Biology Reference
In-Depth Information
frog populations. The tapeworm's decline has been associated with a decline in the
stability of the local frog community (Freeland, 1994).
Each of these documented or potential changes to food webs and ecosystem
dynamics stems directly from the high standing biomass that some alien reptiles
and amphibians are capable of achieving. Direct measurements of biomass or
densities have not often been made for alien populations of reptiles and amphibi-
ans. However, there is a number of herpetological genera with naturalized popu-
lations whose densities are sufficiently high that they are likely candidates for
disrupting trophic dynamics of invaded ecosystems. These include frogs of the
genera
Bufo
,
Eleutherodactylus
,
Osteopilus
,
Rana
, and
Xenopus
and lizards of the
genera
Anolis
,
Carlia
,
Chamaeleon
,
Hemidactylus
,
Lampropholis
, and
Podarcis
.
This list is not exhaustive but merely highlights some of the more promising taxa
for investigation.
Competition with Native Species
As noted above,
Bufo marinus
has depressed reproductive success of rainbow bee
eaters partially through competition for burrow use (Boland, 2004a). Tadpoles of
the same species also depressed growth rates among a variety of native anuran
larvae in pool and pond-enclosure experiments, but inconsistency among trials
leaves unanswered the extent to which competition exerts population-level effects
among tadpoles in natural settings (Williamson, 1999). Other experiments indicated
apparently strong competitive effects between
B. marinus
tadpoles and those of
Limnodynastes ornatus
(Crossland, 1997, cited in van Dam, 2002). No competitive
effect was noted between adult toads and native frogs (Freeland and Kerin, 1988).
The expansion of
Eleutherodactylus johnstonei
across the Lesser Antilles has
been correlated with the decline or replacement of native congeners on several
islands (Hardy and Harris, 1979; H. Kaiser and Henderson, 1994; H. Kaiser et al.,
1994; H. Kaiser, 1997). However, this replacement largely goes hand in hand with
habitat destruction:
E. johnstonei
has a greater physiological tolerance for higher
temperatures and drying (Pough et al., 1977) and greater use than native
Eleutherodactylus
of opened habitats (Stewart, 1977; Stewart and Martin, 1980).
This tolerance seems to facilitate its use of expanding areas of vegetation disturbed
by human activities (H. Kaiser, 1997), apparently at the occasional expense of resi-
dent congeners (Hardy and Harris, 1979; H. Kaiser, 1997).
Competitive effects from larval
Rana catesbeiana
can be varied. They depress
growth rates and survival in larval
R. boylii
owing to exploitative competition
for algal resources (Kupferberg, 1997a). They also inhibit growth rates in larval
R. aurora
by passive exclusion under conditions in which food resources are
clumped (Kiesecker et al., 2001). This happens because larval
R. aurora
avoid tad-
poles of
R. catesbeiana
and, hence, lose access to the clumped food resources
around which the latter invariably gather (Kiesecker et al., 2001). The two mecha-
nisms need not be exclusive: exploitative deficiencies of native
Rana
tadpoles may
