- A systematic survey of dung beetles recorded 20,927 beetles across 29 species on a barrier island in Kerala.
- Despite heavy urbanisation, the islands supported an unexpectedly high dung beetle diversity.
- Researchers, however, warn that urban expansion, habitat fragmentation, flooding, and climate pressures could still threaten these fragile island ecosystems.
Barrier islands are narrow strips of land between the sea and the mainland, shaped and reshaped by tides and wind. They shield coastlines from storm surges, support unique biological communities and play an important role in nutrient cycling. A new study focuses on one such barrier island system in Kerala to examine dung beetles, insects that play a critical role in keeping ecosystems functioning.
Dung beetles break down and bury dung, contribute to soil nutrient cycling, improve soil aeration and hydrology, aid seed dispersal, and reduce parasite and disease loads. This study, at the Vypin-Kadamakkudy Barrier Island System (VKBIS), breaks new ground as it systematically studies dung beetles in a tropical barrier island. It is also the first such initial documentation of dung beetles in Kerala and the first study of the dung beetle subfamily Scarabaeinae in this barrier island system.
“While designing the study, we expected clear differences in species composition, including the possibility of new records, given the island setting. We also anticipated lower species richness and abundance because the area is densely populated and rapidly urbanising,” says Chitra Rajagopal from the Department of Zoology, Nirmala College (Autonomous), Kerala, and the study’s corresponding author. “However, the VKBIS emerged as a surprisingly strong habitat for dung beetles, with species richness comparable to, or even higher than, many other natural ecosystems.”
An urbanising island system
The Vypin-Kadamakkudy Barrier Island System lies along the western coast of Kochi in Kerala. For the study, researchers sampled two subsystems: Vypin Island, the barrier island that separates mainland Ernakulam from the Arabian Sea, and the Kadamakkudy lagoonal island cluster, situated between Vypin and the mainland. Growing infrastructure, especially bridges linking the islands to Kochi, is steadily increasing connectivity with the mainland, pushing the system toward functioning more like part of the mainland than a separate island ecosystem.
The sample collection was carried out between 2019 and 2021, with limited sampling in 2020 due to COVID-19 restrictions. Dung beetles were sampled using dung-baited pitfall traps — plastic basins buried at ground level and baited with cow dung, with a salt-soap solution added as a preservative. The traps were covered to prevent drying and wash-off, and placed at least 100 metres apart across different habitat types, including grasslands, backyards, farms, abandoned plots, and playgrounds, in both subsystems. Sampling was conducted in dry and wet seasons, twice per season, with a total of 480 traps deployed.
Some of the biggest challenges in the study came during sampling. Sourcing dung, transporting trap materials, and moving between sites were often slow due to poor connectivity and narrow roads. Choosing sampling locations was also cumbersome because the landscape is fragmented and heavily urbanised. “In the field, traps were frequently disturbed by people, livestock, or other animals, making it hard to maintain consistent conditions. The tropical climate added further complications, with heavy rain, high humidity, and flooding, especially during the wet season. Securing permissions and cooperation from local communities was another challenge,” says Anu Anto from the Department of Zoology, St. Xavier’s College for Women (Autonomous), Kerala, and co-researcher.
Traps were checked every 12 hours to distinguish between day- and night-active species. The collected beetles were retrieved using sieving and flotation, preserved in ethanol, and then identified.
Dung beetles were grouped by how they use dung — dwellers (which live in dung), tunnelers (which dig it), and rollers (which move it away). Activity patterns were analysed by comparing beetles collected during the day and night. Species showing no significant difference between the two were treated as generalists, while others were classified as primarily diurnal or nocturnal.
The study also assessed diversity using ecological metrics, including species richness, diversity indices, seasonal variation, and how similar or different the communities were across seasons.
Diversity despite pressure
The study recorded 20,927 dung beetles across 29 species. One species, Onthophagus cervus, was predominant, making up over 40% of the total. Two species were not previously recorded from Kerala, Catharsius pithecius and Digitonthophagus bonasus. Most of the recorded species were tunnellers, while there were a few dweller species. Rollers were entirely absent.
Overall, the system showed moderate dung beetle diversity, and sampling estimates suggested the survey captured nearly all species present in VKBIS.
“The most important insight from this study is that tropical barrier islands, even with increasing urbanisation, can be surprisingly strong repositories of biodiversity rather than degraded, species-poor areas,” says K.V. Vinod from the Department of Zoology, Nirmala College (Autonomous), and co-researcher.
Seasonally, dung beetles were more abundant and diverse in the dry months, though all 29 species were recorded throughout the study period. This runs against patterns often reported elsewhere, where wet seasons typically support higher dung beetle diversity.
In terms of activity, nocturnal species dominated, followed by diurnal species, with very few generalists. This pattern held across seasons, although a few species shifted their activity between day and night in response to seasonal conditions.
Dispersal and filtering
But how do beetles reach and persist on islands like these? Researchers say dispersal likely happens primarily through flight. “Active flight is the dominant dispersal pathway for most dung beetle species. Many are strong fliers capable of locating dung over considerable distances, allowing them to colonise isolated habitats such as barrier islands,” says Rajagopal.
At the same time, many of the species recorded here are small-bodied, which may limit how far they can travel. “Secondary pathways may also contribute, including passive transport via drifting vegetation, animal movement, or human activities such as the movement of livestock, soil, and organic matter that can carry beetles or their larvae,” she adds.
In a landscape like VKBIS, where islands are closely linked to the mainland through routine human and animal movement, these pathways likely overlap.
Since the study does not directly compare island and mainland systems, it stops short of making definitive claims about how these communities differ. “But such island assemblages are expected to reflect a subset of species able to disperse and persist under these conditions,” says Anto.
These populations offer a glimpse of how species persist in fragmented, human-altered landscapes while retaining the potential to perform key ecological roles such as nutrient recycling, soil aeration, and seed dispersal.
Resilience with limits
The study shows VKBIS supports a substantial dung beetle community, even under pressure. The dominance of tunnelers and the absence of rollers, however, points to a constrained community structure. The system remains vulnerable.
“Key threats include habitat loss and fragmentation due to rapid urban expansion, reduced dung availability, and the use of veterinary drugs such as ivermectin that can render dung toxic to beetles,” says Vinod. Climate stressors, such as flooding, salinity intrusion and extreme weather, add further pressure. Parts of this system lie within coastal regions projected to be submerged in the coming decades, making the need to document and understand these ecosystems more urgent.
The study suggests maintaining dung availability through livestock, protecting mangroves, reducing excessive use of veterinary drugs, and creating links across fragmented patches. Limiting coastal degradation and investing in long-term monitoring are also essential.
The findings sharpen how these landscapes are understood. “Barrier islands can be surprisingly resilient and ecologically important, supporting diverse communities even under increasing human pressure,” says Rajagopal. The researchers also hope their findings will serve as a stepping stone for future studies on dung beetles in barrier island systems across the world.
Read more: Why do dung beetles matter to climate change?
Banner image: A representative image of Digitonthophagus bonasus dung beetle. Image by Len Worthington via Wikimedia Commons (CC BY-SA 2.0).