Biology Reference
In-Depth Information
13.8 Invasiveness
Cappuccino (2004) found that smaller-sized patches (1 and 9 plants) of PSW had
higher reproductive success (measured as maturation of follicles), and higher root-
to-shoot biomass ratios than larger-sized patches (81 plants). However, larger
patches produced three times more follicles and thus had a greater net production
of seeds. When considered in the context of her earlier work (Cappuccino et al.
2002), the latest results suggest that in general larger swallow-wort patches will
produce large quantities of seed and that at least a portion of these seeds will lead
to the successful establishment of pioneering satellite populations.
These smaller satellite patches appear to invest proportionally greater resources
into root biomass, presumably to ensure establishment, before allocating resources
to vegetative or reproductive structures (Cappuccino et al. 2002; Cappuccino 2004;
Smith 2006). Indeed, Smith (2006) found that PSW had a significantly greater root-
to-shoot biomass ratio than its close relative common milkweed. Likewise, when in
competition with common milkweed, young PSW plants had greater overall repro-
ductive output. While diminishing its competitive ability relative to common milk-
weed, this allocation of resources to reproduction could ensure the presence of a
large seed bank from which satellite populations could be produced (Cappuccino
2004; Lockwood et al. 2007; Myers and Bazely 2003). Given that the seed output
of a single PSW population in central New York was 35,244-62,439 seeds/m
2
(the
higher figure takes into account an average proportion of polyembryonic seeds), the
seed bank of this species can be quite large, and its impact over the course of several
years significant (Smith 2006).
Once established, both swallow-wort species grow profusely and aggressively.
PSW and BSW can rapidly alter the abiotic and biotic features of their understory
and surrounding areas: decreasing sunlight penetration, increasing nutrient acquisi-
tion through large root biomasses, and altering rhizosphere dynamics both through
shifts in the AMF community and the exudation of allelopathic chemicals (Douglass
2008; Greipsson and DiTommaso 2006; Lawlor 2002; Sheeley and Raynal 1996;
Weston et al. 2005). Despite the increasing number of studies on the two invasive
swallow-wort species, there has been little focus to date on three potentially signifi-
cant factors that may influence the invasiveness of the species, namely, allelopathy
activity by tissue leachates or root exudates; adaptive morphological plasticity; and
the genetic diversity of introduced populations.
13.8.1 Allelopathy
Some exotic plant species competitively exclude and eliminate their neighbors in
invaded “recipient” communities, but generally are found to coexist with neighbors
in species-diverse systems in their native habitat. Allelopathy has been suggested
as one of the mechanisms responsible for this success (Callaway et al. 2005,
