Australia and frogs have a long and successful history. Over 200 species of frog currently call Australia home and the fossil records show that the association has persisted through 45 million years of extreme climatic change and environmental stress. In the 1980s, however, it was noticed that frog numbers were undergoing a severe decline, some species even slipping into extinction. This phenomenon was not restricted to Australia and world-wide frog populations seemed to be in rapid decline. Today, frog numbers continue to dwindle despite extensive research efforts aimed at identifying and remedying the causes. Explanations for the decline have included habitat destruction, pollution, pesticide use, competition with introduced species and increased ultra-violet radiation. Another possible explanation, so far strongly supported by the evidence, is infectious disease and it appears that species of fungi and virus have contributed to frog mortality world-wide. In Australia another frog pathogen, a small microscopic parasite related to jellyfish, has also been discovered infecting the livers and brains of native frog populations. Wide-spread infection of native frogs with this parasite seems to be a recent development – the first recorded case occurring less than 40 years ago. Interestingly, this time frame suggests a link with the introduction of the cane toad and its explosive migration into a vast area of northern Australia. The coincidental occurrence of the parasite and the cane toad has led to the assumption that the parasite was co-introduced to Australia with the cane toad and subsequently spread to native frogs. This assumption has recently been put to the test by Dr. Jan Slapeta and colleagues from the University of Sydney. In a study, recently reported in PLoS One, Dr. Slapeta and his team show that the cane toad did not in fact introduce the parasite to Australian native frog populations, but, far from being innocent, they suggest that the mere presence of the cane toad in Australia has increased the prevalence of a native parasite in native frogs via an ecological process known as ‘spill-back’.
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| Scanning electron microscopy of myxospores recovered from the gall bladder of the Striped Marsh frog (Limnodynastes peronii). Myxospores are ellipsoidal, shell valves have ridges and suture line cross-sectioning the spore. Scale bar, 5 µm. Figure from Hartigan et al. 2011 (doi:10.1371/journal.pone.0018871.g004) |
It is not difficult to cast the cane toad as a villain, notoriously ugly, toxic and the bane of furrier and cuddlier native animals, the cane toad is now infamous in Australian history as the archetypal example of ecological meddling gone wrong. First introduced to Australia in 1935 by the Bureau of Sugar Experiment Stations in an effort to control the sugar cane beetle that threatened local cane production, cane toads now have an enormous range over the north of Australia. Initially only one hundred and two juvenile toads, imported from Hawaii but native to Brazil, were released into areas surrounding Cairns, Innisfail and Gordonvale in the far north of Queensland. But, without bothering the local cane beetle population, these cane toads immediately began breeding and hopping. Cane toads can now be found anywhere between Brisbane in the south to Darwin in the north, a range encompassing approximately 1.2 million square kilometers. Remarkably, it is estimated that the toad front now moves 40-60 km per year and that a single toad can travel more than a kilometre in a single night. Although there is no documented instance of a native species, including frogs, being driven into extinction by the cane toad, native predators such as quolls, several species of snakes, and goannas have suffered population declines from deaths caused by predation on the toxic cane toads.
In some respects the toad invasion is good for the native frog population as frog predators will also hunt toads, resulting in less frog predators. However, like the Europeans introducing smallpox to the Americas, the co-introduction of an exotic pathogen with a introduced species has the potential to devastate a native population that is susceptible to, but defenceless against, the pathogen. In this case the suspect is a microscopic parasite of aquatic animals known as the Myxosporea, organisms from the same phylum as the jellyfish (Cnidarians). These parasites have a complex lifecycle, usually involving two hosts, a frog or fish and an annelid worm or Bryozoan (a phylum of small aquatic filter feeders). Although often not producing serious disease in their hosts some species do suffer frightful pathology as a result of infection. For instance ‘whirling disease’ has devastated parts of the north American farmed fish industry. In salmonid fish in particular, the parasite destroys the ability to swim, resulting in an awkward corkscrew-like forward motion, or ‘whirling’. The condition devastates fish populations by massively increasing mortality in infected juveniles. In frogs the parasite can cause kidney disease, and in native Australian fish they have been found infecting the liver and brain.
If native frogs were infected with a new parasite after the introduction of the cane toad then it is obvious that this could be contributing to their decline. So the questions posed by Dr. Slapeta and colleagues was were the Myxosporean parasites introduced into Australia by the cane toad? and, if not, what could explain the increased occurrence of these parasites in native frogs? To answer this question they first collected DNA from Myxosporean parasites infecting Australian frogs and cane toads from Australia, Hawaii and Brazil. Using several genetic markers the team was then able to compare the DNA of parasites from the different sources and establish if they were similar enough to have come from the same species. The findings showed conclusively that the parasite infecting frogs and Australian cane toads was not the same that infected Brazilian cane toads. This strongly suggested that the parasite infecting native frogs was not introduced with the cane toad, a conclusion reinforced by the finding that no Hawaiian cane toads were infected with a Myxosporean parasite. Given that the toads originally introduced to Australia were from Hawaii it therefore seems very unlikely that the parasite was co-introduced.
If the cane toads did not bring the parasites with them why was there a coincidental increase in native infections after the cane toad arrived in Australia? The answer to this question may lay in a ecological effect known as ‘spill-back’. The introduction of pathogen with an introduced species is a well documented process. But, the alternative of this is a native pathogen, in this case a parasite, infecting the introduced species and receiving a population boost due to the increased number of potential hosts. Native hosts are then exposed to a greater number of parasites in their environment which results in higher rates of infection. In this case, the explosion in cane toad numbers after introduction provided a potential bonanza for the Myxosporean parasites. Moreover, the extreme mobility of the cane toads provided a new, fast moving host that could increase the range of the parasite, bringing it into contact with native species not previously exposed. For this to occur it is easy to see that the parasite infecting native frogs and Australian cane toads needs to be the same parasite and, using the same methodology described, above Dr Slapeta and his colleagues showed that this was indeed the case. Although providing some strong evidence for spill-back further work needs to be done to conclusively show the effect at work. For instance, a systematic mapping of the geological occurrences of the parasite would provide further evidence for or against the conjecture, but in the absence of an introduced parasite species the theory is compelling.
So, although cane toads are not strictly the bad guy in this story, they are playing an indirect role in the challenges affecting our native frog population. Moreover, apart from a cool and elegant study that has settled an interesting question, the study illustrates the complexity of a natural ecosystem and just how difficult it is to assess changes that may arise after a exotic species is introduced into a new environment. Particularly one as unique, and isolated, as Australia’s.
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