Rats place tortoises behind bars!

Ile aux Aigrettes is a 26ha islet located in the Mahebourg Bay off the south-eastern coast of Mauritius. The island is a Nature Reserve leased for conservation management to the Mauritian Wildlife Foundation. It is one of the last and best remnants of dry Mauritian coastal forest. In an attempt to preserve this dwindling ecosystem, extensive restoration work has been carried out since 1985. Nearly all of the island has been weeded of invasive plants and replanted with native plants. In 1991 numerous exotic animals such as rats and cats were successfully eradicated. The restoration of the island was paralleled with the re-introduction of a host of endemic plants and endemic animal species (pink pigeon, Mauritian fody, Olive white-eye, Telfair’s skink) and also the analogous Aldabra giant tortoise (to replace the now extinct Mauritian giant tortoises).

In June 2009, there was a suspected sighting of a rat on Ile aux Aigrettes. The presence of rats was confirmed on 29th September, when a female black rat (Rattus rattus) was caught and killed. Rat droppings have also been found in endemic bird feeding stations and subsequently, more rats have been captured and terminated. The rats are believed to have reached the island as stowaways or hitchhikers on one of the numerous boats that visit the island’s recently naturally recreated beach. The negative impact of rats on island ecosystems is broadly documented. Wherever rats arrive, the local bird and reptile populations crash. Mauritian animals are particularly vulnerable to these predators, since they evolved in the absence of any mammalian predators. Currently, rats are believed to be present on at least one third of the island and rough estimate of the population, is approximately 50 individuals. If left unattended, the rat population will increase exponentially and they will colonize the whole island. This will be especially detrimental to the endemic birds and reptiles. Hence, there is a strong emphasis to remove this invasive rodent from Ile aux Aigrettes before it causes any permanent damage and imperils the massive biological, human and financial investment in restoring the island.

To ensure the removal of rats preparations are underway to set a 25x25m grid of rodenticide across the 26ha Island. A total of 393 poison stations will be placed. The bait stations will be monitored on a daily basis for any sign of rat intake (but also by other animals such as hermit crabs, ants, etc) and wherever needed, bait will be replaced. After the completion of the eradication program, the island will be thoroughly monitored for signs of rats: using chew sticks (sticks dipped in peanut oil), searching for rat dropping at the feeding stations and using baited traps. If there is any indication of presence of rats, additional poisoning might be required. Possible routes of reinvasion must be identified, and action must be taken to minimize them.

Although the poison targets mammals, potential secondary impacts upon the resident endemic wildlife have to be minimised. As a preventive measure, it was decided that the 19 adult Aldabra giant tortoises present on the island will be fenced in shaded areas (where they won’t be able to access the bait). Finding and placing the tortoises in makeshift pens prove to be more difficult than envisioned. The first 14 tortoises were found quite easily in one day, but looking for the remaining tortoises in the 26ha forest was like looking for big needles in a big haystack. Besides, some tortoises tested our fences so well we also had escapees for the first three days.

Another difficulty arose because of their size and weight, try shifting a 250kg tortoise that wants to go in completely the opposite direction, across rough ground in a dense forest – it’s not easy. Although some of the tortoises would follow some of their favourite food held in front of them like dangling a carrot in front of a donkey. Tortoises are not particularly fast movers and easily distracted so this has been a slow and sometime complicated process. Creation of the pens has also been a tricky procedure, especially hammering poles through coralline rock to support the fence. Based on trial and error, the pens have now been made escape-proof. The next stage is to create the baiting grid, which will require a lot of machete work through the currently impassable thorny scrub.