Invasive Species - Ecological Systems - page 163

release at various ports). In this case, the probability of dispersal to a given site is
nearly independent of time and distance from the primary colony but instead is
driven largely by human-mediated dispersal opportunity [ 18 ].
Factors Affecting Establishment Success
In addition to propagule pressure, other biotic and abiotic factors have been
hypothesized to explain why some species are better invaders, and why some
systems are more invaded, than others. Attributes associated with highly invasive
species include an ability to rapidly reproduce from small numbers (a high intrinsic
rate of population growth), broad environmental tolerance, and mechanisms of
exploiting human transportation vectors and human-modified landscapes.
A popular view is that generalist species are better invaders than specialists, because
the former can thrive in a broader range of habitat conditions ( niche breadth-invasion
success hypothesis [ 19 ]). As such, traits that enable species to cope with new
environments (e.g., diet breadth, physiological tolerance [ 20 , 21 ]), or proxy variables
that suggest broad tolerance (e.g., latitudinal range [ 22 ]), are generally good predictors
of invasion success. Among vertebrates, brain size also generally predicts invasion
success [ 23 - 25 ], perhaps because it facilitates behavioral flexibility in new
environments (but see [ 26 ]). Similarly, invasive plants tend to be more phenotypically
plastic than noninvasive plants [ 27 ]. Traits associated with reproduction are often
correlated with the post-establishment success (abundance and range size) of plants
[ 20 , 28 ]. However, the most important factor limiting the large-scale distribution of
a species is whether it is valued by humans for domestication [ 29 - 32 ]or,foraspecies
that is not introduced deliberately, whether its life history allows it to be easily
transported by human vectors operating on a global scale [ 33 , 34 ].
Much research on the question of why some communities or systems are more
invasible has addressed the concept of biotic resistance , which posits that
biotic interactions between nonnative species and resident enemies can limit estab-
lishment and post-establishment success. The logical extension of this concept is
that resident species diversity may act as a barrier to invasion - an idea promoted by
Elton [ 1 ] to explain the seemingly disproportionate invasibility of species-poor
systems such as oceanic islands and highly disturbed areas such as agricultural
fields. Most support for Elton's hypothesis is derived from terrestrial plant
communities and is equivocal. Over a range of scales, from small garden plots to
regional landscapes, positive correlations between native and nonnative species
richness have been observed, reflecting shared responses to external variables [ 35 ].
Where negative correlations exist, they are found only at local (m 2 )scalesin
experimental manipulations [ 36 ]. Numerous studies suggest that competition, her-
bivory, and native species richness can strongly inhibit the performance (and impact)
of nonnative plants following establishment [ 37 , 38 ], but little evidence suggests that
these interactions can prevent establishment when abiotic conditions are favorable
and propagule pressure is high. The lesson for managers from these studies is that