Biology Reference
In-Depth Information
How Fast?
There is virtually no information on how quickly herpetological invasions progress.
A few quantitative estimates of range expansion have been provided in the literature
(e.g., van Beurden and Grigg, 1980; Easteal, 1988; B.L. Phillips et al., 2007 for
Bufo
marinus
; Lobos and Jaksic, 2005; Fouquet and Measey, 2006 for
Xenopus laevis
;
Locey and Stone, 2006 for
Hemidactylus turcicus
; Rodda and Savidge, 2007 for
Boiga irregularis
), and one can occasionally find comparative snapshots of invaded
range size at different stages of an invasion (e.g., Percsy and Percsy, 2002b for
Rana
spp.). But these are insufficient to make general conclusions about range-expansion
rates. It is important to recognize that rate of spread can be due both to the invader's
inherent ability to negotiate terrain under its own power as well as to secondary, sal-
tational transport of the species by humans. The latter is likely to be operative in
many herpetological invasions, including those involving species having an amenity
value. This has been a frequent theme among pet-trade introductions in Florida and
Hawaii (e.g., L.D. Wilson and Porras, 1983; Meshaka et al., 2004a), for example,
and was a major cause of the rapid spread of
Eleutherodactylus coqui
around Hawaii
Island (Kraus and Campbell, 2002). Saltational secondary transport by humans can
also be important for accidental hitch-hikers, such as
Anolis sagrei
(T. Campbell,
1996a) and a variety of geckos. Because saltational transport by humans will
increase the numbers of populations (and therefore the numbers of spreading nodes)
of an alien, it can be a major contributor to rate of spread. And, indeed, species arriving
into Florida as cargo stowaways were found to be spreading at a faster rate than
those introduced intentionally (Butterfield et al., 1997), which may reflect that these
species are preadapted to such saltational dispersal. However, intentional introduc-
tions to Florida tend to be more recent than the pool of accidental introductions, so
comparisons may well be confounded by time since introduction. In any event,
range-expansion rates have rarely been measured for alien herpetofauna, and are not
easily measured for some, but there are suggestions that interesting differences in
spread rates may correlate to invasion pathway. To what extent range-expansion
rates might correlate with impacts remains unresearched.
A related issue of some importance is ascertaining to what extent current per-
ceptions that most alien herpetofauna are benign are due to unrecognized lag
phenomena. As noted in Chapter 3, herpetofaunal invasions for which the best evi-
dence of impact has yet been adduced average 62 years old. Some alien populations
that were dismissed as harmless two or three decades ago (e.g.,
Iguana iguana
in
Florida) are now viewed as invasive, and
Eleutherodactylus coqui
appeared to
persist largely unnoticed in Hawaii for approximately one decade before exploding
out of control over a period of only three years (Kraus and Campbell, 2002). These
observations suggest that alien herpetofauna may frequently be subject to long lag
periods before population growth becomes sufficiently high that they generate
concern among scientists or managers. It remains thoroughly unknown what
percentage of alien herpetofauna will exhibit lag phenomena, what the modal time
period of these lags might be, and what accounts for them (e.g., human misperception
vs. real biological limitations).
