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Sri Lanka’s marbled rock frog may not be on brink of extinction, modeling suggests

  • Using a combination of old-fashioned field work and new-fangled computer modeling, researchers in Sri Lanka have found that the critically endangered marble rock frog (Nannophrys marmorata) occurs over a potentially much larger range than previously thought.
  • In a new paper, they suggest that, pending further studies, this finding should qualify the species for removal from the critically endangered list, to which it was added in light of the much smaller range in which it was previously believed to occur.
  • The researchers say the discovery should also prompt more field surveys, and raise the possibility that another frog in the genus, N. guentheri, which is considered extinct, may be rediscovered this way.
  • At the same time, they also emphasize the need to protect the frogs’ habitats from threats such as the encroachment of illegal cardamom and tea plantations.

UDUDUMBARA, Sri Lanka — Twelve years ago, as a young undergraduate, Kanishka Ukuwela was trekking with his colleagues in the Ududumbara area along the 18 Hairpin Bends, a 6-kilometer (4-mile) ribbon of road in central Sri Lanka, and was surprised to find a rock frog by the roadside. They didn’t think it was one of the critically endangered rock frog species, commonly known as the marbled rock frog or Kirtisinghe’s rock frog (Nannophyrs marmorata), but still noted the sighting and the location.

They returned to the region for more surveys in 2008 and again in 2016, this time as a team of young researchers. From that latest survey, they brought back fingertip tissues from the frogs they found to confirm whether it was the same species they’d spotted the previous years.

There are four known species in the genus Nannophrys, all of them endemic to Sri Lanka. (One of the four, N. guentheri, is considered extinct, having not been seen in more than a century.) N. marmorata was thought to be largely restricted to the Pitawala Pathana area of the Knuckles Conservation Forest (KCF). But the confirmation lead researcher Kanishka Ukuwela and team obtained from DNA tests on the tissue samples and measurements taken of specimens in the field established that the species is found across a wider range than previously thought.

That finding, in a paper recently published in the Russian Journal of Herpetology, has led the researchers to recommend that the marbled rock frog be removed from the critically endangered category of the IUCN Red List. The listing, made in 2008, was based on what was then believed to be the very small range in which the species occurred — a range that the new study calculates is potentially 10 times larger.

Tiny rock crevices with enough moist make great daytime hiding places for the marbled rock frog. Image courtesy of Kanishka D.B. Ukuwela.

It took a combination of old-fashioned field work and new-fangled computer modeling to reach that conclusion. Drawing on their own observations and previous sightings, they took the data about the areas where the species was confirmed to have occurred, and fed this into an ecological niche modeling (ENM) application. The application then generated a map of locations with ecological conditions best suited for the species.

“We used ENM to predict that the eastern slopes of the Knuckles mountain range as the region with the highest habitat suitability for this threatened frog species,” Kanishka Ukuwela, an evolutionary biologist with the Department of Biological Sciences at Rajarata University, told Mongabay.

The ENM yielded a potential range, or extent of occurrence (EOO), of 831 square kilometers (321 square miles) — much larger than the previously established EOO of 81.7 km2 (31.6 mi2).

Another measure of range, the area of occurrence (AOO), which is the combined area of locations in which there have been confirmed observations of the species, was calculated as 255 km2 (98.5 mi2), compared to less than 10 km2 (3.9 mi2) previously.

“Since the species was placed in the ‘critically endangered’ category based solely on EOO (EOO<100km2) and AOO (AOO<10km2), the species can now be removed from the category as EOO and AOO are higher,” the researchers write in their paper. They add, “However, it is necessary to confirm the occurrence of N. marmorata in the predicted area before this can be revised.”

“It is still unknown whether the rest of the sanctuary supports N. marmorata’s habitats other than the area identified,” Ukuwela said. “But their possible existence in these areas cannot be ruled out without a through survey of the sanctuary.”

The researchers also say the discovery of the species outside its previously known range highlights the importance of further surveys — and raise the possibility that such a survey could turn up specimens of N. guentheri, the species declared extinct.

The VRR sanctuary in Ududumbara overlooking the Knuckles mountain Range. Image courtesy of Kanishka D. B. Ukuwela.

Amphibians under threat

The potentially expanded range of the marble rock frog means it likely also occurs outside the boundaries of the protected Knuckles Conservation Forest.

“The presence of these frogs adjacent to highly disturbed places such as highways and tea estates may indicate the ability of this frog to tolerate and survive in disturbed habitats up to a certain extent,” the researchers write.

But they emphasize that frogs, and amphibians in general, are extremely sensitive to their environment, with changes in temperature having a huge bearing on them.

“Habitat loss and habitat degradation are the key reasons for population collapse and extinction of amphibians,” Ukuwela said.

He added that throughout Sri Lanka, threats to amphibian survival are on the rise.

A recent amphibian review recorded 18 species as having recently gone extinct on the Indian Ocean island, the world’s highest rate of amphibian extinctions. Sri Lanka is a global amphibian hotspot, with most of the species that occur there found nowhere else on Earth.

Amphibian are the most threatened group of vertebrates globally, with nearly 41% of species facing the risk of extinction.

Researcher Kanishka D.B. Ukuwela examining a pond in Ududumbara in 2008 while searching for marbled rock frogs. Image courtesy of Imesh Nuwan Bandara.

In the case of N. marmorata, conserving the microhabitats it depends on is key to the species’ survival. According to Ukuwela, the frogs of this genus are closely associated with wet rock surfaces and boulders and are easily identified by their flattened bodies that enable them to live under boulders and in rock crevices (hence the name “rock frogs”).

“For their survival, these frogs need a thin film of water on the rock surfaces. The tadpoles don’t swim but live on moist rock surfaces. When rocks warm, this film of moisture evaporates, altering their habitat and the required microclimate for survival,” Ukuwela said.

While much of their habitat remains within protected area, the illegal encroachment of cardamom and tea plantations poses a threat.

“To conserve the marbled rock frogs,” Ukuwela said, “we need to continuously monitor both the habitat and the species.”



Ukuwela, K. D. B., Bandara, I. N., De Zoysa, H. K. S., Rupasinghe, U. A. L. D., & Vandercone, R. P. G. (2020). New localities, distribution and habitat modeling of the critically endangered Sri Lankan frog Nannophrys marmorataRussian Journal of Herpetology27(1), 33–40. doi:10.30906/1026-2296-2020-27-1-33-40


Banner image of an adult male marbled rock frog (Nannophrys marmorata) from Ududumbara in central Sri Lanka, courtesy of Kanishka D. B. Ukuwela.


Dilrukshi Handunnetti is a staff writer at Mongabay. Find her on Twitter: @DilrukshiH

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