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Madagascar’s native fauna defenseless against toxic invasive toads

  • Toxic Asian common toads (Duttaphrynus melanostictus) have spread rapidly around the port city of Toamasina on Madagascar’s east coast, raising concerns that the invasive amphibians could take a severe toll on the island’s unique wildlife species.
  • A recent paper vindicates those concerns: through a genetic analysis of 77 endemic species, scientists found that just one demonstrated clear resistance to toad toxins.
  • A separate estimate published last month suggests there are now over 7 million Asian common toads in Madagascar. Reports suggest they arrived accidentally with mine construction equipment prior to 2010.

Madagascar is facing an invasion. Not military, but amphibian.

Toxic Asian common toads (Duttaphrynus melanostictus) have spread rapidly around the port city of Toamasina on the country’s east coast. The invasion has raised concerns that the amphibians could take a severe toll on Madagascar’s wildlife species, approximately 70 percent of which are endemic to the island.

A paper published last month vindicates those concerns: through a genetic analysis of 77 Malagasy species, scientists found that just one demonstrated clear resistance to toad toxins.

“Our findings stress the importance of the timely investment of resources to monitor and control the spread of this alien species in order to prevent a worsening biodiversity crisis in Madagascar,” the scientists write in the journal Current Biology.

Asian common toads secrete a milky fluid from special glands behind their eyes when threatened. The fluid contains toxins known as cardiac glycosides, which can kill predators, including humans.

Toxin-secreting parotid glands are visible as a large sac behind each eye. Image courtesy of JamesReardon.org.

“If sufficient cardiac glycosides are present, then the affected individual can suffer from an irregular heartbeat, which can lead to … cardiac arrest, and death,” study co-author Nicholas Casewell, an expert in snake venom with the Liverpool School of Tropical Medicine in the U.K., told Mongabay via email.

The situation has drawn fearful comparisons to a devastating invasion of toxic cane toads (Rhinella marina) in Queensland, Australia. The species was introduced from South America in 1935 to control agricultural pests. No one foresaw that their cardiac glycosides would decimate local fauna. For instance, some populations of monitor lizards (Varanus mertensi, V. mitchelli and V. panoptes) declined by 90 percent or more after cane toads invaded their habitats.

“During the cane toad invasion of Australia, animals like snakes and monitor lizards were often found dead in the field with the toad still in their mouths,” co-author Wolfgang Wüster, a herpetologist at the University of Bangor in the U.K., told Mongabay via email.

An Australian monitor lizard killed by cardiac glycosides after ingesting parts of a cane toad, which lies next to it. Image by Beata Ujvari and Thomas Madsen.

Scientists first detected the Asian common toad invasion of Madagascar in March 2014 in Toamasina, but local observations suggest the toads had arrived in the region by 2010. Since then, the toads have spread to an area of more than 100 square kilometers (39 square miles) around the city. An estimate published in May suggests there are now over 7 million of them.

According to Wüster, there has not yet been a study to determine whether Madagascar’s native species are collapsing as a result of eating the toads. However, he expressed concern that the country’s endemic fauna is already cornered due to decades of deforestation, and that death by toad poison could be a knockout blow.

“The outcome of the toad invasion may be more severe in Madagascar than it was for the cane toad invasion of Australia,” he said.

Unfazed by toads: the white-tailed antsangy

Many predators in the Asian toad’s native territory are resistant to cardiac glycosides. Resistance is conferred by mutations in a specific gene segment, and so scientists looked for these in the DNA of 77 species of Malagasy fauna, including birds, snakes, lizards and mammals.

Only the white-tailed antsangy (Brachytarsomys albicauda), a type of rodent, demonstrated clear resistance.

The white-tailed antsangy, shown here in an 1875 work of art by Joseph Wolf and Joseph Smit, was the only Malagasy species whose genome demonstrated clear resistance to the deadly cardiac glycosides secreted by introduced Asian common toads. Image by Joseph Wolf and Joseph Smit (Public domain) via Wikimedia Commons.

“As far as we know, without the resistance-conferring amino acid substitutions [an animal] will not survive ingestion of a toad,” Shab Mohammadi, a specialist in toxin resistance at the University of Nebraska-Lincoln, who was not involved in the study, told Mongabay.

Despite the nearly complete sensitivity to the toxins, Wüster said not every species studied was doomed. For example, though three lemur species showed no sign of resistance, their diet tends to consist of plants and insects, not amphibians.

“Based on the Australian experience with cane toads, the most vulnerable species are likely to be larger predators that [can] eat larger toads with plentiful toxin reserves,” he said. “I would be most worried about larger mammalian predators (e.g., the fossa) as well as some of the larger colubrid snakes, such as Madagascarophis.”

The fossa (Cryptoprocta ferox) is the largest mammalian carnivore in Madagascar. Being both at the top of the food chain and sensitive to cardiac glycoside toxins means the endemic predator could be one of the species most impacted by the Asian common toad invasion. Image by Rhett A. Butler/Mongabay.
Though sensitive to the toad toxin, smaller snakes, such as the Malagasy tree snake (Lycodryas inopinae), may be less at risk than larger snakes. Image by M. Vences.

Hope for these predators could lie in adaptation and evolution.

“A number of species in Australia have employed behavioral measures to circumvent [cane] toad toxicity, including learning to ignore toads as potential prey, and by preying on them in a manner where the poisonous parts are not ingested,” Casewell said.

Crows, for example, have learned to consume only the non-toxic parts of cane toads: the tongues, thighs and intestines.

According to Wüster, researchers in Australia similarly found that red-bellied black snakes (Pseudechis porphyriacus) learned to avoid cane toads and had even evolved smaller heads, which reduces the likelihood of ingesting larger, more toxic, individuals.

“As a result, red-bellied blacksnakes have been making something of a comeback,” he said.

Eradication unlikely

The recent study estimating that the invasion had grown to 7 million toads also tested four methods to eradicate the toads. The researchers came away hopeful after finding the toads have a fatal sensitivity to citric acid.

However, given the size of the invasion, they noted that citric acid would have to be sprayed via helicopter, an expensive proposition. With the other methods depending upon intensive manual labor and disruptive equipment such as fences and traps, the researchers concluded that any successful eradication strategy would face serious logistical challenges.

“The dense human population, poverty, lack of infrastructure and extremely suitable habitat for the toads in the area of their current range — plus the significant costs in becoming operational — are the primary risks,” lead author James Reardon, a herpetologist with New Zealand’s Island Eradication Advisory Group, told Mongabay.

The result of a single night of pitfall trapping of Asian common toads around a small village near Toamasina, in January 2016. Image courtesy of JamesReardon.org.

Reports have suggested that the Asian toads arrived accidentally via shipments of equipment from Thailand for the construction of the Ambatovy nickel and cobalt mine’s processing plant, in Toamasina. Though the mine denies responsibility, its spokespeople point to its involvement in eradication efforts.

“Since 2014, [Ambatovy] has engaged various contractors (incl. University of Antananarivo’s Department of Biology) to capture and destroy animals,” Ambatovy’s communications team told Mongabay via email.

The team said that, besides manual capture, other efforts included the identification of breeding sites, the collection of eggs, the capture of tadpoles, and a community awareness program. “Approximately $250,000 has been paid to contractors,” the team said.

Reardon told Mongabay that eradication was arguably “a lost cause.” He said the best-case scenario would be for conservationists to develop his team’s tools to stop the toads from invading new habitats, but ultimately the country must learn from this experience to improve biosecurity in Madagascar.

Short of this measure, Reardon warned, “the Asian toad will be one of a growing number of species arriving and establishing [itself], unless we can work with the Malagasy government to control these growing risks from international trade.”

An Asian common toad climbs a tree trunk in a village in Madagascar. Image by Franco Andreone.

Banner image: The Asian common toad (Duttaphrynus melanostictus). Image courtesy of JamesReardon.org.

 

Citations:

Marshall, B.M., Casewell, N.R., Vences, M., Glaw, F., Andreone, F., Rakotoarison, A., Zancolli, G., Woog, F., Wüster, W. (2018). Widespread vulnerability of Malagasy predators to the toxins of an introduced toadCurrent Biology28(11): R654-R655.

Reardon J.T., Kraus F., Moore M., Rabenantenaina L., Rabinivo A., Rakotoarisoa N.H. & Randrianasolo H.H. (2018). Testing tools for eradicating the invasive toad Duttaphrynus melanostictus in MadagascarConservation Evidence 15:12-19

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