Download Ecological impacts of invasive cane toads

HOT TOPICS IN ECOLOGY
https://www.ecolsoc.org.au/hot-topics/ecological-impacts-invasive-cane-toads
Ecological impacts of invasive cane toads
An invader's arrival is bad news for some native species, but good news for others
Rick Shine
1. The invasion of cane
toads through Australia
kills many large
predators, that are
poisoned when they try to
eat large, highly toxic
toads.
2. But most native
species are not affected
by toads, because they
soon learn not to eat
them; or else have a preexisting tolerance to the
toad's poison.
3. The decrease in
numbers of large
predators benefits many
smaller species, which
increase in numbers after
cane toads arrive in an
area.
Ecological Society of Australia
https://www.ecolsoc.org.au
Further information about
this topic contact:
Prof Rick Shine
[email protected]
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Board
Dr Rachel Standish
Large predators like this freshwater crocodile (Crocodylus johnstoni) can be in big trouble when cane toads
invade, but most smaller species are less affected by toad invasion. Indeed, the collapse of apex predator
populations is good news for the species previously consumed by those predators. Photograph by G. P.
Brown.
Cane toads (Rhinella marina) were brought to Australia in 1935, and released in
northeastern Queensland. They have since spread westwards (through Queensland, the
Northern Territory, and into Western Australia) and southwards (into NSW). Concern about
the toads’ impacts on wildlife spawned predictions that most native species encountering
toads would be severely affected, and thus decline in abundance. Recent research is
revealing a more complex picture. Toads do indeed cause precipitous population declines
(of >80%) in some large-bodied predator species, due to lethal poisoning when predators
attempt to eat toads. However, populations of smaller predator species are unaffected;
some individuals are fatally poisoned, but most survive (because small toads contain far
less toxin than large adult toads) and learn to avoid toads thereafter. Most Australian birds
and rodents have inherited a tolerance of toad poison from Asian ancestors, and thus are
not affected. Even for heavily impacted predators, populations might eventually recover.
Simplistic predictions on the ecological impact of cane toads in Australia have not been
supported by field studies - indirect impacts of toads often outweigh direct effects. For
example, the abundance of native frogs appears to be unaffected by cane toads, because
negative impacts (e.g. frogs being eaten by toads, poisoned by toads, competing with
toads) are balanced by positive impacts (e.g. toads reducing abundance of frog-predators).
Some native snake species predicted to be vulnerable to toads, have become more
common since toad invasion – presumably because of the disappearance of predatory
goannas. In summary, the impacts of invaders may be complex, and difficult to predict.
Any disruption to one native species is likely to have indirect effects on others. Some
native species are winners not losers as the toad invasion rolls across the landscape.
Status: approved ; Last updated: 02/07/2014
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HOT TOPICS IN ECOLOGY
https://www.ecolsoc.org.au/hot-topics/ecological-impacts-invasive-cane-toads
Ecological impacts of invasive cane toads
An invader's arrival is bad news for some native species, but good news for others
Rick Shine
Supporting Evidence
Title
Aim
Key Results
Beckmann C. & Shine R. (2009) Are Australia's birds at risk due to the
invasive cane toad? Conserv. Biol. 23, 1544–9.
To determine which bird
species are potentially at
risk of consuming toxic
toads and whether the
predicted impact is real,
or due to a dearth of
empirical evidence
Cane toads are sympatric with 172 frog-eating bird species in
Australia, and an additional 8 bird species overlap with the
predicted future range of the toad. Thus many bird species are
potentially at risk, however behavioral observations suggest the
risk level is generally low. Despite occasional reports of Australian
birds being killed when they ingest cane toads, most birds either
ignore toads or survive the predation event
Brown G. P., Phillips B. L. & Shine R. (2011a) The ecological impact of
invasive cane toads on tropical snakes: field data do not support
predictions from laboratory studies. Ecology 92, 422–31.
To evaluate the accuracy
of our a priori predictions
of the impacts of toads
on an assemblage of 12
reptile species in tropical
Australia.
Two main conclusions emerge from the study: (1) fewer snake
species declined in abundance after toad invasion than was
predicted a priori based on toxin tolerances and feeding habits;
and (2) many of the shifts in snake abundance that occurred at
about the time of toad invasion likely were not causally driven by
toad impact
Caller G. & Brown C. (2013) Evolutionary responses to invasion: cane
toad sympatric fish show enhanced avoidance learning. PLoS ONE 8,
e54909.
To determine whether
aversion learning is
occurring in aquatic
ecosystems by
comparing cane toad
naïve and sympatric
populations of crimson
spotted rainbow fish
Cane toad sympatric fish showed reduced attacks on both cane
toad tadpoles and native tadpoles which highlights evolutionary
shifts in the ability of these fish to learn to recognise and
subsequently avoid novel, noxious prey items.
Doody J. S., Green B., Sims R., Rhind D., West P. & Steer D. (2006)
Indirect impacts of invasive cane toads (Bufo marinus) on nest
predation in pig-nosed turtles (Carettochelys insculpta). Wildl. Res. 33,
349–54.
To quantify and
understand the indirect or
secondary impacts of the
decline in the yellowspotted monitor lizard
(Varanus panoptes)
associated with the
arrival of cane toads, and
the predicted
simultaneous impact on
pig-nosed turtle nest
predation by yellowspotted monitors
Yellow-spotted monitors have been reduced to such low numbers
that they are currently no longer a significant predator of pignosed turtle eggs
Letnic M., Webb J. K. & Shine R. (2008) Invasive cane toads (Bufo
marinus) cause mass mortality of freshwater crocodiles (Crocodylus
johnstoni) in tropical Australia. Biol. Conserv. 141, 1773–82.
To quantify the impact of
this massive mortality
event on the freshwater
crocodile population from
the Victoria River.
The impact of cane toad invasion on freshwater crocodiles may
increase with increasing aridity. Subadult crocodiles are
particularly vulnerable to lethal toxic ingestion from eating cane
toads, and with the exception of hatchlings, this size class
showed the greatest decrease in abundance following the arrival
of cane toads
Lettoof D. C., Greenlees M. J., Stockwell M. & Shine R. (2013) Do
invasive cane toads affect the parasite burdens of native Australian
frogs? Int. J. Parasitol.: Parasite. Wildl. 2, 155–65.
To determine if the cane
toad invasion influences
rates of parasitism in
native frogs
Contrary to initial predictions, toad invasion was associated with a
reduced parasite burden in native frogs. Thus, cane toads do not
appear to transfer novel parasites to native frog populations, or
act as a reservoir for native parasites to ‘spill-back’ into native
frogs. Instead, cane toads may reduce frog-parasite numbers by
taking up native parasites that are then killed by the toad’s
immune defences.
O'Donnell S., Webb J. K. & Shine R. (2010) Conditioned taste aversion
enhances the survival of an endangered predator imperiled by a toxic
invader. J. Appl. Ecol. 47, 558–65.
To investigate whether
conditioned taste
aversion (CTA) could be
used to mitigate toad
impacts by modifying
quoll predatorybehaviour
and enhance survival
An aversion to live toads in juvenile northern quolls was
successfully induced by feeding them a dead toad containing a
nausea-inducing chemical (thiabendazole). In both sexes, toadsmart quolls had higher apparent survival rates than did toadnaive conspecifics
Phillips B. L. & Shine R. (2006) An invasive species induces rapid
adaptive change in a native predator: cane toads and black snakes in
Australia. Proc. R. Soc. B 273, 1545–50.
To examine the
possibility that red-belly
blacksnakes display an
adaptive response to the
presence of cane toads
Strong differences were evident between blacksnakes from toadexposed versus toad-naive populations both in their physiological
resistance to toad toxin and in their willingness to eat toads.
Importantly, both of these differences are in an adaptive direction
i.e. there is an increased resistance to toxin and lowered
preference for consuming toads in toad-exposed populations
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HOT TOPICS IN ECOLOGY
https://www.ecolsoc.org.au/hot-topics/ecological-impacts-invasive-cane-toads
Ecological impacts of invasive cane toads
An invader's arrival is bad news for some native species, but good news for others
Rick Shine
Title
Aim
Key Results
Pizzatto L. & Shine R. (2011) Ecological impacts of invading species:
do parasites of the cane toad imperil Australian frogs? Austral Ecol. 36,
954–63.
To explore the risk posed
by Rhabdias
pseudosphaerocephala
lungworms to native
frogs, and to assess the
effects of Rhabdias
exposure on anuran
survival, growth and
locomotor performance
Laboratory experiments showed that infective Rhabdias larvae
can penetrate the body of all seven species of Australian frogs
tested (five hylids: Cyclorana longipes, Litoria caerulea, Litoria
dahlii, Litoria nasuta, Litoria rothii, one myobatrachid: Opisthodon
ornatus, and one limnodynastid: Limnodynastes convexiusculus),
but most did not host the adult worms at the end of the trials, and
none showed major impairment of growth, survival or locomotor
performance. One native tree-frog (L. caerulea) retained high
infection levels with few ill effects, suggesting that we might be
able to use this taxon as a reservoir species to build up local
parasite densities for toad management.
Shine R. & Doody J. S. (2011) Invasive-species control: understanding
conflicts between researchers and the general community. Frontiers
Ecol. Environ. 9, 400–6.
To better understand the
reasons for
disagreements between
scientists and
community groups about
the nature and magnitude
of conservation efforts
aimed at the invasive
cane toad
Appreciating the pressures faced by all participants engaged in
conservation debates can lead to a more constructive and
cooperative atmosphere
Shine R. (2010) The ecological impact of invasive cane toads (Bufo
marinus) in Australia. Q. Rev. Biol. 85, 253–91.
To review the ecological
impacts of cane toads in
Australia
Overall, some Australian native species (mostly large predators)
have declined due to cane toads; others, especially species
formerly consumed by those predators, have benefited. For yet
others, effects have been minor or have been mediated indirectly
rather than through direct interactions with the invasive toads.
Factors that increase a predator’s vulnerability to toad invasion
include habitat overlap with toads, anurophagy, large body size,
inability to develop rapid behavioral aversion to toads as prey
items, and physiological vulnerability to bufotoxins as a result of a
lack of coevolutionary history of exposure to other bufonid taxa.
Shine R. (2014) A review of ecological interactions between native frogs
and invasive cane toads in Australia. Austral Ecol. 39:1-16.
To review current
information on ecological
interactions between
cane toads and
Australian anurans
Overall, the interactions between native frogs and invasive toads
illustrate the diverse ways in which an invader’s arrival can
perturb the native fauna by both direct and indirect mechanisms,
and by which the native species can curtail an invader’s success.
Ward-Fear G., Brown G. P., Greenlees M. & Shine R. (2009)
Maladaptive traits in invasive species: in Australia, cane toads are more
vulnerable to predatory ants than are native frogs. Funct. Ecol. 23,
559–68.
To determine how cane
toad phenotypes differ
from native frogs and
then to explore the
possibility that any
differences might render
cane toads more
vulnerable to a predator
(meat ant) that poses
little threat to native
anurans
Unlike the frogs, (1) toads selected open microhabitats and were
active diurnally, thus increasing encounter rates with meat ants;
(2) toads failed to detect and evade approaching ants; (3) toads
exhibited poor locomotor ability (short slow hops, reflecting their
small size and short limbs); and (4) toads frequently relied on an
ineffective defence mechanism (crypsis) when attacked. In
combination, these traits rendered cane toad metamorphs far
more susceptible to predation by meat ants than were any of the
native frogs tested. That vulnerability presumably reflects lack of
coevolution between cane toads and Australian ants.
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