Biology Reference
In-Depth Information
numbers of native species. Disturbance is well known to facilitate establishment
and spread of many alien species, and it too is correlated with human numbers.
While these correlations can often allow us to roughly predict which areas are likely
to host increased alien species richness, they are silent with respect to establishment
mechanisms and, hence, are not predictive in a manner that can readily be used to
prevent individual future naturalizations.
Spread
Naturalized populations can vary tremendously in their ecological dominance,
ranging along a continuum from those that barely hang on in small numbers at a
single locality to those that spread like wildfire over a large range and become
numerically dominant. Obviously, those at the latter end of the spectrum are clearly
invasive, those at the former end are not, and opinions would differ about where
along the continuum one might divide “invasive” from “non-invasive”. We would
like to have an understanding of why these differences occur, as that would allow
us to predict both the likelihood that any particular species would prove invasive as
well as the relative susceptibility of particular locations to invasion.
A variety of hypotheses has been advanced to explain invasion success (reviewed
in Hufbauer and Torchin, 2007). Ecological hypotheses include the notions that
invaders are preadapted to the new environment, are inherently superior competitors,
have novel adaptive mechanisms giving them a competitive edge over natives, have
escaped from enemies that limit their population sizes in their native ranges, or
interact with other introduced organisms in a positive-feedback loop that promotes
population expansion. As well, ecological attributes of the invaded environment
may serve to promote or to limit introduced species. In particular, the empty-niche
hypothesis suggests that invasive species may use resources ignored or underuti-
lized by natives. Conversely, the biotic-resistance hypothesis posits that natives that
are close relatives of introduced species may serve to limit the expansion of the latter
via competition or increased likelihood of parasite transferral. As well, invasion
may be promoted by genetic changes within the introduced species. Hybridization,
either with closely related natives or among populations of the introduced species
from disparate parts of its native range, may increase genetic variation and allow
for rapid creation of novel genotypes that are better suited to exploiting the new
environment. Founder events may create new genotypes with similar ecological
effect. Alternatively, the novel environment may impose a novel selective regime
that promotes improved competitive ability among the invaders. In particular,
release from enemies may allow energy resources that would otherwise be expended
on defense to be used instead to promote growth and reproduction. Empirical sup-
port for each of these hypotheses is available for one invasion or another, although
examinations of the genetic and evolutionary consequences of introductions have
barely begun. Compellingly testing the empty-niche and biotic-resistance hypotheses
has proven difficult because of the complexity of biotic interactions involved in
