What is an isolated island in an era of global shipping?

Biodiversity on islands is governed by the push and pull of life and death. Species disappear as they become extinct, and colonists from nearby islands create new populations. Bigger islands have more habitats, and thus colonists can diverge to become different species. Smaller islands have less space, so any speciation that does occur will leave the species quite closely related. Islands that are more isolated will be colonized less often, over time, than those that are separated by narrower channels of water. Eventually, island biodiversity reaches an equilibrium point, where there is the proper number of species that an island of a given size and isolation can sustain.

As I wrote in April, the equilibrium theory of island biogeography was “an idea first described in the late 1960s by researchers Robert MacArthur and E.O. Wilson to describe species diversity in islands, and how the isolated terrestrial ecosystems found on islands evolve over time.” It’s first test was carried out in the Florida Keys, where fumigated some islands to wipe out all arthropods. He waited and watched to see how long it took for the insects, spiders, and crustaceans to re-establish themselves. By the end of a year, all the islands had been recolonized by the leggy critters, but the process was faster for those islands that were less isolated, or closer to the mainland.

But the world in 2014 is very different from the world even in the 1960s when MacArthur and Wilson formulated their theory; that world was also markedly different from the one just fifty years before that. Turning to the Anolis lizards that crowd the Caribbean Islands, an international group of researchers have discovered how.

Writing in this week’s issue of Nature, researchers Matthew R. Helmus, D. Luke Mahler, and Jonathan B. Losos point out that Wilson’s 1960s arthropod defaunation study was the most comprehensive experiment ever done on the theory, but it was still fairly small in scale. A more recent natural experiment has taken place in the Caribbean, on a much grander scale: lizards have spread among the islands, remaining native to some islands and exotic on others. By quantifying the spread and speciation of Caribbean anoles, Helmus, Mahler, and Losos were able to re-affirm the basic tenets of island biogeography theory. That is, as the lizards have moved around, those islands with the fewest native species gained the most exotic species, obscuring the historical influence of speciation on island biodiversity. The relationship between species diversity and island size has increased, while the relationship between species diversity and island isolation has decreased.

While island biogeography theory may not apply to metaphorical islands, their findings confirm its application to real islands, surrounded by water.

But we live in a new epoch, the Anthropocene, and species colonizations aren’t facilitated by geological processes or by adventurous individuals navigating the dangerous waters between islands. They’re facilitated by humans. For lizards like anoles, they arrive on new islands as stowaways on cargo ships.

Cartography of the Anthropocene in the Caribbean highlights cities, roads and shipping lanes. Human introductions of exotic species to Caribbean islands via cargo shipping has fundamentally altered natural island biogeography relationships. <em>Credit: Félix Pharand-Deschênes/Globaïa.</em>

Cartography of the Anthropocene in the Caribbean highlights cities, roads and shipping lanes. Human introductions of exotic species to Caribbean islands via cargo shipping has fundamentally altered natural island biogeography relationships. Credit: Félix Pharand-Deschênes/Globaïa.

“Beginning with the Industrial Revolution, multiple indicators of human activity (for example, atmospheric CO2 concentration) slowly increased, but then following the Second World War, increased rapidly,” the researchers write. “The establishment of exotic anoles follows such a curve. Establishment rate increased first after WWII, and again following the end of the Cold War when global shipping more than doubled.”

And if species using different modes of transportation to colonize new islands than they have historically, then we might have to rethink what exactly constitutes an isolated island. Because it is no longer a function of geology or topography. At one time, the islands that were most isolated were physically farther from the mainland and from other islands. Bermuda, for example, is a two and a half hour plane flight away from Miami, with no others nearby. That might explain why it’s got no native Anolis lizards.

Surprisingly, Bermuda has three exotic anole species, but Cuba has none, despite its proximity to the mainland and to other islands. Thanks to the US-Cuba trade embargo, Cuba sees far fewer ships than many of the other islands in the Caribbean. That’s evident in the graphic above, where most of the shipping neatly surrounds the relatively isolated Cuba. The limitation on its ability to sustain multiple anole species is not geological (it’s got some 50 native ones), but is instead a product of international politics. In the Anthropocene, it’s economic isolation that rules island biodiversity. – Jason G. Goldman | 26 September 2014

Source: Helmus, M.R., Mahler, D.L., and Losos, J.B. (2014). Island biogeography of the Anthropocene. Nature 513, 543-546. DOI: 10.1038/nature13739.

Header image: A Cuban Brown Anole (Anolis sagrei) in Florida, where it has become naturalized and then secondarily spread to islands across the Caribbean in ship cargo. Photo by Neil Losin, used with permission.