Monday, November 28, 2011

Supertramps play checkers on remote tropical islands

Biogeographers have a not-so-secret love affair with archipelagos.  Inarguably, islands are sexy.  But, when a bunch of islands get together they become the pieces in an irresistible puzzle of “who lives where and why?”

Consider the facts: Islands are hard to get to; for land-dwelling plants and animals they are stepping stones of potential habitat amidst an endless sea of death.  Therefore, islands close together (such as in an archipelago) should have the same plants and animals living on them, because the hardest part of setting up shop in a group of islands is traversing the distance from the mainland.  However, contrary to expectation, islands in archipelagos often have different biotas and the two-hundred-year-old question is “why?”

You may have heard of Charles Darwin- his answer to the “why” for finches in the Galapagos was evolution by natural selection after a long trip from mainland South America.  Someone you may not have heard of is Jared Diamond, but he also sparked a ferocious debate when he claimed that different bird species in the Bismarck Archipelago live on different islands because they compete with each other.

In 1975, Diamond noticed that certain pairs of bird species make a checkerboard pattern across the archipelago- the two species never live on the same island but do live on neighboring islands.  He suggested that one explanation is that the species are so similar in the types of food or habitat that they use that they compete and one kicks the other off the island.  Of course, there are lots of other reasons why two species don’t occur together on the same island: maybe the two are different and need different types of habitat, or maybe they live in different parts of the archipelago that are two difficult to travel between.  A new study this month in the Journal of Biogeography revisits Diamond’s original bird data from the Bismarck Archipelago and looks more closely at whether his checkerboard patterns really do result from competition.

It’s time for some math.  There were 154 bird species found across all the islands, which means that there are (154 x 153)/2 = 11, 781 possible pairs of bird species.  The researchers found that 1,516 pairs of species followed a checkerboard pattern, but that only 27 of these were between closely related species that are likely to compete with each other.  This number seems small relative to the total number of possible pairs; how do we know that this isn’t the number of checkerboard pairs that would happen just by chance? By using a null model, of course (see previous post), where a computer puts bird species on islands at random and then calculates the number of pairs of species that don’t occur together.  This showed that for several types of birds, there were actually more checkerboard pairs than expected by chance- which means that competition could be the explanation.  Or, it might not be.

When glaciers expanded and the sea-level dropped back in the Pleistocene, many of the Bismarck islands became connected by land, but there were still groups of islands that remained separate.  In many of the checkerboard pairs of species, the species live in different parts of the archipelago that were separated by water during the Pleistocene- this shows that species may not live together because they have not been able to get to the same islands.

Another peculiar commonality of the 27 checkerboard pairs of closely related species is that in almost all cases one of the species lives only on small islands where there aren’t many other bird species.  These ‘supertramps’, as Diamond called them, may be weak competitors that can only live in places not already usurped by other species.  The prevalence of supertramps in pairs of species that form checkerboards means that, despite 35 year of controversy, competition may actually be a good explanation, at least in part, for why islands have different bird species.  As for what’s responsible for the other (much larger) part, geography, evolution, and chance appear to be the major culprits.

You can find this article at:

ResearchBlogging.orgCollins, M., Simberloff, D., & Connor, E. (2011). Binary matrices and checkerboard distributions of birds in the Bismarck Archipelago Journal of Biogeography, 38 (12), 2373-2383 DOI: 10.1111/j.1365-2699.2011.02506.x

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