Biology Reference
In-Depth Information
released into natural waters and established wild populations.The ecolog-
ical impacts of many of these introductions are substantial (Cox 1999).We
shall examine the role of some in extirpation and extinction of native
species in chapter 16.
History of Human-assisted Dispersal
Long-distance human transport of plants falls historically into three
phases: accidental, utilitarian, and aesthetic (Mack and Lonsdale 2001).
The accidental phase, which began with the first colonial settlements in
the Americas,Australia and New Zealand, and many other world regions,
involved the unintentional introduction of ruderal plants—those that
inhabit roadsides and wasteland areas—and crop weeds to these areas from
Europe.Within 50 yr of the first settlements in New England, for exam-
ple, at least 22 European plants had become common yard and garden
weeds. Most of these probably arrived as contaminants in crop seed or
animal fodder. Others reached new continents as seeds in soil used as ship
ballast, which was usually dumped onshore following a transoceanic voy-
age.The utilitarian phase of introductions comprises the numerous delib-
erate introductions of plants for herbal and medicinal use, pasture
improvement, and forestry. The aesthetic phase constitutes deliberate
introduction of plants for horticultural purposes. Obviously, these phases
overlap, and today introductions still occur for all of the reasons cited
above.
With the development of transoceanic ship travel and commerce,
many characteristics of aquatic plants and animals became preadaptations
for accidental long-distance dispersal.These adaptations now function for
dispersal of organisms across ocean areas, along marine coastlines, and to
and from interior seas and large lakes.These characteristics include sessile
growth in shallow marine habitats, which preadapted plants and animals
for growth on the hulls of ships, and tolerance of conditions in ballast
water that came into use by modern transoceanic shipping. Ballast-water
transport has been responsible for introducing about 35 species of fish to
new areas throughout the world (Wonham et al. 2000). Broad salinity tol-
erance has been a major factor in the recent ballast-water introductions of
invertebrates and fish from the Black, Caspian, and Azov seas of Asia to
the North American Great Lakes, for example (Ricciardi and Rasmussen
1998).
The ability to survive as seeds, eggs, larvae, or adults for periods of
weeks or months has enabled many plants and animals to stow away in
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