Why Frogs Are Vanishing From the Western Ghats and What Chytrid Fungus Is Doing to Them

The Western Ghats run for roughly 1,600 kilometres along India's western edge and hold more frog species per square kilometre than almost any other forest system on Earth. Of the approximately 180 amphibian species recorded there, around 65 percent exist nowhere else. Frogs are not incidental to this ecosystem. They sit at a middle tier of the food chain, eating insects that would otherwise overwhelm the forest floor and feeding the snakes, birds, and small mammals above them. When a frog population collapses, the ripple moves in both directions.

Several of the Ghats' most distinctive species, the purple frog, Nasikabatrachus sahyadrensis, which spends most of its life underground and surfaces only to breed during monsoon; the Malabar gliding frog, which steers between trees using webbed feet; the tiny Raorchestes bush frogs found only in specific elevation bands, have already shown steep declines in field surveys. The causes are layered. Habitat loss from plantation agriculture and road-building fragments the forest. Pesticide runoff from tea and coffee estates enters the streams where frogs breed. Climate shifts alter the monsoon timing that many species depend on for reproduction. And then there is the fungus.

Batrachochytrium dendrobatidis, called Bd for short, is a chytrid fungus first formally described in 1999 after it was linked to mass die-offs in Australia and Central America. It does something specific and lethal: it infects the keratin layer of a frog's skin. Frogs are not like mammals. They do not drink water through their mouths in the conventional sense, they absorb it and regulate electrolytes directly through their skin. Bd thickens and disrupts that skin layer, blocking the transfer of sodium and potassium ions. The frog's heart eventually stops from the electrolyte imbalance. It is cardiac arrest caused by a skin infection.

The fungus spreads through water and on wet surfaces. It can survive in soil for weeks without a host. Frogs moving between pools, or water flowing downstream from an infected site, carry it efficiently. Bd thrives in cool, moist conditions, the precise conditions that define the upper-elevation shola forests of the Ghats. A 2019 global assessment published in Science identified Bd as contributing to the decline of at least 501 amphibian species worldwide, with 90 species confirmed or presumed extinct. That single pathogen represents one of the most significant losses of vertebrate biodiversity ever recorded from one cause.

The dominant hypothesis is the global trade in amphibians, for laboratories, for the pet trade, and historically for the use of the African clawed frog, Xenopus laevis, as a pregnancy test in the mid-twentieth century. Xenopus carries Bd without dying from it, and it was shipped to hospitals and research institutions across the world for decades. When those animals were released or escaped, they seeded the fungus into local water systems. Once Bd is in a watershed, it does not leave.

Surveys in the Western Ghats, including work by researchers at the Centre for Ecological Sciences at the Indian Institute of Science in Bengaluru, have detected Bd in multiple frog populations across Kerala, Karnataka, and Tamil Nadu. Prevalence varies by elevation and species, with high-altitude species showing greater vulnerability. The Ghats present a particular problem: their frog communities evolved in relative isolation and have had no evolutionary exposure to Bd. There is no acquired resistance. A population meeting the fungus for the first time can collapse within a single breeding season.

Frogs are one of the few animals that operate as both predator and prey at a scale that keeps insect populations in check across an entire forest. A single adult frog can consume hundreds of mosquitoes and agricultural pest insects per night. Across a healthy population in a monsoon forest, that adds up to a biological pest-control system that no spray programme can replicate at cost or coverage. Frog tadpoles also filter algae from streams, keeping water quality stable for other species. Their absence restructures the stream ecology.

There is a secondary loss that is harder to quantify. The Western Ghats are still yielding new frog species, several are described each year from surveys in underexplored elevation bands. Species are being named and lost within the same decade, sometimes before their ecology is understood. The purple frog, for instance, was only formally described in 2003. Its underground life history is still not fully mapped. Losing a species before its biology is known is a different kind of extinction than losing one that was studied.

Captive breeding programmes exist for a handful of critically threatened species, and some conservation organisations are testing probiotic skin treatments that may help frogs resist Bd. Neither is a solution at the scale of a mountain range. The fungus is already there. The habitat loss is ongoing. What remains is documentation, habitat protection where it is still possible, and strict biosecurity around any movement of amphibians, wild-caught or captive, across state lines and international borders. The frog that disappears from a Ghats stream in October will not be replaced by the monsoon that follows.