Invasion of the Cane Toad: How Fast Can They Conquer?
By Danielle Myers
It might be nippy outside, but the cane toad does not mind. Looking to expand its reign, it has turned to colder climates in Australia. This invasive species might be able to move further than once thought.
First, what is an invasive species?
An invasive species is a non-native or exotic species that has established itself and has begun damaging that environment. They can be very damaging to environments and can hurt species diversity. Besides the cane toad, a few well known invasive species include the European Starling (Sturnus vulgaris) and the Wild Boar (Sus scrofa) which have established themselves throughout North America and impact many native species.
There are many factors that influence how invasive species spread through different environments including reproduction rate, temperature limits, and environmental conditions. However, these factors often do not include the possibility of varying and flexible traits, meaning different populations of a species might be able to survive in varying conditions which is an important factor in determining how invasive species spread. Dr. Samantha McCann and her colleagues aimed to discover if flexibility in temperature factors into the spread of cane toads in Australia.
The Cane Toad: A Brief History
The large and highly invasive cane toad (Rhinella marina) has conquered over 40 countries in the world and could be looking to add more to the list. Originally from Central and South America, it was introduced to Northeastern Australia, which has a similar climate to its native habitat, as pest control for crops, but it has spread across the country to cooler parts over the last 80 years. In Australia, cane toads are responsible for a high number of large predator deaths due to being toxic. Because the toads were introduced, the predators have not adapted to avoid the toads or become immune to the toxicity (Shrine, 2010). The cane toads in Australia can very rapidly adjust to cooler climates by acclimating, meaning adjusting to different environmental conditions, to the lower temperature ranges.
But is acclimating to cooler temperatures unique to Australian cane toads?
In this study, scientists collected cane toads from different parts of Australia and the Big Island of Hawai’i and subjected some of them to cool temperatures. They selected cane toads from Hawai’i as a comparison because the toads were there for approximately the same time as in Australia and they were where the cane toads in Australia were brought from. In addition, the temperature and rainfall ranges are very large in both places.
To test it, they put half of the collected toads in the refrigerator and half were left at room temperature overnight. This is where the toads would become acclimated to either a cooler or warmer temperature. The short period of time in the refrigerator or room allowed the scientists to demonstrate whether or not the toads can quickly deal with the new temperature. Then all the toads were cooled in a container, and they were periodically tested for their righting reflex. The righting reflex is whether or not they can get upright after being flipped on their back. If they were unable to, then they would not be able to live in that cold of an environment because they would not be able to do basic things like move or eat. The temperature that they were at when they failed the righting reflex test was called their critical thermal minimum, meaning it was the lowest temperature they could survive in.
Overall, the study found that in the cane toads from Australia there was a rapid acclimation to cooler temperatures in toads from the cooler and wetter climates, but toads from warmer and drier climates could not adjust to cooler temperatures. However, one of the cool sites in Hawai’i did not see the rapid acclimation like another similar site did, leading the scientists to believe that the difference is due to the varying rainfall at the site locations. The site that did not see the rapid acclimation may be due to drier soil causing the forest floor to be slightly warmer than the other site, accounting for the difference in results between sites.
While the scientists cannot say whether the rapid acclimation in the cane toads from Australia is due to a genetic change leading to adaptation of toads in a particular region or the ability for the toads to live in colder environments from long-term exposure, they can say that their ability to live in colder areas may increase their success of invasion.
Why is all this important?
It is important to know how far the cane toads can spread in order to manage their population and estimate how much impact they might have. With the information in this study, conservationists can predict where and how far the cane toad can move across Australia and in other countries with similar climates where it has invaded. Then, they can try to prevent them from spreading even farther. By preventing their spread, many large predators’ lives may be saved.
Similar studies can also be performed and applied to other invasive species to estimate how far they can spread. If scientists know how invasive species can become invasive, it can make it easier to stop and prevent in the future. There are many factors that cause a species to become invasive, and temperature and climate acclimation may be one of the keys to figuring out new potentially invasive species.
Works Cited
Primary Reference
McCann, SM., Kosmala GK, Greenlees MJ, Shine R (2018). Physiological plasticity in a successful invader: rapid acclimation to cold occurs only in cool-climate populations of cane toads (Rhinella marina). Conservation Physiology 6(1): cox072
Supporting Reference
Richard Shine (2010). The Ecological Impact of Invasive Cane Toads (Bufo Marinus) in Australia. The Quarterly Review of Biology 85, no.3: 253–291.
