How Reintroducing Once Eradicated Giant Tortoises Has Restored an Island's Ecosystem

Photo: Dennis Hansen

A highly controversial technique called 'rewilding', introducing new species to places where others have gone extinct, has been proven to work according to a recent study in Current Biology. Iles aux Aigrettes is a small island of Mauritius which has had no large fruit-eating animals to disperse seeds since the native species of tortoise went extinct.

Photo: Nik Cole

The island's critically endangered ebony trees had also been decimated due to logging, and a survey showed large areas were still ebony-less even though logging stopped 30 years ago. Researchers introduced the exotic Aldabra tortoises to make up for the large void in the ecosystem. Christine Griffiths of Bristol's School of Biological Sciences explained: "Our results demonstrate that the introduction of these effective seed dispersers is aiding the recovery of this critically endangered tree whose seeds were previously seed-dispersal limited. Reversible rewilding experiments such as ours are necessary to investigate whether extinct interactions can be restored."

Photo: Christine Griffiths

Not only did the tortoises eat the ebony seeds and disperse them throughout the area that had no seedlings, but the action of passing through the tortoises' digestive systems enabled better seed germination. The heavily logged areas of the island ended up with widespread ebony seedling growth.

This was a reversible introduction (the tortoises could be removed if they showed signs of damaging the ecosystem further) and its success may lead to further studies to fill environmental voids caused by the extinction of previous species. Professor Stephen Harris, co-author of the study, said: "Ecological restoration projects generally involve the plant community, as more often the animal components are extinct. There is, however, increasing evidence that restoration ecologists should be most concerned with the decline of species interactions, rather than species extinctions per se. Species interactions structure ecological communities, and provide essential ecosystem processes and functions such as pollination, seed dispersal and browsing, that are necessary for the self-regulation and persistence of a community."

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