Islands are not static slabs of rock in the ocean - they are highly geologically dynamic, changing widely over their life cycle of millions of years. In Island biogeography: Taking the long view of nature's laboratories we see how this geological life of islands shape the ecology and evolution of the highly diverse plant and animal life that inhabit them. By looking at how the evolutionary processes of immigration, speciation and extinction patterns change over the course of an island's life span, we are making important new discoveries about island life, and even life on Earth more generally.
In a new paper published in Science, marking the 50th anniversary of Robert H. MacArthur and Edward O. Wilson's seminal work Theory of Island Biogeography, Professor Robert Whittaker et al review the most interesting recent developments in a field that is once again seeing an era of rapid developments and new insights.
Theory of Island Biogeography was "the single most influential contribution to island theory" in the last century, says Whittaker. The MacArthur-Wilson model sought to quantify islands' biodiversity, and even more importantly, to predict their diversity from patterns in the ways animals and plants immigrated, evolved and subsequently became extinct.
However Whittaker and his co-authors suggest that MacArthur and Wilson's insights on biogeographical processes are now being expanded by ideas that incorporate geological and environmental dynamics over much longer time scales than has previously been done.
"Our idea was to put the geological story together with the biological story," explains Whittaker.
As the island proceeds through it's life cycle - from its emergence out of the sea, subsidence and erosion, through to its eventual disappearance - the environmental conditions on the island change, and so do the biogeographical patterns of immigration, evolution and extinction.
Drawing on this emerging body of theory, Whittaker and his co-authors - a team of former SoGE researchers José María Fernández-Palacios, Thomas J. Matthews (BCM and DPhil, 2010), Michael K. Borregaard (Post Doc Researcher, SoGE 2012-14), and Kostas A. Triantis (Marie Curie Fellow, SoGE 2007-8) - observe that:
- Immigration starts low but picks up as the island establishes and grows as the island gains immigrants from other nearby islands in the archipelago.
- Speciation follows, with sufficient time, as empty niches get filled by adaptive evolution. Later extinction begins to rise, as the island fills with species and shrinks in size. Many species colonize younger, nearby islands, helping to seed this process anew.
- In the end, all is lost as the island sinks.
Whilst their models assume extinction is a natural process that happens on islands predictably over time, they also highlight the human impact on biodiversity:
"We don't have precisely known dates for all species extinctions on islands that are caused by humans," Whitaker explains. "But for those birds, invertebrates, mammals and plants for which we can reasonably estimate the timing of species extinctions, we can see that the losses have shown a pattern of increase over time.
"This reflects increased pressures placed by human activity on many island ecosystems around the world."