Behind the biodiversity in the Western Ghats

The Western Ghats (WG), which run along the west coast of India, are one of 36 global biodiversity hotspots. Most life forms here are endemic to the region and found nowhere else on Earth. The Western Ghats sustain millions of people who are directly or indirectly dependent on them for resources such as water and play a vital role in regulating the monsoon patterns.

Researchers and environmentalists have argued that the WG is unique and emphasised the need to protect this fragile ecosystem. However, little is known about the origins of this rich diversity and how it sustained over time. 

Biodiversity

Biodiversity is often measured in terms of species richness—the number of species in an area. The diversity of life arises from a process called speciation that takes place over a few thousand to several million years. Typically, new species can arise when a population splits and becomes isolated until they cannot interbreed anymore.

This can happen due to a geographical barrier such as a river or a valley. Sometimes, species populations can also become isolated due to genetic mutations when individuals between these populations can no longer interbreed. Eventually, they drift apart and become different species. Repeated over millions of years, this process leads to many new species. The extinction of species is an important step in this process.

The rate at which speciation and extinction occur ultimately determines the number of species in a particular region. However, these rates can vary depending on several factors, including the climate. Prolonged periods of stable climate will likely enable species to persist, and populations will often undergo fewer extinctions.

In contrast, rapid and drastic climatic fluctuations can lead to bursts of speciation or extinction events. Time can also determine the number of species in a region. A population that has persisted in a region for a longer duration is likely to undergo speciation many more times than a region that has only recently been colonised by a species group. All these processes can result in high species richness.

Evolutionary history

In a recent study, a team of researchers from the Centre for Ecological Sciences (CES) and the Indian Institute of Science (IISc) shed light on how the Western Ghats support a rich diversity of amphibians. They use a large set of genetic data from over 450 species of frogs to reconstruct a genetic history and try to answer the question: “Why are there so many species of frogs in the Western Ghats?”

They began by inferring the evolutionary age of frogs in the WG by comparing genetic mutations between species. Genetic mutations on specific genes are assumed to accumulate at a relatively constant pace, and comparing such mutations between species, they can tell us when they diverged from their ancestor.

They found that most frog families are about 40 million years old, while some groups of frogs, commonly called the Dancing Frogs and the Night Frogs, were as old as 60 million years. This is significant considering that the Indian plate broke off from the supercontinent Gondwanaland, drifted north, and collided with the Asian plate, forming the Himalayas around 55 million years ago.

Then, they explored whether the Ghats' geography had anything to do with the high species richness. The Western Ghats is a 1600-km-long chain of undulating terrain. Broadly, the Western Ghats are classified into the northern, central, and southern Western Ghats based on their rainfall. The WG also varies in elevation, with high elevations experiencing cooler climatic conditions, especially in the southern WG.

Historically, mountains have been considered a ‘species pump’ where populations of a species would retreat when the climate was extreme and recolonise habitats when climatic conditions were favourable. Repeated retreating and expansion can isolate species, resulting in speciation events. 

Climate history

To examine whether climatic history had anything to do with the rich diversity of amphibians, they then compared the diversification rate across frog groups within the Ghats. They found it to be independent of location within the WG and elevation. “Our findings support the idea that the high frog diversity in the WG is likely a result of gradual species accumulation in stable tropical environments,” said Dr Cyriac, a postdoctoral researcher at CES, IISc, and lead author of the paper published in the journal Evolution.

“Our findings are a result of more than a decade of research done by many students, researchers, and postdocs. The team traversed the entire length of the Western Ghats, covering all elevations, habitats, and hill ranges. Using this biogeographic perspective, we could capture information about most of the frog species in the region and identify subtle variations between closely related species. This was critical in arriving at our inferences," explained Prof Kartik Shankar, the study's senior author, in whose lab the numerous students and researchers undertook this research.

Western Ghats

Impacts of climate change

Amphibians are ectotherms; they cannot maintain an internal temperature like mammals. The new study highlights that amphibians in the Ghats have evolved over a prolonged period of climatic stability. “The rich biodiversity in the Ghats did not arise simply but has taken millions of years of evolution in relatively stable climatic conditions. We hope our work will help people appreciate biodiversity and take steps to conserve it,” said Dr Cyriac.

Rampant anthropogenic impacts such as deforestation and increased atmospheric carbon dioxide have resulted in climate change. While amphibians have been exposed to climate change, the ongoing onslaught has been too quick for many slow-evolving organisms, such as amphibians, to adapt.

"Frogs are great models for understanding the evolution and contemporary climate change. Their small body size and low dispersal rates mean they diversify at time scales, allowing us to understand speciation processes. On the other hand, their sensitivity to the environment makes them important indicators of the effects of pollution and global warming," concluded Prof Shankar.