- More than half of the habitat suitable for muggers falls outside the existing protected areas, leaving much of the species’ potential habitat exposed to human pressures.
- Smaller, concentrated wetland patches support muggers more than large continuous water bodies, implying that conserving smaller wetlands may be as important as protecting large river stretches.
- Instead of creating new protected areas, the researchers call for targeted management in priority habitats with attention to stretches of river that sit between the formal islands of protection.
From the rocky gorges of Hogenakkal to the coastal marshes of Muthupet, the mugger crocodile has long inhabited the rivers, reservoirs and wetlands of the Kaveri river basin in southern India. A new study estimated that while there are about 2,209 square kilometres of potentially suitable mugger habitat in the river basin, only about 38% falls within the existing protected area network, leaving much of the species’ habitat exposed to human pressures.
The study, published in the journal Ecological Applications, assesses mugger habitat across the entire Kaveri (Cauvery) river basin which stretches across parts of Karnataka, Kerala and Tamil Nadu and covers about 81,155 sq. km. in southern India.
“The number that stays with me is 38%. Only 38% of the habitat our model identified as potentially suitable for muggers falls within the existing protected area network. That means roughly six in every ten hectares of meaningful mugger habitat is outside any formal protection, unmanaged, unmonitored, and subject to the full range of human pressures,” says co-author Nikhil Whitaker, the Research Director at Madras Crocodile Bank Trust and Centre for Herpetology.
Building a basin-wide picture
The mugger is one of India’s three native crocodilian species and is found in rivers, lakes, reservoirs and marshes across much of the subcontinent. Although populations have been studied in river systems such as the Moyar, Kabini and Amaravati, researchers say conservation planning has lacked a basin-wide understanding of where suitable habitat occurs and what factors shape its distribution.
“The CRB (Cauvery River Basin) has been reshaped dramatically, with dams, agriculture [fields] covering nearly two-thirds of the land, urban expansion, and roads cutting through what were once continuous riverine habitats,” says corresponding author Rahul Gour, who is also a conservation researcher at the Kenneth Anderson Nature Society. “We had scattered sightings, local surveys, and some knowledge of where muggers had historically occurred, but no basin-wide picture of where they actually persist today, and more importantly, where they could persist if given the chance.”
To build that picture, the researchers compiled mugger records from field surveys, published studies, biodiversity databases and other sources. After filtering the data, they analysed 82 occurrence locations across the basin. They then examined how crocodile presence was related to environmental variables, such as wetlands, hydrology, climate, topography, land use, and human infrastructure.
Unlike many previous habitat studies that examine environmental factors at a single scale, the researchers evaluated each variable across multiple scales, ranging from 500 m to 8 km. The approach allowed them to examine how local habitat conditions and broader landscape features influenced mugger habitat suitability.
“A mugger’s world is not flat. When it basks on a sandbank, it is responding to the immediate microhabitat. But whether it can reach that sandbank at all depends on whether the river stretching 50 km upstream still has enough flow, whether a dam has interrupted connectivity, or whether there is wetland cover spread across a much wider area,” says Gour.
What makes a good mugger habitat?
The analysis identified several factors associated with suitable habitat. One of the strongest predictors was isothermality, a measure related to temperature stability. “Muggers are ectotherms; they rely on external heat for thermoregulation. So consistent, predictable temperatures are not a luxury for them; they are a physiological requirement,” says Gour.
Hydrological connectivity also emerged as an important factor. At broader scales, habitat suitability was strongly associated with flow accumulation, a measure of how much water from a watershed converges through a particular location. “This captures whether a river reach is perennial and well-connected to the larger river network, or is an isolated, seasonally drying stretch that offers little security,” says Gour.
Distance from roads also influenced habitat suitability. Areas farther from roads generally had higher predicted suitability, reflecting how disturbance and human activity shape crocodile distribution.
One finding surprised the researchers. Conventional wisdom suggests that large rivers and expansive wetlands provide the best crocodile habitat. Instead, the model found that muggers were more strongly associated with smaller, more concentrated wetland patches than with large, continuous water bodies. “At first, you would assume more water equals better crocodile habitat. But it likely reflects prey dynamics. Smaller, discrete wetlands concentrate fish and other prey, reduce competition, and offer the shallow [river] margins that juveniles need for cover,” says Gour.
The finding implies that conserving smaller wetlands, seasonal marshes and oxbow lakes along floodplains may be as important as protecting large river stretches.
The researchers were also surprised by the importance of temperature stability. Gour says that in some parts of the basin, thermal conditions appeared to be a stronger constraint on habitat suitability than proximity to water, suggesting that climate change could alter habitat quality even in landscapes where water remains available.
Priority areas beyond protected reserves
Using the habitat model, the researchers mapped potentially suitable mugger habitat across the basin. They then combined these habitat maps with information on human pressures, including settlement density, population density, dam density and non-wetland areas, to identify conservation priorities. The analysis identified approximately 990 sq. km. of priority habitat, including 77 sq. km. classified as very high priority and 167 sq. km. classified as high priority.
Five regions emerged as particularly important conservation priorities outside the existing protected area network. These include the Krishna Raja Sagar backwaters and the river corridor extending toward Ranganathittu Bird Sanctuary in Karnataka, as well as parts of Tamil Nadu such as a corridor stretching from Pennagaram to the southeastern flank of the Stanley reservoir (Mettur Dam), stretches of the Kaveri downstream of its confluence with the Amaravati river, sections of the Kollidam river in Mayiladuthurai district, and the coastal wetlands of Muthupet and Kodiyakarai. Together, these regions cover about 540 sq. km.
Whitaker says these are landscapes that rarely feature in discussions about crocodile conservation. “The mugger is, in a sense, a species that has been protected by accident in places where the habitat happened to fall inside reserves established for tigers or elephants,” he says. “This study makes the case that we need deliberate, mugger-specific conservation attention in the stretches of river that sit between those formal islands of protection.”
Many of these river stretches are also places where people and crocodiles have shared the same waters for generations. In areas of higher human populations, people bathe, wash clothes and carry out other daily activities in these rivers, bringing them into frequent contact with crocodiles. In some places, that coexistence has produced conflict.
The researchers argue that the next step is not necessarily the creation of new protected areas. Instead, they call for targeted management in priority habitats, including monitoring populations, protecting nesting sites from sand mining, restoring wetlands and improving responses to human-crocodile conflict.
Sand extraction from riverbeds is a particular concern. According to the researchers, successful reproduction depends on females being able to find suitable sandy banks with the right physical and thermal conditions for nesting. The loss of these sites could reduce breeding success even in areas where adult crocodiles remain present.
They also argue that conservation planning needs to begin identifying thermal refugia under future climate scenarios. Thermally stable stretches of river, particularly in the forested gorge systems of the central basin, may become increasingly important as temperatures become more variable.
“The mugger is resilient. It has survived millions of years of change. But it is not infinitely resilient to losing the pockets of stable, connected, low-disturbance habitat that remain. The window to act in the Kaveri is still open,” says Gour.
Banner image: A local fisherman and a mugger use the same stretch of the Kollidam river, illustrating the potential for human-crocodile interactions in shared riverine habitats. Image by Jason Dominic Gerar.
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