Biology Reference
In-Depth Information
effects of these frogs has not appeared. More compelling correlational evidence
is available from France, where
X. laevis
was introduced in Deux-Sèvres in the
mid-1980s (Fouquet, 2001; Fouquet and Measey, 2006). Amphibian communities
in ponds containing
X. laevis
closest to the original site of introduction were
found to have lower species richness and diversity than ponds lacking that frog or
having it but occurring farther away (Grosselet et al., 2005). In this case, distance
from introduction site is taken as a rough measure of duration of infestation with
X. laevis
; hence, long association with
X. laevis
is correlated with reduced native
amphibian diversity. Numbers of eggs of native salamanders (
Triturus
sp.) were
also approximately an order of magnitude lower in ponds containing
X. laevis
than in those lacking them. Finally, populations of
Triturus cristatus
from ponds
containing
X. laevis
lacked the smaller size classes present in ponds without that
frog (Grosselet et al., 2005).
It has been noted that populations of
Hyla squirella
and
H. cinerea
in a Florida
hammock were found to decline dramatically upon colonization of the hammock
by adult
Osteopilus septentrionalis
(Meshaka, 2001: 98). Although the mechanism
of decline remains unidentified, it was presumed to be predation, given the known
feeding habits of the alien.
Tadpoles of
Rana catesbeiana
were demonstrated to feed upon eggs and larvae
of the endangered fish
Xyrauchen texanus
in laboratory conditions (Mueller et al.,
2006), and their densities in artificial habitats (human-made levee ponds) can be
sufficiently high that they may be depressing larval recruitment of the fish, but studies
have not yet demonstrated direct impacts on fish in wild habitats. Tadpoles of
Osteopilus septentrionalis
have been demonstrated to prey upon and significantly
reduce average survivorship of native
Hyla squirella
tadpoles under crowded labo-
ratory experiments (K.G. Smith, 2005b) but not under conditions of moderate den-
sity and alternate food availability (K.G. Smith, 2005a).
Individual reports of alien reptiles or amphibians feeding on endangered or
potentially sensitive native species have been reported (Table 3.1) but each of these
reports is based on single or few observations, and depression of native populations
has not been investigated. In other instances (Martínez-Morales and Cuarón, 1999;
Enge et al., 2004c) reasonable concerns have been voiced over the potential for
recent reptile introductions to impact endangered or sensitive native wildlife, but
insufficient time has elapsed to validate these concerns. However, Martínez-
Morales and Cuarón (1999) speculated that already-depressed populations of
several endemic birds and mammals on Cozumel might be due to introduced
Boa
constrictor
.
In sum, predation impacts from alien herpetofauna are frequently invoked and
have been clearly demonstrated in a few instances. Anecdotal observations (Table 3.1)
suggest they may be of frequent occurrence, but population-level effects are diffi-
cult to demonstrate and may be difficult to distinguish from other causes (witness
bullfrogs in the western United States). There is an additional difficulty in that there
is typically a narrow window of opportunity after an invasion begins during which
predation impacts can clearly be demonstrated by direct observation and measure-
ment. But this is precisely the stage of an invasion during which study is, in general,
