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

Prosperous Bay Plain and potential airport development

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6. Prosperous Bay Plain and potential airport development

    • 6.1. Nature of the ecosystem

To most people, Prosperous Bay Plain appears as a desolate area lacking interesting biological features. There are no watercourses, no substantial peaks, few substantial rocks, no trees and little obvious animal activity. Furthermore, the small shrubs and herbaceous plants show obviously low diversity.

As already indicated, this apparent poverty is partly a product of human activity (Section 3.3). Seabirds that once bred here and some native landbirds that foraged here have been eliminated by introduced predators (especially pigs, cats and rats). The native shrubs (perhaps mainly Boxwood, Scrubwood, St Helena Tea and Salad Plant) have been eliminated by alien herbivores and direct destruction by people. Surface rocks have been systematically removed, drastically reducing the availability of retreats for invertebrates, and scars have been left in many places by people and their machines.

Appearances are often deceptive, and before the start of our survey in 2003 we had analysed the reports of the Belgian entomologists and had pointed out that Prosperous Bay Plain was an area where endemic animals adapted to arid habitats were concentrated (Ashmole & Ashmole 2000a: 116). We naturally focused on the large number of species (around 20) and genera (5) that were apparently endemic to PBP; several more were restricted to the wider area that we now call the Eastern Arid Area (EAA).

However, in focusing primarily on number of species, we had not fully grasped the implication: that this was a miniature but mature desert ecosystem. It has been colonised over a period of several million years by representatives of invertebrate groups capable of crossing a large oceanic barrier and also of exploiting the opportunities offered by the challenging environment of PBP. The colonists originated mainly in Africa, but the process of colonisation of such a remote island – considered in detail for Ascension Island by Ashmole & Ashmole (1997: 559) – leads to development of a community very distinct from those on the continent. Continental deserts have been well studied, but a mature desert on an isolated oceanic island is a rarity, and is worthy of detailed study on purely scientific – as well as conservation – grounds.

It can be argued that the desert ecosystem of Prosperous Bay Plain has survived more nearly intact than any other major habitat on St Helena. Figure 1, based on our observations, is a preliminary food web for the Central Basin of PBP. In conjunction with the taxonomic analysis provided in the accompanying Guide to Invertebrates of Prosperous Bay Plain, it demonstrates the great extent to which the key links in the system are provided by endemic species, or in some cases non-endemic but probably native ones.

The most important species in the latter group is Samphire Suaeda fruticosa, which forms the basis – in terms of primary production – for much of the community. Few people with knowledge of St Helena would think of this plant as a key element in the island’s flora, and the Belgians never accepted it as native, although they found many endemic insects associated with it. We are convinced that it is native. Quentin Cronk (2000 and pers. comm.) is prepared to accept this conclusion, and points out that St Helena is by far the most southerly part of its range, making this a particularly interesting population. It appears to be the primary or sole foodplant of the noctuid moth Cardepia subvelata, the geometrid Scopula separata, the heteropteran Hirtopsallus suedae and about ten species of endemic beetles!

The predators and parasites depending on these herbivores are less easy to detect, and it is very likely that there are more to be discovered. We have inserted in Figure 1 the names of specialised predators (especially Hymenoptera and Diptera) providing non-obvious links in the web, but the more general predation of spiders and the other invertebrate carnivorous groups is also of great importance.

All the plant and animal communities on such a remote island inevitably lack groups that are not effective long-distance dispersers, even if they would be adapted to the island conditions. In its pristine state in 1501, the desert ecosystem of Prosperous Bay Plain lacked reptiles, key predators in warm continental deserts. However, several endemic landbirds were present, as well as the many seabirds (now extinct or excluded from the main island) that nested in the surrounding area.

Native bird predators adapted to conditions on PBP included the St Helena Hoopoe Upupa antaios (Ashmole 1963, Olson 1975). In 1995 we found bones of the hoopoe in the Central Basin of PBP, and it is likely that this bird was a significant predator in the PBP ecosystem, its long bill playing a part in determining the depth of burrows used by the wolf spiders on the plain. The two extinct ralliform birds Aphanocrex (or Atlantisia) podarces and Porzana astrictocarpus may also have been present, at least around the seabird colonies. Sadly, the only surviving native avian predator is the Wirebird Charadrius sanctaehelenae, which has around one tenth of its total population on the plain (McCulloch 1991). The hoopoe and rails have been in some sense replaced by the Indian Myna Acridotheres tristis, and waxbills Estrilda astrild fill an ecological niche that may once have been occupied by unknown endemic passerine birds.

Mammals were lacking in the original ecosystem, but introduced mammals now play a significant role. Pigs and goats have been removed and donkeys only occasionally graze in the area, but House Mice are abundant, and their arrival probably had significant effects on the native invertebrate fauna; they are now prey for feral cats. Rabbits are also present in some areas, and may influence the plant communities.

For ecologists, the complex interrelations of the species shown in Figure 1 are of prime interest, and the system deserves much more detailed investigation than we have so far been able to undertake.

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