Biology Reference
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Inderjit et al. 2006a). Although allelopathy has not yet been clearly implicated in
association with the field dominance of any invasive species, evidence suggests that
many invasive species produce an array of secondary plant products or allelochemi-
cals that are released through decomposition of plant material or directly exuded
from roots or shoots into the soil rhizosphere (Inderjit et al. 2006b).
Cappuccino (2004) first reported that PSW root extracts inhibited the germina-
tion of radish seeds and showed broad antifungal activity. DiTommaso et al.
(2005b) suggested that the purported allelopathic activity of root exudates could
indirectly affect competitive interactions of plants through the alteration of the
structure of the rhizosphere community. Interactions between the allelopathic activ-
ity of invasive plant species and mycorrhizal associations of affected plant commu-
nities can have marked effects on the population dynamics of invaded sites (Roberts
and Andersen 2001; Stinson et al. 2006).
Both swallow-wort species have been found to have high concentrations of cyto-
toxic secondary products in their roots, stems, and leaves (Capo and Saa 1989; Lee
et al. 2003; Nowak and Kiesel 2000; Staerk et al. 2000, 2002). In unpublished labo-
ratory work, N. Cappuccino demonstrated that the foliage of PSW produces phyto-
toxins when these tissues are ground and their chemical constituents extracted
using water (N. Cappuccino, personal communication). Other investigators have
postulated that the dense monocultural stands created by swallow-worts following
establishment may have resulted from the exudation of root-released allelochemi-
cals that limit growth of neighboring species. The decomposition of swallow-wort
foliage and stems underneath a dense stand may contribute to seedling suppression
from the effects of both allelopathy and limited light reaching the soil surface due
to a mulch effect from swallow-wort plant material (Weston et al. 2005).
Our own findings in laboratory simulations of allelopathic activity in agar gel
box assays and Parker bioassays suggest that the role of allelopathy may be limited
in contributing to the establishment and interference of either swallow-wort species
and their respective effects on nearby competitors (Douglass 2008). Indeed, PSW
root exudates in particular resulted in substantial (up to 40%) reductions in the root
length of a number of indicator species, and leachates of leaf tissues of both swal-
low-wort species caused similar reductions in both root and shoot length of indica-
tors. However, both stimulation of indicator species growth and autotoxicity were
observed during the experiments, and a comparison of inhibitory effects with com-
mon milkweed (generally not considered to be invasive) found that the swallow-
worts did not exhibit significantly greater negative allelopathic abilities than the
related nonnative species.
13.8.2
Phenotypic and Genetic Diversity
Ellstrand and Schierenbeck (2000) proposed that hybridization (both inter- and
intraspecific) could play an important role in enhancing the invasiveness of intro-
duced species. In particular, they suggested that hybridization between populations
