Biological Inventory of Saba

Abstract:

Saba is the smallest of the three Windward Islands in the Netherlands Antilles, with an area of 13 km2. Saba is the northernmost island within the Lesser Antilles' inner curve. The island is actually the top of a dormant volcano. Typical of the island are the steep cliffs, deep ravines running radially and sheer cliff coasts. The highest top is the Mt. Scenery, commonly called “The Mountain”. About 1200 people live on Saba in four villages all situated on the south- eastern half of the island. The northwestern half is not inhabited. Only a small part of the land area is used for agriculture or cattle breeding. In the south there is a stone crusher facility.

The climate is tropical and according to the system of Köppen it falls between a savanna- and monsoon climate, however the big differences in altitude cause a large variety of climatologi- cal conditions. Above 450 meters the rainfall and humidity gradually increase until they reach their maximum on the top of The Mountain. The top of The Mountain is nearly always veiled in clouds. The diversity of plants and plant-communities is caused by these variations in climatological conditions.

With 520 species the small island of Saba possesses practically the same number of species of wild plants as the much larger island of St. Maarten. The number of ferns is especially large. Saba has no island-endemic species among the plants. However, the geographical distribution of six species and one variety is limited to only a few islands, and 4.6% of the species is en- demic to the Lesser Antilles and the Virgin Islands. The Bryophyte flora consists of 48 leaf mosses and 31 liverwort species.

Expressing the differences in climatological conditions as the altitude increases, a series of plant communities are found, ranging from Croton thickets to (secondary) rainforest and elfin woodland. The elfin woodland is of a regionally rare type. The palm brakes and the ravine rainforest are practically undisturbed by human activities. They belong to the few virgin vegetations of the Netherlands Antilles. The tree-fern brakes are a special type of secondary vegetation, which develops under conditions of high humidity. The secondary rainforest zone was seriously disturbed in the past. There are still small cultivation plots here. Wherever the vegetation is left alone however, new rainforest is developing. The rainforest is one of the most species-rich plant communities. This is also where the greatest number of leaf mosses are found on Saba. The evergreen and seasonal formation zones were also disturbed in the past, however the vegetation is recovering. The great variety of forest formations on The Mountain makes this area highly attractive to visitors. The uninhabited area in the northwest of the island is important to the survival of various small island populations and has a high scenic value.

Saba's fauna has very few species compared to a similar area on the mainland, but this is to be expected from a relatively isolated small island.
Among the vertebrates the birds form the largest group, represented by 26 local and breeding species. In addition 36 migrating species are present every year on a temporary basis. Amphibians and reptiles are the next largest group of vertebrates with eleven species.

Bats are the only mammals on Saba that were not introduced by humans. This group is represented by five species. There is one island endemic among the vertebrates: the lizard Anolis sabanus. One species, the Red-bellied Racer Alsophis rufiventris is limited to Saba and St. Eustatius and is listed on the “Red List of Threatened Animals” of the IUCN. Various vertebrates are endemic to the Lesser Antilles and the Virgin Islands, either at the species level or the subspecies level: two bats, nine birds, one amphibian and one reptile. The gecko Sphaerodactylus sabanus, the bat sub-species Natalus stramineus stramineus and the Trembler Cinclocerthia ruficauda pavida have a geographical range, which is limited to only a few islands. Among the birds and reptiles there are species of which the population has declined because of hunting or the gathering of the eggs, like for instance the Rednecked Pigeon Columba squamosa, the Audubon's Shearwater Puffinus lherminieri, and the green Iguana Iguana iguana. A few bird species are regionally almost completely limited to the habitat of the rainforest and the mountain formations, e.g. the Purple-throated Carib Eulampis jugularis, the Trembler Cinclocerthia ruficauda pavida and the Blue-crowned Euphonia Euphonia musica flavifrons, which has not been seen since 1952. The possibly endangered Bridled Quail-dove Geotrygon mystacea and Red-billed Tropicbird Phaeton aetherius mesonauta breed on Saba. Of the latter the breeding population on Saba is the largest of the Caribbean region. Of the invertebrates not much more is known than 86 names. The Mountain Crab Gecarcinus ruricola is endangered through hunting.

In the past the nature of Saba was mainly impacted by activities related to agriculture and cattle breeding. Today, the most important threats are destruction and degradation of habitat by development in general, especially because of the lack of regulations.
Since the nineteenfifties there is a lot of interest in the conservation of nature and the beautiful scenery. The necessity to preserve the top of The Mountain is mentioned many times, accompanied by specific plans. A group of people even promoted the idea to declare the whole island of Saba a conservation park. Up until today however, the terrestrial nature on the island remains unprotected.

Several studies have produced a wealth of data, yet not enough for optimum nature conservation. Further vegetation research of the top of The Mountain and the northern part of the island is necessary, as well as further study of the status of the island populations of regionally rare and/or endangered species. Also additional knowledge of the invertebrates is needed.

For now a few general conclusions can be drawn with respect to conservation of the biodiversity. For this conservation it is necessary to secure large contiguous areas. For small islands this can be problematic, but there are still a lot of possibilities on Saba. In particular the uninhabited northwestern part of the island and The Mountain above 450 meters would qualify. Specific conservation is required for the breeding places of the Audubon's Shearwater, Tropicbirds and other seabirds on Green Island and Diamond Rock.