
5 minute read
ALIENS ON OUR DOORSTEP
Shining a light on the strange skin and big eyes of the world’s secret invaders
Invasive species pose a serious threat to the fragile ecosystems of our planet. From escaped pets to government-backed schemes, what may at first appear to be a minor mistake can have grave consequences in the future. There are two amphibian species that go head to head for the title of most impactful on our worlds ecosystems, the American bullfrog (Aquarana catesbeiana) and the cane toad (Rhinella marina).
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The story of the cane toad
One of the most famous cases of non-native species that continue to cause devastation to this day, is the cane toad (Rhinella marina). The species was introduced to several countries in the 1930s to control numbers of cane beetles which were eating farmers crops. With almost no understanding of genetics at that time, the choice to introduce these amphibians to new countries soon turned into a major problem.
Gabriel Cohen is a PhD student at the University of Zurich, who dedicated much of his MSc research at Sao Paulo University around invasive species. He explained: “Both bullfrog and cane toad introductions happened in Brazil and Australia at the same time, in 1935. The cane toad was introduced to cane plantations because they wanted a species from the amazon to control the cane beetle populations. The thinking was ‘it comes from a humid area, it’ll survive on the plantations, but they will die if they enter the desert, so the desert will be used as a natural fence.’ What they didn’t know was, based on the tree of life on the Rhinella genus, is that this lineage of toads actually originates from very dry regions in South America. They later colonised the Amazon.”
“They discovered that they have genes that allowed them to survive in very dry places, but these were just inactivated. When they were brought to Australia, these genes were selected back into the population. Now they have colonised much of Eastern and North Australia.”
In regions where the cane toad has spread, significant declines in reptiles and mammals have been reported. The northern quoll (Dasyurus hallacutus), yellow-spotted monitor (Varanus panoptes) and King brown snake (Pseudechis australis) populations have suffered drastically because of predating on the poisonous toads. Saltwater crocodile (Crocodylus porosus) populations have risen drastically with fewer monitor lizards to eat the eggs and various skink populations have dipped due to competition with the toads for prey items.
Bullfrogs and cane toads
Although cane toads are often the most ‘talked about’ invasive amphibian, there is a species which has already caused destruction in over 50 countries worldwide. The American bullfrog is considered the most widespread invasive amphibian on the planet and populations have even established themselves here, in the UK. Gabriel’s most recent paper aimed to genetically track the populations of American bullfrogs, to original populations in the USA. By tracking these paths of introduction, Gabriel hopes to identify which international trade routes are having the greatest impact and hopefully advise future imports and exports.
Gabriel told Exotics Keeper Magazine: “The bullfrog was sent to more than 50 countries because people wanted to breed them most commonly for the food market. It was sent to these places because they have economical value and that’s how they become so ‘invasive’. The cane toad on the other hand, has much more adaptive power which allowed it to colonise new areas. When it comes to ‘most invasive’, it depends how you define invasive species.”
Due to their enormous size, most frog meat we eat across the world has come from American bullfrogs. Whether these are in the form of frog legs in Europe or soups in South Asia, people are estimated to consumer over three billion bullfrogs each year.
“We are still eating bullfrogs” Explained Gabriel. “Some countries are still producing a lot, such as Brazil. Taiwan province is also one of the biggest producers of bullfrogs. In these areas they’re always being accidentally released. I’ve been to many breeders in Brazil and when frogs do escape, they don’t really care about it. We call these ‘feral populations’ and you see them in the places close to where they have escaped. In most cases, they don’t really colonise new areas, they stay close to the insects and small rodents where humans live.”
Many more people are aware of cane toads and their invasive capabilities as being a more considerable threat to wildlife than the bullfrog. However, this is largely due to their toxicity. In Australia, cane toads pose a serious threat to domestic pets so the general public are well-informed on the ecological damage that the species can cause. Although bullfrogs might not possess the same toxins that cane toads have, they do carry something much more dangerous.
Gabriel continued: “Bullfrogs are carriers of Bd or the Chytrid fungus as well as ranavirus. They usually don’t suffer from it, but they do carry it. We can liken this to superbacteria as a result of using too many antibiotics. Bullfrogs can bring more and more mixed strains of these diseases into the environment which native species can’t cope with.”
“The bullfrog is also ambitious with their feeding. They have been reported to eat swallows whilst they’re flying! They eat small mammals and even small turtles. They cannibalise on other bullfrogs too. Someone at my lab published a paper1 in 2012 after he had this hypothesis that because the bullfrog calls are so high-pitched, they were actually interfering with other species. So, they went to a place in Brazil where there was a population of native frogs with no bullfrogs and a second place where there were bullfrogs. They recorded the calls, then took the recordings to the new location and played it back. Eventually they concluded that the native frogs were actually changing the pitch of their calls, so they didn’t have to compete with the bullfrog calls. It can happen, with time, that this population becomes unable to recognize the original frequency of its species and that can influence speciation. This could eventually create an entirely new species if they cannot recognise the original population’s calls.”
While there are lists of potential damage that could be caused due to new bullfrog populations appearing across the globe, the ‘invasiveness’ of this species is almost entirely driven by human consumption and commercialisation. Whether the frogs have appeared as stowaways in groceries, bred deliberately for food, or sold commercially as tadpoles, human activity is clearly to blame.
Speaking on his upcoming scientific paper, Gabriel told Exotics Keeper Magazine: “The biggest focus in the research is to understand the paths of introduction. We wanted to know how many populations they are and how they are related to other countries. For example, one of the findings was that the population in Cuba are the same population found in China and most of Asia. Whereas they are very far removed from the populations found in Central and South America, even though geographically, they’re close.”
“One of the main reasons it’s important to know the genetic structure of these populations is so we understand the paths of the transactions we are making now. If one country decides it’s time to control an invasive population, they can identify where they are coming from. Also, even though it has never been tried with bullfrogs, if you know the genetic structure of populations you can help target eradication schemes. This is something that has been very successful with other species. If you know that one cluster has a lot of diversity, but another has very little diversity then you don’t want those clusters to cross ways. Diversity means they are more adaptable. Therefore, if you want to focus on eradicating a population you should focus on the ones that are less variable, so you have a higher chance of getting rid of them.”
Although there are lots of researchers working tirelessly across the globe to help control invasive amphibians, it does seem like an uphill battle. Amphibians are generally prolific breeders. Whilst attempts to remove spawn from waterways may be an effective eradication method in some cases, it is very clear to see how an already adaptable animal with hundreds of offspring could quickly repopulate an area. Hopefully, with greater scientific knowledge and understanding of the grave threat that chytrid poses, restoring our natural ecosystems will soon take precedence over our eating habits and commercial interests.
1. https://royalsocietypublishing.org/doi/ full/10.1098/rsbl.2012.0412