Island species are at greater risk from human activities, the researchers warn
Rare species on islands are at risk of being lost forever because they have been generally overlooked by current conservation models, a study suggests.
Although islands had less diversity of species compared to mainland sites, a greater proportion were unique to the remote habitats, researchers concluded.
Yet the impact of human activities was relatively greater on islands because space was at a premium, they added.
The findings appear in the Proceedings of the National Academy of Sciences.
The team from Germany and the US wrote: "Islands are well-known centres of range-restricted species and thus high levels of endemism.
"However, they are also acknowledged for their lower species richness compared to mainland areas," they added.
"Hence, an index combining both endemism and species richness can provide insight into the question of relative conservation value of islands and mainlands."
But to date, the team observed, no study had focused on the differences between mainland areas and islands.
While some islands, such as the Galapagos archipelago and Madagascar, were well known for their biodiversity richness, the team said the habitat's biological value had not been quantified.
"Normally, you want to focus on the most diverse places to protect a maximum number of species," said co-author Holger Kreft, a post-doctoral fellow from the University of California, San Diego.
"But you also want to focus on unique species that occur nowhere else."
To understand the level of endemic species found in particular areas, the team used a measure of biodiversity that weighted rare species more heavily than widespread ones.
When they calculated the level of weighted biodiversity, they then compared island ecosystems with continental habitats.
Using this measurement, the team found that islands' populations of flora and fauna were eight to nine times as rich.
The team observed: "Island floras and faunas are usually recognised to maintain a high degree of endemism because of their geographic isolation and the limited interchange with neighbouring mainland or island biota."
"Islands are important and should be part of any global conservation strategy," Dr Kreft added.
"Such a strategy wouldn't make any sense if you didn't include the islands."
The team also noted that islands were at the centre of "past and imminent species extinctions, stressing even more the need for information on both biodiversity and specific threats in this part of the world".
Lead author Gerold Kier, project leader at the University of Bonn, warned that threats to islands' biodiversity were likely to rise more sharply in the coming decades.
"That threat is expected to accelerate particularly rapidly on islands where access to remaining undeveloped lands is comparatively easy," he explained.
As a result, expanding farmlands, deforestation and other changes in how human populations use land were likely to be more stark than on the mainlands.
"We now have new and important data in our hands," said Dr Kreft, "but still have no simple solution for nature conservation."