Biology Reference
In-Depth Information
cargo carried by ships, railroads, and land transport.The rapid naturaliza-
tion and spread of alien plants in North America during the late 1800s,
for example, was probably promoted by the expansion of railroad systems
and commerce (Mack 2003). The enormous growth of air cargo now
enables many alien species to reach new areas in a matter of hours.
A little-appreciated mode of dispersal of alien species is via public or
private mail and parcel post systems. Regardless of federal and state laws
prohibiting the transport of designated species across national or state
lines, it is easy to mail such species almost anywhere. The growth of
e-commerce has increased this risk. Kay and Hoyle (2001), for example,
found that almost every aquatic or wetland plant that carried a state or
federal designation as a noxious weed could be ordered from an aquatic
plant nursery somewhere. Plants federally designated as noxious weeds in
the United States could be obtained from nurseries in both North Amer-
ica and Europe.
Dispersal Capabilities and Invasiveness
Reaching a new region and establishing a population there are only the
first phases of the invasion process (Shigesada and Kawasaki 1997). Some
alien species sooner or later undergo an expansion phase that introduces
them into suitable habitat throughout the new region. Many other adap-
tations function to enable this expansion or invasion phase and determine
its pattern and speed (National Research Council 2002). For seed plants,
biologists have searched with only moderate success for characteristics
that would serve as predictive tools for plant invasiveness. Invasive plants
are quite varied in their characteristics, and the attributes that create inva-
siveness are still not clearly understood.
Many aggressive alien weeds are self-fertilizing, apomictic (meaning
that they produce seed without fertilization), or have vigorous modes of
vegetative reproduction (Brown and Marshall 1981). The climbing ferns
(
Lygodium
spp.) that are rapidly spreading in Florida, for example, possess
gametophyte stages in which self-fertilization is common (Lott et al.
2003). This enables single sporophyte plants, established by wind-carried
spores, to reproduce once they become established in a new location.
Several seed plant features show a high statistical relation to invasive-
ness. For woody plants, these include small individual seed mass, short
generation time, frequent large seed crops, and high mass-specific relative
growth rate (Rejmánek 1996a; Grotkopp et al. 2002). Small seed size is in
turn correlated with small nuclear DNA content and small cell volume,
