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


Special significance of the Central Basin



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6.2. Special significance of the Central Basin

As indicated in Section 3.1, the Central Basin of Prosperous Bay Plain (Plate 4 a, b) has an unusual geological history. It has been formed over a period of several million years and it still extending slowly southwards while being cut away by erosion in the north. The stability of the area, coupled with the low rainfall and lack of luxuriant plant communities, has enabled the native invertebrates to become highly adapted to this special environment. A few endemic species (for instance the pseudoscorpion Sphallowithius excelsus) may have their total populations entirely confined to this miniature desert, barely 1 km in diameter and about 60 hectares in area, and several of the wolf spiders undoubtedly have their main population centres there.

The deep accumulation of dust and grit is the main feature of the basin environment. This substrate makes it feasible for the wolf spiders and some kinds of beetles and other insects to dig burrows. These conditions are not fully replicated elsewhere, although the commoner of the wolf spiders, Hogna nefasta, can also be found in a few other places that bear some similarity to the environment in the floor of the basin (Sites 4, 5, 10 and 11). In surrounding areas on PBP and elsewhere in the drier parts of the island there are communities of animals in which most of the species live under small stones or larger rocks, as indicated above, but the substrates suitable for burrow-dwellers are generally lacking. The diverse wolf spider community of the Central Basin is a remarkable biological resource. Its value is enhanced by the fact that the spiders’ eyes reflect torchlight, providing an intriguing spectacle for visitors (providing that they come at night!).


Although no decisions have been made about access, it seems sensible to base this analysis on the long runway option, which is the ‘worst case’ scenario from the point of view of the invertebrates. If the medium length runway is chosen the damage will be significantly less. However, it should be remembered that apart from the impact of the runway itself, construction activities involving heavy plant have a potential to cause extensive damage. Furthermore, access roads, fuel storage area, work camp, terminal building and other facilities all constitute major intrusions on the area that will be suffered under any runway option.

Threats to the habitats could appear even at the early stage of tendering for the airport job. It seems likely that there will be numerous engineers investigating the area and damage can be caused from uncontrolled vehicle access with associated ground compaction.

Plates 1 and 3 show the approximate footprint of the proposed long runway and associated terminal area, kindly provided by Atkins, and Plate 2a shows a montage in a view from the Peaks. We have not seen maps of proposed access routes.

The major impact of the runway itself is on the ridge running north-south immediately above the eastern sea cliffs. Levelling for the runway will destroy the natural substrate of this entire ridge (Plates 5e, f). Our Sites 4, 6, 17 and 21 will be obliterated and Sites 2, 3, 7 and 12 will be close to the edge of levelled or filled areas. Most of these sampling sites have no special a priori value, but they represent a set of habitats that will be gravely at risk.

The northern part of the ridge (Plate 5f) includes much decayed, cindery lava well represented around our Site 6. This site has most of the species typical of the plain in general that do not make burrows, and also has the mogoplistid cricket and the non-endemic spider Gamasomorpha insularis, two species characteristic of the exposed eastern fringes of PBP (Species maps 4 and 14). Further south (Plates 5e, 8) there is compact whitish stony ground best represented at Site 21, which – considering the degree of exposure and scarcity of plants – has a surprisingly rich fauna.

However, although the impact on the invertebrate communities in the whole of this area will be devastating, we are somewhat encouraged to see that a substantial levelled platform will be left on the eastern (seaward) side of the paved runway. Since the original habitats in this area (especially around our Site 21) are smooth, flat areas fully exposed to the prevailing easterly winds, the general character of the area may be rather similar at the end of construction to its present state. In Section 7.3.1 we suggest that restoration of this area may be feasible.

Under the long runway option (which we were not aware of when we undertook the field work) there will be a major impact on Dry Gut. In general, we anticipate that the damage to the invertebrates will be increased only in proportion to the extent of habitat lost. However, the generic endemic spider Bonapruncinia sanctaehelenae – known from only two juvenile individuals found by the Belgians – may have been collected in the area above Dry Gut that will be filled with rubble if the long runway is constructed. The Belgians suggested that this cryptic species – adapted to a substrate of sand and gravel – might have a total range of only a few tens of square metres. It must therefore be considered gravely threatened by airport development. However, we suspect that its range is actually rather greater and that it may also occur on other gravelly terraces in the same general area, for instance on the slopes of Bencoolen to the south of Dry Gut.

The loss of Site 4 is also unfortunate, since this gully – which offers a variety of habitat types – is the only place outside the Central Basin where the Obscure Wolf Spider Lycorma was recorded and where subfossil remains of the Giant Earwig were found (in 1995). However, gritty habitats similar to the flatter part of Site 4 are available to the west of the proposed terminal (the pale areas visible in Plate 1). We did not sample these, but they are likely to be valuable in terms of fauna. Care in construction of the terminal and any associated access road would pay dividends here.

If a runway is to be established on the eastern part of Prosperous Bay Plain, special care will be needed in planning the access route(s) to avoid damaging the Central Basin. If a route down from Woody Ridge is chosen, it could continue along the high ground around the south rim of the Central Basin. This is marked with a heavy line on the maps (see especially Plate 2b) and also with stakes on the ground. The positions of the stakes are listed in Appendix 2. A road running near the crest of the ridge need not cause serious environmental damage, but there is a risk of pollutants from the road draining into the Central Basin. For this reason – and to minimise its visual impact – it may be best to keep the road just to the south of the ridge summit.

Care is also needed at the western end, close to our Site 5, since the gully here (shown in Plate 6b) is the main input of surface water to the Central Basin. The catchment for the gully is quite small (see Plate 3) and includes silty deposits where we found bones of storm petrels in 1995. It would be very unfortunate if pollutants from the road drained into the basin.

Alternatively, if there is to be a route from Bradleys and around the north side of the Central Basin, this should run as close to the rim of the Fishers Valley canyon as possible and should be fenced off at the outset to prevent any construction vehicles from straying on to the floor of the basin.

Our greatest concern relates to the Central Basin itself. Fortunately, current development plans – unlike proposals made in the 1980s (see Caesar 2001 and Plate 5h) – do not involve runway construction in the main part of the Central Basin. The principal damage will be in the southeast corner, where the break in the basin rim is the point of origin for the gully running southeastwards towards the Gill Point area. On the basin side of the break in the rim is the pale area visible in Plate 1 to the north and northeast of our Site 12. This has a substrate of whitish grit that provides prime habitat for the Lurking Wolf Spider Trochosippa, a species whose entire population appears to be confined to the Central Basin (Species map 10). Since the small colony in the north of Site 2 is also at risk, the damage to this species is likely to be significant. Site 12 is our only recorded location for the generic endemic tachinid fly Atlantomyia nitida, but this species may actually have somewhat wider distribution.

Most of the other species typical of the Central Basin, for instance the pseudoscorpion Sphallowithius, the other three wolf spiders, the salticid spider Pellenes (Species map 7) and the heteropteran bug Nysius sanctaehelenae (Species map 15) have their main population centres close to and to the west of Stone Hill (see Plate 3) and will thus be at serious risk only if vehicles are allowed access to that area. It is therefore clear that if airport development proceeds, the most strenuous efforts should be made to safeguard the integrity of that part of the Central Basin that is not actually within the area where runway works are required.






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