Investigating the claim that cane toads enhance the local ecology and biodiversity in Australia
Toowoomba Grammar School
It is claimed that human introduced species into Australia have had a positive effect on the local ecology because the introduced species add to the biodiversity which is beneficial for the environment.
In the early 1900’s, the Australian Government introduced a number of species, including foxes, pigs, rabbits, goats, camels and cane toads (Ponnamperuma, 2006). Along with introduced plant and fish species, many of these introduced species were imported as a biological control solution to eradicate pests which were native to Australia (Ponnamperuma, 2006).
The introduction of the cane toad (Bufo marinus), has become one of Australia’s most significant introduced species. The Australia Government imported approximately 100 cane toads from Hawaii in 1935 as a biological control solution (Alford, 1994). After reproduction, approximately 3,000 young cane toads were released at Gordonvale near Cairns (Alford, 1994). The aim was to control the population of the sugar cane pests such as the greyback cane beetle (Dermolepida albohirtum) (Australian Department of the Environment, 2010). Cane toads proved to be unsuccessful in controlling the cane beetles. Since their introduction, the cane toad population has exploded and has spread uncontrollably too many other states, as seen in figure 1 (Cheng, 2013). It is estimated that there is now 1.5 billion cane toads that have invaded Australia (Australian Department of the Environment, 2010). That is roughly 60 cane toads for every 1 Australia citizen. The cane toads that have adapted and evolved have lead the invasion front in which they extend the invasion boundary by 40km – 60km per year (Masterson, 2018). The boundaries extend significantly each year due to the population living in high density. This allows the female cane toads to reproduce twice per year, laying between 9,000 – 35,000 eggs (Masterson, 2018). Each tadpole that survives birth, evolves and adapts to the Australian conditions.
The aim of this scientific report is to analyse the causes, effects and possible solution of the explosive cane toad population in Australia. The claim that human introduced species into Australia have had a positive effect on the local ecology because the introduced species add to the biodiversity which is beneficial for the environment, will be evaluated.
The following research question will also be answered: how have the introduction of cane toads in Australia affected the local ecology and biodiversity?
Evaluation of Cane Toads
Cane toads are having an extremely detrimental effect on the local ecology and biodiversity in Australia. This is primarily due to their physiological adaptation. Cane toads most significant physiological adaptation is their parotoid glands. These are external glands and are located behind the eyes, as seen in figure 2. These glands contain large poison sacs connected to the surface via ducts (Tyler, 1994). When threatened, the cane toad will excrete the poison to the surface of its skin (Tyler, 1994). The poison contains chemical substances including marinobufagin, dehydrobufotenine, adrenaline and noradrenaline. Research suggests the toxins from cane toads have more than one function (POI Australia, 2014). Firstly, they act as a persuasive deterrent to predators such as dogs, snakes and birds. Secondly, they may protect the cane toad against bacteria and fungi (Tyler, 1994).
The poison that is excreted from cane toads is extremely detrimental to many animal populations in Australia. It affects both marine and terrestrial animals. The poison mainly effects the predators of the cane toad, including snakes and native frogs. The poison, once consumed, sticks to the gums and tongue of the victim (Greencross Vets, 2018). The toxin is then rapidly absorbed across the membranes of the mouth (Greencross Vets, 2018). Symptoms depend on the amount of toxin absorbed and the length of exposure time. If the victim remains untreated for more than 15 minutes of exposure to the poison, they are likely to succumb to its affects and die (Greencross Vets, 2018).
Snakes are known to be one of the major predators of cane toads. It is estimated that 49 species of snakes are potentially impacted by the cane toad. The majority of these species are unable to withstand the toxin produced by cane toads (Phillips, 2004). A team of biologists from the University of Sydney began to experiment and record the effect that the cane toads had on the python species along the Queensland coast. The arrow in figure 3 represents the arrival of the cane toads. As usual, the distribution of snakes per kilometre initially increased, however then decreased due to the pythons dying from cane toad poison (L, Benjamin, 2011). Based on their data, which is summarised in figure 3, the researchers concluded that there was a significant decrease in abundance of the python species after the cane toads had arrived (L, Benjamin, 2011). As discussed, it is evident that the introduction of cane toads has had a detrimental effect on snake species. Therefore the local ecology of snake species is impacted negatively.
The main competitor of cane toads are the native Australia frogs. This is due to both species having relatively similar niches (Kearney, 2014). These species main niche ranges from sand dunes and coastal heath, to the margins of rainforests and mangroves (Kearney, 2014). However, they are most abundant in open clearings in urban areas, and in grassland and woodland areas (Kearney, 2014). The competition for food, habitats and resources in the ideal areas of the ecosystem has created problems for both species. Due to the cane toads broad food web and its poison, it has decimated the local ecology and biodiversity mainly where native frogs interact (Pash, 2014). This creates effects such as limited food supplies and resources for native frogs. This in turn forces native frogs to seek alternative habitats (Pash, 2014).
In a study conducted by Michael Crossland (1995), he discovered that many native frogs ate young cane toad tadpoles. This directly led to cane toad poisoning, resulting in many deaths for the native frog population in Australia (Crossland, 1995). The extent of this problem is significant due to the fecundity of cane toads is higher than native frogs (Shine, 2006).
Both snakes and native frogs play a significant and important role in the food chain. They are both secondary consumers. However, snakes are predators of native frogs, and without them the snake population would suffer as they are the primary food source, as seen in figure 4 (Roberts, 2015). Top order consumers such as birds would also be affected if the snake and native frog population decreased. This is because birds’ main prey is snakes and native frogs (Roberts, 2015). As discussed above, the cane toad is affecting both the snake and native frog population. Therefore, cane toads also affect the food chain because they create a knock on effect throughout the food chain as seen in figure 4.
From the case studies discussed above, there is likely to be far less biodiversity and local ecology in Australia due to the introduction of cane toads, unless solutions are implemented.
One possible solution to combat the issues associated with the increasing cane toad population is genetically modified organisms (biological control). The development of a virus capable of lethally infecting the tadpoles of cane toad is considered viable in terms of investment. It is suggested that the intent would be to allow the virus to be innocuous to toads and other wildlife, however it would carry cane toad?specific genes to irradicate the species (Hyatt, 2010). When tadpoles are infected with the virus, adult proteins encoded by the cane toad?specific gene would be expressed within the tadpoles, therefore inducing an autoimmune response to the protein (Hyatt, 2010). The survival rate of tadpoles would then be significantly diminished. The Australian Animal Health Laboratory demonstrated that this virus could kill up to 100% Bufo marinus tadpoles (Hyatt, 2010).
Another possible solution is to allow native Australian animals, such as snakes and frogs to evolve. Although this approach would take many years and many generations, it would effectively enhance the biodiversity and local ecology in Australian ecosystems. By allowing many predators to evolve and become immune to its poison, the cane toad population would diminish over time. The cane toad poison would not kill its predators, enabling the predators to control the cane toad population effectively without human assistance.
It is evident that the cane toad population is continuing to thrive in Australian ecosystems due to their detrimental effect on local ecology and biodiversity. It is concluded from the research conducted that the Australian Government must implement solutions, such as the two discussed above, to reverse and control the effect of the cane toad. If solutions are not implemented, the cane toad population will continue to thrive and adapt, resulting in their distribution increasing throughout mainland Australia. However, if these solutions are implemented, the local ecology and biodiversity in Australia will be enhanced by controlling the cane toad population. From the research conducted, it is proven that cane toads are in fact detrimental to, rather than enhancing the local ecology and biodiversity in Australia.