Biology Reference
In-Depth Information
across the state as a whole (K.G. Smith, 2006a). This spatial contrast probably
results from two factors: the recency of many introductions has likely not yet
allowed homogenization effects to spread very far, and the climatic gradient in
peninsular Florida may not allow many established southern species to access more
northerly latitudes. This is the only study I know to quantify regional effects of
herpetological introductions.
Evolutionary Effects
Evolutionary effects from invasive reptiles and amphibians are primarily of interest
in terms of how they impact native faunas. Such effects have been demonstrated in
a few cases, are frequently to be expected, but have been little studied to date.
Evolutionary changes have been noted for the alien invaders themselves in a few
instances. With the possible exception of the last example below, all changes dis-
cussed here have or are presumed to have a genetic basis.
Genetic Changes
Hybridization with congeners is a frequent outcome of rampant transport of organ-
isms (cf., Levin et al., 1996; Rhymer and Simberloff, 1996; Mooney and Cleland,
2001; Low, 2003: 261-272; Largiadèr, 2007), and the same consequence has been
documented for a number of alien reptile and amphibian introductions. Such
hybridization may lead to loss of native allelic or genomic identity, outbreeding
depression (Rhymer and Simberloff, 1996), or, in the extreme case, loss of native
species due to wholesale genetic swamping by the invader (e.g., Echelle and
Connor, 1989). Clearly detrimental impacts on native reptiles and amphibians
resulting from introgressive hybridization of alien genomes have been demon-
strated for only a small set of species. Nonetheless, these effects have frequently
been grave and this seems one of the more damaging impacts attending herpeto-
logical introductions.
Among amphibians, populations of the salamander
Ambystoma tigrinum
across
the western United States have experienced widespread introduction of larvae of
eastern forms of this species used as fishing bait (Lowe, 1955; Espinoza et al.,
1970; Bury and Luckenbach, 1976; Collins, 1981). Genetic contamination of native
populations has been documented in Arizona, where genetic introgression threatens
the endangered
A. t. stebbinsi
(Storfer et al., 2004), and in California, where the
endangered
A. californiense
is extensively threatened with the same (Riley et al.,
2003). In the latter case, hybridization appears to be promoted by habitat alteration,
with alien alleles preponderating in unnatural, perennial ponds. This pattern derives
from differential success of hybrid genotypes and has resulted in a complex mosaic
hybrid zone (Fitzpatrick and Shaffer, 2004).
