- Experiments showed that tadpoles of the Indian bullfrog, an invasive frog species that has spread to six of eight human-inhabited islands in the Andamans, rapidly predated upon tadpoles of native frog species that are found only in the Andaman archipelago.
- The results highlight the need to manage invasive Indian bullfrog populations. Researchers suggest screening at entry points and removal of small populations in South Andaman and Little Andaman may be effective strategies.
- Multiple factors such as the presence of other predators, alternative food sources, anti-predator defense strategies and the presence of vegetation as refuges can influence the survival of native tadpoles in natural settings.
The invasive Indian bullfrog, native to the Indian subcontinent, is currently an invasive species in the Andaman archipelago in the Bay of Bengal. Yet, little is known about the impact it could have on native species. Now, a study has revealed that tadpoles of the Indian bullfrog quickly devoured native tadpoles when they were housed together.
Part of the Indo-Burma biodiversity hotspot, the Andaman archipelago consists of 300 tropical islands with 40 percent of its reptiles and amphibians endemic to the area — residing only in the islands. As a result, these species are vulnerable when faced with invasions by exotic species.
Since 2009, the range of the Indian bullfrog, Hoplobatrachus tigerinus, has expanded exponentially in the Andaman archipelago. It has now gained a foothold in six out of eight of the human-inhabited islands.
“Unless managed effectively, invasive populations of the Indian bullfrog are likely to spread to the Nicobar Islands, which are currently uncolonized by the bullfrog, and expand in new locations of the Andaman Islands,” said Nitya Mohanty, lead author of the study and post-doctoral fellow at Stellenbosch University, South Africa.
The large-bodied frog, measuring about 160 mm from snout to vent, commonly inhabits plantations and agricultural fields, preying on small native vertebrates. Its diet is similar to that of large native frogs. Tadpoles are carnivorous, consuming other larvae and occasionally engaging in cannibalism.
Mohanty conducted an experiment in the Andaman Nicobar Environment Team field station located in Wandoor, South Andaman Island to find out the chances of survival of native or endemic frog tadpoles when they encounter bullfrog tadpoles. He collected clutches of bullfrog eggs and those from two other native species that are also found in its breeding pools: Chakrapani’s narrow-mouthed frog, Microhyla chakrapanii and the Andaman tree frog, Kaloula ghoshi.
When tadpoles emerged, he randomly mixed Indian bullfrog tadpoles with those of the two native species in circular treatment pools filled with pond water in three different combinations: single-species, two-species, and all three species together. In each pool, density was kept at 30 tadpoles per treatment. The tadpoles were provided with food (leaf litter and algae) regularly and the pools were monitored to detect the time it took for the tadpoles to develop forelimbs, their body lengths upon completing metamorphosis (turning in to adult frogs), and survival of each species.
Rapid extermination of native tadpoles
When housed with the two native species, the voracious bullfrogs went on a killing rampage: they gobbled up all of the native tadpoles within the first week in 83 percent of the pools. And by the third week, the tadpoles in the remaining pools were also consumed by the bullfrogs. But Indian bullfrog tadpoles did not appear to benefit from preying on the native frog species in terms of their growth rate, time to metamorphosis and metamorph body size.
Although Mohanty’s team had expected high levels of predation by the bullfrog tadpoles, they were surprised at how fast they exterminated native tadpoles. They also expected to observe competitive interactions between the native and bullfrog tadpoles. “But, since the bullfrog tadpoles ate all the native tadpoles, there was no competition left to observe,” Mohanty remarked.
The survival of bullfrog tadpoles was low even in pools containing only members of their own species. In fact, on average only three bullfrog tadpoles survived—the rest were cannibalized, said Mohanty.
Native tadpoles fared better without bullfrogs present: on average three in every four made it to metamorphosis. According to Mohanty, this could be because “native tadpoles have not evolved anti-predatory strategies against invasive bullfrog tadpoles, as the bullfrog was only introduced to the Andamans very recently (in 2000).”
Controlling bullfrogs, complex predator-prey interactions
Mohanty acknowledges that inferences from these experiments have several limitations. “In natural settings, these interactions between bullfrog tadpoles and native tadpoles are likely to be influenced by the presence of other predators like dragonflies; and the availability of alternative food sources like mosquito larvae.”
Nevertheless, the findings highlight the need to manage to bullfrog population in the Andaman Islands. For his thesis, Mohanty modelled the invasive spread of bullfrogs under several management scenarios. The results suggested that the “most effective way to start management is to immediately deploy screening mechanisms between islands on the Andaman archipelago and at the Nicobar Island.”
This, he said, “should be followed by removal of the small populations present on South Andaman and Little Andaman, stressing that “ultimately, the Andaman and Nicobar Islands need a biosecurity agency to screen and prevent the entry of more non-native species.”
This is an important study with “huge implications in managing the native biodiversity of the islands,” said Karthikeyan Vasudevan, senior principal scientist at the Centre for Cellular and Molecular Biology, Hyderabad, who was not involved in the study. He agrees that the population of the Indian bullfrog in the Andaman and Nicobar Islands needs to be managed.
“If a control mechanism is implemented to remove the bullfrog,” explained Vasudevan, “the success can be measured from the variables that are influenced by bullfrogs, such as the increase in abundance of other frogs, insects, and other vertebrates.”
Srinivas K. Saidapur, a reproductive biologist and professor at Karnatak University, Dharwad district, who was also not connected to the study, said that the results are interesting and that his team has also conducted similar unpublished experiments with bullfrog tadpoles as predators and Microhyla ornata tadpoles as prey. The latter is a frog species found in South Asia and the Andaman and Nicobar Islands.
“We found that the Microhyla tadpoles being transparent are suited to escape predation,” he said, explaining that M. ornata are found in the surface waters or mid-water column whereas bullfrog tadpoles mostly dwell at the bottom of ponds but come up to occasionally gulp air.
Saidapur added that the presence of aquatic flora and other structural refuges could minimize the larval predation rate. Further, some prey tadpoles possess certain anti-predator defense strategies like remaining still for long periods and increased burst speed when attacked. Consequently, Saidapur believes that it is unlikely that Indian bullfrog tadpoles will completely eradicate M. ornata tadpoles.
Like Mohanty, Saidapur points out that during the breeding periods, frog eggs and tadpoles are also predated upon by other predators such as aquatic predators such as crabs or visiting birds and snakes, for example. In ephemeral ponds where frogs breed, desiccation is another threat which can cause the death of eggs and tadpoles, he noted.
Mohanty, N. P., & Measey, J. (2019). No survival of native larval frogs in the presence of invasive Indian bullfrog Hoplobatrachus tigerinus tadpoles. Biological Invasions, 21(7), 2281-2286.