Image copyright SPL Image caption Black rats are now found throughout the globe, primarily as a result of human introductions

Scientists have developed a way to identify island habitats that face the greatest threat from invasive pests, such as rats and feral cats.

A team of research developed a priority list of islands that they hoped would help governments and conservationists to allocate resources.

Invasive alien species are recognised as one of the main threats to native biodiversity, especially on islands.

The findings have been published in the Conservation Biology journal.

The study - funded by the UK government - was compiled from a range of data sources relating to more than 2,000 islands within 11 UK overseas territories.

Damage limitation

"The main idea behind the study is that we have invasive vertebrate species on most of the islands around the whole of the world, and they are doing a lot of damage to many endemic and globally threatened species," explained co-author Steffen Oppel, senior conservation scientist at the RSPB Conservation Science Centre.

"Knowing where to start in terms of removing the invasive species is quite difficult."

Invasive species are defined as non-native species that have been introduce to a habitat, usually as a result of human activity, and have become established, often posing a real threat to the long-term survival of native species.

"We compiled data that allowed us to prioritise which islands we could achieve the greatest biodiversity benefits by eradicating invasive vertebrate species. Quite simply, you cannot do everything at once and you have to allocate resources," he told BBC News.

Eradication programmes, although logistically challenging, have become one of the tools used by conservationists when it comes to tackling invasive programmes.

For example, co-ordinators of a long-running project to remove brown rats from the sub-Antarctic island of South Georgia are hopeful that they will have eradicated all the rodents, accidentally introduced during the 18th Century, by 2015.

The island is home to a number of species not found anywhere else in the world, such as the South Georgia pintail, a sub-species of duck. However, the species was threatened by the rat population because the rodents were eating the birds' eggs and young chicks.

First of a kind

Dr Oppel said the study was the first of its kind to consider a broad range of taxa to include all native vertebrate species.

"Previous prioritisation efforts either only considered birds or just a few islands in a very regional context," he said.

"We did this for a very large number of islands, more than 2,000, from the sub-Antarctic to the tropics."

He explained how the prioritisation list was compiled: "In all of the territories, we consulted people working there and compiled data on the islands themselves - its size, number of people living there and we got data on all of the native vertebrates, including their population and conservation status. We also compiled data on all of the invasive species on each island.

"It seems like very basic information but for many islands, especially in places like the Falkland Islands, there are hundreds of islands where no-one lives. So it was quite a large undertaking to get this data for every island within the UK overseas territories.

From this, the team was able to produce a list of 25 "priority islands" for invasive species eradication.

At the top of the list was Gough Island in Saint Helena, located in the South Atlantic Ocean. It is home to seven globally threatened species, but just one invasive alien vertebrate - the house mouse.

"Mice on Gough Island are known to eat albatross and petrel chicks," Dr Oppel observed.

"You might think that an albatross chick is 10 times the size of a mouse, but the mice eat them alive - they just nibble away. If you have several mice biting chunks out of the same albatross chick, it just dies.

"Albatrosses can only breed every two years and only have one chick. If none of those chicks survive then the population can decline quite rapidly."

However, he was keen to stress that the list could only act as guidance: "It is very important to remember that this is not a list that prescribes people to do something or act as a roadmap of what will be done next.

"But we have nailed the scientific baseline where eradications would be the most beneficial to globally threatened species."