- A new study in the Western Ghats examines the genetic diversity and population genetic structure of the freshwater Kempholey night frog.
- Developmental pressures in the region can lead to fragmentation of the frog’s habitat, which can reduce the connectivity of populations and lead to reduced genetic diversity.
- Researchers suggest designing conservation strategies focused on enhancing gene flow among the various populations at the stream level.
A new study in the Western Ghats has found that, contrary to popular belief, river drainage boundaries were not strong barriers to the movement of aquatic Kempholey night frog. Instead, genetic variation among different species was mainly shaped by the distance between them. The Kempholey night frog, endemic to the Western Ghats, prefers headwater habitats and has a limited ability to disperse, which influenced the genetic differentiation of the frog, at a localised level, the study showed.
Little is known about the genetic diversity of freshwater frog species living in the Western Ghats. Only five studies so far have assessed fine-scale genetic variation in Indian amphibians. For aquatic or stream-dependent species, it was believed that river basins act as barriers and influence genetic variation. This study examines the genetic diversity and population genetic structure of the widespread freshwater Kempholey night frog (Nyctibatrachus kempholeyensis) and finds that river basins were only minor barriers. These findings will aid in the conservation of the species.
“The results of this study also have implications for inter-river basin water transfers. Such methods can change the community structure of aquatic biota, introduce exotic species, and can also erode the genetic integrity of differentiated populations,” says Priti Hebbar, Assistant Professor at the Manipal Insititute of Technology, Manipal Academy of Higher Education, Bengaluru, who led the study.
Consequently, Hebbar suggests avoiding linking river basins for water transfers. A river basin is comprised of multiple stream orders, which are designated as first-order, second-order and so on, explains Hebbar. They join to form bigger orders.
Distributed across Kerala and Karnataka, spanning approximately 400 kms., the Kempholey night frog is found in first, second and third-order streams. Its population trends are unknown and the species is listed as data deficient by the IUCN.
There are 34 frog species of the genus Nyctibatrachus, all endemic to the Western Ghats. Mountain gaps were shown to be barriers to the dispersal of Nyctibatrachus species.
“This study provides an overview of genetic patterns across the range of N. kempholeyensis that illuminates the connection between its biology and the geographic context for its population genetics and it provides a solid basis upon which meaningful management decisions can be made,” says herpetologist Aaron Bauer, professor at Villanova University, United States, who was not involved in the study.
“Despite the critical role that frogs play in tropical ecosystems, and their tremendous diversity, they have been understudied, particularly with respect to species in North America or Europe,” notes Bauer, stressing that “frogs are major contributors to the high biodiversity of the Western Ghats and the area is the most significant hotspot for amphibian diversity in India and South Asia more broadly.”
According to Abhilash Nair, a researcher at the University of Helsinki, who was also not connected to the study, this research is crucial for effectively conserving this poorly studied species. “The findings suggest the need for conservation strategies that consider the high genetic structure in this species and try to enhance gene flow among populations,” says Nair.
The Western Ghats, a 1,600-km.-long chain of mountains along India’s west coast, shelters about 230 amphibian species, around 90% of which are endemic to the region. Over 41% of global amphibian species are facing extinction. To protect them, it is vital to understand the genetic variations within and among amphibian species which influence their fitness and survival.
Rivers are not a barrier but distance matters
The team explored the genetic diversity, and structure of N. kempholeyensis at two spatial scales between the years 2013 and 2019.
At a larger scale, the team sampled the streams of west-flowing river basins like Kali, Bedti, Aghanashini, Sharavathi, Nethravathi and Suvarna as well as streams of east-flowing river basins like Tunga, Kaveri and Kabini of the Karnataka and Kerala region of the Western Ghats. They collected tissues from tadpoles and adults and extracted and sequenced DNA. Using markers from both the mitochondria (an organelle inherited from the mother) and nuclear DNA, they analysed the phylogeographical (link between geography and genetic diversity) and population genetic structure at multiple geographic scales.
At a regional scale, the team assessed the fine-scale genetic variation of the Kempholey night frog across four river basins in the central Western Ghats by analysing small pieces of repeating DNA known as microsatellites.
The findings suggest that the Kempholey night frog is distributed in the first and second-order streams of a river basin and appears to be a headwater specialist, Hebbar says. In an earlier study, the team found that the Kempholey night frog prefers slow-moving streams and hence it is not found near rivers, which are deep and wide, explains Hebbar. “It perhaps moves along the stream corridors. The region’s heavy rainfall joining the streams allow its movement.”
The researchers were surprised to find that river drainage boundaries were not barriers to the movement of the Kempholey frog. “Generally, for aquatic, stream-dependent species, river drainages act as barriers. But here, geographical distance between the populations mattered more than the river drainages,” Hebbar points out.
“The limitation of this frog to the headwater streams in these river basins helps to account for why distance is more important,” Bauer points out. “Since lower, larger, deeper river courses are inappropriate habitat, they cannot simply spread along waterways to occupy the whole of a river system,” he explains. “However, the degree of divergence between populations is limited, suggesting a relatively recent expansion of the species.”
Conserving the Kempholey night frog
G. Ravikanth, a senior author of the study, says that the increased genetic divergence observed in peripheral populations should be considered when planning for conservation efforts. “Additionally, conservation strategies should be designed to enhance gene flow among these populations at the stream level,” adds Ravikanth, Associate Professor at the Suri Sehgal Centre for Biodiversity and Conservation, Ashoka Trust for Research in Ecology and the Environment (ATREE), Bangalore.
The genetic distinctiveness of populations holds significant importance in delineating conservation units, state the researchers. “Based on our phylogeographical and population genetic analyses, we propose that the populations originating from the Bedti and Aghanshini river basins, as well as the population from the Kabini river basin, should be regarded as separate management units,” suggests Ravikanth. “These populations exhibit clear differences in mitochondrial haplotypes, suggesting historical isolation of maternal lineages. Particularly noteworthy is the population in the Kabini river basin, which not only lacks geographical overlap with all other populations but also resides at the species’ distribution range periphery,” Ravikanth notes. Mitochondrial DNA haplotypes refer to specific regions of DNA from the mitochondria that cluster with other mitochondrial sequences to show the phylogenetic origins of maternal lineages.
The headwater habitat of the Kempholey night frog affects conservation decisions, says Bauer. “Headwater habitats in general can be subject to human-mediated destruction or degradation so amphibians limited to these habitats may be at higher risks than those that are more widespread within river systems,” he warns. “Further, the poor dispersal ability of these frogs has contributed to their regional-scale population structuring so that some populations are represented by unique haplotypes and therefore conservation actions limited to only small areas will not be sufficient to preserve genetic diversity in the species as a whole,” Bauer explains.
The Western Ghats have faced sustained developmental pressures such as timber extraction, agriculture and roads and railway expansion. Converting streams for agriculture can wipe out habitats for the Kempholey night frog. “Habitat fragmentation can reduce the connectivity of the amphibian populations leading to the reduction in genetic diversity,” explains Hebbar. “Hence, it is necessary that such activities be kept at the minimum, preserving core forest areas.”
In the future, more genetic studies from amphibians of the Western Ghats are needed for a better understanding of the genetic diversity of the species, stresses Hebbar. “Studies on how landscape influences the movement of amphibians can also be helpful in conserving them.”
Banner Image: N.kempholeyensis with eggs. Photo by Aravind CK.