Substitute Tortoises Help Repair an Island Ecosystem

Ile aux Aigrettes tortoise photo
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For thousands of years, on the small island of Ile aux Aigretts in the Indian Ocean, ebony trees and giant tortoises shared a symbiotic relationship. The trees provided food for the tortoises, who, in turn, distributed their tough seeds throughout the island, allowing them to flourish. With the arrival of settlers, however, the tortoises were made extinct and the trees were all but cleared entirely. But now, in an effort to regrow the endangered ebony forests, conservationists have introduced an exotic tortoise to serve as a substitute, a process known as 'rewilding' -- and for the first time ever, there's empirical evidence that it can save an ecosystem.In the 1980s, after years of deforestation on the island, it was decided the few remaining groves of endangered ebony forest should be protected -- but without their tortoise partners to eat and distribute their seeds, the forests failed to regrow. So, in 2000, conservationists introduced Aldabra tortoises, a foreign species, to serve as 'taxon substitutes' to the island's extinct tortoises -- and, over a decade later, it seems to have helped.

This is the first time empirical evidence has been put forth showing the rewilding efforts can pay off.

From Science Daily:

The introduced Aldabra tortoises not only ingested the large fruits and dispersed large numbers of ebony seeds, but the process of passing through a tortoise's gut also improved seed germination, leading to the widespread, successful establishment of new ebony seedlings in the heavily logged parts of the island.

"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," says Christine Griffiths of Bristol's School of Biological Sciences, co-author of a study on the process. "Reversible rewilding experiments such as ours are necessary to investigate whether extinct interactions can be restored."

Researchers involved in the study, which was published in the journal Current Biology, says the findings highlight the importance of inter-species relationships on the health of ecosystems -- and that ensuring their preservation calls for more than protecting a single species from extinction.

With so many endangered species hovering on the brink of extinction, much more than their fates alone hang in the balance. Ecosystems, by definition, are composed of a variety of plant and animal species whose very existence, to varying degrees, relies on the health of all the others. And, though an extinct species can never be truly replaced, the timeless interplay of species can continue, at least for now.