Biology Reference
In-Depth Information
least likely, either because the invasion is not noticed or because it is not perceived
to be a concern. More often, the swiftness with which native prey can disappear
makes hypotheses of predation impact merely liable to ex post facto inference
instead of direct demonstration. Nonetheless, the numerous suggestive or compel-
ling examples make it likely that population suppression via predation represents
one of the more common ecological impacts from alien herpetofauna.
Removal of Native Predators
A second effect involves destruction of native predators via introduction of species
bearing novel defensive mechanisms. This is documented for the cane toad (
Bufo
marinus
), a Neotropical anuran that attains large size, defensively secretes quanti-
ties of highly toxic bufoteneins from its skin, and attains high population densities
where introduced. It appears to have had dramatic effects on many native predators
in its introduced range in Australia because of the naivety of native Australian
predators to that species and its toxin. There are several reports of native snakes,
lizards, turtles, crocodiles, birds, and mammals dying after ingesting toads
(Breeden, 1963; Rayward, 1974; Covacevich and Archer, 1975; Stammer, 1981;
Ingram and Covacevich, 1990; Shine, 1991; Tyler, 1994; S. Burnett, 1997; van Dam
et al., 2002; Fearn, 2003; Phillips and Fitzgerald, 2004; Doody et al., 2006a) or
experiencing population crashes or community changes subsequent to arrival of
toads (Pockley, 1965; Shine and Covacevich, 1983; S. Burnett, 1997; McRae et al.,
2005; Doody et al., 2006a, b; Shine et al., 2006). At least 26 native Australian
vertebrate species have experienced such toad-induced mortality (C. Lever, 2001).
These reports tend to be anecdotal or inferential but the studies by Doody et al.
(2006a, b) contained pre-invasion abundance estimates for
Varanus panoptes
,
V. mertensi
, and
V. mitchelli
and demonstrated significant population declines syn-
chronous with arrival of toads, as did the independent study of Griffiths and McKay
(2007) for
V. mertensi
. B.L. Phillips et al. (2003), using ecoclimatic, dietary, and
toxin-sensitivity information, systematically assessed risk to Australia's snake spe-
cies from cane toads and concluded that 43% of Australia's non-scolecophidian
snake fauna (i.e., excluding the burrowing blind snakes) are potentially threatened
by the toads. Identical conclusions placing much of Australia's large herpetofauna
at risk derive from a similar analysis for the remaining Australian taxa of large rep-
tiles (J.C. Smith and Phillips, 2006).
Unpublished data suggest that some
Varanus
populations can survive invasion
by cane toads (van Dam et al., 2002). In the invasion area studied, most
Varanus
consumed toads and were killed by doing so; however, those few lizards that sur-
vived the invasion did not eat toads, and this allowed for long-term recovery of
lizard populations.
Varanus
from populations having long exposure to toads also
refuse to attack toads (van Dam et al., 2002). Both these observations argue for
strong selective pressure against toad consumption by some predatory lizards,
which may lead to eventual recovery of native populations. However, further data
