Biology Reference
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be worsened by also decreasing activity levels (and, hence, amount of time spent
feeding) in the presence of
R. catesbeiana
(Kiesecker et al., 2001). Severity of
competitive effects may vary with environmental conditions between aquatic habi-
tats (Adams, 2000), a confounding factor that has yet received no detailed treat-
ment. Laboratory trials have also found survival of native European
Rana
tadpoles
to be considerably reduced in the presence of larval
R. catesbeiana
, even when
densities of the latter were low (Laufer and Sandte, 2004). This appeared to result
from direct competition for food inasmuch as larval bullfrogs displaced native tad-
poles from food resources and larval predation was never observed.
Similar competitive effects have been found with
Osteopilus septentrionalis
intro-
duced to Florida. Tadpoles of this species depressed growth rates and delayed metamor-
phosis in native
Bufo terrestris
and
Hyla cinerea
when raised together in a laboratory
setting; they also led to reduced size at metamorphosis in
B. terrestris
(K.G. Smith,
2005a). When raised together in mesocosm experiments
O. septentrionalis
decreased
survival rates, growth rates, and size at metamorphosis of
B. terrestris
, although those
effects were reversed when tadpoles were raised in the presence of predatory newts
(
Notophthalmus viridescens
), which preferentially preyed upon the alien tadpoles
(K.G. Smith, 2006b). Although these results are suggestive, competitive impacts of
O. septentrionalis
in natural systems remain experimentally uninvestigated.
Pearl et al. (2005b) documented unexpectedly frequent rates of interspecific
amplexus between
Rana catesbeiana
and native
R. aurora
and
R. pretiosa
in the
Pacific Northwest of the United States. They hypothesized that, should males of the
two natives be limited in breeding pools, sexual interference by frisky
R. catesbeiana
might serve as a hindrance to population recruitment, although the importance of
such a mechanism remains to be demonstrated.
A variety of alien lizards has been presumed to competitively displace native
species, judging from historical patterns of changes in species abundance and
geographical patterns of species assortment (Case and Bolger, 1991; Case et al., 1994).
Exclusion of the long-resident geckos
Lepidodactylus lugubris
by recently established
Hemidactylus frenatu
s in urban and suburban niches in several locations in the Pacific
appears to result from behavioral interference (Bolger and Case, 1992) and consump-
tion of juveniles by the newcomer (Bolger and Case, 1992; McCoid and Hensley,
1993b), but especially by enhanced ability of
H. frenatus
to exploit food resources
(Petren and Case, 1996). This exploitative exclusion is dependent upon dense concen-
trations of insects attracted to human light sources and the structural simplicity of
building surfaces (Petren et al., 1993; Petren and Case, 1998). However,
L. lugubris
also avoid
H. frenatus
(Bolger and Case, 1992; S.G. Brown et al., 2002), and this
avoidance may make
L. lugubris
more susceptible to predation subsequent to invasion
by
H. frenatus
(S.G. Brown et al., 2002). Although
L. lugubris
itself may be a human
introduction across much of the Pacific (Moritz et al., 1993), making this an example
of displacement of one alien lizard by a more recent introduction, it does illustrate the
potential for competitive exclusion to result from alien lizard introductions. A similar
mechanism may be occurring between two alien geckos in Texas. There, resident
H. turcicus
are being displaced by more recently arrived
Cyrtopodion scabrum
, and
the displaced species exhibits a dietary shift in sympatry that is consistent with strong
