Why do some islands have more species than others?

New research on island birds confirms key aspects of a 60-year-old theory

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a bird flying to a small rock island

Photo by Lesly Juarez on Unsplash 

The biodiversity of islands around the globe has fascinated and inspired scientists for hundreds of years. Islands are frequently home to unique species and are hotspots of biodiversity. But not all islands are equally rich — larger and less isolated islands harbor more species. 

In the 1960s two researchers, Robert MacArthur and E.O. Wilson, developed the theory of island biogeography, predicting that the number of species on an island depends on a balance between colonization, evolution of novel species in the islands, and extinction, and that these processes are determined by the size and isolation of the island. 

Since then numerous studies have found the same pattern, but a test of their predictions at a global scale had not been performed until now.  Earlier this year, an international team of ornithologists, evolutionary biologists and mathematical modellers, led by Luís Valente from Museum fur Naturkunde and Naturalis Diversity Center, published a new model to explain species richness in the islands. 

The researchers compiled a new dataset with DNA sequences from 596 species of terrestrial birds from 41 archipelagos (island chains) around the world. Their dataset combined data from samples from their own field trips, research collections, and field samples from colleagues with sequences from GeneBank. This dataset was then used to develop and to apply to their new analysis methods, a dynamic model that was able to predict global relationships that govern variation in biodiversity. In doing so, they confirmed two key aspects of the original island biogeography theory proposed by MacArthur and Wilson.

Understanding island biodiversity is important for island conservation, but has implications beyond it — it can allow us to better evaluate the effect of human actions when imposing barriers to species dispersal, and at a large scale it can contribute to the understanding of biodiversity around the planet.