Aldabra

Cabris pan

Land of giants

Any baby turtles emerging in daylight normally have no chance of making it safely to the sea. Fortunately, this one got a helping hand from ZARP co-worker Dennis Hansen...

Any baby turtles emerging in daylight normally have no chance of making it safely to the sea. Fortunately, this one had a helping hand from ZARP co-worker Dennis Hansen…

Lying in the western Indian Ocean and undisturbed by humans, Aldabra is a place where wild nature still holds sway. The coral atoll – one of the largest in the world – is home to endemic birds, fruit-bats, and the largest population of giant tortoises found anywhere on the planet. The atoll encloses an enormous lagoon, which floods and empties through a series of narrow channels with the rise and fall of the tide. The lagoon hosts pristine mangroves that host baby turtles and act as an important nursery for many fish.  The reef surrounding Aldabra is unparalleled: here – undisturbed by fishing – groupers and other large fish can achieve their full adult size, and the reef is incessantly patrolled by large numbers of black-tipped reef sharks. Running this gauntlet is the largest population of green turtles in the region. A few females haul out every night to dig their nests on the sandy beaches, and the hatchlings must struggle past a long line of waiting predators if they ever wish to return as fully-grown adults. In short, Aldabra is a true Eden, and one that is carefully guarded and protected by the Seychelles Government in the form of the Seychelles Islands Foundations (SIF).

 

Zurich Aldabra Research Project (ZARP)

A group of researchers from the University of Zurich is helping to uncover the secrets of Aldabra. We currently fund a project officer and our work is focussed on the history, current activities and future prospects of the tortoises. These animals are key to the atoll’s terrestrial ecosystem. When present at such high densities large herbivores often act as ecosystem engineers because of their unique ability to exert very large impacts on the species around them, which in turn can influence fundamental processes such as biogeochemical cycles and soil structure. These giants once populated all of the islands in the region, but the arrival of humans – sometimes in the form of permanent inhabitants and sometimes in the form of hungry sailors calling in to re-supply their vessels – caused them to become extinct everywhere except Aldabra. Aldabra survived due to its remote location and petitions from no lesser figures than Charles Darwin, Joseph Hooker and Richard Owen. It seems that even in the 19th century, when pristine ecosystems were less of a rarity, Aldabra still had special resonance.

Goals

A unique three-letter code is branded onto the shell of each individual. This does not harm the tortoise in any way.

A unique three-letter code is branded onto the shell of each individual. This does not harm the tortoise in any way.

ZARP has several specific goals: first, to estimate the size, timing and duration of the genetic bottleneck in the tortoise population. Genetic bottlenecks occur when populations are forced to low densities for brief or extended periods. The severity and duration of a bottleneck are critical in determining how much genetic variation is lost. To determine this, we are marking and sampling as many of the tortoises as possible on one of the smaller islands that make up the atoll. So far, work is going well, with several hundred tortoises now sporting a unique three-letter code that enables us to identify them. Our second goal is to understand the links between climate and vegetation. SIF have been monitoring rainfall for the last 50 years, and worryingly, there seems to be a downward trend. This might have impacts on the productivity of the atoll, which ultimately might affect the tortoises. To estimate changes in productivity we are measuring tree rings and analysing satellite images. We are also helping SIF to analyse their long-term tortoise database. This can help to reveal changes in tortoise population density and provide clues as to what limits the tortoise populations on different islands.

Finally, around 30 tortoises now have a GPS transmitter attached to their shells. These regularly record precise locations, allowing us to better understand how tortoises use the island and the different types of vegetation.

Links

SIF: the organisation that protects and manages the atoll

ZARP: see how our work is proceeding and read the latest updates

e-obs: a small company that provides transmitters for animal movement studies