The invertebrates of Prosperous Bay Plain, St Helena a survey by Philip and Myrtle Ashmole September – December 2003 Commissioned by the St Helena Government and financed by the Foreign and Commonwealth Office

3.2. Vegetation

The energy base for the animal community of Prosperous Bay Plain is provided mainly by the primary production of the few plant species that grow there. This is supplemented to a small and unknown extent by dispersing insects and a small amount of plant detritus brought in by the wind.

Much of PBP has a bare rock substrate with very little development of soil. Where there is a relatively fine-grained substrate it is likely to have high concentrations of mineral salts (see previous section). These salts evidently provide major constraints on the growth of plants, and those that grow there constitute a halophytic (salt tolerant) community.

Throughout much of the plain, plants are few, scattered and low growing. Large areas appear on first sight to be completely lacking in plants, and in many other parts plants cover less than 10% of the ground; only in a few gullies does vegetation cover approach 100%.

The most significant plant species is Samphire Suaeda fruticosa (family Chenopodiaceae), a non-endemic but undoubtedly native low-growing woody shrub that is dominant in the dusty areas of the plain and in gullies. However, in many rocky areas (especially in the west) the dominant plant is the introduced perennial Creeper or Hottentot Fig Carpobrotus edulis (Aizoaceae). ‘Creeper waste’ covers almost the whole of Horse Point Plain and large areas on the lower slopes of Woody Ridge, as well as much of the southwest rim of the Central Basin of PBP and elsewhere. Creeper is less dominant in the more arid eastern parts of the plain, but occurs sporadically even around the cliff-tops. Although Creeper and Samphire now occupy rather distinct habitats, it is very possible that Samphire occurred more widely on rocky substrates before the arrival of Creeper, and that its range contracted as that of Creeper extended.

Another important member of the Chenopodiaceae is the apparently introduced Saltbush Atriplex semibaccata, which seems able to grow even in places too dry for Samphire. It is especially successful on dry slopes, including those of the Central Basin. On the floor of the basin there are several other introduced chenopods, not all securely identified.

Apart from the Creeper, the family Aizoaceae is represented by two annual species. One is the endemic Babies’ Toes Hydrodea cryptantha, which exploits relatively flat places where water is available on a temporary basis and can be found both on silty deposits of dust and grit and on volcanic cinders. A particularly good site for Babies’ Toes is below the Signal House near King & Queen Rocks. The second annual species is the introduced Ice Plant Mesembryanthemum crystallinum, which grows in similar places and may compete with Babies’ Toes.

Scrubwood Commidendrum rugosum (Compositae), which now has only a few scattered individuals, may have grown extensively along the cliff tops in the past. It was recorded in 1807 by the botanist Burchell as growing in patches on Prosperous Bay Plain. We imagine that it may have grown especially where there is potential condensation from updraughts along the cliffs. St Helena Tea Frankenia portulacifolia (Frankeniaceae), a wiry endemic shrub, survives in a few small colonies in such places along the cliff tops.

It is not clear that Prosperous Bay Plain ever supported much more extensive shrubby vegetation. However, Burchell recorded the Boxwood Mellissia begoniifolia (Solanaceae) growing with Scrubwood in clumps on PBP. This species, rediscovered by Stedson Stroud in 1998 near Lot’s Wife, is a prime candidate for reinstatement on PBP. The Salad Plant Hypertelis acida (Aizoaceae) may also have occurred on the seaward side. It was once known as ‘Longwood Samphire’ and was found by Burchell “in Longwood towards the sea”. This endemic shrub has been neglected by conservationists and may need protection from rabbits, but would probably flourish in areas subject to condensation if given the chance. Sadly, there is no opportunity for reinstatement of the extinct Dwarf Ebony Trochetiopsis melanoxylon (Sterculiaceae), which may also have grown in the area.

Four alien shrubs now occur sporadically on PBP and could spread further in the future. Two species of prickly pear cactus, the White Tungy Opuntia cochinillifera and the Red Tungy O. vulgaris have a foothold in the area, the former occurring as a few scattered individuals on the floor of the Central Basin and the latter on the rim and inner slopes of the basin in the northwest. Lantana Lantana camara occurs as isolated bushes. Wild Tobacco Nicotiana glauca is mainly a colonist of gullies, and is most widely distributed on the seaward eastern slopes and cliffs. Wild Coffee Chrysanthemoides monilifera has not yet invaded PBP to a significant extent but might do so in future.

Among significant non-woody species is the small indigenous grass Eragrostis cilianensis, which grows in sometimes prostrate tufts in extremely exposed places. Although very inconspicuous in dry periods, it covers substantial areas and must provide an important food resource for insects such as the homopteran Balclutha saltuella.

Two other interesting species are the endemic French Grass Euphorbia heleniana (Euphorbiaceae), a small prostrate plant that occurs sporadically as single individuals or small groups; and the Pagoda Plant Cotula coronopifolia (Compositae), which is commonest on cliff tops.

Lichens are a significant component of the plant community of PBP. They grow most luxuriantly on slopes and rock faces exposed to moist air generated as the southeast trade winds rise along the sea cliffs.

• 3.3. Impact of humans

For millions of years before 1500 ad, Prosperous Bay Plain had been subject to only slow geological and ecological change. Erosion of the upper surface had been slowly lowering the plateau, and the Central Basin had been gradually increasing in size, but the landform had remained basically the same. The Pleistocene ice ages doubtless affected the wind and rainfall regime, but the PBP plateau was isolated from the drastic changes caused at the base of the surrounding cliffs by changes in sea level associated with alternating cool and warm periods. Ecological changes will have occurred at long intervals, as a result of colonisation by additional species of plants and animals.

Discovery of St Helena in 1502 brought massive ecological changes to all parts of the island. On Prosperous Bay Plain these changes were in some ways less drastic than in the moist central parts of the island, where almost all of the native vegetation was destroyed, but they were nonetheless profound. The introduced herbivorous mammals (mainly goats and donkeys) doubtless caused significant changes in the natural vegetation, though Prosperous Bay Plain must always have been a sparsely vegetated area. Introduction of goats, pigs, cats and rats had fundamental effects, leading to elimination of native landbirds and rapid decline of the seabird colonies of the area. The collection of seabirds and their eggs for human food, along with continued predation by feral cats, led to their elimination from all accessible sites within the last few decades.

Removal of surface rocks from the plain has also had a major impact on the desert ecosystem over a long period. The effect will have been mainly on the invertebrate animals. Although some of these live on or under the scattered plants, many of them hide under rocks during the daylight hours, emerging to forage by night. Since most of the underground cracks are filled with mineral salts, loose rocks on the surface are a crucial resource. Deeply bedded rocks offer few refuges suitable for animals, though ants can often excavate passages along the rock surface and below it. The uncontrolled but systematic collecting of rocks as building material, which has been going on for many years, has drastically reduced the availability of retreats. In some areas on the plain, and on adjacent places such as Horse Point, almost no movable rocks are left. It is very likely that reduction in the numbers of surface rocks is a significant factor in the disappearance of the Giant Earwig and the Giant Ground Beetle. We are concerned to see evidence of continuing disturbance from stone collecting in some parts of PBP.

The floor of the Central Basin of PBP, however, is mainly dusty or gritty, and the dominant endemic animals there are spiders that are not dependent on loose rocks. Instead, they are burrow dwellers, and are thus vulnerable to the effects of vehicles compacting the dusty soil.

Human occupation of the island also resulted in the building of forts at all strategic points, and there are the remains of some of these around PBP, together with the mule tracks used in servicing them. Target practice in the second world war has left pits where shells have fallen, along with fragments of mortar bombs and pieces of shrapnel. In the 1960s about 70 aerials were installed on PBP, and although now dismantled, their concrete bases are still dotted over the area. There are signs of small scale mining of gypsum and phosphate.

Also occurring sporadically are excavations probably connected with site evaluation for a potential airport, including a shallow trench extending for some 250 m across the Central Basin, making a significant scar (see Plate 5h).