Review of the management of feral animals and their impact on ...

with cane toads <strong>and</strong> <strong>the</strong> level <strong>of</strong> competition is not likely to be <strong>of</strong> great significance
except in certain habitats.
Competition for food is not thought likely at <strong>the</strong> adult stage. Dietary studies show that
toads mainly eat beetles, ants <strong>and</strong> termites, <strong>and</strong> that <strong>the</strong>y are not obvious competitors
with native frogs. But toad tadpoles (‘toadpoles’) do compete with native tadpoles in
small pools. This competition would be unlikely to lead to extinctions but it might
reduce frog numbers at some sites.
Predation
Although cane toads are occasionally reported eating frogs, snakes, lizards, birds,
mice <strong>and</strong> o<strong>the</strong>r vertebrates, dietary studies, based upon dissecting large number <strong>of</strong>
toad stomachs, indicate that beetles, termites <strong>and</strong> ants are <strong>the</strong>ir main foods. It seems
very unlikely that any prey species has become rare because <strong>of</strong> toad predation at
ei<strong>the</strong>r <strong>the</strong> adult or tadpole stage.
Summary
Cane toads are <strong>of</strong>ten claimed to have a devastating <strong>impact</strong> upon biodiversity.
Laboratory experiments demonstrate that toads <strong>and</strong> <strong>the</strong>ir eggs <strong>and</strong> tadpoles are
poisonous to many predators. But surveys <strong>of</strong> toad-invaded areas show that nearly all
predators survive alongside toads, albeit sometimes at reduced densities. The evidence
suggests that predator populations sooner or later learn to avoid toads. This does not
mean that <strong>the</strong> <strong>impact</strong> <strong>of</strong> toads should be downplayed. Impact should not be measured
only by <strong>the</strong> loss <strong>of</strong> species. Feral <strong>animals</strong> <strong>of</strong>ten have a wide range <strong>of</strong> <strong>impact</strong>s upon
ecosystem functioning. Cane toads are sometimes <strong>the</strong> most abundant vertebrate within
a habitat, <strong>and</strong> <strong>the</strong>ir <strong>impact</strong>s can be wide ranging, as noted above. The cumulative
effects <strong>of</strong> cane toad poisoning, competition <strong>and</strong> predation can represent a substantial
<strong>impact</strong> upon ecosystem functioning, especially when it leads to reduced numbers <strong>of</strong>
predators such as quolls, goannas <strong>and</strong> snakes.
The <strong>impact</strong> upon nor<strong>the</strong>rn quolls is <strong>the</strong> most serious <strong>impact</strong> <strong>of</strong> toads yet recorded. In
some regions <strong>of</strong> Queensl<strong>and</strong>, quolls occur alongside toads, although mainly in rocky
<strong>and</strong> upl<strong>and</strong> situations where quolls probably have little contact with <strong>the</strong>m. In o<strong>the</strong>r
areas, quolls disappeared soon after toads arrived <strong>and</strong> <strong>the</strong>y have never returned. The
nor<strong>the</strong>rn quoll is now listed federally as endangered. Permanent declines in
populations <strong>of</strong> frog-eating snakes are ano<strong>the</strong>r significant <strong>impact</strong>. Declines in goannas
may be a third <strong>impact</strong>, but <strong>the</strong> evidence for this is less convincing: goanna densities
are not easy to monitor.
Fish
3.7.40 Common Carp (Cyprinus carpio)
Summary
Carp are widespread throughout sou<strong>the</strong>rn Australia where <strong>the</strong>y dominate <strong>the</strong> aquatic
fauna. Although perceived as <strong>the</strong> number one introduced fish threat to inl<strong>and</strong>
waterways, <strong>the</strong> extent <strong>of</strong> negative <strong>impact</strong>s this species has is still unclear. They have
been implicated in reduction <strong>of</strong> water quality, destruction <strong>of</strong> aquatic vegetation,
spreading disease <strong>and</strong> parasites, undermining banks <strong>and</strong> competition for food
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