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Biology Issue Report: The impact of the introduction of Cane toads into the Australian habitat.
Cane toads (Rhinella Marina) were introduced to north-eastern Australia in 1935 in an attempt to control the sugar cane pests damaging the crops of farmers.
Cane toads are large toads native to Central and South America. They are voracious predators, eating a diet consisting mainly of arthropods. The main habitat of cane toads was originally thought to be humid, tropical conditions, however it is now thought that they are able to tolerate a much broader range of environments and climates. This has enabled them to quickly thrive in north-eastern Australia.
The numbers of cane toads in Australia have increased rapidly since their introduction into the environment and many people are concerned about the threat that they may pose to the native wildlife of Australia. The aim of this report is to examine the nature of the threat to the native wildlife and to explore whether their numbers can be reduced.
The spread of cane toads
The increasing numbers of cane toads in Australia has led to concern in both the scientific community and amongst the general public about the threat that they pose to Australian ecosystems. The spread of cane toads in north-eastern Australia has occurred at an unprecedented rate however it is difficult to accurately determine the true extent of the species’ dispersal due to the inherent margin of error that exists when attempting to conduct a species census (Shine 2010). In 2010 Jane Elith and her colleagues used the case of the cane toads to explore whether there is an accurate method by which the future distribution of invasive species can be determined, taking into account the shifting range of many invasive species (Elith, et al. 2010). They concluded that an ecophysiological model is the most accurate way to predict the future extent of the spread of an invasive species such as cane toads (Elith et al.2010). This model considers both the species’ characteristics and the environment into which it is expanding when determining their possible future spread. This highlights one difficulty with the current suggestions regarding the rate at which cane toads are spreading in Australia. It has been estimated by some researchers that cane toads are spreading at a rate of 5-6km a year in northern Australia; however it can be argued that these suggestions do not take into account the changing climates into which the toads are spreading. Whilst some habitats, such as tropical climates are particularly suited to the cane toads, others, for example dry habitats, are more challenging environments for cane toads. Therefore their spread into different habitats may be more uneven than predicted.
Implications of the spread of cane toads
The diet of cane toads consists predominately of invertebrate species, particularly ants, termites and beetles. This has led some to question whether the introduction of cane toads into the Australian habitat may have a damaging effect on the populations of such invertebrates. In 2006 M.J Greenlees and his colleagues carried out experiments to explore the effect that the presence of cane toads in small enclosures had upon native invertebrates (Greenlees et al. 2006). In their experiment they introduced large and small cane toads and the equivalent biomass of native frogs to different controlled enclosures and used pitfall traps to measure the number of invertebrates found in the enclosure after the introduction of both cane toads and native frogs. The results of their experiment suggested that the numbers of invertebrates in the enclosures of both the cane toads and the native frogs decreased by a similar amount, and they concluded that the cane toads had a similar effect to native frogs on the numbers of invertebrates. This result is displayed in Figure 1.3 which shows that the richness and abundance of invertebrates in both the cane toad’s enclosures and the Cyclorana and Litoria native frogs’ enclosures was of a similar level (Greenlees et al. 2006). This experiment provides some evidence that the perceived threat of cane toads to the Australian native species may not be as great as previously feared.
The most serious threat posed by cane toads to the native species of Australia can be argued to come from the possibility of lethal toxic ingestion of cane toad poison by predators. Cane toads are poisonous, to varying degrees, throughout their lifecycle and bufonid poisons are particularly dangerous to some of Australia’s native animals. Previous to the introduction of the cane toad, many native Australian species had no evolutionary exposure to bufonid poisons, and therefore no immunity to them. Many of Australia’s native predators prey on anurans and have not yet learned to distinguish between native frogs and cane toads. When mouthed or ingested the toads secrete a toxin from their shoulder glands which can prove lethal to many predators. There have been many anecdotal reports in Australia about the deaths of predators such as freshwater crocodiles, northern quolls, goannas and northern death adders, due to the ingestion of cane toad poison. These anecdotal reports are hard to quantify however and cannot be used to provide reliable evidence for the effects of cane toads on large predators.
The northern quoll (Dasyurus hallucatus) is a marsupial carnivore, native to Australia, and found predominately in western and northern Australia. The northern quoll is a predatory species that is particularly threatened by the invasion of cane toads into their habitat. Under the Environment Protection and Biodiversity Conservation Act 1999 the northern quoll is listed as an endangered species and this is the result of the local population extinctions that have occurred as a result of the lethal toxic ingestion of cane toads by this species. Radio tracking of quolls in various studies has shown that the presence of even a few cane toads in the habitat of quolls can decrease the population of quolls in these areas. Without intervention the spread of cane toads could result in the extinction of one of Australia’s smallest mammalian predators.
Cane toads are disliked by many people because of the perceived threat that they pose to the native wildlife of Australia. This threat has been featured in numerous news reports such as the article in Figure 1.4 and has led to growing concern amongst the general public about the threat that cane toads may pose to humans. In order for cane toad venom to have an effect on humans it needs to be absorbed through the mucous membranes of the mouth, nose or eyes. Cane toad venom is a bufotoxin which contains cardiac glycosides. Cardiac glycosides are compounds which contain the sugar glycoside which has an effect on the force of the cardiac muscle’s contraction (Cohen et al. 2003). When absorbed into the human body the toxin can also cause gastrointestinal problems. Cane toad poisoning has led to the death of some people and because of this many Australians feel threatened by the spread of cane toads into their community. This dislike of cane toads has made community toad-culling events popular amongst the general public. The Australian government has also introduced leaflets aimed at raising awareness about the dangers of cane toads and how to protect families against them (Department of Environment and Conservation, n.d).
Can the spread of cane toads be halted?
The Australian government has concluded that any attempts to cull cane toads on a large scale would be unfeasible and that there is no prospect for national eradication of cane toads in Australia (Department of Sustainability, Environment, Water, Population and Communities, 2011).
The Australian government has turned its focus away from preventing the spread of cane toads, and towards the protection of endangered species such as the northern quoll. One program, called the Territory Wildlife Park introduced a captive breeding and research program in 2006 to prevent the species’ extinction (Territory Wildlife Park, n.d). The program has also introduced a project to train their captive quolls to avoid cane toads (O’Donnell et al. 2010). After training the quolls to avoid cane toads through aversion therapy, the ‘toad-smart’ quolls and some ‘toad-naïve’ quolls were released into the wild wearing radio collars. The survival rates of both types of quoll can be seen in figure 1.6. This table shows that the male and female toad-smart quolls had a much greater mean survival time in the wild, it also suggests that females are less likely to be poisoned by toad ingestion. Male quolls appear to attempt to ingest the toads more frequently with 7 toad related deaths accumulatively in the males, and the toad-naïve male quolls had the biggest proportion of toad-related deaths at 5 deaths. Figure 1.7 shows that the toad-smart quolls had a greater daily survival percentage than the toad-naïve quolls and this provides evidence to support the argument that it is better to invest time and money in projects that aim to help native wildlife live alongside cane toads than to try to eradicate the toads completely.
The most recent research on methods that could be used to halt the spread of cane toads has focussed on whether there is a way to prevent the toads reaching specific areas during arid seasons. In 2015 Mike Letnic and his colleagues identified ‘invasion hubs’, areas that cane toads initially colonise and then use as a base from which to expand their population (Letnic et al. 2015). They hypothesised that if the access of cane toads to areas like this was restricted, then they would be unable to establish a successful population in previously unpopulated areas. To test their hypothesis the researchers maintained toad-proof fences around dams where toads had previously been eradicated. They then measured the numbers of dead toads around the fences in both dry and wet seasons and observed the numbers of live toads in the area. The results of their study were encouraging and suggested that toads are attracted to these ‘invasion hubs’ and when their access to these is denied their mortality rate rises and population numbers decrease (Letnic et al. 2015). This study is particularly important because it highlights a method that could be used to prevent the invasion of cane toads in areas of particular ecological interest.
Review of sources
The first source I will review is the paper ‘Effects of an invasive anuran [the cane toad (Bufo marinus)] on the invertebrate fauna of a tropical Australian floodplain’ by M.J. Greenlees et al (2006). This research aims to provide quantitative data exploring the effect that invading cane toads are having upon the native invertebrates of Australia. The authors highlight the fact that the effect of cane toads upon the native ecosystems is poorly understood due to a lack of experimental data. In the light of this they conducted a simple experimental trial that produced data that could be analysed to provide an indication of the ecological effects of this invasive species. The researchers used simple methods such as controlled enclosures and pitfall traps to conduct their experiment. Whilst it can be argued that the controlled enclosures used in this experiment will not accurately reflect the true ecology of natural habitats, the researchers acknowledge this in their paper and suggest that their experiment be viewed as the first step towards more detailed research. I found this experiment to be thorough in their control of variables and in their analysis of the results. I believe that because the researchers are from the University of Sydney, a well-respected institution, their experiment can be viewed as trustworthy and their results credible.
The second source I will review is the website http://www.canetoadsinoz.com/invasion.html (Cane toads in Oz, 2011). This website was created by Richard Shine and his ‘TEAM BUFO’. Shine is a Professor of Biology at Sydney University and is one of the key scientific figures in the field of cane toad research. The website created by Shine and his team examines all aspects of cane toads and their threat to Australia’s native wildlife. Whilst the website is undoubtedly more informal than a scientific paper, it can still be used as a comprehensive tool with which to gain more information about cane toads. It can be argued that websites such as this are more effective at engaging the public and informing them of environmental issues because, in comparison to scientific papers, the easily accessible website is more likely to be read and used as a source of information by the public.
The spread of cane toads in Australia is an issue that has concerned many people. The scientific research surrounding this issue is particularly interesting because it gives an insight into how the true reality of a threat from an invasive species may be very different from the original perceived threat. Cane toad poison has undoubtedly had a lethal effect on native predators, causing some to become endangered. However not all species appear to have populations detrimentally effected by the presence of the toads. This shows the extent to which natural selection and adaptation allows organisms to cope with environmental threats. Populations may decrease, but they also increase if natural selection allows a species to recover.
The spread of cane toads in Australia is now too great to prevent and so scientists are experimenting with methods that allow Australian ecosystems and native species to be conserved and protected alongside the presence of cane toads. These early studies are beginning to provide positive results that suggest that whilst the spread of cane toads may be lethal to some species, a focus on conservation may limit the danger and help to preserve the unique species living in the Australian ecosystem.
Cane Toads in Oz, 2011. Available from: http://www.canetoadsinoz.com/invasion.html. [26 February 2015]
Cohen, R.A., Gowda, R.M., Khan, A. ‘Toad venom poisoning: resemblance to digoxin toxicity and therapeutic implications’, Heart, 89:4 (2003): e:14
Department of Environment and Conservation, Protecting children and pets from cane toads, Government of Australia. Available from: http://archive.agric.wa.gov.au/objtwr/imported_assets/content/pw/vp/toad/cane_toads_protect.pdf. [26 February 2015]
Department of Sustainability, Environment, Water, Population and Communities, 2011. Threat abatement plan for the biological effects, including lethal toxic ingestion, caused by cane toads, Government of Australia. Available from: http://www.environment.gov.au/system/files/resources/2dab3eb9-8b44-45e5-b249-651096ce31f4/files/tap-cane-toads.pdf. [26 February 2015]
Elith, J., Kearney, M. and Phillips, S., ‘The art of modelling range-shifting species.’, Methods in Ecology and Evolution, 1(2010): 330–342.
Greenlees, M. J., Brown, G. P., Webb, J. K., Phillips, B. L. and Shine, R. ‘Effects of an invasive anuran [the cane toad (Bufo marinus)] on the invertebrate fauna of a tropical Australian floodplain.’, Animal Conservation, 9 (2006): 431–438
Letnic, M., Webb, J. K., Jessop, T. S., Dempster, T. (2015), ‘Restricting access to invasion hubs enables sustained control of an invasive vertebrate’. Journal of Applied Ecology. doi:10.1111/1365-2664.12390
O’Donnell, S., Webb, J.K., Shine, R. ‘Conditioned taste aversion enhances the survival of an endangered predator imperilled by a toxic invader’, Journal of Applied Ecology, 47 (2010): 558-565
Shine, R. ‘The Ecological Impact of Invasive Cane Toads (Bufo Marinus) in Australia’, The Quarterly Review of Biology,85:3, (2010): 253-291
Territory Wildlife Park. Available from: http://www.territorywildlifepark.com.au/about/quoll.shtml. [26 February 2015]
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