Fears about the impact of invasive, poisonous toads in Madagascar appear to be justified, suggests new research. The amphibians are driving a spike in deaths of a native snake species.
Asian common toads (Duttaphrynus melanostictus) are native to southern Asia, but were introduced to eastern Madagascar around 2010, probably as stowaways on a ship. Since then, the toads have been spreading from the point of arrival by about 2.5 kilometres per year, prompting anxiety among researchers that they could threaten Madagascar’s native wildlife just as Australia’s indigenous species came under threat after cane toads were introduced in the 1930s.
on a wide range of Madagascan predators found little evidence of resistance to the Asian common toads’ lethal toxins, suggesting the island’s unique fauna was particularly vulnerable to the invasion.
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at the University of Porto in Vairão, Portugal, and her colleagues were interested in the toads’ movement patterns, since it was unknown if they could infiltrate patches of forest. Working near a forest close to the seaport town of Toamasina, the team fitted eight toads with radio transmitters and tracked them.
During this work, team member , also at the University of Porto, noticed carcasses of native Malagasay cat-eyed snakes (Madagascarophis colubrinus).
It is rare to find carcasses while working in humid tropical regions, says Crottini, since they decompose rapidly. The researchers suspected the toads were responsible as they found more dead snakes, sometimes with dead toads nearby or in the snakes’ mouths.
The team never found toads inside the snakes’ gut, hinting that the toxins kill remarkably quickly.
“It was very sad to find the dead bodies of these beautiful native predators,” says Crottini.
The team conducted forest surveys to gauge cat-eyed snake abundance, and to see how many were dying. They estimate that at least 5.7 per cent of the snake population was being killed by toad poisoning every month. At this rate, the population could halve within a year, raising the risk it could soon be wiped out.
This conclusion – combined with the team’s confirmation that the toads are entering forests – shows that they are starting to harm native biodiversity. The invasion adds another stressor for fauna already suffering dramatic declines from ongoing deforestation.
Additionally, if snakes dwindle across Madagascar, rodent populations could swell, elevating people’s risk of catching animal-borne diseases from rodent waste or parasites.
at the University of Stirling, UK – who was the lead researcher on the 2018 genetics study – says seeing his team’s dire predictions confirmed is “bittersweet”.
Crottini and her colleagues argue that the cat-eyed snakes could be an indicator of more widespread impacts. “I fear they are right,” says Marshall, “but desperately hope that the amphibian-eating species in the area can adapt either behaviourally or physically to cope.”
There may be some reasons for hope, though. The toads may have a difficult time reproducing as their range expands. The eastern side of Madagascar is characterised by steep terrain with few ponds for toads to breed in.
Going forward, keeping track of Malagasy snakes could be crucial. “We really need to start monitoring them,” says Crottini. “We are still basically describing the diversity, and there is zero data on abundances.”
Biological Invasions
