WHAT EFFECTS ARE CANE TOADS PRODUCING TOWARDS THE SUSTAINABILTY OF ECOSYSTEMS AND THE SURVIVAL OF NATIVE SPECIES

Chelcy Jordan
SUST 1000
October 10 2012

The introduction of invasive species can often lead to unpredictable alterations and a shift within natural ecosystems. Cane toads were originally introduced to Australia in 1935. They were brought into Australia to prey on specific pests that were attacking sugar cane plants. Cane toads are highly adaptive and can easily adjust to climate changes and feeding grounds. The growth of reproduction in cane toads has formed a species invasion on the habitat of native animals. Immense competition and a shift in ecosystems are a result of their significant growth rate. What effects are cane toads producing towards the sustainability of ecosystems and the survival of native species? Many animals, and plant species have been either finding new ways to adjust to changes caused by the introduction of cane toads. Cane toads have impacted natural ecosystems so drastically that they have left a strenuous effect on native species ability to maintain their own survival. This is crucial when looking at the effects cane toads have on sustaining plant and animal life within ecosystems. Dramatic changes in an ecosystem could potentially fail due to inadequate nutrients and energy distribution. It is important to examine ways in which cane toads affect ecosystems so environmentalists can predict potential negative shifts and try to develop methods of control that decrease this invasive destruction.
Ruchira Somaweera, Jonathan K. Webb, Gregory P. Brown,
& Richard Shine (2007) state, “In practice, scarce resources for conservation need to be allocated to control the invasive species most likely to have major ecological impacts; taxa that have minimal or transitory ecological impact warrant less investment. Accordingly, predicting the nature, magnitude and duration of ecological impact of an invasive taxon is a high priority for wildlife managers.”
The survival of native species is necessary, especially in Australia. Australia accounts for about 75 percent of the world’s biodiversity. It is estimated to be home to over a million native plant and animal species, some of which are not found anywhere else in the world (The Environmental law fact sheet, 2001). The Environmental law fact sheet (2001) argues that the vitality of the earth is reflected in the diversity of its inhabitants. The fact that we have so many species in trouble is a sign that we have an unbalanced environment. Cane toads pose a major threat to the sustainability of ecosystems and have decreased the chance of survival for many native species. The growth in the cane toad population has led to an increase in their overall rates of consumption. This has had a declining affect on the invertebrate taxa population. Cane toads have left several native species vulnerable to ingesting deadly toxins and have also caused a shift in the behavioral attitudes of other native species due to immense competition.
The consumption habits of cane toads on invertebrate taxa have a negative effect on plant communities and cause a shift in long-term effects of vegetation. Matthew J. Greenless, Gregory P. Brown, Jonathan K. Webb, Benjamin L. Phillips & Richard Shine (2007) mention that “Cane toads have voracious appetites, with individual toads consuming up to 200 invertebrates in a single meal, and these animals can attain population densities in excess of 2000 individuals per hectare. Plausibly, then, toads may deplete invertebrate biomass and biodiversity and thus negatively impact any taxon (e.g. of frogs, lizards, birds or mammals) that feeds on ground- dwelling invertebrates.” As the population of cane toads continues to increase so do their levels of food consumption. Cane toads generally eat ants, beetles and termites. During the dry season many cane toads like to relocate towards water holes where native insects live. These invertebrate taxa have not yet evolved to maintain their survival against the mass reproduction of cane toads. Richard Shine (2006) explains, “The native invertebrate fauna may lack a history of evolutionary exposure to anuran predation during the dry season.” Because cane toads must consume a substantial amount of prey to maintain their massive rate of reproduction, they have led to a huge decline in the diversity of invertebrate taxa. This is an issue because the potential decline of seed-harvesting ants could cause a shift in plant community dynamics, which leads to long-term vegetation changes. The role of seed-harvesting ants is crucial for the growth Australia’s areas of agriculture. Seed-harvesting ants are responsible for taking seeds resting on the topsoil underground. These ants aid in the growth of crops, and if they were to continue to decline, this may reduce levels of crop production. The effects of cane toads and their appetite on ecosystems are anything but positive. They are making it harder for native invertebrate taxa to survive. Because of this, cane toads are causing a major loss of plant species and other animals that feed on invertebrate taxa as well.
The invasion of cane toads into the wildlife of Australia has left native animals sensitive to death by poison. Cane toads have glands that secrete toxic material when they are tampered with. Cane toads are toxic during every stage of their development; from the time they are eggs right up until adulthood. Richard Shine (2006) notes that because their toxins are highest in eggs they are considerably dangerous for vertebrate predators. Native species that naturally prey upon amphibians are not adapted to the poisons released by cane toads, which leads to increased mortality rates. Matthew J. et al. (2007) agree that toxins present in the skin of toads (and especially in the parotid glands) kill many animals that attempt to eat these invasive anurans. One animal that has been widely affected by the invasion of the cane toad is the fresh water crocodile. Australian fresh water crocodiles are used to feeding on amphibious creatures but are unable to adapt to the poisonous secretions that are dispersed from the cane toad. There have been extensive cases of crocodiles being seen eating cane toads, even though their bodies are not immune to their toxins. The invasion of cane toads has allowed for fresh water crocodiles to become very vulnerable and studies show that many of them are dying off because of these specific amphibians. The mortality rates of crocodiles due to cane toads vary from region to region, and some cases have been about as high as 77% (Ruchira Somaweera, Jonathan K. Webb, Gregory P. Brown
& Richard Shine, 2006).
Figure 1. Crocodile density. Adapted from “Invasive canetoads (Bufo marinus) cause mass mortality of freshwater crocodiles (Crocodylus johnstoni) in tropical Australia,” by Webb, J. K., & Shine, R., 2006, Biological Conservation, 141, copyright 2006 by University of Sydney. Adapted with permission.
What figure 1 shows us is the decline in density of crocodiles over time in specific regions, for both hatchlings and non-hatchlings. Figure 1 suggests that there is a decline of crocodiles in every region. Jonathan K. Webb and Richard Shine (2006) express that even though there has been a ongoing decline of crocodiles before the introduction of cane toads, crocodile density has been significantly declining in regions where the cane toad was introduced first. This suggests further that cane toads are negatively having a dramatic effect on the mortality rates of crocodiles. The deadly effects that cane toads have on crocodiles are a prime example of how they pose a major threat to the sustainability and survival of other native species within a set ecosystem. Lastly, cane toads provide competition between other terrestrial vertebrates. Cane toads may cause other vertebrate, such as lizards, birds and other native frog species to compete for habitat and food. The dry-season plays a huge part in the competition for food between cane toads and other vertebrates. During times where water holes are low animals re-locate in order to find more pools, causing large swarms of diverse species to come together. This is an issue because food becomes limited, and species battle to consume the necessary amount of prey in order to maintain their survival. Richard Shine (2006) suggests that food is not the main causal of competition between cane toads and other vertebrates. He talks about how their presence itself has reduced normal activity in other vertebrate. The presence of cane toads has caused a disruption in the reproduction of other native frog species. There is some sort of confusion that exists between species when trying to mate. Female and male frogs may attempt to mate with frogs of different species, but because they are not used to the way in which each other mate this can often lead to “fitness costs” (Shine, 2006), which includes drowning. Other frog species may not be the only native species affected by the competition of cane toads. Cane toads might occupy the nesting burrows of bee-eaters. It was reported that 33% of all nesting attempts were ‘lost’ due to cane toads (via either nest predation or nest upsurpation), and that average chick production per nest fell from about 1.2 to 0.8 after toad invasion (Shine, 2006). Native animals are forced to search for new habitat locations. The competition of cane toads has caused an enormous disruption within Australian ecosystems, leaving other native animals incapable of proper reproduction and of obtaining food.
Cane toads pose a major threat to the sustainability of ecosystems and have led to a decrease in the chance of survival for many native species. The population of cane toads is rapidly increasing throughout various regions of Australia. This is an issue because many of Australia’s species account for a large percentage of diversity in the world. An upset to their natural ecosystem can have a significant impact on the agricultural and overall social well being of humans. The introduction of cane toads to Australia has brought about many concerns. Cane toads are eating large sums of invertebrate and aiding in the loss of their diversity. By taking away from the diversity of invertebrate they are extracting plant species basic forms of nutrients. Not only have cane toads led to a decline in invertebrate species, but they have also led to high mortality rates in preying vertebrate. Some vertebrate that originally fed on certain amphibians are having a hard time evolving the necessary immune system required to fight off their toxins. Finally, cane toads impose competition upon native species. They take over their natural habitats and extensively consume their food. Other vertebrates are forced out of the natural conditions in which they need to carry out their normative behavior. Competition of cane toads constrains the functions of native species, and in the process leave a damaging affect on the progress of an ecosystem. There is much research being conducted on the introduction of the cane toads. Scientists are trying to find ways in which they can decrease their population rates and control their effects on other native species. Michael R. Crosslands, Gregory P. Browna, Marion Anstis, Catherine M. Shilton, & Richard Shine (2006) believe, “despite these obstacles, clarifying the ecological impact of invasive organisms is critical to identifying priority habitats and species for conservation, as well as for predicting the longer-term consequences of the invader’s arrival.” Cane toads are invasive species that are dramatically disturbing the balance of ecosystems in Australia and aiding in the loss of native species, but as the structure of cane toads becomes clearer scientists hope to find a way to maintain their extreme rates of reproduction.

References
Crossland, M. R., Brown, G. P., Anstis, M., Shilton, C. M., & Shine, R. (2006). Mass mortality of native anuran tadpoles in tropical Australia due to the invasive cane toad (Bufo marinus). Biological Conservation, 141, 2387-2394.
Environmental law fact sheet. (n.d.). Protection of Native Animals and their Habitat. Retrieved October 30, 2012, from www.edo.org.au/edosa/files/fs11_native_animals_300611.pdf
Greenlees, M., Brown, G. P., Webb, J. K., Phillips, B. L., & Shine, R. (2007). Do invasive cane toads (Chaunus marinus) compete with Australian frogs (Cyclorana australis)?. Austral Ecology, 32, 900-907. Retrieved October 10, 2012, from the Ebsco database.
Pizzato, L., & Shine, R. (2011). You are what you eat: Parasite transfer in cannibalistic cane toads. Herpetogica, 67(2), 118-123. Retrieved October 10, 2012, from the Ebsco database.
Shine, R. (2006). The Ecological Impact of Invasive Cane Toads (Bufo Marinus) in Australia. Chicago Journals, 85(3), 253-291. Retrieved October 10, 2012, from the JSTOR database.
Somaweera, R., Webb, J. K., Brown, G. P., & Shine, R. (2006). Hatchling Australian freshwater crocodiles rapidly learn to avoid toxic invasive cane toads. School of Biological Sciences, 1, 502-517. Retrieved October 10, 2012, from the Ebsco database.
Webb, J. K., & Shine, R. (2006). Invasive canetoads (Bufo marinus) cause mass mortality of freshwater crocodiles (Crocodylus johnstoni) in tropical Australia. Biological Conservation, 141(7), 1773-1782. Retrieved October 30, 2012, from http://www.sciencedirect.com/science/article/pii/S0006320708001511