Flashback Feature: Aliens on our doorstep
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. Two amphibian species go head to head for the title of most impactful on our world's ecosystems, the American bullfrog (Aquarana catesbeiana) and the cane toad (Rhinella marina).
Cane toad (Rhinella marina)
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 the 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 to 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, that this lineage of toads 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.
A cane toad (Rhinella marina) pictured in Queensland, Australia
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 on 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 consume 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 is 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 paper[1] 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 them 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 the 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 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 less variable ones, 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.
American bullfrogs (Aquarana catesbeiana) in a frog farm
Invasive Species in the UK
Here, in the UK we also have many non-native species that have managed to form populations within our native ecosystems. We have a handful of populations of American bullfrog as well as various other carriers of the Chytrid fungus which could spell ecological disaster for our native wildlife. Unfortunately, without a tale of grave significance tied to the escaped pets that now call the UK home, much of the damage caused by these species is generally unrealised.
Jim Foster, Conservation Director at the Amphibian and Reptile Conservation Trust (ARC) told Exotics Keeper Magazine, “it’s probably fair to say the issue with non-native species generally isn’t well understood. There are certain exceptions to that, such as the grey squirrels and red squirrels but looking at other taxa, it is less well-understood that non-native species can have a negative impact on native wildlife. However, one thing we’ve noticed is when you do explain what the issues are, people do pick it up and they do really care.”
“Where we have seen an improvement in the media treatment in the last few years is the reporting of disease. For example, chytridiomycosis and ranavirus have been covered by the media and I think people know that ranavirus is a threat that’s facing frogs in Britain.”
Of course, the ecological impact of invasive species varies on a case-by-case basis. Whilst Jim suggests that media coverage of non-natives has increased, this does not mean that public understanding will naturally grow alongside it. For many residents in the UK, identifying between a smooth newt (Lissotioton vulgaris) and an alpine newt (Ichythosaura alpestris) or a common frog (Rana temporaria) and a marsh frog (Pelophylax ridibundus) would prove extremely difficult.
Equally, there are instances where a species that has gone extinct in the UK can be reintroduced. ARC’s recent work with the pool frog (Pelophylax lessonae) is proving to be extremely successful. After this species went extinct in the UK, identifying whether this was a native species created another difficult task.
Jim continued: “Defining what constitutes a ‘native species’ is sometimes quite difficult. The convention generally is that if presence has been confirmed since the last ice age, it is counted as native. Some people argue we should go back further to the Pleistocene era but that seems to be quite a marginal view at the moment.”
“The pool frog is a really good example where that’s been looked into in great depth. A whole range of research went into establishing that the pool frog was native. Using analysis of its call, the male vocalisations in spring, the genetics and so on. It takes a lot of effort to go to that extent, but it was absolutely necessary in the case of the pool frog. There were examples of pool frogs and edible frogs which were known to have been introduced to Britain, but then there were some populations that were considered to be native. The research confirmed beyond all reasonable doubt that there were some native populations.”
“With other species, for example the moor frog, agile frog and European pond turtle they are natives that have gone. Whether we should reintroduce those natives is a BIG question. You need to look at whether it’s appropriate to do so and whether it’s technically feasible to do so and I think that’s where our concern is with recent publicity. Those issues haven’t really been addressed, people think we can just breed them up and release them, but you need to start way before that point. It’s good to see people caring so much about these lost species but putting that theory into practice takes a long time to get right, as we found with pool frogs.”
A pool frog (Pelophylax lessonae)
Bullfrogs in the UK
Bullfrogs have also made their way over to the UK and have brought with them potentially very harmful pathogens. Jim Foster, Conservation Director at ARC explained: “Since the year 2000, there have been three confirmed breeding populations of North American Bullfrogs in England. All of those have been subject to eradication efforts, which is encouraging that the authorities have acted to try to remove those populations and broadly, with some success. Particularly our concern in Britain is disease transmission and the first time Batrachochytrium dendrobatidis (Bd), one of the funguses that causes Chytrid was found in Britain was in a population of North American Bullfrogs on the Kent/Sussex border. Now, we can’t prove for sure that the bullfrogs introduced the pathogens there. It could be that the pathogen was already present in the wild but we do know in other parts of the world that they are a harbinger for Bd.”
“It’s import to Europe now has been banned through legislation. Back in the 90s and early 00s it was still traded and 5-6inch long tadpoles could be bought at garden centres and a year or two later these bullfrogs would emerge. They would mostly die but on rare occasions they could establish populations.”
Although it appears that Bd is spreading much slower in Europe, there are two potential ways of looking at it. Firstly and perhaps more optimistically, the cooler temperatures might be combatting some pathogens and slowing down the spread of the disease. Alternatively, it might be that higher biodiversity in the tropics and neotropics has accelerated the spread of the disease and herpetologists are therefore more aware of species decline.
An American bullfrog tadpole (Aquarana catesbeiana)
Captive breeding programmes and correct regulation
Areas of the world that are biologically rich, whilst hosting a high human population are often impacted more so by irresponsible pet ownership. For example, Florida is now home to over 500 non-native plant and animal species. Green iguanas (Iguana iguana), Burmese pythons (Python bivvitatus) and Argentine tegus (Salvator merianae) have now established themselves in the Everglades after escaping from the pet owners. Lionfish (Pteros volitans) imported from the Pacific have established themselves throughout the Caribbean and just this year have crossed the Amazon Basin into Brazil, where they are feeding on native fish without being predated upon due to their venomous spines. Cuban tree frogs (Osteopilus septentrionalis) hitched a ride on imported goods in the 1920s and now feed on native tree frogs.
Here in the UK, our climate does not allow for many of the most popular exotic pets to survive for long in the wild but that does not mean that we can be relaxed about our biosecurity measures in our own homes.
Jim continued: “I wouldn’t categorise the issue as irresponsible pet ownership. I do know of some deliberate releases which of course are irresponsible, but the issue is much more nuanced. We see a huge value in people keeping reptiles and amphibians, the trade of these just needs to be done properly. Animals can escape and of course, there’s an argument there to ensure vivariums are secure to prevent escape but also to prevent incursions. If you’re a zoo or an established collection you probably have much more resources to ensure you have secure vivaria. We have regular reports that exotics have escaped and for many of those that are kept indoors, they won’t last very long if they get out. Now, that’s a welfare concern but the potential to establish a population might be limited.”
For ARC, the major concern is to prevent the spread of disease. Whilst reptile and amphibian enthusiasts might be the first ones out of their seats to jump on board with conservation efforts and actively seek out wild herptiles, they are also more likely to be exposed to diseases harboured in captive collections.
Jim continued: “What native species need generally is more habitat, better habitat and the threats to their populations removed, rather than breeding up and releasing more animals. Breeding up and releasing animals is useful in some circumstances. For example, we have a captive breeding project for sand lizards. However, it’s a carefully managed programme, we only do a few releases each year and we work closely with the landowners to make sure the conditions are correct.”
“One thing that pet owners can do is just spread the word. Amphibian and reptile owners are very enthusiastic and just explaining how fantastic it is to see common frogs or common toads in their breeding ponds is great. Encouraging people to go out and look at these wonderful wildlife spectacles is a brilliant way to support them. If people specifically want to get involved in the conservation of native species, we offer opportunities for people to get directly involved through our projects. People can also help with monitoring efforts, that’s useful and it’s something easy for people to get involved with. If people want to go a step further, they can help protect areas that might be under threat by writing to planning authorities. Sadly, that’s still a major issue for many of our species which are threatened by development.”
Lion fish (Pterois sp.)
Our role
There is a whole myriad of ways in which animals can arrive in places that are unsuitable for them. Whether it is a deliberate introduction to tackle an existing problem such as the cane toads, or sheer negligence within a commercial trade like the bullfrogs, understanding the value and potential risks involved with moving live animals is crucial to preventing further damage. As exotics keepers, we play a key role in education, inspiration, and conservation in herpetoculture. Whether it’s a first-time reptile owner showing their friends and family a beloved leopard gecko or a lecturer from one of the world's top zoos, we all have a part to play in the spreading of positive and beneficial information. Ensuring that we keep our animals responsibly and teach others to do the same, is crucial to inspiring another generation of conservationists to help the global efforts to save the world's ecosystems.
A bucket of cane toads collected in Australia to be destroyed