Island biodiversity: stable but vulnerable

The biodiversity on islands is the result of an equilibrium between the immigration of new species and the extinction of established ones. This idea was proposed exactly 50 years ago by the founders of Island Biogeography Theory, Robert H. MacArthur and Edward O. Wilson. But it had never been proven – until now that is. Ecologists from the Museum für Naturkunde in Berlin (Germany), Stony Brook University (USA) and the University of Groningen have discovered that the bat species on the Greater Antilles islands were in a state of equilibrium before humans arrived some 4000 years ago. Then extinctions became prevalent. The researchers calculated that it will take nature eight million years to restore the original diversity. The results were published in Nature Ecology & Evolution on 9 January.

Edward O. Wilson was right. That is the news exactly 50 years after the famous ecologist (who specialized in ants) wrote his textbook together with Robert MacArthur. ‘Their idea that island biodiversity would reach an equilibrium lost its popularity in recent years’, says Luis Valente from Berlin, the first author of the paper who is currently visiting Groningen.

It is very hard to prove that there ever was an equilibrium on islands. ‘Nowadays, people will say that the dynamics of islands prevent an equilibrium from being reached’, explains University of Groningen evolutionary ecologist and co-author of the study, Rampal Etienne.

Speciation

It took careful analysis of the DNA of all bat species on the islands and a study of fossil bats to reconstruct the past biodiversity. DNA and fossil data obtained by Liliana Dávalos, the other co-author of the study and an expert on Neotropical mammals, show when new species arrived from the mainland or formed on the islands. The fossils also show how many species have disappeared. Furthermore, the researchers developed a computational model that allowed them to analyse all this information. This model not only accounts for the processes of immigration and extinction that MacArthur and Wilson described, but also for speciation, the formation of new species on the island.

As a result we now know that 13 out of 37 bat species have disappeared since humans arrived in the Greater Antilles. Ten out of these 13 lost species were endemic to the islands. ‘And our data allows us to estimate how long it would take for biodiversity to restore to the original values’, says Valente. The answer is eight million years. ‘This estimated recovery time puts a new perspective on the damage that was done to the biodiversity’, says Etienne. Some researchers argue that nature will restore itself. ‘But in this case, that seems unlikely. Those islands will probably be gone in eight million years’ time.’

And these results do not just apply to bats. ‘We have some preliminary data on birds which seem to be in the same range’, says Valente. It shows how careful we should be with island biodiversity: once it’s gone, it won’t come back. MacArthur and Wilson were right in their prediction that an equilibrium would be reached. But the new analysis shows that once disturbed, the biodiversity may be lost forever.

Reference: Luis Valente, Rampal S. Etienne & Liliana M. Dávalos: Recent extinc tions disturb path to equilibrium diversity in Caribbean bats. Nature Ecology & Evolution (2017) doi:10.1038/s41559-016-0026

See also the blog by Liliana Dávalos on the Nature Ecology & Evolution website.

An article on this paper was featured on the National Science Foundation website for several days after publication.