The São Tomé ibis (Bostrychia bocagei), also known as the dwarf olive ibis or the dwarf ibis, is a critically endangered bird that is endemic to São Tomé off the western coast of Central Africa in the Gulf of Guinea. Discovered in the 1800s by Francisco Newton, it was once thought to be a subspecies of the larger olive ibis, but is now classified as a distinct species. Living with 19 other species of endemic birds on the island, the dwarf ibis is one of 10 that are threatened for extinction. B. bocagei prefers the southern forests almost exclusively living there in nests that are high up in the trees to avoid predators. Aside from predation, other common threats to the species include overhunting and habitat loss as a result of overexploitation of resources by the people of São Tomé. Although no specific conservation reforms have been enacted to help B. bocagei, indirect efforts to help biodiversity on São Tomé in general have been instituted, such as the ECOFAC program and the establishment of a national park.
Description The São Tomé ibis measures 248 mm on its wings, 75 mm on its bill, 52 mm on its tarsus, and 95 mm on its tail, on average. The head is a dull olive color with black surrounding the eyes and the base of the bill. The tectrix (wing coverts) and mantle of the bird are slightly bronzed. Most of the time, the dwarf ibis is silent, but when it is disturbed, it gives off differing coughed grunts. When going to roost, it lets out a harsh honking. Its call could also be described as a stressed Walalalala.
Behavior B. Bocagei chooses to nest between 8m and 16m up in the trees in the forests. The nests were composed primarily of twigs and other biomass material, with leaves and feathers lining the inside. The nests had diameters of between 330 and 370 mm x 260–360 mm.
Habitat and ecology
The São Tomé ibis was discovered in the late 1800s by Francisco Newton, a Portuguese collector, inhabiting the island of São Tomé off the coast of Western Africa. Alongside 19 other endemic species of birds, the São Tomé ibis is one of the 10 that are threatened for extinction, but little has been done to protect the island's very high biodiversity. It is found in primary forest below 450 m altitude, being seen almost exclusively in the southern forests. The decision of the ibis to nest higher up is related to predation avoidance. The nests appear both higher up and away from running water, most likely to avoid snakes and introduced mammals that could exploit the eggs, such as rats and Mona monkeys. Recently, it has been seen more and more in the newly created Obô Natural Park, which spans the majority of the western side of the island. It forages for food on the forest floor with sparse undergrowth, especially where the ground has been disturbed by feral pigs, and in swampy areas bordering watercourses.
Threats The extremely small population of São Tomé ibises contains between 50 and 249 individuals. Due to this, it is listed as Critically Endangered on the IUCN Red List. A neighboring subspecies of the São Tomé ibis on the island of Príncipe became extinct in the last century, indicating that small habitats like São Tomé make ibises more vulnerable to threats and extinction. The São Tomé ibis is facing problems that are both environmental and anthropogenic that contribute to its status as critically endangered.
Anthropogenic In total, approximately 75% of the native flora of São Tomé has been lost since its colonization in the 1500s. Living in the Anthropocene, the São Tomé ibis suffers from anthropogenic factors, including habitat destruction, overhunting, and urbanization. In efforts to urbanize the small island, railroads and roads have been built, bringing down forests with it. Additionally, dams are being constructed to provide electricity to the surrounding communities. The most significant part of urbanization is the development of oil-palm concessions. Oil-palm concessions are the licensing of specific land by governmental organizations to companies to build oil palm plantations. Aside from oil-palm plants, cacao, coffee, quinine, and cinnamon plantations have been built on the island. While building these plants, the habitat of all surrounding wildlife is destroyed, damaging the dwarf ibis population along with the surrounding ecosystems. All of the efforts mentioned above to attempt to urbanize the island have come with their drawbacks, including the habitat destruction of the dwarf ibis. These anthropogenic factors have been in response to an influx of inhabitants on the island. When more people visit the island, more resources are required to sustain them, explaining the additional increase in logging and contributing to the limiting of the ibis's available habitat and breeding range.
Overhunting also plays perhaps the most important role in the endangered status of the dwarf ibis. As a large bird that forages on the forest floor and displays island tameness, it is very vulnerable to human hunters. Whether it is hunting for sport or accidental killings, the ibis population is decreasing in ways that are not attributable to simply the overexploitation of resources and the environment.
Environmental Aside from anthropogenic threats, there are several environmental factors that threaten the population of the dwarf ibis in the form of invasive species, which dominate the food web of São Tomé and can compete with the ibis for resources or act as predators. Non-native predators include feral cats, feral dogs, Mona monkeys, black rats, least weasels and African civets.
Conservation
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