Biology Reference
In-Depth Information
are needed to determine how general this result is across Australia's diversity of
native predators, and no research has yet investigated the effect such strong selec-
tive pressure has had on genetic diversity within the varanid populations. Ecological
studies at the expanding front of toad invasion in Northern Territory are underway
(R. Shine, University of Sydney, personal communication, 2007), so more direct
evidence of population-level effects may be forthcoming.
Bufo marinus
were also introduced to Kayangel Atoll in Palau and to Ponape
and Kosrae in the Federated States of Micronesia in a deliberate attempt to control
Varanus indicus
, which were considered undesirable because of their propensity to
kill chickens (Gressitt, 1952; W.B. Jackson, 1962; Dryden, 1965). Introduction of
B. marinus
did result in a dramatic reduction of
Varanus
in Kosrae, with some dead
monitors found with toads in their mouths (Dryden, 1965). The toads have been
credited with apparent monitor declines on Guam (McCoid et al., 1994a), Ponape
(W.B. Jackson, 1962), and Palau (Thyssen, 1988) as well. Similar results have been
said to attend the introduction of toads to New Guinea (Pippet, 1975) and the
Solomon Islands (Cain and Galbraith, 1957). Anecdotal reports of poisoning of
native wildlife from ingestion of cane toads also come from Bermuda (Davenport
et al., 2001) and Fiji (Gorham, 1968).
In laboratory experiments, eggs and larvae of
Bufo marinus
can be toxic to an
array of native invertebrates and tadpoles (Crossland, 1998a, b; Crossland and
Alford, 1998; Crossland and Azevedo-Ramos, 1999; Punzo and Lindstrom, 2001),
and that toxicity can increase ontogenetically (Crossland, 1988b). In experiments
carried out in artificial ponds, these results were extended to demonstrate that
presence of
B. marinus
eggs and tadpoles significantly depressed survival of native
Limnodynastes ornatus
tadpoles, presumably via poisoning of the latter. This
depression of
L. ornatus
, in turn, led to enhanced survival of native
Litoria rubella
tadpoles due to release from predation by the former (Crossland, 2000). Survival of
L. ornatus
,
L. tasmaniensis
,
L. terraereginae
, and
Notaden bennetti
was also some-
times depressed in independent pool and pond-enclosure experiments (Williamson,
1999). These results are suggestive of changes liable to occur in native anuran com-
munities from introduction of
B. marinus
, but direct examination for similar
effects under entirely natural circumstances has not occurred. Larval
B. marinus
can be toxic to a few native Australian fish species as well (Crossland and
Alford, 1998; van Dam et al., 2002) but are typically rejected as food (Lawler
and Hero, 1997; van Dam et al., 2002), so seem unlikely to exert any significant
effects on native fish populations.
Wider Changes in Ecosystem Dynamics
The widespread loss of terrestrial vertebrates occasioned by the introduction of
Boiga irregularis
and other vertebrates to Guam led to ecosystem-wide trophic
changes (Fritts and Rodda, 1998). The dominant vertebrate biomass on Guam now
consists of alien species, there is an increased number of predatory links in the food
