Alien invasive species threatening Seabirds in the Grenadines

Mass migrations and movements of humans worldwide over many millennia have enabled the transport of species, both purposefully and incidentally, into new regions. Some of these species, such as domesticated goats, sheep and cattle, enabled the survival of human populations as they settled into new areas. Others, such as mice, rats and ants were brought by accident on ships.

While an introduced species (also known as alien or exotic) is an animal that has been transported to a new area by humans, an invasive species is an introduced species that is harmful to native flora and fauna. The introduction of non-native species, such as rodents, to seabird nesting islands is a global problem that has resulted in seabird disappearance from many islands. In fact, the destruction by invasive species accounts for half of bird extinctions worldwide over the last five centuries, but is particularly pronounced for seabirds which nest on isolated islands.

Seabirds like this Brown Booby nest on the ground, making their chicks and eggs vulnerable to introduced predators. Photo: EPIC

Despite the huge variety of seabirds worldwide, they all share common characteristics. In particular, they spend most of their lives at sea, and they must return to land to lay eggs and raise chicks. Seabirds are colonial nesters, which means that they nest together, ranging from several to millions of breeding pairs. Seabirds have evolved to life on remote, predator-free islands and have not developed defence mechanisms to non-native species, which can injure and kill them and their eggs. Introduced species can also alter or destroy the terrestrial habitat which seabirds require to nest and raise chicks, and directly compete with other local species for food and habitat.

Prior to human habitation in the Caribbean, seabird populations thrived by nesting on predator-free island havens, which provided them both with the nesting habitat and protection required to nest and raise their chicks. For example, the only native mammals naturally present on the seabird islands were bats, while all other mammal species, such as mice, rats, cats and goats were brought by humans.

Invasive species can interrupt the entire food web and productivity of ecosystems. Seabirds are ecological connectors; their guano transfers nutrients between the ocean and the land to improve the growth of plants on the island and coral reefs surrounding their colonies. When invasive species reduce seabird populations, it can also reduce the health of islands and surrounding waters.

Within the Grenadines, feral goats are a particularly harmful invasive species that is present on numerous seabird islands, such as Diamond Rock, Frigate Island, and Battowia. Goats are abundant on these offshore islands, where they were originally placed to graze and fend for themselves. However, these goats contribute significantly to erosion by eating vegetation that holds soil in place.

The displaced soils can then wash into the sea, smothering and killing surrounding reefs which fish need to survive. Goats can also scare seabirds off their nests, leaving chicks and eggs exposed to the hot tropical sun. In as little as a few minutes, an egg or young chick can perish from exposure, while unprotected chicks and eggs can also be trampled by grazing animals. Other invasive species, such as mice and rats can directly prey upon adults, chicks and eggs.

The very characteristic that makes seabirds vulnerable – their tendency to nest on isolated island colonies – is also their biggest chance for recovery, allowing conservation efforts to be focused and contained.

Eradication of invasive species from seabird islands has proven to be a successful seabird conservation strategy in many locations worldwide. One such success story involves Redonda, a sister isle of Antigua and Barbuda, which had become infested with rats and feral goats. Within a short time of rat and goat removal from Redonda, native vegetation began to flourish and many species of reptile and seabird began to rapidly recover.

Similarly, dramatic and positive results are possible for Grenadines seabird colonies. Environmental Protection in the Caribbean (EPIC) and Science Initiative for Environmental Conservation and Education (SCIENCE) encourage the public to protect seabird nesting areas by not letting livestock or pets into these sensitive areas. Together we can protect these natural treasures for future generations and reap the benefits of a healthy ecosystem.

This project is made possible through support from USAID and the U.S. Fish and Wildlife Service.