In 1981, American biologist Paul Ehrlich proposed the ‘rivet-popper’ hypothesis, likening species in an ecosystem to the thousands of rivets that hold an aeroplane together. Remove one, and it may keep flying. Remove several, and it may still stay in the air. But take out a critical rivet, and the plane could collapse.“The trouble is, no one knows which species is that critical. Therefore, it is wise to protect all of them,” says K S Seshadri, researcher at the Ashoka Trust for Research in Ecology and the Environment (ATREE), Bengaluru. His work focuses on amphibian behavioural ecology, especially frogs of the Western Ghats. And frogs, say ecologists like Seshadri, may well be that rivet.The species is what a group of researchers from Chennai-based Care Earth Trust (CET), an organization engaged in biodiversity conservation, is studying in the city’s first-ever frog survey. “We aim to cover Greater Chennai Corporation limits,” says Rajkumar K P, wildlife biologist at CET, who is doing the study along with Vivekananthan M under the guidance of Ranjit Daniels.Preliminary surveys have documented eight species. “Chennai’s urban area is said to have at least 15 frog species, and yet no proper studies have been done on these creatures, their role in the ecosystem or the threats they face, so far,” says Rajkumar.In any ecosystem, urban or otherwise, frogs play a major role in controlling insect populations, says Rajkumar. The frog’s larvae are found in ponds, lakes or other aquatic ecosystems. “Adult toads and frogs mostly feed on insects. In the aquatic system, their larval forms, known as tadpoles, also feed on mosquitoes and other insects that lay eggs in aquatic ecosystems.” By controlling insect populations, frogs help control disease spread.Any pollution in the aquatic system causes stress on frogs from the tadpole stage onward. “If the oxygen level is poor in that ecosystem, the eggs decay. Also, if there is chemical pollution, their development is affected,” says Rajkumar.“In amphibians, their skin is permeable and plays an important role in gas exchange. Therefore the pollutants in the aquatic ecosystem will affect adult individuals by causing psychological stress, affecting respiration, causing diseases and they slowly die off. Over time, some species may adapt to withstand pollution and stress of urbanization. These aspects will also be studied during the survey.”Studies from Australia, Brazil and USA show that urban noise can interfere with the way frogs communicate. During the breeding season, male frogs use calls to attract mates, but traffic and other city sounds can drown out their calls, affecting breeding and reducing genetic diversity over time. Light pollution too can cause physiological stress. “The studies indicate that some frogs adapt by changing their calls to overcome noise pollution,” says Rajkumar. “This needs to be explored further in the India scenario.”That’s why species surveys are important, says Seshadri. “They help us understand why or how biodiversity manages to survive in cities. Little is known about the impact on biodiversity, especially on smaller, less charismatic organisms such as amphibians.”Frogs, he says, are a critical link in the food chain. “A healthy ecosystem requires all components such as insects, frogs, snakes and birds to make it functional. Researchers need to think about how their work can be taken to policy makers and nudge them to factor biodiversity into the equation when developing the city.”Toad tallyGreater Chennai Corporation city limits will be divided into grids based on building density and the availability of green spacesSurvey sites will include areas with 5% to 20% greenery, OSRs, parks, educational campuses, wetlands, burial grounds, gardens and remnant vegetation patchesFieldwork will be conducted mainly during the monsoon (talks on with authorities)Researchers will search for amphibians across microhabitats, from stagnant pools and leaf litter to wetlands and lake edges. Record of numbers, types, habitat and climatic conditions will be madeSchools and colleges may later be involved through a citizen science programmeThe findings will create baseline data to track how urbanization affects Chennai’s frog and toad populations over the next decadeCommon toads and frogs found in ChennaiThe Common Indian Tree Frog (Polypedates maculatus) builds foam nests over temporary pools, ponds or water tanks and lay their eggs inside the foam. Once the eggs develop into tadpoles, they drop into the stagnant pools below and begin feeding in the aquatic ecosystem, where they complete their development.The Skittering Frog (Euphlyctis cyanophlyctis) is a member of the family Dicroglossidae. They are capable of moving rapidly across the water surface for a distance of two to three meters, and their movement resembles that of a flat stone skipping across the water. This is made possible by their body shape and the webbing between their digits.
