Biology Reference
In-Depth Information
assessing predictions based on community-level parameters. Unsurprisingly,
mechanism importance will vary with the biological particularities of each invading
species, so generalizations have been difficult to clearly identify. It is important to
recognize too that some of these hypotheses are not mutually exclusive, and the
proposed mechanisms may interact with each other synergistically (Blumenthal,
2005; C.E. Mitchell et al., 2006). Attempts to integrate several of these specific
hypotheses into more general theoretical frameworks have recently been made
(Shea and Chesson, 2002; Facon et al., 2006). These synthetic perspectives provide
a variety of specific predictions (C.E. Mitchell et al., 2006; Hufbauer and Torchin,
2007) whose future testing may better explain the diversity of outcomes of species
introductions, potentially making identification of high-risk invaders more
successful.
The difficulty of testing these ecological and genetic hypotheses has resulted in
more attention being directed toward identifying characteristics of the introduced
species themselves that might prove predictive of invasiveness. Unsurprisingly,
many of the same features important in favoring establishment of species also tend
to explain invasiveness, in particular, degree of climate-matching between native
and introduced ranges (Duncan et al., 2001; Forsyth et al., 2004; Thuiller et al.,
2005), and an assortment of life-history or other biological variables (Pheloung
et al., 1999; Duncan et al., 2001; Kolar and Lodge, 2001, 2002; Williams et al.,
2002; Daehler et al., 2004; Forsyth et al., 2004; Rejmánek et al., 2005; Pyšek and
Richardson, 2007). As with predicting establishment, however, it is clear that gen-
eralities will not obtain across all taxa (Hayes and Barry, 2008).
It is easy to misinterpret the status of an alien population in its early stages of
spread. A species ultimately recognized as invasive can often appear non-invasive
at that time. Few individuals are encountered, and population growth and spread
can be difficult to detect during this “lag-phase”, when population sizes are dou-
bling but appear quiescent because of low total numbers. Slow doubling rates,
which are typically associated with slow maturation rates and long life spans, can
make a species appear non-invasive for one or more human lifetimes. Because
it is difficult to perceive the growth pattern without explicit measurement and
quantification, complacency about such a species can be easy. Consequently,
management responses are frequently delayed until the invasion is logistically
difficult or impossible to stop. This has the practical effect that many alien inva-
sions become managerially dichotomized into two stages: (1) “it's not a problem”,
and (2) “it's too late to do anything”. The middle ground of the lag phase, when
human control activities could prove most effective, is often squandered because
we are maladept at recognizing it. This seriously undermines efforts to meaningfully
control many invasive species and has been a frequent problem for herpetological
invasions (see Chapter 4).
An invasion will progress more rapidly if it involves many separate populations
rather than only a single one (Moody and Mack, 1988; Mack and Moody, 1992).
This can occur either because a species is introduced independently to multiple
localities or because a single invasive population further expands to multiple sites
with human help. As multiple populations become established, each expands at
