South Georgia rat cull begins again 1 March 2013 Last updated at 15:37 By Lorna Stewart BBC Radio Science Unit An "alien invader" eradication scheme of unprecedented scope will resume on Friday on the sub-Antarctic island of South Georgia. Three former air ambulance helicopters will begin dropping 100 million poisonous pellets in a bid to eradicate brown rats from the island. The rats, which first arrived in the 18th Century as stowaways on sealing ships, are a threat to native species. The three-month mission is the second phase of a four-year project. The first phase saw successful extermination of brown rats from a much smaller portion of South Georgia in 2011; the second aims to clear more than half of the remaining area. Brown rats are an "invasive species" in South Georgia, and with no natural predators on the island the population exploded soon after their arrival. Now their appetite for the eggs and chicks of ground-nesting birds such as the South Georgia pipit and pintail is threatening the survival of these endemic species. The helicopters will criss-cross the island, distributing the bait pellets with mathematical precision to cover each and every square metre. Every single rat must be eradicated for the programme to be a success, said project leader Prof Tony Martin from the University of Dundee, who spoke to BBC News before setting off. Killing 99.999% is a failure. If we don't get every last one, we may as well not have gone there in the first place," he said. We have to eradicate, not control, them. Clearing an entire island of an invasive species is hugely ambitious but the terrain of South Georgia allows the team to work in stages. Glaciers, which the rodents will not travel across, cover much of the island - meaning that the rat colonies live in isolated pockets and can be tackled separately. But time is of the essence. The glaciers are retreating and if the separate colonies are allowed to mingle, eradication will become much more difficult. So far Prof Martin's team have faced gale-force winds and blizzards, but with the weather improving they are reported to be ready to resume the largest-ever species eradication project.