Biology Reference
In-Depth Information
(e.g., Greenlees et al., 2006; Woolbright et al., 2006). Thus, many reptile and
amphibian species are likely candidates to facilitate subsequent alien predator
establishment by serving as a dense food source. Concern has been expressed that
this phenomenon could facilitate establishment of introduced snakes in Hawaii
(Kraus et al., 1999; Kraus and Cravalho, 2001; Loope et al., 2001), but this form of
ecological “priming” has been uninvestigated except for the
Boiga
and
Natrix
cases
discussed above. As introductions of additional herpetological predators and their
prey continue to increase this phenomenon may become more widely noticed.
Dense populations of alien reptiles and amphibians could potentially affect
nutrient-cycling dynamics within ecosystems, but this effect has been little investi-
gated to date. It has been proposed that two alien frogs (
Eleutherodactylus coqui
and
E. planirostris
) could serve as nutrient sinks in Hawaii by depletion of inverte-
brate biomass and disruption of ecological pathways (Kraus et al., 1999). This
speculation was based on known high population densities of the frogs, their high
invertebrate-cropping rates, and the lack of native predators (and paucity of alien
predators) to feed on them. One study (Beard and Pitt, 2006) lent some support to
this conjecture, finding that in a dense population of
E. coqui
frogs were consumed
in very low amounts by mongoose (
Herpestes javanicus
) but not at all by rats
(
Rattus rattus
and
R. exulans
) or cane toads (
Bufo marinus
). These are the only
predators available to prey on these frogs in most of Hawaii. Studies in their native
Puerto Rico have shown
E. coqui
to affect nutrient cycling dynamics in forest plots
by reducing aerial invertebrates and leaf herbivory and by increasing primary pro-
ductivity and leaf decomposition rates (Beard et al., 2002, 2003). These effects
resulted from high predation rates on aerial insects and fertilization of soil by frog
feces. Identical effects were found in the invaded range of
E. coqui
in Hawaii, as
were reductions in numbers of herbivorous and leaf-litter invertebrates and
increases in new leaf production by the invasive plant
Psidium cattleianum
in one
invaded site (Sin et al., 2008).
Similar ecosystemic impacts are considered likely to result from the invasion of
Bufo marinus
in northern Australia. In this system, a four-fold increase in amphibian
biomass has been documented as toads invade virgin territory (Greenlees et al.,
2006). Because the toad is largely invulnerable to predation by native species, the
increase in amphibian biomass is expected to serve as a nutrient sink (Greenlees
et al., 2006), although possible effects on primary productivity and decomposition
rates would also merit investigation.
Another change to community dynamics is attributed to colonization by
Bufo
marinus
. High prevalence of a native tapeworm in the Australian anuran
Litoria
pallida
declined after invasion by cane toads, apparently because the high density
of toads interfered with transmission of the parasite to its definitive snake host,
Liasis childreni
(Freeland, 1994). The tapeworm's life cycle originally involved
transmission of eggs from snake feces to frogs via consumption of infected food.
Cyst-bearing frogs were then consumed by the snakes, completing the worm's life
cycle. The creation of high-density populations of voracious toads shunted most
worm eggs to that alien species, which was shunned as a food item by the snakes,
breaking the life-cycle of the tapeworms and reducing their prevalence in native
