Photograph courtesy of wiki commons
By Jenna Gersie
In the early twentieth century, sugar cane farmers in Queensland, Australia, found that the Frenchi beetle (Lepidiota frenchi) and the grey-backed cane beetle (Dermolepida albohirtum) were destroying their sugar cane crops. In search of a biological way to manage these pests, Reginald Mungomery, an entomologist who worked for the Queensland Bureau of Sugar Experiment Stations, was sent to Hawaii in 1935 to collect cane toads (Rhinella marina), which are actually native to South and Central America. Having heard successful reports of increased sugar cane production with the introduction of cane toads in Hawaii, Mungomery, in perhaps one of the worst ecological mistakes of the past century, imported 101 of these anurans to Australia.
The 101 cane toads—51 females and 50 males (one male died in transit)—were kept briefly in captivity to breed. In August 1935, 2,400 cane toads were released into the Little Mulgrave River and surrounding sites in Gordonvale, Queensland. Unfortunately, the introduced cane toads had no actual effect on controlling the beetle populations; the beetles prefer to remain at the tops of the stalks of sugar cane, much higher than a cane toad can jump.
Cane toad populations began to grow immediately upon their release in Australia. From the 1940s to the 1960s, cane toad populations began to expand at about 10 kilometers per year, but now, cane toad populations are expanding south and west at 50-60 kilometers per year. They have traveled through the Northern Territory and crossed the border into Western Australia, and they have expanded southward out of Queensland to New South Wales.
Cane toads have parotoid glands on each shoulder that squirt out poison when the cane toad is threatened or handled roughly. At each stage of life—egg, tadpole, toadlet, and adult—cane toads are toxic. As a result, there is no known predator of cane toads in Australia, and animals like crocodiles, goannas, tiger snakes, dingoes, and quolls are killed by the cane toad’s poison. Cane toads, therefore, are not only out of control in their population growth, but they are also a threat to native Australian wildlife, both because they utilize resources that other animals need and because their poison kills any unfortunate animal that tries to eat them. The many problems that this invasive pest poses are clear.
The original release site of cane toads in Gordonvale is less than fifty kilometers from where I now live in the rainforest near Yungaburra. Therefore, it is not surprising that when I walk back home through the forest each night, numerous cane toads, large and small, sit in the pathway and hop away as I approach.
Dr. Sigrid Heise-Pavlov, Professor of Rainforest Ecology at the School for Field Studies, Centre for Rainforest Studies, researches cane toad pest management with her American university students. After dinner, once it is dark and the resident bandicoot (affectionately known as “Boris”) is cleaning up any scraps of food that have fallen on the ground, Siggy and the students don their gumboots, raincoats, and headlamps, and head out into the forest to hunt for cane toads. The cane toads are collected in a bucket, euthanized according to methods outlined in Siggy’s research permit, and dissected. Siggy is particularly interested in the stomachs and lungs of these cane toads.
In her paper “Effect of Rhabdias pseudosphaerocephala on prey consumption of free-ranging cane toads (Rhinella marina) during Australian tropical wet seasons,” Siggy and co-authors Karena Paleologo and William Glenny, former School for Field Studies students, analyze the presence and potential impact of a species of lung nematode in cane toads. R. pseudosphaerocephala is a species of lung nematode that is specific to cane toads; it has no effect on native anurans. Cane toads came to Australia with this lung nematode and it can potentially be used as yet another biological control agent. Laboratory experiments executed elsewhere in Australia have shown that this lung nematode reduces growth rates and survival in metamorph cane toads, potentially due to reduction in prey consumption and/or parasite-induced anorexia. Additionally, the impaired lung function as a result of the parasite’s presence affects the aerobic activity of cane toads needed to access rich foraging sites. Reduced prey consumption or certain prey selection could therefore be a consequence of infection of cane toads by the lung nematode. This means that there is more food available for native frogs, and this is what Siggy and her students are most interested in.
Siggy and her students aimed to discover the changes in prey consumption of free-ranging cane toads infected by the lung nematode. After dissecting the cane toads, the Arthropods (invertebrates with exoskeletons, including insects and arachnids) and Non-arthropods in the stomachs of the toads were identified. The number of lungworms present in each toad was also counted. The researchers found that 81 percent of the collected toads were infected with the lung nematode. They observed that the presence of the lung nematodes did not affect the quantity of prey items consumed, but they did find that increased infection intensity resulted in decreased prey item diversity. Therefore, infection may indeed reduce the ability of cane toads to access rich foraging sites, leaving more tasty ants and beetles for the frogs that are supposed to live and thrive in Australia.
Of course, more studies must be done to determine ways to reduce the impact of cane toads on native Australian wildlife. But for now, let’s hope they’re leaving enough food behind for the native frogs, like these orange-eyed tree frogs that I saw last night!
Photograph by Jenna Gersie