Invasive Species are killing off Native Marine Birds

Por Marcelo Flores, Biólogo Marino
Núcleo Milenio de Ecología y Manejo Sustentable de Islas Oceánicas, ESMOI

The relationship between mankind and birds dates back to the early days of the development of humans as a species and has manifested itself in falconry, in the use of pigeons in communication, in raising of fowl as a food source, in the use of feathers and bones for adornment and in rituals related to birds, some of which, although ancient, are still observed today.   Birds have also served as inspiration for the arts,  such as Tchaikovsky’s “Swan Lake” or the Alfred Hitchcock film “The Birds”.

In spite of all that, in many cases, humans have caused extinctions due to indiscriminate hunting and the introduction of other animals in areas beyond their natural range of distribution, intentionally or inadvertently, such as rats, dogs, cats and pigs.   These contribute to the depredation of sea birds, chicks and eggs.  Many of these foreign species establish themselves and disperse in their new environment, becoming, what is called scientifically, Invasive Exotic Species (IES). They affect the ecosystem and, in most cases, end up replacing the original species.

Rapa Nui has not been an exception.  History registers the arrival of foreign species along with the first Polynesian colonizers who brought chickens and the Polynesian rat (Rattus exulans) as a food source and to trade for other products.  It is also probable that later European and American sailors introduced other animals, such as dogs, cats, rabbits and goats, during their visits.  A sketch by Pierre Loti from 1872, on view in the Father Sebastian Englert Archaeological Museum, shows the presence of cats and domestic fowl as commonplace on the Island.  Another IES is the Argentine ant, considered one of the 100 most damaging invasive species, which is thought to have arrived between 1980 and 1990 by ship.  It is believed that the chimango caracara (Milvago chimango), a South American falcon, called Manu Toke-toke in the Rapanui language which means “bird thief”, was introduced sometime between 1920 and 1930 to control the plague of European rats (Rattus spp.) which had arrived unintentionally from the continent. However, the plan didn’t work because the chimango caracara is not a raptor-hunter but an opportunistic predator, which will even steal food from people. The result was that the introduction of the chimango caracara as a means to eliminate the rats had an undesired effect and today both species, chimango caracara and rat, prey on the native fauna.

Since 2013, a research project has been underway to identify the invasive species on Rapa Nui, especially those which can affect the nesting of native sea birds. For the purposes of the study, the nesting colony of the red-tailed tropicbird, or Tavake hiku mea-mea (Phaethon rubricauda) which nests in the cliffs of the Rano Raraku volcano, was selected.  Through use of camera traps and direct observation, we were able to detect the presence of five species which interact with the red-tailed tropicbird during its nesting period: the chimango caracara, the cat, the dog, the rat and the Argentine ant (Linepithema humile). During incubation, depredation by chimango caracaras was registered.  The chimango caracaras and ants later prey on the newborn chicks. When the fledglings are getting ready to leave the nest and the birds are adult, depredation occurs with cats and dogs. Rats have been observed inspecting nests, but without direct interaction with the red-tailed tropicbird. These situations have caused heavy stresses on the tropicbird and the number of eggs which manage to survive to become juvenile birds is low, around 30%.

All the invasive species registered in this study have been “subsidized” by humans. They have permanent sources of food supplied by people in the garbage dumps and rubbish bins and through those who consciously feed chimango caracaras, dogs and cats during their picnics. Of the five invasive species specifically studied in this project, those that require immediate attention are the chimango caracara and the Argentine ant. The former is a predator of the hens and eggs while the Argentine ant is found all over the Island, attacking eggs as soon as they hatch and the chicks in their first hours.

Once the data has been able to be analyzed, we hope to contribute with useful information to the revision of the Management Plan for the Rapa Nui National Park and propose measures for control and/or eradication of IES. To achieve this task, it is  essential to count with the collaboration and active participation of the entire community of Rapa Nui.