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

Note on interim and final versions of the report

In December 2003, at the end of three months of fieldwork on St Helena, we submitted an Interim Report. It seemed sensible to ensure that suggestions for the conservation of Prosperous Bay Plain were available during the ongoing negotiations about the potential construction of an airport. These recommendations were made on the strength of field observations and preliminary identifications of species, and have not changed in any significant way. A substantial interval between submission of the preliminary report and this final report was required because of the need for study of invertebrate specimens by entomologists specialising in different groups.

This Final Report includes the information provided in the interim one, which may be best discarded. However, people who have read it will find little new in Sections 1, 2 and 3 of this final report. Section 4 is new, and has a taxonomic summary of the findings for the groups of invertebrates that have significant implications for conservation. Details of all the species recorded from the plain and photographs of many are included in the parallel document: Guide to the Invertebrates of Prosperous Bay Plain, St Helena, submitted to the St Helena Government at the same time as the present report. Sections 5 and 6 contain much new material, and the recommendations in Section 7 have been significantly augmented. The new plates include photographs of many of the sampling sites.

Summary

Prosperous Bay Plain (PBP) has only recently been widely recognised as having special environmental value. Entomologists on two Belgian expeditions in the 1960s carried out comprehensive studies on the invertebrates of St Helena. Analysis of their results by Ashmole & Ashmole (2000a: 116-118) showed an extraordinary concentration of endemic invertebrates on Prosperous Bay Plain, and demonstrated that this area is the main evolutionary centre on St Helena for animals adapted to arid habitats.

The present survey was designed to investigate the current richness of the area in terms of endemic invertebrates, to assess the risk to these animals inherent in potential airport development and to suggest ways of minimising its impact.

Intensive investigations at 22 representative sites in the ‘Eastern Arid Area’ (comprising Prosperous Bay Plain and immediately adjacent areas) confirmed that it is an extraordinary hotspot of endemicity. A total of 35-40 animal species and six genera that have been recorded in this limited area occur nowhere else in the world (Table 1). Some 25 of these species were recorded during the 2003 survey. In addition, 51 species endemic to St Helena have been recorded from the EAA though they are not restricted to it. All of these species are described – and many of them illustrated – in the Guide to Invertebrates of Prosperous Bay Plain, St Helena, which forms a companion document to this report.

Up to ten of the species found in the EAA for the first time during the 2003 survey may prove to be new to science. They include several spiders (two of them lycosids), a thysanuran, a cricket, a wasp, a wingless tineid moth and two flies, while a psocopteran represents a new endemic genus. Several of these new species will be formally named and described within the next year or so. Furthermore, several non-endemic species were found on St Helena for the first time and are considered to be probably native. A bonus was provided by the rediscovery, alive, of the endemic snail Nesopupa turtoni, described in the 19^th century from fossils found near Sugar Loaf but long assumed to be extinct.

Neither the Giant Ground Beetle nor the Giant Earwig was found in 2003, and both may well be extinct. The apparent absence of the large beetle Helenomelas basilewskyi and the spider Benoitodes caheni (both members of endemic genera) also gives cause for concern, but in the case of the beetle seasonal factors may account for its apparent absence. The other previously known endemic species not recorded in 2003 include several mites (Acari), but the samples of these have not yet been fully studied. Several of the other ‘missing’ species are either rare or extremely local (and thus likely to be missed in a short survey) or may not be adult in the season used for the survey. It seems likely that few of the endemic species of PBP have succumbed in the last half century.

Prosperous Bay Plain is by no means uniform, so the main habitats within it are described in terms of their invertebrate communities. In some cases, it is possible to find analogues of threatened PBP habitats in places within the EAA but outside the plain itself. However, conditions in the Central Basin of PBP, with its level dusty floor, are not replicated elsewhere on St Helena. This area has unusual geological features and is of outstanding biological interest. Observations made during the current survey indicate that it represents a mature desert ecosystem, a miniature analogue of the great continental deserts such as the Namib some 2000 km to the southeast (Seely 1992). As such, it deserves rigorous protection and international recognition as a remarkable product of island evolution. If effectively preserved, it can become a recognised part of the island’s natural heritage, valued by tourists and a continuing source of pride to the local community.

Massive construction and development activity inevitably poses threats to the natural environment, and construction of an airport in the east of PBP would destroy a substantial fraction of the rocky habitats available to the invertebrates. Furthermore, the latest proposals for the runway imply destruction of the easternmost part of the Central Basin. It is accepted that this destruction is inevitable if the proposed airport development proceeds, but there is also the possibility of additional and unnecessary ‘collateral damage’ unless strict guidelines and controls are developed and enforced during the whole development process.

The recommendations are framed with this danger in mind. Although some of them extend beyond the limits of purely entomological matters, we believe that our intensive work in the area, as well as background knowledge, justifies us in bringing broad issues to attention.

Mechanisms are suggested for monitoring and limiting the environmental impact of construction activity, and for restoration of the area that will be levelled east of the proposed runway.

Ideas are also put forward for two types of mitigation. First, it is suggested that as a way of ensuring long-term care for the endemic plants and animals of the island, an endowed terrestrial conservation post should be established within the St Helena National Trust. Second, ecological restoration initiatives are outlined, relating both to native plant communities on Prosperous Bay Plain and to seabird communities in the vicinity.

It is our hope that in this way, airport development on Prosperous Bay Plain can be carried forward in full conformity with the spirit of the St Helena Environment Charter of 2003. This would demonstrate the value of such instruments and might ensure that healthy and diverse communities of animals and native plants are maintained in a scenically attractive landscape, in this austere and unique part of St Helena.