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

7.3. Ecological issues

7.3.1. Background

Pollution associated with construction of the airport (and its associated facilities and access roads) poses a significant threat to invertebrate populations. Seepage through the porous rocks will be a hazard, and there is likely to be drift of pollutants into the basin from the southeast, since the prevailing winds are from that direction. Control of dust during construction projects has sometimes been achieved by spraying of oil (Caesar 2001), but this practice should be explicitly prohibited in or near the Central Basin.

The endemic plants of St Helena are a major concern of conservationists on the island and elsewhere. On Prosperous Bay Plain the only common endemic is Babies’ Toes Hydrodea cryptantha, a widespread but sporadic annual. Its range will be significantly reduced by airport construction but the population of the area should not be endangered. The same applies to French Grass Euphorbia heleniana, though it is much less conspicuous. The threat is more serious to relict stands of St Helena Tea Frankenia portulacifolia and Scrubwood Commidendrum rugosum, which occur in only a few places on the plain. These species have much reduced populations on St Helena and all remaining stands should be preserved if possible.

The natural plant communities of Prosperous Bay Plain have developed in an arid and saline environment, in substrates of low fertility. Conventional landscaping often involves establishment of plant species alien to the area concerned and use of watering, fertilisers and pesticides. All of these are totally inappropriate on Prosperous Bay Plain, and it is our considered opinion that there should be no use of ‘amenity’ planting. The airport is being constructed in a naturally barren and austere environment, and any temptation to disguise this fact should be resisted (but see Section 7.5 for discussion of possibilities for ecological restoration). Plants brought into the area from other parts of the island are likely to be accompanied by inappropriate animals, and the need to care for such plants would be likely to lead to use of watering, fertiliser and pesticides. Vigilance on this matter will be required not only during the construction period but for the indefinite future.

We are particularly concerned at the possibility of use of grasses and other inappropriate plants to stabilise the large banks that will be created by filling of areas now below runway level, mainly on the landward side, and in Dry Gut under the long runway option. These banks will be subject to erosion during the rare episodes of heavy rain, but physical (engineering) solutions rather than biological ones should be given priority in guarding against this (but see also Section 7.5).

There is an opportunity for mitigation of the impact of construction in the large levelled area that will be created on the eastern (seaward) side of the proposed runway. Since the existing ground surface within most of the runway footprint is itself relatively featureless and level, it may be possible, after levelling, to reinstate substrates comparable to those now present. Further investigation of the practicalities will be required during the pre-construction phase, but it seems likely that in some places (for instance the pale yellow gravelly areas around our Site 21) it may be feasible to scrape off the top layer of the existing substrate in a manner analogous to preservation of topsoil, and to spread this material over the final surface. In this way seeds from the native plants (for instance Samphire Suaeda fruticosa and Babies’ Toes Hydrodea cryptantha) could be made available for colonisation of the new surface, and some animals might survive and recolonise the levelled ground.