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 per cent 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.