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.
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