Biology Reference
In-Depth Information
(
Passer domesticus
), European starling (
Sturnus vulgaris
), and cattle egret
(
Bubuculus ibis
) showed exponential population growth that enabled them
to colonize suitable habitat from coast to coast in 50 yr or less (Cox
1999). The Eurasian collared dove (
Streptopelia decaocto
) is now spreading
through the United States and Mexico in a similar manner. In Italy, gray
squirrel (
Sciurus carolinensis
) populations introduced to a broad-leafed
woodland area in 1970 are now spreading exponentially and are likely to
spread throughout broad-leafed forests of Europe and Asia (Bertolino and
Genovesi 2003).
Inadvertent Human-assisted Dispersal
Humans have assumed major roles as unknowing agents of dispersal of
plants, animals, and microbial species. On a local scale, seeds, spores, and
other propagules are often dispersed in mud on tires of motor vehicles
and shoes of hikers (see, e.g., Jules et al. 2002). Plant seeds are carried by
almost all types of transport vehicles, so the dispersal of many invasive
plants tends to follow railroad lines and highways. On a broader scale,
human-assisted dispersal is responsible for the vast majority of interconti-
nental and interoceanic introductions of alien species. Cargo ships and air-
craft carry propagules of all types of organisms across previously insur-
mountable barriers.They provide rapid mechanisms of dispersal to which
some species are preadapted and to which others may evolve adaptations.
Perhaps the most common inadvertent means of introduction of alien
plants to new continents has been via crop seed contaminants. In the
northeastern United States, 6-7% of the alien plants that became estab-
lished before 1900, and are now thoroughly naturalized, were introduced
via contaminated crop seed (Mack and Erneberg 2002). Many of these
species introductions were truly inadvertent, their seeds being accidentally
included because of inadequate techniques of seed cleaning. In some
cases, however, they were deliberately added by seed merchants to increase
the volume of their seed sales.
Cropland Weed Adaptation for Dispersal by Humans
Many cropland weeds have capitalized on dispersal as crop seed contam-
inants.These species have evolved races that show patterns of growth and
reproduction that mimic those of crop plants (Barrett 1983).These forms,
known as agroecotypes, are largely restricted to stands of cultivated crops.
Agroecotypes are often weedy races of the crop plant itself, with the
