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Friday, May 04, 2007

The Song of the Dodo


Two of the genres that are among my favorites are natural history and travelogues. So it's no surprise that I found The Song of the Dodo: Island Biogeography in an Age of Extinctions by David Quammen one of the best books I've read in a while. Originally published in 1996 it retains the status of a classic of sorts having won several literary awards and remains in print and on store shelves in a newer 2004 printing that I picked up a couple months ago.

Basically, the book traces the intellectual pedigree of the field of island biogeography starting with Alfred Russell Wallace, a first rate naturalist in his own right, and more famously the "co-discoverer" of evolution (although Quammen has a little axe to grind on that score) with Charles Darwin. From there Quammen traces the development of the field through the 20th century which really picks up momentum in the 1960s with the work of Robert MacArthur and Edward O. Wilson. He brings it up to the present day as the field morphs from being primarily descriptive (what organisms live on what islands) to a theoretical and quantitative one (why are those animals found in their distributions on those islands) to the field's application beyond islands to the problem of habitat fragmentation and its consequences for the future.

While that may sound a little dry and challenging to some readers, Quammen makes the intellectual journey an enjoyable one as he describes the theories, papers, and studies in the context of his visits to the famous and not so famous islands and locales of habitat fragmentation. From traipsing through Brazil to get a look at species of New World monkey (Brachyteles arachnoides) isolated in rainforest bordered on all sides by clear cut where he helps collect droppings, to the Galapagos, Mauritius (once home to the Dodo, Raphus cucullatus, whose song if it had one is now lost, hence the title of the book), Guam, Hawaii, Madagascar, Tasmania (and his search for the extinct? Tasmanian wolf (Thylacinus cynocephalus), Aru in the Malay archipelago to view the birds of paradise (Cicinnurus regius and Paradisaea apoda) which was Russell's last stop before heading home, to the islands Komodo where he relates the sometimes dangerous coexistence of people and dragons (Varanus komodoensis), Quammen makes each site come alive while illustrating some theme in the larger intellectual story he's telling. Although not the main story he's telling, I've always been fascinated by the stories of human "first contact" with island species and how that contact typically decimated the species through what Quammen calls their "ecological naivete" evidenced by their shortage of defensive adaptations that creatures on larger land masses must evolve in order to survive. In part I liked the book because he does a good job of documenting many of these cases and giving us a sense of what's been lost.

But the main story line is also a fascinating one that at its core relates how the fundamental ideas of species distribution on islands were slowly formulated in the twentieth century. In particular Quammen points to the distillation of 100 years of observation in a 1967 book titled The Theory of Island Biogeography by MacArthur and Wilson that describes the area and distance effects. In epitome the area effect shows itself in that small islands tend to harbor fewer species than large islands with the underlying causes being fewer immigrations and more extinctions (due to what he refers to as stochastic factors or seemingly random occurrences that can decimate a smaller population such as drought, fire, or volcanic eruption) with the area of the island being determinative of the quantities. The distance effect shows itself in the fact that more remote islands are the home to fewer species than islands closer to the mainland since there are fewer immigrations. These ideas then lead to the so-called "species-area relationship" that attempts to quantify these effects through an equation (known as the species-area equation). Biogeography had matured into a full theoretical and quantitative science.

The really interesting part however, is when these ideas began to be applied not just to islands but other ecosystems as they were in a seminal paper by Jared Diamond in the mid 1970s. The implication of this application is simply that as habitats are fragmented they may end up supporting fewer species than larger contiguous blocks. In other words, although efforts to preserve species through the creation of parks and reserves is noble, their creation may end supporting fewer species than we think due to the inability of species to migrate and "extinction" of the local population due to these random events which probabalistically have more impact than they would if the populations were larger. As a hypothetical example, assume you were to survey your lawn and count the number of species of organisms that inhabit an average square yard. If you then cut out one of those squares and isolated it from the rest of yard, the idea is that if you surveyed it again later it would contain fewer species because of the area and distance effects. Quammen then details the raging scientific debate that ensued regarding not only if the application of biogeography to the design of preserves was appropriate but if so, just how large preserves should be to maintain what would become known as the "minimum viable population" or the number of animals required in order for a population to survive in the long-term, however that's defined.

Quammen interviews many of the participants, or should I say characters since several are quirky to say the least, in the debate on all sides but certainly seems to take the view that at least in the larger perspective, attempts to connect habitats and enlarge nature preserves are what is needed to avoid the problems inherent in fragmentation.

The ideas and the debate fascinating but Quammen also brings to them a life in a way that allows the reader to not only understand the concepts but think deeply about their implications. For my money you can't ask more of a book than that.

5 comments:

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