Island Biogeography in the Era of Humans

In some ways, islands provide a ready-made laboratory for studying evolution. Thanks to their isolation from each other and the mainland, islands offer an ideal venue for speciation, with Darwin's finches on the Galapagos islands being perhaps the most famous example. Since Darwin's time, biologists have developed a theory of island biogeography to explain how species diversify as they disperse across islands, and the theory has held up well in the face of numerous tests — it's even become standard fare in undergraduate ecology courses. Since manipulations on a large enough scale to test the theory are generally challenging (and perhaps unethical), most of the experiments have been based primarily on observation. In some cases, though, human activity causes changes in a population which can be treated as though it were an experimental manipulation, giving researchers a chance to predict what will happen in response and check their predictions. Recently, a team of scientists used human-driven changes in lizard populations in the Caribbean islands to test the theory of island biogeography; they found that it's predictions weren't quite right, but the mismatch could be explained by adding a surprising factor to the theory — economics.

The theory of island biogeography makes a couple of straightforward predictions based on an island's size and how isolated it is. Islands which are easy to reach will be colonized by many species, while those that are more difficult to get to will find themselves home to fewer guests. Isolated islands, on the other hand, will only be colonized by a few species, and, as a result, all of the species on those islands will be descendants of the handful of original settlers. The more isolated an island is, the lower its species richness will be. An island's size also affects its biodiversity, since larger islands will have a wider variety of habitats, so species which arrive on the island will diversify to fill up the available niches. All in all, the theory predicts that an island's size sets a maximum for how many species it can host, while its isolation and local speciation on the island will decide how many species it actually has. Since the theory is framed in terms of isolation and area, it's also a useful tool for addressing questions in conservation biology, where a species' habitat may be reduced to distant patches, which are, effectively, islands.

To test the theory, a team of researchers took advantage of human-driven changes in the populations of Anolis lizards on the Caribbean islands. Until recently, each island was host to its own endemic lizards, but human activity has changed the lizards' distribution on the islands. The team wanted to see how well these changes fit with the predictions of island biogeography. For example, are new lizard populations more likely to be established on species-impoverished islands, such as large islands which can support high species diversity but only have a few species on them? Yes. And since each island is hosting more and more species, the diversity limits set by island area are playing a bigger role in determining the distribution of species. A surprising finding, however, was that species richness is increasing even on isolated islands. The theory predicts that biodiversity should be lower on isolated islands and should increase only slowly, since they're difficult to reach. So why are the lizards colonizing isolated Caribbean islands so quickly?

The answer turns on the meaning of 'isolation'. In their first analysis, the team used geography to determine how isolated an island was. The further away an island was from the mainland or other islands, the more isolated it would be, since the lizards would have to cross more water to get there. The entry of humans introduced a new dynamic into the process — shipping. Like so much of the planet, the Caribbean islands are criss-crossed by a shipping network which also serves as an unintentional transport route for other species. When the researchers recalculated each island's isolation score based on maritime shipping data, the theory's predictions fit nicely to the data. Islands which weren't as well connected by shipping lanes — that is, which were economically isolated — had less biodiversity and a lower rate of increase. Shipping, which depends on economics and politics, has superseded geography's role in determining how speciation happens on islands, at least in the Caribbean. The researchers suggest that the theory of island biogeography should be expanded to include the impact of economic isolation, which is as sure a herald of the Anthropocene as any I've heard. Interesting times, indeed.

Ref
Helmus, MR, Mahler, DL, and Losos, JB. Island biogeography of the Anthropocene. Nature 513:543-546. (2014) doi:10.1038/nature13739

Image credits
The anole image is distributed by Wikimedia Commons under a CC-BY-SA license.