Location and General Description
These three islands are located in the Gulf of Guinea off the west coast of Africa and form part of a volcanic chain that continues inland as the Cameroon/Nigerian mountains (Cameroonian Highlands Forests ecoregion ). The volcanoes date back to the Tertiary and are considered inactive. Príncipe is the closest to mainland Africa and has an area of 128 km2. São Tomé lies further out and covers approximately 836 km2, and Annobon is the furthest from the coast, with an area of 17 km2.
The volcanic plugs and mountainous parts of the islands range up to elevations of 948 m on Príncipe, 2,024 m on São Tomé, and 695 m on Annobon. An area of low land at the base of the volcanoes is the only relatively flat land on the islands (Jones and Tye 1988). The volcanic soils of basalts and phonolites, which are more than 3 million years old, are relatively fertile and have been used for plantation crops in the past.
The islands are within the wet tropical belt. On São Tomé, annual rainfall ranges from 1,000 mm in the northeast to more than 4,000 mm in the southwest. Mean annual temperatures range from a maximum of 30° to 33°C to a minimum of 18° to 21°C, with little seasonal variation and high humidity all year. On Príncipe, rainfall patterns are similar; whereas on the island of Annobon rainfall is somewhat less.
The islands were uninhabited in 1470-71 when the Portuguese discovered them. Colonization began in the early sixteenth century when São Tomé became the world's largest sugar producer and, after this crop's decline, the island grew to be an important slave trading post. Annobon became a Spanish colony in the eighteenth century and now forms part of Equatorial Guinea. In the nineteenth century, coffee and cocoa plantations were established on São Tomé and Príncipe and Africans were moved to the islands to work on the estates. These islands have remained populated and became an independent country in 1975.
The islands were not mapped within White's (1983) Phytogeographical Classification of Africa, but have many elements in common with the Lower Guinea Forest Block of the Guineo-Congolian regional center of endemism. The original vegetation of both islands comprised forests of various types, including lowland and montane forests on the wetter side of the islands, and mossy forest at the highest altitudes. The rain shadow side of the islands supported a drier forest type, which has been extensively cleared for farmland and estates. Although some of the forest areas have been heavily damaged in the past, there is significant regeneration to secondary forest in many areas.
The biological values of the islands have been described in a number of recent publications (Collar and Stuart 1988, Jones and Tye 1988, Jones 1994, Atkinson et al. 1991, 1993). Endemism at the generic, specific and sub-specific level is extremely high for such a small land area. As with island faunas elsewhere, the high taxonomic rank of endemic forms indicates that the process of relictualization has occurred on these islands for some millions of years.
Typical island adaptations, such as gigantism and dwarfism, occur in genera that are found elsewhere in Africa. The São Tomé olive pigeon (Columba thomensis, VU), the São Tomé giant sunbird (Nectarinia thomensis, VU) and the giant begonias (Begonia crateris and B. baccata) are all examples of species that have large size compared to similar continental species. The dwarf olive ibis (Bostrychia bocagei, CR) is an example of a species that is much smaller than other members of its genus. Unusual physiological and behavioral adaptations have also occurred such as in the São Tomé weaver (Ploceus sancthithomae), which has become a forest-dwelling tree-creeping species atypical of weavers.
Colonization occurred as species migrated from the African mainland. Thereafter, species have undergone speciation, or have become relictualized as their relatives became extinct on the mainland. Significant radiations exist in a number of groups (e.g. the terrestrial gastropod genus Bocageia and the plant genus Calvoa). There appears to be evidence of older and then more recent colonization of the islands by endemics. Thus the giant sunbird appears to be an older arrival than Newton's yellow-breasted sunbird (Nectarinia newtoni), which more closely resembles its mainland relatives. The taxonomic status of many species remains subject to debate
The flora of these islands is highly distinctive; there are 37 endemic plant species on Príncipe, 95 on São Tomé (along with one endemic genus), and 20 endemic species on Annobon (Figueiredo 1994, WWF and IUCN 1994). Only 16 of the region's endemic plants are shared by more than one island. This emphasizes the high degree of isolation under which their floras evolved and it indicates that each island received its flora separately from the mainland. The Rubiaceae, Orchidaceae, and Euphorbiaceae are characteristic of the islands' flora having high generic diversity and high numbers of endemics (Figueiredo 1994). Significant endemic radiations among other genera (e.g. Begonia and Calvoa) are also found. The Pteridophyte flora of the islands is also considered particularly rich (Figueiredo 1998). The islands are distinguished as Centers of Plant Diversity (WWF and IUCN 1994)
These islands have high importance for bird conservation. There are seven endemic bird species on Príncipe. São Tomé possesses a total of sixteen endemic bird species and two endemic genera. On Annobon, there is a monotypic endemic genus. Two or more of the islands share an additional four endemic bird species. All of the birds are forest dwellers. A number of endemic subspecies also exist. Some of these would almost certainly be elevated to full species if subjected to further taxonomic study. In a global review of priority areas for bird conservation, Príncipe and São Tomé were both classified as critically important on the basis of the numbers of restricted range bird species occurring together (Bibby et al. 1992). Four bird species were recently rediscovered after having been unobserved for more than 60 years and are all threatened (Hilton-Taylor 2000): the dwarf olive ibis (Bostrychia bocagei, CR), Newton's fiscal (Lanius newtoni, CR), the São Tomé canary (Neospiza concolor, CR) and the São Tomé short-tail (Amaurocichla bocaii, VU). The islands are classified as Endemic Bird Areas (Stattersfield et al. 1998).
The islands have few indigenous mammals. The shrew Crocidura thomensis (VU) is the only endemic terrestrial mammal, and is found on São Tomé. São Tomé also boasts two endemic bat species of which Myonycteris brachycephala (EN) is notable as being the only known mammal with an asymmetric dental formula (Hilton-Taylor 2000). A further three endemic subspecies of this bat are also found, of which Príncipe and Annobon have one endemic subspecies each (Juste and Ibanez 1994). One species of bat has been discovered only in the past few years, the endemic Chaerephon tomensis (VU) (Hilton-Taylor 2000).
Rates of endemism are also high in other taxonomic groups. Among the Rhopalocera (Lepidoptera), there are 13 single island endemic species on São Tomé and 6 on Príncipe (Pyrcz 1992, Wojtusiak and Pyrcz 1997). Other Lepidoptera groups (e.g. Geometridae) also show high levels of endemism. Terrestrial gastropods show rates of endemism above 75% on all three islands, with several endemic genera and a monospecific endemic family comprised of the São Tomé door snail (Thyrophorella thomensis). This species is notable for a hinged operculate flap attached to the main shell, a unique feature among the Gastropoda (Gascoigne 1994b). Of the 24 reptile species recorded for these islands, only 6 are non-endemics. It is likely that non-endemic geckos and skinks were introduced with freight (Broadley, pers comm.).
Many of the species on these islands have small populations and feature in the world list of species threatened with extinction (Hilton-Taylor 2000).
There is currently estimated to be 40 km2 of primary forest on Príncipe, and 240 km2 on São Tomé. Over larger areas of both islands, secondary forest vegetation is regenerating on old plantations. On Annobon, much of the forest, with the exception of the high peaks of Santa Mina and Quioveo, has been modified by humans but remains important habitat for endemic species. Many of the endemic species have adapted to modified habitats on the cocoa and coffee plantations because of the use of shade trees to protect crops. Some agencies have proposed to protect the remaining areas of primary forest on São Tomé and Príncipe as national parks. A law establishing procedures for the proclamation and management of a protected area system was passed in 1999. The proposed parks would protect the largest remaining habitat blocks, including areas of primary forest. São Tomé and Príncipe are in the final stages of preparing to declare protected areas to be known as the Parques Naturais d'Ôbo, which will cover a total of 293 km2. The entire island of Annobon was recently ratified as a protected area.
Concerns exist over the protection and status of the remaining areas of drier forest on São Tomé, and for the long-term survival of species confined to lowland forests of these islands. The remaining lowland forest habitats are being gradually cleared for agriculture. Knowledge of many species of conservation concern is so poor that it is difficult to assess whether the proposed national parks on São Tomé and Príncipe will adequately protect these populations.
Types and Severity of Threats
During the sixteenth century, a large area of dry forest in the north and northeast of São Tomé was cleared for sugar cane production. After the decline of this cash crop at the end of that century, some of this forest recovered.
From the middle of the nineteenth century, large coffee and cocoa plantations were established on both Príncipe and São Tomé, which led to the widespread modification and destruction of primary rain forest. Rain forest in the north of Príncipe was also severely modified during a campaign against sleeping sickness from 1911-16. However, many endemic species adapted to the shade forest found in plantations. After the 1930s, and especially following independence in 1975, many plantations were abandoned, and there was some regeneration to secondary forest. Since the mid-1980s, land reforms have led to the development of market gardening and consequent land conversion from coffee and cocoa plantations. Some secondary forest areas have also been cleared once more for agricultural use. This is of some concern because it will put pressure on endemic species that have adapted to secondary and plantation shade forest, and it will also increase pressure on remaining primary forest areas.
Agricultural practices on Annobon have traditionally been based on a forest agricultural system that was less damaging to biodiversity than the large-scale plantations of Príncipe and São Tomé. However, there remains a danger that agricultural encroachment in the primary montane forest zones of Pico Quioveo and Pico Santa Mina will result in irreversible damage to these habitats.
Island biodiversity is particularly susceptible to the introduction of non-native species. On all three islands, a number of terrestrial mammals, both domestic and wild, have been introduced over the centuries (Dutton 1994). It is now impossible to evaluate the damage they have caused. Recent introductions of terrestrial gastropod species have been recorded on all three islands (Gascoigne 1994a), and it is probable that species from other taxonomic groups have also been recently introduced.
Little direct exploitation of the endemic terrestrial wildlife occurs. Medicinal plant use is almost exclusively concerned with non-endemic species, and limited hunting activity is restricted to the introduced primate Cercopithecus mona, feral pigs and the common non-endemic bat species Eidolon helvum. The endemic São Tomé green pigeon (Treron australis virescens) and the São Tomé olive pigeon (Columbus thomensis) are occasionally shot.
A cause for conservation concern is the capture of the African grey parrot (Psittacus erithacus) on the island of Príncipe for the international pet trade. Although the Príncipe population is no longer considered an endemic subspecies, recent studies indicate that it shows distinct behavioral adaptations and genetic variation (M. de Melo, pers. comm.) that differentiate it from mainland populations. It is not known whether the level of exploitation is sustainable (Juste 1996).
Justification of Ecoregion Delineation
The forested oceanic island ecosystems of São Tomé, Príncipe and Annobon contain many endemic species. Given the size of the islands, their similar geological history and vegetation, and the fact that some of the endemic species range across a number of islands they are regarded as a single ecoregion.
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Prepared by: Angus Gascoigne
Reviewed by: In progress