The ecology of biological invasions: past, present and future - Invasive Plants: Ecological and Agricultural Aspects - page 10

inition of 'niche space' has been expanded to include 'enemy-free space'. For
example, Maron and Vilà [55] predicted that generalist enemies might offer
resistance to invasion, resulting in a form of biotic resistance that is not strict-
ly competition-based. When the number of individuals introduced (inoculum
size) is low, heavy generalist pressure can render a community resistant to
invasion. However, successful establishment might occur if native enemies do
not show a numerical response, and if the abundance of an invader increases
above some threshold level. For example, the density of Sesbania puniea in
South Africa was not reduced by the weevil Trichapion lativens , even after
98% its seeds were damaged [56-58]. Although the BRH has been criticized,
many of the hypotheses outlined below assume that communities are inher-
ently resistant to invasion, and seek to identify circumstances in which inva-
sions are favored. One key hurdle to testing the BRH, and one that is common
to many alternative hypotheses, is that instances of unsuccessful establishment
are rarely observed, while successful spread and proliferation are highly time-
dependent. Regardless, the fact that modern definitions have switched to stage-
based terminology reflects a growing belief that most species are not able to
invade new communities successfully.
Resource fluctuation hypothesis (RFH)
A popular explanation for the success of various terrestrial plants is the
resource fluctuation hypothesis [59]. According to this model, resource fluctu-
ations promote invasions by creating resource openings for new species, or
reducing potential competition from residents at certain times. Davis et al. [59]
described two mechanisms for increases in resource availability: (1) if
resource use by resident species declined, or (2) resource supply increased. In
the first instance, disturbance could reduce the number of resident individuals,
thus increasing resource availability [60, 61]. For example, Davis and Pelsor
[62] used plant removal to test this hypothesis, and found that the plant
Rudbeckia hirta flowered at a much higher rate in weeded compared to
unweeded plots. Resource increase (e.g., nitrogen addition) has similarly been
shown to increase the success of invaders [59, 63, 64].
There are many examples of invasions facilitated by disturbance and/or
resource availability [59-61, 65]. Chromolaena odorata, a noxious invader
from Central and Southern America in the Western Ghats of India, is causing
severe damage to plantation crops and inhabits disturbed forest (partial cut and
clear-cut) area, forest nurseries and plantations. Western Ghat is the only place
in India listed for plant diversity as one of 26 'hot spots' of the world [66].
Disturbance (e.g., forest cutting and fire) plays an important role in C. odora-
ta success. After flowering, C. odorata shoots dry up and become a highly
combustable fire hazard. After fires have freed resources and removed most of
the competing vegetation, C. odorata quickly re-sprouts and rapidly domi-
nates.