Understanding frog diversity with the highs and lows of the Teesta river valley

Scutiger sikkimensis is found in cold waters in the high elevations in Sikkim Himalayas. Photo by B. Chettri.

  • A scientific survey has mapped the diversity of frogs along the elevation gradient of the Teesta river in Sikkim.
  • Maximum species richness was observed along mid-elevations where rainfall and temperature conditions were conducive to the amphibians.
  • Scientists are concerned that with most amphibians restricted to a narrow range along the elevation gradient, they are vulnerable to a wavering climate. Additionally, many species are harvested for meat and purported therapeutic use.
  • Experts suggest in-depth molecular investigations in taxonomy to unravel more information on species diversity.

As the Teesta gurgles down its way from its snow-fed origin in North Sikkim (5200 metres) to the plains in the south (300 metres), the river and its swiftly flowing tributaries create microhabitats around boulders, logs, mosses, and leaf litter that shelter a diverse species of frogs. 

But researchers surveying the Teesta river valley for frogs weren’t expecting to see much diversity as they made their way from the humid and wet lower elevations to the harsh cold and dry conditions that prevail higher up along the river’s course in Sikkim in the eastern Himalayas.

Frogs, unlike birds and mammals, are ectothermic amphibians unable to regulate their temperatures internally. They need to warm up from external sources. “So as we went higher up, we expected the species diversity to decline because of the harsh climate conditions and environmental fluctuations,” Basundhara Chettri of Sikkim University’s Department of Zoology, told Mongabay-India.

“However, we recorded the maximum diversity of frogs between 1000m and 1500m, which is the middle elevation zone. The diversity of species dips towards higher and lower elevations; the decrease in frog species diversity is more pronounced in the higher elevations, but we still found two unique frog species beyond 3500 metres,” added Chettri.

Chettri and study co-author Bhoj Acharya observed 1368 individuals of amphibians representing 25 species from 11 genera. They carried out 1236 hours of visual encounter survey and 27 km of night stream survey between 300–4600 metres across April 2009-August 2010 and April 2013-August 2015 outside protected areas.

The recently published study recorded one Vulnerable and three Near Threatened species as per the IUCN Red List of Threatened Species. “Seventy-six percent of the species favoured the elevation range up to 1500 m because of the conducive rainfall and temperature conditions that influence moisture availability. Frogs need to keep their skin moist so they can absorb dissolved oxygen from the atmosphere when they are out of the water,” explained Acharya, of the Department of Zoology, Sikkim University.

Beyond 3500 metres, the researchers found only two species: the Boulenger’s lazy toad (Scutiger boulengeri) and Sikkim alpine toad (Scutiger sikimmensis). “S. boulengeri were by the riverside but sheltered by the hot springs’ warm waters and the moss-covered vegetation. But S. sikimmensis is found in the cold water of high elevations,” added Chettri.

However, over a period of time, the wavering temperature and rainfall patterns over the Himalayas may mean the difference between life and death for a section of the amphibians. As most frog species in the survey area lie in a narrow range along the elevation gradient, they are vulnerable to climate change. “These species would be unable to extend their ranges in a warming climate because they have adapted to a specific small area with a set of climate conditions,” observed Acharya.

Researchers sample the Teesta river valley along the elevational gradient. Photo from Basundhara Chettri.

Teesta river is one of the largest tributaries of the river Brahmaputra, which is often called northeast India’s lifeline. The Teesta river basin in Sikkim is projected to experience drastic changes in climate extremes in the 21st century. Officials are worried about the unprecedented melting of glaciers and the development of glacial lakes with an increasing volume of water. These concerns tie in with the assessments of rapidly changing climatic conditions in the Hindu-Kush Himalayan region, in general.

Of immediate concern is frog harvests by local communities, particularly intense in the species-rich mid-elevation zone. “Hunting of frogs is done for two reasons mainly – meat and medicine. They use frogs to cure many ailments that are not validated scientifically,” said Chhetri, justifying a higher IUCN risk category for the over-exploited species before they become locally extinct. For example, Nanorana gammii consumed by the local community is an IUCN red-listed Near Threatened species, and Megophrys robusta, harvested extensively for consumption, is still in the Not Evaluated category. 

The survey uncovered 13 species that were endemic to the Himalayas, including the Sikkim paa frog (Nanorana liebigii), Himalayan cascade frog (Amolops himalayanus); 12 were non-endemics. Endemic species were mainly concentrated in the higher elevations (2000-2500 m), while non-endemics showed a peak at a much lower elevation (1000-1500 m). Non-endemics include the bulky Indian bullfrog (Hoplobatrachus tigerinus) and Boulenger’s lazy toad (Scutiger boulengeri).

Herpetofaunal expert Abhijit Das, who was not associated with the study, said each species is an essential chapter in the story of evolution. “We are still discovering more species. We have 430-odd amphibian species in our country, of which we have added more than 50 percent of the species in the last 15 years. So it seems that when nature is facing the greatest threat, we are discovering our species, so it appears as if the processes of extinction and discovery are happening at the same rate,” Das of Wildlife Institute of India (WII), told Mongabay-India.

Batting for more attention to herpetofaunal research (reptiles and amphibians) in India, Basundhara Chettri also warned of the emerging environmental challenges due to a boom in pharmaceutical manufacturing pushed by business-friendly policies such as the North East Industrial and Investment Promotion Policy (NEIIPP).

“Pharmaceutical companies discharge effluents in the rivers, and that needs to be looked into from the environmental conservation perspective, especially if we want to preserve microhabitats around the rivers and streams,” said Chettri.

Das said steps to survey amphibians are crucial because to save a species, you need to know the name and ecological data of the species. “Amphibians are covert species, and we do not have species-specific plans like that for tigers and elephants; we need to save the species that are lesser-known and covert. We need to know where they exist, and we need to know the regions where they exist in hyper diversity, and then we try to save those zones where maximum species occur so that we can prevent mass extinctions,” he said.

Nanorana liebigii is an endemic species in the Sikkim Himalayas. Photo by researchers.

Das added that detailed taxonomic investigations might reveal more diversity prompting updates on the current survey-based study. “The general pattern looks the same, and there may be a mid-elevation peak, but with more detailed taxonomic investigations, the species diversity they have recorded may undergo a change. For example, the species they have recorded as one species now may consist of several new species (cryptic diversity). Now that they have found out the trend, students and researchers may look into the molecular aspects,” Das suggested.

Sikkim stands out as the smallest but biologically most diverse Himalayan states in India. Wedged between Nepal and Bhutan, it sits at the crossroads of many biogeographic realms (Palaearctic, Oriental) and regions (Indo Malayan, Indo Chinese). Besides, the Teesta’s swift tributaries form the primary habitat for many frogs, some of which may still be unknown to science. The river originates from the snow-fed Tso Lhamo Lake, at 5200 m, and flows in north-south direction till it enters the West Bengal state of India near Melli at 300 m. “The whole of Sikkim is intersected by the river. Accordingly, climate changes from hot tropical to arctic conditions within 150 km linear distance from Melli to Tso Lhamo. So within a very small zone (150 km) we cover a huge range of elevation from 300 metres to up to 5000 metres,” observed Bhoj Acharya.

While the mid-elevation peak in species count is an important observation, there is another aspect to diversity, which is called the unique evolutionary trait, that’s also crucial. “Amphibians and reptiles are cold-blooded animals, and there may be only a few species distributed at a high elevation, but those species being cold-blooded carry unique traits. From an evolutionary perspective, those forms are extraordinary and enigmatic. Losing one species means that you are losing a very important story from your scenario,” Das stressed, adding, “We can’t lose our species diversity. At the same time, we can’t lose the diversity of unique species.”

Banner image: Scutiger sikkimensis is found in cold waters in the high elevations in Sikkim Himalayas. Photo by B. Chettri.

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