Islands seem to have it all: ample sunshine, white sand beaches, and species you can't find anywhere else on Earth. Since Charles Darwin first traveled to the Galapagos Islands and British naturalist Alfred Russel Wallace to the Malay Archipelago in the mid-19th century, ecologists have believed there is something special about islands. A new study provides some of the first empirical evidence that island biodiversity really is different from that of the mainland. The findings have implications not just for how evolution and natural selection operate in these insular areas, but also for how conservation efforts can best protect them.
Darwin and Wallace noted many instances of the uniqueness of islands. For example, when Darwin analyzed bird specimens from the Galapagos after arriving back in England, he noticed that many neighboring islands hosted their own species of finch. Despite this recognition, the so-called theory of island biogeography didn't take off until the 1960s. Back then, ecologists Robert MacArthur and E.O. Wilson began to study species diversity on islands in an attempt to predict how many kinds of organisms a recently formed island could support. They predicted that islands closest to the mainland would be the least unique and that the islands with the highest biodiversity would have been separate from the mainland for the longest period of time. The notion that islands were ecologically and evolutionarily different from the mainland due to their isolation was mostly uncontested until 2005, when a group of international ecologists published a study in Nature indicating that the number of unique species in mainland areas such as the Amazon basin and central African rainforests rivals that of many islands.
In the new study, Adam Algar, an ecologist at the University of Nottingham in the United Kingdom, and colleagues decided to measure exactly how unique islands were by analyzing their rate of species turnover—a value that compares the number of species present in two different locations. Measuring biodiversity by species richness simply accounts for the number of species in a particular area, whereas species turnover allows scientists to compare the number and identities of species between two places, Algar says.
Algar and colleagues measured species turnover using Anolis lizards and Terrarana frogs, two groups of New World animals that underwent rapid evolutionary change and split into a variety of new species whenever they moved to a new environment. The researchers counted the species of Anolis lizards and Terrarana frogs on different Caribbean islands using published species lists. To get a representative measure of Anolis and Terrarana species turnover in the tropical Central and South American mainland, the team created island-sized, nonoverlapping samples. The researchers randomly oriented and superimposed these island shapes on maps of the Central and South American tropical mainland and identified the species living in each area using the same sources as for the islands. They computed species turnover by calculating the number of Anolis lizard and Terrarana frog species unique to pairs of different areas, and then factoring in variables such as the distance between the two areas and any environmental differences, such as elevation and rainfall.
Species turnover between a mainland sample and an island was much higher, on average, than between two mainland areas or two islands, the team reports online today in the Proceedings of the Royal Society B. A higher level of species turnover on islands indicates that islands had more unique species than similar-sized areas of the mainland. Anolis lizards and Terrarana frogs were more likely to form new species on islands because they were isolated by salt water and couldn't return to the mainland to breed with other members of their species, the team concludes. This meant that that the species on two islands were more likely to be different from each other than were the species in two mainland samples.
"Islands are unique from mainland areas," Algar says. "There really is something special about island environments that we hadn't recognized before."
The study has limitations, Algar says. For one, he and his colleagues used only two animal genera to measure species turnover. Furthermore, the mainland samples were randomly selected, rather than focusing on areas with high numbers of endemic species (with flora and fauna can only be found in a particular geographic area). Algar is currently planning a follow-up study to compare species turnover between islands and areas of the mainland that have many endemic species.
Besides improving our understanding of how biodiversity evolves on islands, the results could help guide conservation. It's much easier to preserve species diversity in a representative section of a mainland ecosystem than by preserving one island of an archipelago, Algar says. Because each island has a unique array of plants and animals, conservationists need to protect the entire island chain.
"The study provides additional, comprehensive evidence that islands are important cradles of biodiversity," Lauren Buckley, a biologist at the University of North Carolina, Chapel Hill, writes in an e-mail. "Species on islands are more unique than would be expected based on environmental differences and geographic separation."