Indian bullfrogs take to invasive behaviour early in Andamans

In experiments, bullfrog tadpoles ate up all tadpoles of two endemic frogs

Cannibal: An Indian bullfrog tadpole eating a smaller bullfrog tadpole.

Indian bullfrogs introduced in the Andaman islands are invasive, and eat native wildlife including fish and lizards. Now, experiments reveal that the frogs take to this invasive behaviour early in their lives. Even in the developmental stages, the large bullfrog tadpoles eat other native frog tadpoles, finds a study.

The Indian bullfrog Hoplobatrachus tigerinus (native to the Indian subcontinent) has rapidly invaded the Andaman islands after it was introduced there in the early 2000s. In human-dominated areas, it now shares space with other native (and often endemic) frog species. The bullfrogs are prolific breeders: they have short breeding seasons, and each egg clutch can contain up to 5,750 eggs. Its tadpoles are carnivorous and eat other tadpoles (including their own species).

To discern the impacts that bullfrog tadpoles have on native frog tadpoles, researchers including Nitya Prakash Mohanty (of the Andaman and Nicobar Environment Team and Stellenbosch University's Center for Invasion Biology) used a series of experiments. They first collected egg clutches (four each) of Indian bullfrogs, and native endemic frogs

Microhyla chakrapanii and

Kaloula ghoshi. Once the tadpoles emerged, they mixed the clutches and randomly assigned individuals to seven different ‘treatments’ or combinations in circular plastic pools containing 30 tadpoles each. One treatment contained equal numbers of all three species, three treatments comprised tadpoles of two species and three consisted of tadpoles of a single-species. The team replicated these experiments for up to four times for different treatments, observing 25 pools in total. They monitored the pools daily to detect metamorphosing tadpoles and recorded the survival of tadpoles in each pool every week.

Their results, published in

Biological Invasions, reveal that Indian bullfrog tadpoles – which grew to be the largest (around 20 millimetres) – also grew the fastest. The survival of both endemic frog tadpoles reduced to zero when bullfrog tadpoles were present. In the three-species treatment too, all individuals of the endemic frog tadpoles in most pools were eaten by bullfrog tadpoles within the first week itself. The proportion of bullfrog tadpoles surviving was greater in the presence of both endemic frog tadpoles. This is worrying because other native frog species – many of which are only being described – could also be affected, said Dr. Mohanty. “Humans play a huge role in the invasive success of these frogs and urgent management actions including screening at ports of entry could help prevent their spread to other islands,” he added.