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Andreone, F., Channing, A., Drewes, R., Gerlach, J., Glaw, F., Howell, K., Largen, M., Loader, S., Lötters, S., Minter, L. and Pickersgill, M., 2008. . AMPHIBIANS OF THE AFROTROPICAL REALM. |
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Due to their permeable skin few amphibians are able to tolerate dry or salty conditions, a characteristic that has prevented most species from colonising oceanic islands. The most diverse oceanic island amphibian fauna is found in the Seychelles, an archipelago of about 115 granitic and coralline islands in the Indian Ocean, some 1,600km east of mainland Africa, and northeast of the island of Madagascar (Figure 1). Eleven native amphibian species have been recorded from the Seychelles islands, comprising both recent colonists and ancient endemics. Recent colonization by one species, the Mascarene Grass Frog Ptychadena mascariensis (LC), is probably the result of human introduction (Vences et al. 2004) in the 1800s, whilst the endemic Seychelles Treefrog Tachycnemis seychellensis (LC) is believed to be descended from a natural colonist from Madagascar (Vences et al. 2003). The remaining spe- cies are all ancient endemics that have probably existed on the island since their isolation from the Indian landmass some 65 million years ago. These species comprise the endemic frog family Sooglossidae and seven species of burrowing caecilians (Order Gymnophiona). Sooglossidae are an exceptional family in several respects. Four species are currently recognized in two genera, although recent morphological and mo- lecular data suggests that a further three species remain to be described and that the genera need to be redefined. They are all small frogs, with Gardiner's Frog Sooglossus gardineri (VU) among several species up for contention as the world's smallest frog (with adults being as small as 9mm in length). All sooglossids are found in the damp forests of the two highest islands, Mahé and Silhouette. These habitats have only seasonal or fast-flowing stems, and the sooglossids have abandoned the normal frog life-cycle in favour of ter- restrial development. The Seychelles Frog Sooglossus sechellensis (VU) lays its eggs on land and the female carries the tadpoles until they develop into frogs. Sooglossus gardineri is even more specialized, with the male guarding terrestrial eggs that hatch into 3-mm long froglets; the entire tadpole stage is passed within the egg. Thomasset's Seychelles Frog Nesomantis thomasseti (VU) has recently been discovered to have a very similar breeding strategy, but nothing is known of the reproductive behaviour of the Seychelles Palm Frog S. pipilodryas (VU). This latter species is the most recently described of the Seychelles amphibian species, having been discovered in 2000 (Gerlach and Willi 2002). It lives in the axils of endemic palms where it lay protected from discovery by the dense spines of the palm leaves. It is the only arboreal sooglossid, the other species all being associated with crevices in boulder fields or the leaf-litter and root-mat of the forest floor. The caecilian fauna of the Seychelles islands comprises six species in three endemic genera. Not surprisingly, and as with caecilians in general (see Essay 1.3), these are the least well known of all Seychelles amphib- ians. Their burrowing habits make them difficult to locate and study. For most species, their distributions are reasonably well defined and there are some observations of breeding habits. Different species appear to show the full range of reproductive strategies, form aquatic larvae, thorough to ter- restrial larvae and direct development. The limited ecological data available indicate that Grandisonia alternans (LC) is a widespread species occurring in all habitats and Hypogeophis rostratus (LC) is a similar generalist, although more associated with lowland habitats. In contrast, Praslina cooperi (VU) and some of the small Grandisonia species appear to be specialists of the high forest. Current research into this group is attempting to develop monitoring methods and to identify aspects of their ecology that are of importance to their conservation. Half of Seychelles amphibians are threatened due to their naturally restricted ranges and deteriorating habitats (five species are categorised as Least Concern, 5 Vulnerable and one Endangered). The sooglossid frogs and the caecilian Praslina cooperi have particularly restricted ranges, being associated only with the damper rain-forests. Habitat deterioration is a significant threat to the caecilian Grandisonia brevis (EN), which has a re- stricted range and occurs in habitats that are suffering from ongoing invasion by alien plant species. In addition, there is some suggestion that changes in rainfall patterns may have impacts on some species, which may result from mid-year declines in rainfall restricting the activity and distribution of the rain-forest specialists (Gerlach 2000). Losses of several areas of marsh habitats have probably caused population declines in some species, such as another caecilian Grandisonia alternans (Gerlach 2000). There have been a number of successful conservation programmes in Seychelles, concentrating mainly on birds. Critically Endangered species have been rescued from extinction by limited habitat management, but mostly by inter-island transfers. These programmes have been to the benefit of spe- cies such as the Seychelles Magpie-robin Copsychus seychellarum and the Seychelles White-eye Zosterops modesta. Such options are not available for the amphibians due to their close dependence on their high forest or marsh-land habitats. Consequently, preservation of their habitat is essential; most species are present in protected areas, although habitat management is still required to control the spread of invasive plant species. The island with the highest diversity of amphibians (Silhouette) is currently unprotected and designation of this island as a national park would significantly enhance amphibian conservation in Seychelles. To date, there has been no evidence of any sudden amphibian decline in Seychelles, although the potential impact of an invasion by chytridiomycosis would be considerable. Monitoring programmes are in place for the frogs, and are being developed for the caecilians. Updates on research and conservation progress can be found at http://members.aol.com/jstgerlach/herps.htm |
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