A microscopic aquatic fungus is responsible for the heaviest biodiversity loss ever found to be due to a single pathogen, according to an article published in the journal Science on March 28.
The microorganism causes chytridiomycosis, an infectious disease that affects amphibians worldwide. In the past 50 years, it has caused a decline in the populations of at least 501 species of amphibians. In some cases, they have dwindled to less than 10% of their original distribution, and 91 species are believed to have become completely extinct.
"We consider this quantification conservative. The pathogen has probably caused the decline of many other species unknown to science. This phenomenon may be particularly relevant in the Neotropical region [all of mainland South America, Central America and the Caribbean as well as part of Mexico and the United States], where there are many undescribed species," Benjamin Scheele, first author of the article, told. Scheele is a postdoctoral fellow at Australian National University in Canberra.
The researchers estimate that at least 6.5% of known amphibian species have undergone a decline caused by the fungus.
"It's a very large number. We have records of pathogens since the time of the dinosaurs, and without question, this is the deadliest disease that has ever struck wildlife in all time," said Luís Felipe Toledo, a professor at the University of Campinas's Biology Institute (IB-UNICAMP) in Brazil and coauthor of the study.
Toledo and his PhD student Tamilie Carvalho are the only Brazilians in the group of researchers from 16 countries who conducted the study, which was supported by São Paulo Research Foundation - FAPESP.
The findings presented in the article are based on a comprehensive survey of the literature, queries submitted to experts, and consultation of the IUCN Red List of Threatened Species.
"In the global survey outlined by the article, Brazil is the negative standout: at least 50 species or populations have been affected, 12 have become extinct, and 38 have undergone decline. Some populations are showing signs of a recovery, but others have disappeared for good," Toledo said.
The Atlantic Rainforest is the most affected biome in Brazil, he added. Most of the extinction records relate to the region from Espírito Santo to Paraná. "There are some places where we know many species have disappeared, such as Boracéia [on the coast of São Paulo state], Serra dos Órgãos [in Rio de Janeiro state], Itatiaia [on the border between Rio de Janeiro state and Minas Gerais state] and Caparaó [on the border between Minas Gerais state and Espírito Santo state]. But this doesn't mean other regions haven't been affected. We simply lack as good a sample as we obtained in the Atlantic Rainforest biome," he said.
Chytridiomycosis is caused by two fungi, Batrachochytrium salamandrivorans, which affects only salamanders and has never been recorded in Brazil, and B. dendrobatidis, which is found on all continents and affects all three amphibian groups: anurans (frogs and toads), newts and salamanders, and caecilians.
The pathogen's spores penetrate the skin of adult hosts, which become unable to breathe and die from cardiac arrest. In tadpoles, the fungus parasitizes the region of the beak and denticles, hindering feeding and growth.
According to the survey published in Science, the worst-hit group is the anurans, which account for 89% of all amphibian species and have suffered the largest proportion of severe declines (93%) because they are so abundant. The tropical regions of Australia and Central and South America are the most affected, while the number of declines in Asia, Africa, Europe and North America is "remarkably low".
The main victims are the species with a limited geographic distribution, large bodies, habitats in waterlogged areas, and perennial aquatic habits. This is because Batrachochytrium releases its spores in water, and the spores are able to swim until they infect a host.
According to Toledo, some anuran genera are especially susceptible to infection. An example is Atelopus, with species occurring in Central and South America from Costa Rica to the Brazilian Amazon.
"The declines peaked in the 1980s, as we showed in a previous study, but the disease was discovered only in 1998, making our research more difficult because when we noticed the declines or extinctions, we had no idea of the cause," Toledo said.
The hypothesis defended by most experts and published in Science in 2018 is that a virulent strain of the fungus originating in Asia arrived in Central America in the last century and spread to South America. The process is thought to have been facilitated by the transportation of amphibians for human consumption or for sale as pets.
"In Mesoamerica, where we believe amphibians had no prior contact with the fungus, many species have been wiped out. In Brazil, where the disease has occurred since at least the nineteenth century, some animals have developed resistance, and the impact appears not to have been so catastrophic," Toledo said.
In their latest article, the scientists write that "the unprecedented lethality of a single disease affecting an entire class of vertebrates underscores the danger of the spread of new pathogens in a globalized world". For the authors, effective biosafety policies and an immediate reduction in wildlife commerce are "urgently needed" to prevent the spread of new pathogens.
"Given that the possibility of mitigating chytridiomycosis in the wild remains uncertain, new research and intensive monitoring with emerging technologies are necessary to identify mechanisms of species recovery and develop new mitigation actions for species in decline", Scheele said.
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