The silent killer
A microscopic fungus is responsible for one of the most catastrophic losses of biodiversity in recorded history. Known as Batrachochytrium dendrobatidis, or Bd, this pathogen causes a deadly skin disease in amphibians called chytridiomycosis. First formally identified in 1998 from amphibians dying in Australia and Central America, the fungus has since been detected in over 60 countries. Genetic studies suggest the most virulent global lineage may have originated on the Korean peninsula and spread worldwide through the trade of amphibians, such as the African clawed frog (Xenopus laevis), which can carry the fungus without showing symptoms.
The fungus attacks keratin, a protein found in the outer layers of an amphibian's skin. For frogs, this is a lethal assault. Their permeable skin is a barrier and an important organ for respiration and, critically, for absorbing water and essential electrolytes like sodium and potassium from the environment. As the fungus multiplies, it thickens the skin and severely disrupts its ability to transport these electrolytes. This leads to a catastrophic drop in electrolyte levels in the blood, Finally causing the frog's heart to stop.
An ecological catastrophe in Costa Rica
The global impact of chytridiomycosis is staggering. The disease has been implicated in the decline of over 500 amphibian species, with 90 species driven to confirmed or presumed extinction and another 124 suffering population declines of more than 90%. The peak of these declines occurred in the 1980s, even before the pathogen was scientifically described.
Costa Rica's diverse amphibian habitats were hit particularly hard. The country became a focal point for the crisis with the disappearance of the brilliant golden toad (Incilius periglenes). First described in 1966, this toad lived in a tiny, 4-square-kilometer area of high-altitude cloud forest near Monteverde. In 1987, over 1,500 toads were observed. By 1988, only about ten were seen. The last confirmed sighting of a lone male was on May 15, 1989. The species was officially declared extinct in 2004. While an El Niño-related drought in 1987 likely stressed the population, the arrival of the chytrid fungus is considered a primary driver of its rapid extinction. The collapse of amphibian populations in Costa Rica and neighboring Panama had cascading effects, including a subsequent spike in human malaria cases as mosquito populations, once controlled by amphibians, went unchecked.