Invertebrates of Prosperous Bay Plain

4. Invertebrates of Prosperous Bay Plain

• 4.1. Endemic, native and alien species

Endemic species are those that occur nowhere else. A St Helena endemic, therefore, must have evolved its unique characteristics after colonising the island from a continental area such as southern Africa. Often, species closely related to the island endemic can be found in that area of origin.

Some invertebrates, however, may be native to St Helena (in the sense of having reached the island without the help of humans) but not be endemic. They have not diverged evolutionarily from their founding stock, and members of the same species can still be found on the continent. This seems to happen mainly in species that are such good colonisers that any tendency for the island population to diverge genetically from its parental stock, is prevented by frequent arrival of individuals bearing a complement of genes typical of the continental population.

During two expeditions in 1965-67 the Belgian entomologists from the Musée Royale de l’Afrique Centrale at Tervuren carried out comprehensive studies on the invertebrates of St Helena. Analysis of their results by Ashmole & Ashmole (2000a: 116-118) emphasized the high diversity of endemic invertebrates (mainly insects and spiders) on Prosperous Bay Plain, and demonstrated that this area is the main evolutionary centre on St Helena for invertebrates of arid habitats. The animals there, descendants of colonising stocks that reached the island by travelling more than a thousand miles across the ocean, now constitute a unique community of mainly endemic species, the product of up to eight million years of evolution, isolated from the rest of the world.

The reasons why Prosperous Bay Plain is a major centre of endemicity are fairly clear. For more than half the life of the island, the area has been stable, arid and open, with only sparse vegetation. It offered quite different ecological and evolutionary opportunities from the lush and humid Peaks, and developed a distinctive suite of animal species adapted to life in a dry, saline and exposed environment.

The special nature of the current fauna has been highlighted during the 2003 work by the absence from our samples of the great majority of the introduced soil animals that we encountered continually in our 1995 sampling of moister habitats in the central part of the island. In those areas, alien species were far more prevalent than they are on Prosperous Bay Plain. However, the work in 2003 also shows that two alien species not recorded by the Belgians - a widow spider and a tenebrionid beetle - are now common on the plain and several others are present in smaller numbers.

Table 1 provides a complete list of the endemic invertebrates recorded from the Prosperous Bay Plain and the rest of the Eastern Arid Area of St Helena (see Section 2.1.1. for definition) and allows comparison of the species previously known from the area with those recorded during the 2003 survey.

In summary, about 74 species endemic to St Helena had previously been recorded in the EAA. Of these, three (two species of Helenactyna and one of Pellenes) are no longer relevant because of taxonomic changes, and 39 (including a few only provisionally identified) were found in the 2003 survey. This leaves 32 endemic species unaccounted for.

The missing species are made up as follows: Pseudoscorpiones 1; Araneae 9; Acari 5; Orthoptera 2; Dermaptera 1; Coleoptera 9; and Hymenoptera 5. These species are considered individually in Section 4.2. However, it should be mentioned here that they include the Giant Earwig Labidura herculeana and the Giant Ground Beetle Aplothorax burchelli, which are probably – though by no means certainly – extinct. The absentees also include a number of extremely rare species that could have been missed, a few others that may never have had resident populations in the EAA, and several conspicuous beetles that are probably present as adults only in the early months of the year.

On the positive side was the rediscovery of the snail Nesopupa turtoni, an endemic species previously known only as a fossil. Furthermore, up to ten other species were found that may prove to be endemic and new to science. One of these (a psocopteran) is considered to represent a new genus. The others include two or more wolf spiders, one cricket, a large wasp, one or more small moths and two flies.

Table 1 also shows that eight endemic species previously known from elsewhere on St Helena were found on the EAA for the first time. Perhaps more significant are ten non-endemic species found on the EAA during the survey that were not previously known from St Helena. Several of these are probably native.

In addition to the endemic and native non-endemic species, there are now on the plain a number of invertebrates that are probably introduced, some of which have seem to have spread onto the area only in the last few decades. They include predatory species that have doubtless affected the populations of some native invertebrate animals.

Among these, the most significant are probably the large centipede Scolopendra morsitans and the Brown Widow spider Latrodectus geometricus. The centipede has been on the island for more than a century and is an aggressive carnivore against which the endemic species may not have effective defences. The widow spider has evidently reached the plain since the 1960s, since the Belgians did not find it, but it is now almost everywhere except on the most exposed sites (Species map 2). It makes very sticky webs under large rocks, and may have inflicted heavy casualties on other invertebrates normally living in these sites. The tenebrionid beetle Hemasodes batesi, which has also apparently arrived in recent decades and is now numerous, may compete with native tenebrionids, but it is impossible to be sure. Other alien animals include several species of cockroaches and ants.

An intriguing feature of our results is the finding of several non-endemic species that are known as good aerial dispersers in the barren habitat at the top of the eastern cliffs. The most obvious example is the Death’s Head Hawkmoth, Acherontia atropos. We found an individual resting on a bare rock at Site 6, in a barren habitat lacking resources that would of any use to this nectar and honey-loving species. There is at present a resident population on the island, but we strongly suspect that this individual had recently made a landfall after flying from Africa (one was found on Ascension – where there is no resident population – in 1973). Another interesting find at this site was of significant numbers of the cicadellid bug Balclutha saltuella, which has not been found on St Helena previously. This grass-feeding species is a well known aerial disperser, so it seems possible that a propagule (male and female or fertilised female) had arrived recently. A similar case is that of the extraordinarily minute lathridiid beetle Adistemia watsoni, a widespread scavenger that we trapped at Site 21 and which is also new to the island.

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