A bullfrog invasion in the Andamans is threatening biodiversity

In December 2004, an unexpected wall of water swept across the Indian Ocean, leading to death and devastation across several countries, including the slender sliver of land in Bay of Bengal – the Andaman archipelago. The 2004 tsunami was one of the worst natural disasters in recorded history, its ruinous thumbprint yet to be erased.

A few years earlier, however, unknown to anyone for years, silently but surely, a different ripple had slunk across the islands – the march of the Indian bullfrog (Hoplobatrachus tigerinus). According to the researchers studying the invasion, the introductions seem to have been in 2000-’01 though rapid expansion has clearly taken place from 2009 onwards.

“It is very frightening,” said Karthikeyan Vasudevan, senior principal scientist at the Hyderabad-headquarteredCentre for Cellular and Molecular Biology. Just last week, Vasudevan got a text message that the large Indian bullfrog, which is an alien invasive species, had moved to the Little Andaman, south of Andaman chain of islands.

The first report of the bullfrog on the islands was in October 2011 from the Middle Andaman island. “In 2008 no one even knew it. Now it is almost everywhere,” said Vasudevan.

This is a rare phenomenon, a biological invasion, that scientists are seeing as it unfolds – kind of like watching a car crash in slow motion. All invasive alien species go through a cycle of introduction, naturalisation and expansion – a process being played out at present on the islands. Such species often have severe impacts on endemic flora and fauna, by disturbing delicate ecological balances (impacts such as competition for resources, predation etc).

Since the 17th Century, invasive alien species have contributed to nearly 40% of all animal extinctions for which the cause is known, a factor second only to habitat destruction. On islands, the situation is even more serious. According to a global IUCN study, invasive vertebrates were present in 60% of all islands with highly threatened vertebrates. More often than not, these alien species are spread by human interventions – through travel, or as novelty items or pets.

People moved the bullfrog from island to island

Nitya Mohanty, a doctoral student at the Centre for Invasion Biology at the Stellenbosch University, South Africa, who is studying the spread of this bullfrog through human assistance, heard local people reporting about the bullfrog when he was working on another invasive species – the spotted deer. “The bullfrog was much more widespread than it was previously thought to be. Since scientists generally don’t get to document the initial stages of an invasion, it was a study worth undertaking,” said Mohanty, who is examining the factors that are spreading it.

The 2004 tsunami had messed with the fish stock in the Bay of Bengal that led to losses for fisherfolk. This, in turn, led to the import of fish seed as a boost to livelihoods. “It is possible that the bullfrog was brought in with fish seed in big tanks, which were given as subsidies for farmers,” said Vasudevan. This theory is as yet only a speculation.

Once an undetected introduction happened, it spread stealthily, like a rumour on the grapevine, noticed only after it has reached a critical mass. In 2015-2016, when Mohanty was surveying sites for invasive species, 59 sites out of 91 surveyed included the bullfrog and it was not present on three islands. “Now we are redoing the survey, and we see that it has reached Little Andaman – 60 km across Duncan passage (Rutland island to the North and Little Andaman to the South).”

According to him, the bullfrog could have crossed the wide saltwater barrier when people took it across. They may have either wanted to keep the frogs as novelty species in their water bodies, or they would have wanted to eat them. It is a hefty species – 150 mm in length with chunky thighs.

The Indian bullfrog is a good five times larger than native frogs found in the Andaman islands. Mohanty is analysing the diet of the frog to find out what they have been feeding on since invaders can impact local biodiversity by either outcompeting for resources if diets with native species overlap or by feeding on native species. “We found native frogs in the stomachs of bullfrogs, the blind snake (Typhlops oatesii), which is endemic to the Andamans, the emerald gecko (Phelsuma andamanensis), skinks, some rodent species which we haven’t identified yet. It is clear that the bullfrog feeds on large prey from invertebrates to mammals,” exclaimed Mohanty.

The bullfrog is not alone

There have been earlier invasions in the Andaman group of islands. From the spotted deer (Axis axis), which is also known as the chital, to the house sparrow, giant African snail and common myna – all of these species have made forays into these islands. Island ecosystems are of course especially sensitive to disturbances because new species, complete unknowns, tip the species balance into unpredictable directions. For example, the spotted deer, as reported by Mohanty, Vasudevan and team, affected forest floor and semi-arboreal lizards approximately five-fold, by reducing vegetative cover in the understory of the forest.

“In terrestrial island ecosystems, you have species that are endemic; they have no other home than the island. We might lose a species very quickly if we don’t care for their habitat. The impacts of an invasion on an island are far more critical than on mainland, where we might not lose a species because of an invasive,” explained Vasudevan.

“The challenge is that the entire paradigm of conservation is so focused on the mainland,” he added. “This regime for conservation cannot be copied for the islands. The spotted deer, which is protected by law in India, is a problem in the islands. It isn’t easy to get permission to regulate its population.”

One of the critical aspects of a biological invasion is the speed with which it takes root. Eradication of invasive alien species on islands is possible at early stages of invasion but the more it spreads the tougher it gets. Mohanty and Vasudevan set out to investigate what would be the best way to deal with the problem.

They began with breaking down the problem – how to map the distribution of four invasive species (common myna, house sparrow, Indian bullfrog and giant African snail) across large areas. These four species are synanthropic – found around human habitations. So, logically speaking, locals would know these species. Using questionnaires and interviews about whether they had seen it in their household and around, the team built a map of presence of these species.

“The people here have a small farm or a chicken coop or grow vegetables, they go fishing. They have a connection to the land. So they know,” said Vasudevan. These public surveys cannot confirm absence. The maps ferreted out some very interesting trends that can illuminate the paths these species might have taken, while conquering the islands.

The house sparrow was closely associated with port areas, understandably because that is where the grains are and ferries may have aided in the sparrow’s movement. The common myna was associated with roads. “It is possibly hitchhiking on vehicles or using the disturbance to move into the surrounding forests,” said Vasudevan.

The snails, though, are everywhere, probably because of frequent human-mediated spread through agriculture and construction sectors. “In the Andamans you have to grow all vegetables and these snails are very voracious. They are consuming these very precious vegetable crops,” said Vasudevan. The giant African snail is a superstar invasive – it is generalist by nature, is able to tide over bad environmental conditions successfully, breeds prolifically and has no natural predators.

Citizen science to control snails

This snail species is a global problem and citizen science initiatives to track its distribution have been used on mainland India as well. A couple of years ago, NA Aravind, associate professor at the Suri Sehgal Center for Biodiversity and Conservation, Ashoka Trust for Research in Ecology and the Environment, used citizen science and modelling to develop risk maps of the giant African snail under various climatic scenarios for all of the Indian mainland. Nearly 50% of the data used to develop these maps came from crowd-sourced information.

“If you want to control it, you need the data first. You need to have large-scale data, within a short span of time. Very few people in India are working on either snails or invasive species. So one way is to engage volunteers or local people. Citizen science is faster and economical,” noted Aravind.

Mohanty and team corrected this data collected from public surveys by comparing it to observational data that the researchers themselves collected (from a subset of the total sites, not all). “The primary usefulness is that we don’t have to camp out there and survey for so many species – birds and a snail and a frog,” said Mohanty. His team applied recently developed analytical models to invasive species data to correct the false positives.

But it matters where this public information comes from. For the Andaman surveys, the team questioned farmers, pond owners, and plantation workers. “You need the right kind of people, who will answer from deep knowledge collected over time. Our informers have surveyed invasive populations unknowingly for years,” observed Mohanty.

C Anil, professor and chief scientist at Goa’s National Institute of Oceanography, who is an expert on marine invasions, has been engaging fisherfolk as partners in surveys for their knowledge of natural history. “They know what to get, when and where. They go with an agenda for catching particular fish. They track changes, which lead us to understanding food web dynamic changes. They can give a lot of insights that you cannot get from satellite images or observational surveys,” Anil said.

Mass mobility across the planet will only get smoother and faster. And humans will continue to transport thousands of species, knowingly and unknowingly, to areas they would never have reached by themselves. Field research combined with data generated from citizen science and sifted with analytical tools will hopefully be able to keep pace with the march of the invasive alien species.

This article first appeared on Mongabay.