Biology Reference
In-Depth Information
might offer the greatest potential for successful long-term control in North America
(Tewksbury et al. 2002).
13.7
Characteristics and Patterns of Invasion
13.7.1 Dispersal and Establishment
Cappuccino et al. (2002) demonstrated that 50% of PSW seeds landed within 2.5 m
of their release points, but that seed weight was inversely correlated with dispersal
distance so that lighter seeds dispersed further. However, in a more recent study,
83% of seeds produced by a PSW plant landed directly beneath the parent plant
(Ladd and Cappuccino 2005). Moreover, 51% of seeds placed on the soil surface
germinated and resulted in emergent seedlings while 71% of seeds buried at a soil
depth of 1 cm resulted in emergent seedlings. These high rates of emergence could
contribute to the ability of the two swallow-wort species to rapidly and successfully
establish satellite populations, with maximum dispersal of seeds found to be up to
60 m from the parent plant (Ladd and Cappuccino 2005). First-year PSW seedlings
also have unusually high survivorship (71-100%) when compared with many other
herbaceous plant species. This particular study was performed in an undisturbed
old-field community, suggesting that while the swallow-worts can be invasive in
disturbed habitats, they can also become established in intact natural plant commu-
nities (Ladd and Cappuccino 2005). The ability of swallow-worts to invade appar-
ently stable plant communities is remarkable given the conventional wisdom in the
field of invasion ecology that intact native communities will be “ecologically resist-
ant” to invasive species (Elton 1958).
The production of polyembryonic seedlings by swallow-worts has not been
found to be a significant competitive advantage in the presence of neighbors, but
was beneficial in the absence of plant competition (Cappuccino et al. 2002).
Polyembryony is likely to be most beneficial in disturbed habitats because of the
enhanced ability of multiple seedlings to successfully establish in the absence of
native vegetation and with increased light availability (Cappuccino 2004;
DiTommaso et al. 2005a; Hotchkiss et al. 2008).
13.7.2 Plant-Plant Competition
Direct competition with monocots was found to significantly decrease the average
size of PSW seedlings, especially for seedlings produced from large-sized seeds
(Cappuccino et al. 2002). All but two of the seedlings grown in the presence of
grasses were smaller than expected, while 90% of those grown in the absence of
competition were of above average size. PSW can have strong drought tolerance as
