Wednesday, June 23, 2010

A Little Patch of Home

Yep, I love me some stories about islands. Have I said that before? Probably, especially considering that's what my masters degree was about. Anyway, this is a new study published in the journal Ecology looking at how land birds colonized the Lesser Antilles.

I can, and have, written long and in depth on this topic, but I'll try to quickly sum up the Equilibrium Theory of Island Biogeography (ETIB). First you need an isolated habitat, island or otherwise -- they are great for study because they are relatively small, replicated, and isolated. The whole formation of a theory thing started when scientists noticed that as the size of a patch increased the number of species increased (not always necessarily the number individuals of a species), this is known as the Species-Area Relationship. Yeah, pretty logical right? Let's keep going with the concept though. Let's say you have a brand new island, species will immigrate to the island -- how many, which ones, and who stays? All that depends on the island itself -- how big it is and how isolated it is from the source population (usually the mainland but also other islands). An island far from the mainland is harder to get to and so has fewer species. A small island is harder to survive on and so also has fewer species. Get where I'm going with this? Now the ETIB says that there is an equilibrium point between immigration and extinction -- a balance between these two processes that determines the number of species on an island. Over time immigration increases the number of species on the island, but as you get more species on an island there are more species that have the possibility of going locally extinct (see the embedded graph).

This study looks at not-so-much the equilibrium point itself, but rather the colonization of island by bird species.

As you might guess, birds are pretty good colonizers, considering that they can fly. But also think about bird species themselves, some are widespread and undifferentiated while others are pretty limited in their distributions and can be rather specific. On islands, because of their size and isolation, this can can mean some species will exhibit morphological or genetic differentiation between individual island populations and/or the mainland source population. This study looks at ecological and geographic distributions of source population to see how recent/continuing colonizers, old colonizers, and non-colonizing/mainland species are different from each other. When you are thinking about these categories, think about them in terms of time and how long it takes a population to genetically differentiate.

In terms of ecological and geographic distribution, the author found that recent colonists tend to inhabit open habitats and are pretty abundant and widely distributed on the islands they are found. Conversely, old colonists tend to inhabit forests and their abundance and distribution depends on the species and on the island. If an old colonist has recently spread to other islands then they act like recent colonists in that they have relatively greater abundance and broader habitat distributions. In general, most of these data showed that the distributions of source populations is broader for recent colonizers than for old colonizers.

To read much more on this topic, here's the article:
Ricklefs, Robert E. (2010) Colonization of the Lesser Antilles by land birds. Ecology: Vol. 91(6), 1811-1821. (DOI: 10.1890/09-0682.1)

(image from birdquest.co.uk and algebralab.org respectively)
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