Biology Reference
In-Depth Information
What Makes Ecosystems Invasible?
Clearly related to lag phenomena is an understanding of the degree to which alien
herpetofauna will remain restricted to urban or other highly modified landscapes or
eventually prove capable of invading more-or-less native habitats. Alien herpetofauna
in Florida have been argued to be strongly associated with disturbed, primarily urban,
areas (L.D. Wilson and Porras, 1983). However, virtually all colonizations in
Florida (as in most other locales) originated in urban/suburban areas, so insufficient
time may have elapsed to assess to what extent the introduced herpetofauna are
capable of colonizing native habitats. This is likely true for most jurisdictions inas-
much as the majority of herpetofaunal invasions are quite recent (Fig. 2.1). More
species are now known to be invasive in natural habitats in Florida and elsewhere
than when Wilson and Porras wrote, suggesting that we can not expect many urban
releases to stay put in the long term. And there are few ecological reasons to expect
such a pattern to widely hold, despite the fact that many alien reptiles and amphibians
clearly are capable of exploiting disturbed habitats created by humans. If lag
phenomena are common among these species, then we should see a large increase
in number of natural-area invasions over the coming years. There is currently no
compelling treatment of this very basic question.
As noted in Chapter 2, small islands appear to be more readily invasible than
larger landmasses. It has also been suggested that the high rate of herpetofaunal
invasion of South Florida is partly because that region is ecologically insular, being
surrounded on three sides by water and on the fourth by the frost line (e.g.,
Butterfield et al., 1997). But the reasons why islands and island-like mainland areas
might be more amenable to herpetofaunal invasion are not yet known. It has been
proposed that Guam was especially vulnerable to the brown treesnake because it
has a moderate climate suitable for many foreign species, had high densities of
many prey species, lacked competing predators, was sufficiently small as to be
quickly overrun, and was distant from any other islands that could have provided
refuges or recolonization sources for native wildlife (Fritts and Rodda, 1998; Rodda
et al., 1999b). Clearly, such attributes apply to many oceanic islands and may serve
to explain why such landmasses would be especially vulnerable to herpetofaunal
invasions, but systematic tests of these hypotheses have not been conducted for
alien herpetofauna. In short, a host of biotic peculiarities characterize island biotas,
and it is not clear which of them (or all in concert) allow for easy establishment or
invasibility.
Attempts have not yet been made to determine whether size of naturalized her-
petofauna correlates negatively or positively with size of native herpetofauna,
although that question has been addressed for some other taxa (e.g., Lonsdale,
1999; Sax, 2002). Nor is it clear to what extent ecosystem invasion by herpeto-
fauna is due to the exploitation of untapped resources vs. superior competition for
already utilized resources. The result of these knowledge gaps is that it remains
unknown whether or why particular geographic regions or ecosystems might be
more liable to invasion than others, although several hypotheses are available for
testing (see Chapter 1).
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