Aldabra Island Xeric Scrub

Aldabra Atoll is home to the world’s largest population of giant tortoises, at approximately 152,000, and is one of the few areas in the world where the dominant grazer is a reptile. Giant tortoises are the largest herbivore on the atoll and through their grazing activities, they act as ecosystem engineers, manipulating plant distributions through seed dispersal and nutrient cycling. The intense grazing creates “tortoise turf”, an assemblage of low-growing herbaceous species, grasses, and sedges. 

The ecoregion occupies the island of Aldabra, one of the largest atolls in the world. This is an isolated coral atoll approximately 400 km northwest of Madagascar, 680 km east of the African Mainland, and 1,100 km southwest of the main island group of the Seychelles. The atoll comprises four main islands (from largest to smallest - Grand Terre, Malabar, Picard, and Polymnie) and numerous lagoon islets. As is characteristic of atoll island systems, the four islands form a rough circle that encloses a large, shallow lagoon, and are thus separated by four channels feeding to the Indian Ocean. 

Topographically, the island surface is rugged, as prolonged weathering has eroded much of the limestone into pits and fissures. There are also areas of raised lagoon sediments, coastal beaches, sand dunes, and undercut limestone cliffs. The climate is tropical with an average annual temperature of 27°C. Rainfall is variable from year to year, and it averages about 1,200 mm per year. There is a wet season from November to April and a drier season from May to October. 

There are two structurally different types of xeric vegetation. The first is dominated by dense, almost monospecific stands of Pemphis acidula thicket that covers areas close to the saline water table. The second is a mixed scrub region, generally very dense but quite open in some places, and covers most of the rest of the atoll. The mixed scrub is composed of low trees, shrubs, herbs, and grasses.

The terrestrial flora comprises about 9 species of ferns and 178 flowering plants, 38% of which are believed to be endemic. There are also many endemic species of invertebrate. Significant breeding populations of green turtles and hawksbill turtles use the beaches of the atoll for nesting. Aldabra supports an endemic subspecies of gecko, Abbott’s day gecko, and a large population of Bouton’s fishing skink. 

Aldabra has two endemic species of land bird, and 10 endemic sub-species including the Aldabra drongo and the Aldabra white-throated rail, the last of the flightless birds for which the western Indian Ocean islands were once famed. A successful reintroduction of the bird to Pïcard Island has significantly improved the rails’ conservation status. The other nine endemic sub-species comprise Aldabra sacred ibis, Aldabra blue pigeon, Aldabra turtle dove, Aldabra coucal, Aldabra nightjar, Aldabra bulbul, Souimanga sunbird, Aldabra white-eye, and the Aldabra fody. 

Aldabra is an important regional breeding site for several species of seabirds. The greater and lesser frigatebirds both breed on Aldabra, in the second largest colony of frigatebirds in the world. Aldabra is a regional stronghold for the red-tailed tropicbird and red-footed booby. Aldabra atoll is also a breeding site for the greater flamingo. 

In 1981, Aldabra became a special reserve (350 km²) under the National Parks and Nature Conservancy Act of the Seychelles. In 1982, the reserve was listed as a World Heritage Site. More recently it has been gazetted as a Ramsar site (439 km²). There is a research station on Picard Island. The Reserve Warden and support staff live at the station and apart from visiting scientists and tourists, there are no other people on the atoll. 

Attempts to grow commercial crops were abandoned before large-scale habitat alteration occurred, although several commercially and incidentally introduced plant species still occur. Green turtle numbers are growing, making Aldabra one of the only growing populations of this species in the world. The tortoise population has recovered to reach carrying capacity on Grande Terre, the largest of the islands. 

There are no introduced birds on Aldabra. In the past, rats, cats, and goats were introduced to these islands. The black rat is a significant predator of bird nests. Rats also cause substantial damage to native vegetation by stripping bark and eating leaf and flower buds. They have been responsible for the decline in numbers of several native bird species. 

An alien coccid insect parasitizes many native plant species and does significant damage to native plants, but the introduction of a ladybird beetle appears to have controlled infestations. The presence of several introduced bird species on the nearby Assumption Island has been a source of concern for many years. There exists a very real possibility of these species colonizing Aldabra. The impacts of climate change are growing threats to semi-arid oceanic islands including sea level rise, ocean acidification, and increase in drought frequency. The latter would result in decreased vegetation, resulting in habitat changes impacting species, particularly tortoises. 

The priority conservation actions for the next decade will be to: 1) investigate and predict the impacts of climate change on habitats and species through habitat and climate modeling; 2) continue the eradication of alien species and prevent further invasions of alien species; and 3) promote ecotourism to create financial support for ongoing conservation programs without impacting the environment.

Citations

1. Burgess, N., Hales, J.A., Underwood, E., Dinerstein, E., Olson, D., Itoua, I., Schipper, J., Ricketts, T. and Newman, K. 2004. Terrestrial ecoregions of Africa and Madagascar: a conservation assessment. Island Press.
2. Haverkamp, P.J., Shekeine, J., de Jong, R., Schaepman, M., Turnbull, L.A., Baxter, R., Hansen, D., Bunbury, N., Fleischer-Dogley, F. and Schaepman-Strub, G. 2017. Giant tortoise habitats under increasing drought conditions on Aldabra Atoll—Ecological indicators to monitor rainfall anomalies and related vegetation activity. Ecological Indicators. 80, pp.354-362.
3. Seychelles Islands Foundation. 2016. Aldabra Atoll Management Plan 2016. [Online]. [Accessed 11 April 2018]. Available from: https://www.sif.sc/sites/default/files/downloads/Aldabra%20Atoll%20Management%20Plan.pdf
4. IUCN. 2017. Aldabra Atoll - 2017 Conservation Outlook Assessment. [Online]. [Accessed 11 April 2018]. Available from: https://www.worldheritageoutlook.iucn.org