Biology Reference
In-Depth Information
turtle
Actinemys marmorata
(Hays et al., 1999), and the snake
Thamnophis eques
(Schwalbe and Rosen, 1988; Rosen and Schwalbe, 1995, 2002). Similar declines in
native herpetofauna concurrent with introduction of bullfrogs have been noted in
Germany (C.R. Boettger, 1941; Thiesmeier et al., 1994). Because of the bullfrog's
catholic, opportunistic diet (Bury and Whelan, 1984) and numerous observations of
predation on sensitive species (Table 3.1), declines have most often been attributed
to bullfrog predation. This interpretation is bolstered by scarring and tail loss seen
on affected natives and by skewed population structures consistent with predation
on juveniles (Schwalbe and Rosen, 1988; Rosen and Schwalbe, 1995). Furthermore,
experiments have confirmed bullfrogs to mediate their negative effects in part via
direct predation (Kiesecker and Blaustein, 1997). However, bullfrogs can also
induce behavioral changes in microhabitat use by natives that decrease the latter's
survival and growth rates (Kiesecker and Blaustein, 1998). Further, a variety of
other factors, including habitat modification or loss (Moyle, 1973; Hayes and
Jennings, 1986; Jennings, 1988b; Fisher and Shaffer, 1996; Adams, 1999, 2000;
Kiesecker et al., 2001; Davidson et al., 2002; Rosen and Schwalbe, 2002), presence
of alien fish (Hayes and Jennings, 1986; Jennings, 1988b; Rosen et al., 1995;
Kieseker and Blaustein, 1998; Adams, 1999; Adams et al., 2003; Maret et al.,
2006), commercial exploitation (Hayes and Jennings, 1986; Jennings and Hayes,
1985; Jennings, 1988b), disturbance regimes (Jennings and Hayes, 1994; Doubledee
et al., 2003; Maret et al., 2006), diseases (Rosen and Schwalbe, 2002), and toxi-
cants (Hayes and Jennings, 1986; Rosen et al., 1995; Davidson et al., 2002) can also
be involved in declines of native species or interact synergistically to exacerbate
bullfrog effects. This complexity frequently makes parsing the exact contribution
of bullfrog predation to native-species declines problematic. Despite such compli-
cations, predation by bullfrogs has likely played a central role in declines of several
native reptile and amphibian species in the western United States. It has been
claimed that
R. catesbeiana
has led to decline of native
Rana
in the region around
Florence, Italy (Touratier, 1992b) and of native fish in the Aquitaine of southwestern
France (Touratier, 1992a), and concern has been expressed about their potential
effects elsewhere in Europe (e.g., Albertini and Lanza, 1987; Stumpel, 1992). But
in none of these cases has any of the above-mentioned forms of evidence been pro-
vided. Concerns have also been expressed about the potential threat of bullfrogs to
the endangered snake
Opisthotropis kikuzatoi
, endemic to Kumejima Island,
Ryukyu Islands, Japan. The threat comes both from the frog's potential to directly
prey upon these small snakes but also because it is known to eat the endangered
freshwater crab,
Candidiopotamon kumejimense
, the only known food source for
the snake (Ota et al., 2004a).
Three of six dissected
Xenopus laevis
in an introduced population in southern
California were found to contain one or more of the endangered tidewater goby
(
Eucyclogobias newberryi
) as food items (Lafferty and Page, 1997). The high fre-
quency of occurrence of the endangered fish in this small sample of stomachs, in
concert with the high densities at which
X. laevis
can occur in California, led to the
supposition that the alien frog might serve as a substantial cause of mortality for the
fish (Lafferty and Page, 1997). However, further work to identify population-level
