(ORDO NEWS) — Schlegel’s Madagascar snakes are dying en masse from poisoning by poisonous black cicatricial toads that entered the island about ten years ago. As a result, some local populations lose six percent of individuals a year and risk extinction in just a few years.
Other Madagascar snakes are under threat, as well as local predatory mammals and birds. As noted in an article for the journal Biological Invasions, the same story could be repeated in Madagascar as in Australia, where the number of native predators has plummeted due to the spread of invasive aga toads.
Anyone with an interest in conservation knows the damage done to Australia’s ecosystems by the invasive aga toad (Rhinella marina). In the 1930s, these amphibians were brought to the continent to control sugarcane pests – and since then they have spread widely across the north and east of Australia.
Local predators, who have never encountered toads before and are not immune to their poison, try to eat them and die from poisoning. Not surprisingly, due to the invasion of invasive amphibians, the number of many Australian snakes, monitor lizards and marsupial predators has declined sharply.
Now this story seems to be repeating itself in Madagascar. Here, as in Australia, there have been no true toads (Bufonidae) for millions of years. However, thanks to humans, Asian black-rubber toads (Duttaphrynus melanostictus) entered the island around 2010.
Most likely, they arrived by sea with a cargo of ships bound for the port city of Toamasina on the east coast of Madagascar. In a little over a decade, the toads have successfully colonized Tuamasina, its suburbs, and surrounding farmland.
In total, amphibians have colonized an area of about 500 square kilometers and continue to capture new lands: according to scientists, the boundaries of their range are expanding by an average of 2.5 kilometers per year.
Zoologists fear that Madagascar predators, like Australian ones, will begin to hunt invasive toads and die en masse from their poison. Studies confirm that they have no immunity to toad venom. Of particular concern is the fate of some local species of snakes specialized in eating amphibians.
A team of herpetologists led by Angelica Crottini from the University of Porto set out to find out if the black-rubber toads could populate the rainforests of Madagascar, where snakes that prey on amphibians live. The focus of their attention was the small forest of Analabe, located west of Toamasina.
This is one of the few areas of tropical rainforest that have survived in the region (the rest were burned by local residents for the needs of agriculture). Despite the fact that the area of Analabé is only seventeen hectares, eighteen native amphibian species and sixteen native reptile species live here.
Black-rubber toads appeared in the vicinity of Analabe between 2014 and 2016. During counts in the forest, Crottini and her colleagues counted eighteen representatives of this species of different sex and size.
In 2018-2019, researchers captured eight toads near Analabe, fitted them with radio transmitters, and tracked their movements for an average of 12.4 days. All tagged individuals spent most of their time in the forest and its edges.
One of the tagged toads was killed by Schlegel’s Madagascar snake (Madagascarophis colubrinus). At the same time, the snake did not have time to swallow its prey and quickly died from poisoning.
After finding the corpse of a toad on a radio transmitter and seeing a dead snake nearby, Crottini and her colleagues decided to find out how the appearance of invasive amphibians in Analaba affected this species of snakes.
First, the researchers estimated the abundance of M. colubrinus in the forest area. In the winter and spring of 2019, they counted 54 snakes on five 150-meter transects in a few weeks. Based on these data, the approximate size of the local population was calculated: 43 individuals.
Schlegel’s Madagascar snake (Madagascarophis colubrinus), which died from poisoning, with a black-rubbed toad (Duttaphrynus melanostictus) in its mouth
Among the 54 snakes that scientists found during the counts, there were nine dead snakes with no visible injuries or damage. Most likely, they died from toad poisoning. In four cases, a dead toad was found in the stomach or mouth of a snake, or near its corpse.
The authors estimate that more than two snake snakes die each month from invasive amphibian poisoning in Analaba.
This corresponds to a monthly death of 5.7 percent of the local population. Perhaps the real losses are even higher, since the researchers could not find all the poisoning victims. For comparison, the mortality of native predators from a toads in Australia is almost half that.
For the first time, Crottini and her colleagues were able to confirm that Madagascar snakes really die en masse from poisoning by black cicatricial toads. At the same time, the mortality rate among them is so high that many local populations can disappear in just a few years.
As a result, entire ecosystems will suffer – and even people, because the reduction in the number of snakes will lead to an increased reproduction of pests and disease vectors, such as mice and rats.
However, the authors admit that the situation in Madagascar may not be as severe as in Australia. The fact is that black-rubbed toads breed in ponds and lakes, and the preserved tropical forests of Eastern Madagascar are located in areas with difficult terrain.
There are many fast-flowing rivers and streams, while stagnant water bodies are rare. Perhaps this will limit the spread of invasive amphibians.
Earlier, we talked about how scientists and volunteers tried to stop the toad invasion of Taiwan. Until now, the island has been free from these dangerous invaders, but at the end of last year they were noticed in the vicinity of the city of Zhaodun. An operation to trap the toads was immediately launched on the spot.
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