A BioInvasion Tale from Down Under: the Cane Toads of Australia
February 9, 2022
A BLOG POST FROM THE SCIENCE, POLICY, AND MANAGEMENT DISCUSSION GROUP
AN INITIATIVE OF THE GCC INVASIVE SPECIES WORKING GROUP
BY CAMERON BRASWELL
Dr. Daryl Trumbo, a research scientist in GCC Faculty Affiliate Dr. Meryl Mims’ lab, shared a tantalizing lecture of the cane toad invasion of Australia with the Biological Invasions Discussion Group. Although I’ve heard of cane toads before, I never quite knew just how invasive they are. In fact, through Dr. Trumbo’s discussion along with additional media, it was hard to believe how detrimental of an effect this species has on Australia’s natural habitat.
First introduced in the 1930s, cane toads were supposed to control the greyback sugarcane beetle population. At the time, sugar cane was a huge agricultural commodity that was initally transported from South America to Australia by the First Fleet in 1788. Australia's native sugarcane beetle began colonizing the fields and devouring these crops. In response, the Australian government desperately searched for a solution. Based on a similar invasion of sugar cane beetles in Hawaii, cane toads became their first choice. On paper, cane toads didn’t seem like a bad option. With their ravenous hunger for bugs, the Australian government shipped a mere one-hundred cane toads to the western coast of Australia. There was one big problem though: the sugarcane beetle population didn’t change, and the cane toad numbers exploded! One hundred multiplied to one thousand, a thousand rocketed to fifty thousand, and so on. Their initial purpose was to help control the sugarcane beetle population. However, these insects regularly lived too high on the sugarcane for the toad to reach them, let alone predate them. Instead, the cane toads laid waste to the native terrestrial invertebrates, which increased their breeding rate. Pretty bad, right?
Oh! I forgot to mention – cane toads, as eggs and through development into the adult stage, are severely poisonous when consumed. Their skin glands contain bufotoxin, a moderately potent poison secreted in the skin of many anuran amphibians. When consumed, it causes sodium ions to aggregate within the cells, while pushing potassium ions into the extracellular matrix. Mostly affecting muscle groups, this causes arrhythmia that leads to eventual cardiac arrest. While this is happening internally, the muscles throughout the body begin tensing up until paralysis overcomes the would-be predator. A mere bite of a cane toad egg, tadpole, or adult is overwhelmingly fatal. Lack of predation during juvenile stages due to bufotoxins only fortifies their rapid population growth and size. Cane toad predators - crocodiles, fish, and some mammals - have experienced drastic population decline since the cane toad introduction to Australia. For instance, an approximate 77% decline in freshwater crocodile populations was observed on the Victoria River (located on the Northern Coast). This is one of the most recent freshwater ecosystems within the expanding cane toad range.
Biodiversity decline has been tracked through every Australian region that cane toads inhabit, and they’re spreading quickly! A concept termed the Olympic Village Effect may explain why these organisms are spreading as fast as they are. When scientists compared preserved cane toads from the 1960s to now, they saw around a 25% increase in leg length. Not only is this an obvious sign of evolution, but this also means they are traveling longer distances in a shorter time frame. The so-called Olympic Village Effect refers to the fact that toads with longer legs and higher fitness will breed with other toads of the same stature, due to the distance they’ve traveled. Therefore, they produce an overall brood with increasingly fit genotypic and phenotypic traits.
Physical characteristics aren’t the only thing scientists are comparing from past lineages though. Beyond leg length, genome tracking and analysis have been a major focus of academic institutions within Australia. Genotypic similarities, as well as familial branching, have been studied for decades in an attempt to better understand the rapid evolution over the mere hundred years that cane toads have inhabited Australia. Dr. Trumbo went into detail breaking down analytical techniques and single nucleotide polymorphisms (SNP) tracking across different genomes! Better yet, he unveiled a potential genetic-oriented solution to potentially solve this issue. Unlike humans, amphibians’ sex chromosomes are the same in males (ZZ), while females have one copy of two different chromosomes (ZW). Enzymes named “Y cutter proteins” can be used to shred the W chromosome in females. The long-term goal of introducing this enzyme is to decrease the number of female eggs that are fertilized. In fact, we should see a massive increase in male cane population, and the exact opposite for females.
Cane toads have made a giant impact on Australia’s biodiversity and ecosystems. Furthermore, they’ve created such a problem for the surrounding population, they’re highly publicized! Movies, podcasts, shows, even cartoons tell not only the story of the cane toad but also attempt to reinforce the negative implication this species has on the habitat. As I said before, I had heard of cane toads before but had never been exposed to the incredible damage they’ve inflicted. Dr. Trumbo did an amazing job articulating the fascinating, funny, and disappointing reality of a toad that will forever be remembered in Australia.