introduced to the northeastern United States and Canada in the
1800 s for ornamental and medicinal uses. Purple loosestrife has adapted read-
ily to both natural and disturbed wetlands. It is replacing many of the native
plant species and its environmental impact is enormous.
The role of a species within an ecosystem is a direct result of changing com-
munity processes and lead to alternatives of community composition and
structure. Vegetative reproduction is often important for establishment and
spread of many species in terrestrial environments and also for dispersal in
aquatic habitats. Disturbance of successively advanced communities and then
slow recovery rate promote the invasion of many introduced species [21].
Common habitats for invasive species can be grouped as natural areas, road-
sides, wet lands, lakes/ponds, and agricultural areas. Not all non-native (intro-
duced or alien) species are harmful. In fact, the majority of species cultivated
in agricultural lands in the US are introduced species.
Intentional introductions
Alien or exotic plants are introduced intentionally for a great variety of pur-
poses. A large proportion of important crops are grown in areas outside their
natural distribution for economic reason and as a way to feed the world popu-
lation. Exotic plants can be introduced for many uses, including food use,
forestry use, soil improvements, ornamental plants, cover crops and other uses.
Numerous ornamental plants have also been introduced in the US and some of
the species are invading natural areas (Tab. 3). In India, many plants have been
introduced as cover crops or for other purposes (Tab. 4). Introduced exotic spe-
cies, however, can pose a risk to biodiversity when they naturalize and pene-
trate conservation areas. Tall fescue ( Festuca arundinacea Schreb.), a native
European, has been planted as a pasture grass in North America. It has natu-
ralized and invaded remnant prairies, replacing the once diverse natural herba-
ceous community [22].