Sea level changes over the past 20,000 years have left a strong imprint on present-day island biodiversity, finds a study of flowering plants published in Nature this week.

Current understanding of island biodiversity either assumes that islands are geologically static areas where immigration or emigration of species occurs, or more dynamic areas that are influenced by geological and tectonic changes over millions of years. Neither of these assumptions accounts for the oscillating climate present during most of the Late Quaternary (about 20,000 years ago to present day), during which time the melting of ice caps has raised sea levels by more than 100 metres, dividing up landmasses and reducing island area. Despite the obvious consequences of sea level change on the geography and biodiversity of islands, the effects of these past sea level changes on present-day island biodiversity had not been assessed until now.

Patrick Weigelt and colleagues analysed the effects of island area, isolation, elevation and climate on key components of flowering plant diversity on 184 islands worldwide over the past 20,000 years - including the Seychelles and the Hawaiian Islands. They find that sea level changes had a measurable effect on island biodiversity. Specifically, changes in island characteristics - especially in the area of an island - significantly influenced present-day diversity of endemic species (species that are native to a given area and found only there). The authors show that islands that were larger roughly 20,000 years ago possess a greater number and proportion of endemic species today than one would expect from their current size and remoteness. Conversely, the number and proportion of native species is primarily determined by present island characteristics.

The authors propose that future island research should incorporate the effects of past geographic dynamics on inter-island dispersal and diversification.