Biology Reference
In-Depth Information
example,
Chrysanthemoides monilifera
(a South African native) and
Acacia
longifolia
(an Australian native) both exhibit enhanced flower and seed
production in the region to which they are not native.
Many invasive plants have also benefited from escape from fungal and
viral enemies. Mitchell and Power (2003) surveyed fungi and viruses
infecting 473 species of alien plants in the United States and in their
native ranges. In the United States, these plants, on average, were attacked
by 84% fewer fungi and 24% fewer viruses than in native regions. In these
cases the fungi and viruses included both those that had been introduced
to the United States with the host plant and species that had become asso-
ciated with the plants after their introduction. For the 401 plant species
that were not deliberately planted or heavily used by humans, the inci-
dence of fungal enemies was much lower than for the 71 species heavily
used by humans. This was believed to reflect the fact that heavily used
plants had been introduced more frequently and in greater numbers than
species not heavily used. In addition, species that were designated as nox-
ious by many states or listed as invasive in natural communities by many
states and nongovernmental organizations tended to show the greatest
release from fungi and viruses.
Release from constraints other than predators and parasites can also
occur. At the beginning of this chapter, we considered the case of diffuse
knapweed and its allelopathic relationships with grasses in its native
region and in North America (Callaway and Aschehoug 2000). Release
from coevolved relationships with Eurasian grasses may thus contribute to
the invasiveness of diffuse knapweed in North America. Russian knap-
weed (
Centaurea repens
) and spotted knapweed (
C. maculosa
) also owe a lot
of their invasive capability to similar allelopathic behavior (Goslee et al.
2001; LeJeune and Seastedt 2001; Ridenour and Callaway 2001). For
spotted knapweed, the allelopathic agent has recently been identified as
one of the isomers of the organic molecule catechin (Bais et al. 2002).
Many plants contain the (+)-catechin isomer, which is an antioxidant that
protects tissues against oxidative damage and microbial attack.The (-)-cat-
echin isomer, on the other hand, is a highly phytotoxic molecule. Spotted
knapweed produces both isomers, with the (-)-catechin isomer being
released into the soil. This molecule causes rapid death of the roots of
many grasses and broad-leafed herbs, including perennial grasses native to
North America (Bais et al. 2003). As for diffuse knapweed, Eurasian
grasses appear to show greater resistance to spotted knapweed allelopathy.
Similar relationships hold for many alien animals.Torchin et al. (2003)
examined the parasites of 26 species of invasive animals, including marine
