Tuesday, September 9, 2014

A Tale of Two Islands

Despite its title, this story is not exactly about two islands. It’s more about the eight-legged inhabitants of two groups of islands. But fear not arachnophobes, body size and parasitism do not feature in today’s plot. Instead we travel to the Hawaiian Islands in the middle of the Pacific Ocean and the Mascarene Islands in the Indian Ocean to unravel a mystery.

The Hawaiian and Mascarene Islands are similar in many ways. The islands that comprise both archipelagos are generally of a similar size and elevation. Both were formed recently in the last 10 million years by volcanoes pushing their way to the ocean’s surface. Both suffer from similarly mild oceanic tropical climates that support a lush flora and fauna, including many species found nowhere else in the world. A few of these endemic species belong to a group of spiders called Tetragnatha. Hawaii boasts 38 different species of Tetragnatha spiders, but when CNRS scientists from the Université Paul Sabatier in Toulouse, France spent three years looking for spiders in the Mascarene Islands, they only found 3 Tetragnatha species. And so we confront a mystery: why the ten-fold difference in diversity? Shouldn’t evolution proceed in similar ways on two island chains that appear, at first, so similar?

A quick comparison of the locations of Hawaii and the Mascarenes provides a clue. While the Mascarenes are a quick 725 km jaunt from Madagascar, Hawaii is stranded in the middle of the Pacific, 4000 km away from any source of land-based life. So, how does this help solve the mysterious diversity disparity? Consider why isolation might allow the Hawaiian Islands to evolve more Tetragnatha spider species. First assume that more distance means fewer spiders are able to balloon their way across the intervening ocean.  ↑ distance = ↓immigration is a primary tenant of island biogeography. If Hawaii received very few colonizing spiders, then the original few Tetragnatha colonists probably had many opportunities to use habitats and resources that weren’t currently being used by other spiders and so were able to evolve into many different species. (Hawaiianplants and birds are famous for this.) However, in the Mascarenes, if other types of spiders arrived before the Tetragnatha, these opportunities to evolve may not have existed. Furthermore, a constant influx of spiders from Madagascar and subsequent fraternization between Mascarene spiders and the new colonists would make it difficult for the genes of Mascarene spiders to become different from the genes of Madagascar spiders. Without genetic differences, new species cannot form.

Is this a general phenomenon? Do the species that colonize isolated islands subsequently evolve into more species than they would on less isolated islands? That’s not something we can conclude from just one example, which is why scientists test general ideas by looking for lots of examples. Ideas about evolution inspired by the natural world can also be tested in more artificial settings- for example, by watching how viruses and bacteria evolve when they’re put into different situations. Real islands, like Hawaii and the Mascarenes, are often referred to as ‘natural experiments’ because their isolation from other land allows us to compare one to another to test how ecology and evolution play out in different situations. This is the core of biogeography- using the natural world to understand how geography shapes life around us. But, it can be tricky to make conclusions when islands differ in more than just the way that we are interested in, or when factor X (e.g. ‘isolation’) may affect factor Y (e.g. ‘evolution of more species’) in several ways (e.g. ‘new ecological opportunities’ and ‘reduced gene flow from colonists’). I like this paper because it shows how observations of nature inspire “rules” of biogeography that we think are generally true. Now, can we figure out how to test the logic of these biogeographic rules with an un-natural experiment? Could we disentangle the effects of new ecological opportunities from reduced gene flow for the process of evolution on isolated islands? Bring on the microbes and computers!

You can find this paper at:

Casquet J., Corinne Cruaud, Frédérick Gavory, Rosemary G. Gillespie & Christophe Thébaud (2014). Community assembly on remote islands: a comparison of Hawaiian and Mascarene spiders, Journal of Biogeography, n/a-n/a. DOI: http://dx.doi.org/10.1111/jbi.12391