Urbanisation in the Western Ghats could be altering frogs, says study

In India, urbanisation and land-use changes are driving forest loss across biodiversity hotspots, including the Western Ghats which support over 250 amphibian species. A new study based in Udupi, a Tier-2 city at the foothills of the Western Ghats, suggests that urbanisation may not necessarily lead to species loss, but may be reshaping frog communities.

The study, published in Urban Ecosystems, suggests that urbanisation is altering amphibian communities by filtering out species according to certain traits such as body size, reproduction and habitat use.

Species with specialised traits – such as arboreal (tree-dwelling) or fossorial (burrowing) frogs, direct-developing species (that hatch directly as frogs, bypassing the tadpole stage) or those with a larger body size – were associated with less-urbanised habitats farther from the city centre. In contrast, generalist species with more adaptable traits showed greater urban tolerance.

“The study’s most novel finding is that urbanisation acts as a trait filter rather than simply a diversity filter,” said Aravind NA, Senior Fellow at the Ashoka Trust for Research in Ecology and the Environment (ATREE) and the study’s supervising author.

While traditional biodiversity assessments may conclude that urbanisation has either little or drastic impact based on species counts, trait-based analyses can reveal how landscape changes restructure amphibian communities, Aravind explained. “This finding is particularly important because it highlights hidden biodiversity changes that may precede measurable species losses,” he added.

Although around 40% of amphibian species are threatened with extinction globally, little is known about how amphibian communities respond to urban pressures, especially in the Global South.

Urbanisation favours generalists

The study was conducted on laterite plateaus – unique rock outcrop ecosystems – in Karnataka’s Udupi district. Although they support endemic and threatened biodiversity, these plateaus remain understudied and “many of the [Western Ghats] foothills are now considered as wasteland”, said Madhushri Mudke, the study’s lead author and currently postdoctoral researcher at the Centre for Wildlife Studies. This classification leaves laterite plateaus prone to land-use changes, with several being lost to agriculture, urban expansion and mining.

“To see how species are distributed on the foothills, we decided to do a gradient approach,” said Mudke. The team studied frog communities at 23 sites radiating outwards from Udupi’s city centre towards the forest, including eight forest sites, 13 city sites and two at the city-forest edge. Over two monsoon seasons in 2018-19, they recorded 947 individuals across 19 frog species.

“Although urban, edge, and forest sites supported broadly similar levels of species richness, the species occupying these habitats differed substantially,” said Aravind. Species count and diversity peaked at the urban-forest edge, suggesting that moderately urbanised landscapes can retain significant biodiversity, though the authors advised caution when interpreting patterns given the small number of edge sites sampled.

Unlike metropolitan cities, Udupi has retained its diverse natural habitats, including sacred groves, which may explain why amphibian diversity remained stable across sites, they noted. Mudke added that the inclusion of sacred groves in the study shows “these spaces preserved through decades of cultural practices are also important and need to be recognized.”

At each site, researchers also recorded environmental variables like temperature, elevation and humidity, and anthropogenic pressures such as roads, artificial light, and noise. They then used statistical modelling approaches to study associations between environmental variables and species traits.

According to their analysis, specialist frogs were associated with higher elevations, cooler microclimates, and greater microhabitat availability farther from the urban centre. “These species often depend on specific microhabitats, stable moisture regimes, intact vegetation, or specialised reproductive environments that are frequently lost during urban development,” said Aravind.

Generalist species have broader ecological tolerance and greater flexibility, and can exploit a wide range of habitats, including artificial or modified environments, he added. “As a result, urbanisation tends to favour generalists while selectively excluding specialists, leading to functional homogenisation of amphibian communities,” Aravind said.

Microhabitats support frog communities

Researchers also found that microhabitats such as ephemeral pools, leaf litter and rocky crevices may buffer some of the impacts of urbanisation, highlighting the need to maintain diverse microhabitats to support frog communities in rapidly-urbanising environments.

Microhabitats provide refuge, control temperature and humidity, and provide breeding opportunities. “Even one or two degrees of change will affect amphibians” as they are highly sensitive to environmental changes and act as ecosystem indicators, Mudke said.

Frogs belonging to the Indirana genus, for example, produce tadpoles that depend on moist rocky patches for survival – a tiny microhabitat otherwise considered of no use. Removing these rocky patches could “remove a whole trait that is important for the survival of one species,” she explained.

Karthikeyan Vasudevan, herpetologist and scientist at CSIR-Centre for Cellular and Molecular Biology found the study “informative” as it shows that “intense human-use areas have paramount diversity.” He was not associated with the study.

But since the study was conducted in a single Tier-2 city over a relatively short period of time, its findings cannot be generalised to other cities, Aravind pointed out, adding that the differences observed in community composition while “statistically significant” were “relatively weak”.

According to Vasudevan, elevation may be a confounding factor as elevated areas are less urbanised and more forested due to lower accessibility. Detectability could also be a concern, as some species may be easier to observe than others, he added. Vasudevan also suggested studying deformity and scale mass index (a measure of an animal’s body weight relative to its specific size) to obtain a measure of their body condition, especially in urbanised areas.

Biodiversity-sensitive urban planning

“Most of this work feeds into regulating how we look at development,” said Mudke. “We have to look at cities as areas that can retain microhabitats because they are not just home to biodiversity, but they also act as spaces for ecosystem functioning,” she said.

The study’s findings highlight the significant biodiversity that Tier-2 cities like Udupi and lateritic plateaus hold which, Mudke added, needs to be recognised. They also provide guidance for biodiversity-sensitive urban planning in tropical cities. “All our [India’s] Tier-2 cities are growing rapidly and this sort of study gives insight into how things can be better planned,” said Vasudevan.

Aravind emphasised the need for long-term monitoring to determine whether the observed communities are stable or will undergo further change as urbanisation progresses. Further studies could look at the individual effects of urban stressors such as artificial light or noise pollution.

“It would also be valuable to examine whether urbanisation reduces genetic connectivity among amphibian populations and whether similar trait-filtering processes occur across other tropical cities,” Aravind said. “Such research would improve our understanding of how urban development shapes biodiversity over longer timescales and across broader geographic regions,” he added.

This article was first published on Mongabay.