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

Habitats in the Eastern Arid Area

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5. Habitats in the Eastern Arid Area

    • 5.1. Introduction

At each of the study sites on the plain (see Plates 1, 2b and 3) an assessment was made of several characteristics – aspect, altitude, form, substrate, vegetation and the invertebrate species present. Details of the sites are provided in Appendix 1, and photographs of most of them are included in this report (Plates 7-8). Particular attention was paid to the possibility that habitats liable to destruction by airport construction would be matched by similar habitats outside the affected area, thus reducing the risk of extinction of species as a result of habitat destruction. We list below our very tentative list of habitat types, realising that much more detailed study would be needed to define them precisely, but also aware that there is in reality a habitat continuum within which subdivisions must be somewhat arbitrary. Although various quantitative techniques can be used to compare sites, we find that interpretation of the results is generally more difficult than with verbal descriptions.

    • 5.2. Habitat types

5. 2.1. Barren cindery areas

Rugged and largely barren, cindery habitats of more or less decayed lava are especially common in the northeast part of PBP (e.g. Site 6). This area includes miniature “mesas” – small raised areas with flat tops and rubbly cliff sides – and also some shallow dusty depressions that obviously hold water after heavy rain. Conditions here grade into those to be found on the steep rubbly slopes that form much of the outer boundary of the plain, especially on the eastern (seaward) side and in the ravine leading down to Prosperous Bay in the north (e.g. Site 13). Both animals and plants are hard to find in these areas, but trapping showed that Site 6 has most of the species typical of the plain that do not make burrows. It also has the mogoplistid cricket and the non-endemic spider Gamasomorpha insularis (Species maps 14, 4), both species characteristic of the exposed eastern fringes of PBP. The occurrence at this site of non-endemic species that may have recently arrived from Africa is discussed in Section 4.1.

5.2.2 Cliff tops, crags and rocky gullies

A somewhat distinct habitat is provided by exposed and “draughty” rocky places with loose rocks lying on bedrock or pockets of grit but with hardly anything that could be called soil. Sites 3 and 7 are the best examples on PBP, but the rockier parts of Sites 4 and 6 are somewhat comparable. Further north in the EAA, Site 9 has similar character. These sites have rather low numbers of animals and also low diversity, probably because of the lack of soil.

5.2.3. Exposed stony areas

Level and very exposed places with substrates mainly of small stones are characteristic of the southeastern high part of PBP and the summit of the ridge along the southern edge of the Central Basin. Sites 17 and 21 are the best examples.

These exposed flat areas with small stones are perhaps the habitat most at risk from airport development. The long eastern part of PBP, from Signal House south to the Gill Point waterfall, bordering the sea cliffs and swept by the trade winds, will be entirely destroyed if the long or medium runway is built. In topographic terms there is some parallel with the eastern border of Horse Point Plain, but the substrate and vegetation there are very different. We suspect that the closest parallels with Sites 21 and 17 can be found on the Barn, and regret – by hindsight – that we did not work there. Site15 within the Central Basin, again with a stony substrate but on an outwash fan below a gully, is physically somewhat similar to these sites, but is less exposed.

The most noticeable native invertebrates on these windswept sites are the endemic anthicid beetle Anthicodes fragilis, which shelter under small stones and emerge at night to feed, the endemic thysanuran Ctenolepisma sanctaehelenae, and dictynid and oecobiid spiders which spin tiny webs under the stones. Site 15 proved to have the same abundant species, and may mitigate the potential loss of the others. It is notable, however, that these sites are not especially rich in the species that must concern us most. Ones that are relevant are Tarphiophasis leleupi, which we found at three sites subject to destruction (4, 17, 21) and only one (15) that will survive. This species, however, may be easier to find in summer. Homoeodera scolytoides, recorded only from Site 17 and with an element of doubt, seems to be one of the scarcest of the endemic anthribids and was found nowhere else.

5.2.4. Level areas with deep dust

This habitat occurs mainly in the Central Basin, at Sites 8, 24 and the lower part of 1. Excavation at Site 8, at the point at which the exploratory trench crosses the most established vehicle track, showed that the friable dusty material was fairly dark brown below the surface and uniform in texture down to a metre depth and probably beyond. This material has probably been deposited by a combination of wind and water. We frequently saw small dust devils swirling across the floor of the Central Basin from east to west. These must gradually transport light fragments of decayed lava from the higher ground in the east of the plain. Occasional flooding of the area after heavy rain doubtless sorts and re-deposits this material to some extent. Erosion gullies are minimal in this area – because the ground is almost level – and are only strongly developed close to the rim of the Fishers Valley canyon to the north.

The dominant shrub is the indigenous Samphire, which tends to grow in clumps slightly raised above the general surface. There are also areas mainly with Ice Plant and two or three chenopod species, and small patches of the endemic Babies’ Toes. There are substantial unvegetated areas.

The dominant invertebrate species here is the large (>2 cm) burrow-dwelling wolf spider Hogna nefasta. At least two other endemic wolf spiders are also present, and there are also endemic jumping spiders (Pellenes inexcultus) that spend the day under rocks but emerge to forage at night. The endemic pseudoscorpion Sphallowithius excelsus occurs here, as does the endemic lygaeid bug Nysius sanctaehelenae.

A small level sandy patch included in the sampling area at Site 4 (liable to destruction by airport development) was physically similar to parts of the basin, and two of the wolf spiders occurring in the latter were also found at this site. That area appears to lack, however, the spider Trochosippa that we call the ‘lurker’, which is locally common in the dusty parts of the Central Basin.

Two more sites with dusty (or gritty: see 5.2.5) substrates were found outside PBP. Site 11, just north of Fishers Valley canyon, is of interest since it gives the impression of being a part of PBP that became separated long ago by the development of the canyon. The wolf spider Hogna nefasta was found here, but the rarer species were absent. Further north, the Samphire-dominated dusty Site 23 close to Holdfast Tom seemed to lack burrowing lycosids.

On Horse Point our main sampling site (10) was on highly compacted dust with many small rocks embedded in it. Although we found two non-adult spiders that were probably Hogna nefasta, this was clearly not a good site for that species. However, in some parts of Horse Point, for instance near the dump, where the substrate is sandier, Hogna is still well established, in spite of the fact that the habitat is now dominated by Creeper and bears little resemblance to the original habitat, which probably bore mainly dry Ebony/Gumwood thicket.

5.2.5. Gritty areas

White grit, rather coarser than that found at sites 8 and 24, has probably formed by disintegration of the white rock on high ground east of the Central Basin around Site 21. It has blown into the basin and forms the main component of the substrate at and to the east of Site 22, and in the north of Site 2, as well as the area between these two sites over the eastern saddle of Stone Hill. On this type of ground we have been puzzled by a phenomenon that we initially thought was seepage – almost a springline – but which we are now convinced is caused by condensation.

In this area, in October-November, the ground regularly became visibly damp a few hours after dark. Small patches of Babies’ Toes and prostrate chenopods grow in the dampest places, and some invertebrates are concentrated there. We found a previously unrecorded ephydrid fly, Psilopa species, on Babies’ Toes in one such place near Site 2, and the gritty substrates without Samphire seem especially favoured by the ‘lurker’ lycosid Trochosippa, which occurs in a totally unvegetated area of white grit north of Site 2 and in a similar area east of Site 22.

We are concerned at the likely loss or gross disturbance of a high proportion of these white gritty areas, which lie just west of the proposed runway (see Plate 1). The ‘watershed’ just north of Site 17 is at the eastern edge of the white grit area and is an area with a high concentration of sodium in the substrate (Brown 1981 Map 3, Site 54).

This habitat shares many of the invertebrate species with the areas discussed in 5.2.4.

5.2.6. Creeper dominated areas

In several parts of the Central Basin there is a transition from a dusty habitat to one in which there are many small rocks (~10 cm diameter) at the surface. This transition in the substrate is mirrored by that in the vegetation, where there is often a sharp change from Samphire to Creeper. In these rocky places (e.g Site 10, Site 14 and the upper part of Site 5, and also on the steep rim of the western part of the basin) the Creeper has invaded an area and become dominant, and the native plants are almost entirely suppressed. Some endemic animals still survive, however, often including scarabaeid beetles in the genus Mellissius (and sometimes the lycosid spider Hogna (see 5.2.4)

Perhaps more significant is the fact that the creeper mat is a favoured habitat for the tineid moths that form the least understood component of the PBP ecosystem. Although Edith Wollaston did a careful study of the tineids in the wetter parts of St Helena in the 1870s, the species of the arid parts of the island remain largely unknown. Our collections include one (presumably new) species with vestigial wings and others that seem to prefer to scuttle through the vegetation rather than to fly with their short wings. We found these tineids at many of our sites, but they seemed especially abundant in Creeper mats. Unfortunately it has not yet been possible to work out how many species are present, but it seems likely that these moths evolved as members of the fauna of the litter in the Gumwood forests that occupied most of the area where Creeper now grows.

5.2.7. Rocky slopes

The steep rim in the eastern part of the Central Basin, sampled at Site 12 and in the upper parts of Sites 1 and 2, has rather loose substrate with many medium-sized rocks (~5-30 cm). There are some patches of Creeper but also Samphire bushes, Saltbush and additional small plants. Alien species such as Brown Widows, the jumping spider Hasarius adansoni (Species map 6) and the beetle Hemasodes batesi (Species map 19) are common here, but so also is the apparently native beetle Gonocephalum. It was also in this area that we found the fly Atlantomyia nitida, an ancient endemic in the family Tachinidae that probably parasitises the endemic grasshopper Primnia sanctaehelenae. The first probable sightings were during our stay, but specimens were only obtained later by Edward Thorpe. Since the grasshopper can be found all over the EAA it seems unlikely that the fly has a restricted distribution, but more study of this intriguing species is badly needed.

5.2.8. Low altitude sites

Two sites near sea level at Prosperous Bay (Sites 18 and 19) were sampled because of the possible use of this area for landing of heavy plant and as a base for air-sea rescue. Site 19 was some distance inland in a rocky gully, and it proved to have fauna similar to that of gullies at higher elevations. Site 18, however, was immediately behind the beach at Prosperous Bay, and at this site we obtained specimens of a new species of lepismatid thysanuran that was otherwise found only at Bryan’s Rock (Site 9).

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