Turtle Power to the Rescue

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After centuries of invasion by non-native species, returning native species to the islands of Mauritius is restoring the environmental balance lost. But one non-native species of reptile, the giant Aldabra tortoise, also seems to be helping correct the ecological imbalance left by the extinction of Mauritian tortoises centuries ago.

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Mauritius was once nearly mammal-less. The only furry, warm-blooded creatures on the island were bats. In the absence of mammals, reptiles and birds filled many of the island’s ecological niches.

But with the arrival of humans, as early as the 10th century, the unique native reptiles began to disappear. Rats, dogs, cats and other mammals that hitched rides with human explorers overwhelmed the natives animals. Humans also hunted the native creatures, particularly the infamously extinct dodo bird and two species of giant tortoise.

The rest of the island’s ecosystem suffered for lack of the giant reptiles. The tortoises used to eat the fruit of native ebony tree (Diospyros egrettarum) and help to spread their seeds to sunny spots. Without the shelled seed scatterers, the ebony seedlings sprouted directly beneath their parents. To make matters worse, non-native rats devoured the tree’s seedlings, and humans cut the trees for timber and firewood, according to the Mauritian Wildlife Foundation.

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Mauritian ebony trees are similar to avocados and mangoes in that they are all large fruits dependent upon large animals to disperse them. In its native southern Asian homeland, the mango still has the elephant to disperse it’s seeds, but the avocado and Mauritian ebony tree are fruits without a partner.

In The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and Other Ecological Anachronisms, author Connie Barlow, examines the possibility that the avocado co-evolved with the giant ground sloth that once roamed the North America. After the giant sloth died out, the avocado seemed doomed as well, but luckily for guacamole lovers, humans began dispersing the avocado far and wide.

The Mauritian ebony tree is now being spread by a surrogate fruit-disperser as well. In 2000, a group of Aldabra tortoises (Aldabrachelys gigantea) was introduced to Ile aux Aigrettes, a small island in Mauritius’ Mahebourg Bay. A total of 19 tortoises now roam the island and are helping to disperse the ebony seeds, according to the paper, “Rewilding with Taxon Substitutes,” recently published in the journal Current Biology.

A 2007 study had found that the ebony trees had failed to re-colonize areas were they had been wiped out by logging. But this recent study, conducted by researchers are the University of Bristol in cooperation with the Mauritian Wildlife Foundation, found that the tortoises were helping to reestablish the ebony trees in the previously ebony-less areas. The seeds even germinated better after taking a trip through a tortoise’s guts.

“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,” said lead author Christine Griffiths of the University of Bristol in a press release.

Previous efforts to restore the Mauritian ecosystem had focused on plant and bird species. In 1997, a nursery was built on Ile aux Aigrettes for production of endangered native plants. The nursery now produces about 30,000 plants per year. Most of these are planted on Ile aux Aigrettes during the rainy season from February to April.

So far, 20 endangered endemic plant species have been planted on the island, including the Round Island Hurricane Palm, of which only one adult tree remains.

The pink pigeon is also making a comeback on Ile aux Aigrettes. In 1994, a pink pigeon aviary was set up on the island. Now, there are over 75 pink pigeons flying free.

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While reintroducing individual species is important. Restoring the interactions of different species within ecosystems may be a more important long term goal, said co-author Stephen Harris also of the University of Bristol in a press release.

“Ecological restoration projects generally involve the plant community, as more often the animal components are extinct,” said Harris.

“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,” said Harris.

IMAGE 1: An Aldabra Giant Tortoise at Ile aux Aigrettes Nature Reserve, Mauritius (Wikimedia Commons).

IMAGE 2: An Aldabra tortoise on Ile aux Aigrettes (University of Bristol Press release; Nik Cole).

IMAGE 3: An Aldabra tortoise eating in the Bristol Zoo (Wikimedia Commons).

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