On a Mediterranean island off Italy, a conservation project aimed at removing an invasive rodent produced a change that could be seen in the seabird colony. Zannone had been among the larger Italian islands targeted in the country’s expanding rat eradication programme. The work was part of a wider effort to remove black rats from islands where they posed a threat to native wildlife, particularly seabirds that nest in burrows. On Zannone, the eradication was completed in 2007. The number of breeding pairs of Scopoli’s shearwater subsequently rose from 27 in 2007 to 80 in 2016, giving conservationists a clear example of how removing an introduced predator could alter the fortunes of an island bird colony.
Black rat eradication transforms Italy’s islands for native wildlife
Black rats, or Rattus rattus, have been removed from a growing number of Italian islands since the late 1990s. The work was supported in many cases by the European Union’s LIFE programme, which helped fund projects designed to protect native island ecosystems. By the time of the review, rats had been eradicated from 14 Italian islands, with projects under way on five others.Islands offered a particular advantage for this kind of intervention. Their isolation can make eradication more manageable than it would be on a mainland landscape, while also helping to protect the result once the invasive animals have been removed. The projects varied considerably, however. Some islands were tiny, while others covered hundreds or thousands of hectares. Some were uninhabited, while others contained homes and small communities. Terrain was another consideration, with steep or inaccessible areas making conventional bait placement difficult.
Zannone tests new methods for black rat eradication
Zannone was larger and more difficult to treat than the very small islands where some of Italy’s earliest eradication projects had taken place. According to the Occasional Paper of the IUCN Species Survival Commission titled ‘Black rat eradication on Italian islands: planning forward by looking backward’ rats had begun to be eradicated from islands exceeding 100 hectares from 2005 onwards, with Zannone among them.The island’s uneven terrain also influenced how bait was distributed. On Zannone, bait stations were used at a lower density than during some earlier operations. In areas that could not easily be reached, rodenticide bait blocks were hand-broadcast from a helicopter. The blocks were placed inside biodegradable bamboo dispensers before being distributed.As conservation teams gained experience, techniques were adapted to the physical conditions of individual islands rather than relying on one standard method. Larger islands presented different logistical problems from small, relatively accessible sites, while the presence of other animals also had to be considered when selecting bait and delivery methods.
What makes black rats a threat to burrowing seabirds
Scopoli’s shearwaters are burrow-nesting seabirds, and rats can be particularly damaging to species that breed in this way. The Italian review points to evidence from several Mediterranean islands where declines in burrowing seabirds had been associated mainly with introduced predators, especially rats. The species affected included Scopoli’s shearwater, Yelkouan shearwater, Balearic shearwater and European storm petrel.The difference was also visible in breeding success. When data for Scopoli’s and Yelkouan shearwaters were pooled, islands without rats recorded a considerably higher breeding success than islands where rats remained. The figures given in the review were 0.78 on islands without rats compared with 0.14 on islands with rats.
Rat-free islands saw Scopoli’s shearwater colonies grow
The Zannone figures are among the clearest in the review. After the rat eradication in 2007, the Scopoli’s shearwater colony increased from 27 breeding pairs in 2007 to 80 pairs in 2016.In simple numerical terms, that was an increase of 53 pairs. The colony was therefore nearly three times its earlier size by 2016. The source presents the figures as an example of the effect rat removal had on shearwater colonies, rather than as an isolated population count detached from the wider eradication programme.Zannone was not the only island where a colony expanded following rat removal. At La Scola, the review records a rise in the Scopoli’s shearwater colony from between 60 and 100 pairs in 2001 to between 150 and 250 pairs in 2010, ten years after eradication, as reported. These examples helped illustrate why shearwater populations were considered when Italian conservationists decided which islands should receive priority.
Conservationists turned to biosecurity after removing island rats
Eradication did not mean the conservation work could simply stop. Rats can return to islands, either by swimming from nearby land or islands or through human transport. The review records reinvasions on several Italian islands and notes that some occurred where neighbouring rat populations remained.The distances involved could be surprisingly small. In the cases examined, reinvading rats had travelled from nearby islands or the mainland, with the maximum distance recorded at 320 metres and the average at about 219 metres. That made biosecurity an important part of maintaining an island after an eradication project had been completed.The researchers also describe how better understanding of rat movements and quarantine practices made it possible to attempt projects in places where reinvasion had previously been considered too risky. Monitoring boats, ports and other possible routes of entry became part of the wider effort to keep islands rat-free.
More than 1,500 hectares of Italian islands were cleared of rats
By the time of the review, completed projects had left more than 1,500 hectares of Italian islands free of rats, while ongoing or planned work was expected to increase that area substantially. About 15% of the Italian population of shearwater pairs covered by the assessment had been released from rat predation.The Zannone result therefore sits within a much larger conservation programme. The island itself provided a relatively contained setting in which removing an invasive predator could be followed by changes in a native seabird colony. Changes to European biocide rules made some later eradication projects harder to carry out, particularly where aerial bait distribution was required. Italian authorities interpreted the regulations in ways that restricted how rodenticides could be distributed, leading to disputes and delays on some islands.
