Spotted deers spell doom for mangroves in Andamans

This proliferating population of spotted deer is damaging mangroves in the Andaman Islands, reports an April 2024 study by researchers from the Wildlife Institute of India (WII). In their study, published in the Journal of Wildlife Science, the researchers report a decline in the diversity of mangrove species in islands with a high spotted deer population. In contrast, in areas where there are no reported spotted deer, the researchers found a greater number of mangrove species.

The findings signal an urgent need for policies aiming to eradicate spotted deer in order to conserve “mangroves and marine natural resources unique to the island ecosystem,” the researchers write in their paper.

Invasive Species

Scientists term organisms that are not native to a region as ‘invasive species’. These organisms – like the spotted deer in the Andaman Islands – may mean havoc for the local ecosystem. A popular example is that of the water hyacinth, an aquatic plant native to South America. Introduced to India in the 19th century by a British officer, its thick cover quickly colonised Indian water bodies and is held responsible for stunting the growth of native plants and affecting the fishing industry.

According to K Sivakumar, a professor of ecology at Pondicherry University, island habitats like the Andamans are particularly vulnerable to invasive species. Other than the spotted deer, the Andamans are at risk from several other invasive species, viz. elephants, the common myna and the house sparrow. The latter two have been reported to be competing with native starlings and parakeets by a 2013 study from the Salim Ali Centre for Ornithology and Natural History, Tamil Nadu.

The Study

In the 2024 Journal of Wildlife Science study, Nehru Prabakaran, a scientist at the Wildlife Institute of India, led a team to compare the diversity of mangroves across different regions of the Andaman Islands. In regions where there were no spotted deer, the team found a greater diversity of mangroves in the seedling and sapling stages. In contrast, regions with reported spotted deer populations had a greater diversity of mangroves only in the larger tree stages.

The authors concluded that the spotted deer might be grazing on younger mangrove seedlings and saplings, thus preventing their growth. This puts 25 mangrove species native to the Andaman Islands at risk, the authors note, adding that the two most affected species were Exeocaeria agallocha and Avicennia marina.

Further, in regions that had a relatively higher number of spotted deer, the most dominant mangrove species was Ceriops tagal, a species that the spotted deer find least palatable. Put simply, the invasion of the Andaman Islands by the spotted deer is changing the diversity and distribution of mangroves in the region.

In the long run, this would lead to a diminishing of the “overall diversity and structural integrity” of the mangrove ecosystems, Prabakaran and co-authors wrote in their paper.

Impacts

Prabakaran told this reporter that the Andaman archipelago is one of the most mangrove-diverse regions of the country. With that diversity under threat from the invasive spotted deer, there might be a threat to the larger ecosystem of these islands, Sivakumar, the Pondicherry University ecologist, added.

Mangroves contribute to ecological sustenance in several different ways, of which Sivakumar pointed out three. One, they reduce the impact of tsunamis by interrupting both the height and velocity of the incoming waves. Two, mangroves act as habitats for many different organisms, including a wide variety of fishes and birds. Finally, mangroves are extremely efficient at sequestering and storing atmospheric carbon.

Carbon sequestration is the process of capturing carbon dioxide from the atmosphere, while carbon storage is the long-term confinement of the sequestered carbon. According to one estimate, mangroves might be sequestering carbon at a rate ten times higher than tropical forests. Owing to their extraordinary ability to sequester and store carbon, mangroves have been thought to help avert climate change significantly.

By reducing the number and diversity of mangroves in Andamans, the increase in spotted deer population might be spelling doom for how coastal communities and ecosystems are impacted by natural disasters and climate change. Even when the islands are not under threat from natural disasters, grazing of mangrove species might be robbing different plants and birds of their habitats. This, in turn, might reduce the diversity of fauna in the Andaman Islands.

Further, Prabakaran added that “mangroves in protected areas like the Mahatma Gandhi Marine National Park are a crucial source of marine and fishery resources ensuring the sustainability of local livelihood.” Thus, rescuing these forests from spotted deer is critical also for safeguarding the livelihood of local communities.

Management and Control

As early as 2001, Sivakumar and his team had recommended to the islands’ authorities to eliminate the spotted deer population. “Somehow, they couldn’t take our recommendation seriously at that time,” he said. Since then, the population of spotted deer in the islands has grown by leaps and bounds. Prabakaran and his team's recent study demonstrates the consequences of this unchecked growth.

Both Prabakaran and Sivakumar suggested that it is time to control the spotted deer population and give the mangroves a chance at survival. This could be achieved in two ways: by relocating the deer, or by culling them.

While it might sound controversial, culling has been adopted as a measure to control invasive species populations elsewhere in the world. For example, in 2020, the Royal Society for Protection of Birds and the Tristan da Cunha authorities led an operation to cull invasive house mice on Gough Island, where the mice were threatening the Tristan Albatross and the Gough Bunting – birds native and endemic to the island. In 2023, authorities of the Kosciuszko National Park, Australia, started culling invasive wild horses that were threatening endangered species in the park.

In fact, relocation might not even be a feasible option in the case of spotted deers in the Andamans. Not only is capturing and transporting them an arduous task, but their relocation may also introduce to their new habitat diseases that they may be carrying. For a successful relocation, the deers would need to be first quarantined and then released into the wild – a process that would be both expensive and logistically challenging.

“I am an animal lover,” said Sivakumar, “but we cannot be emotional in this matter.” There may be no other alternative, he adds.

Sayantan Datta (they/them) is an assistant professor at the Centre for Writing & Pedagogy, Krea University, and an independent science journalist.