The first meeting of the BiK-F/Senckenberg Museum science book club took place outdoors in the garden, where we enjoyed one of the first warm and sunny days of the year whilst we ate lunch and listened to birdsongs. Although several members of the book club were traveling and could not attend this meeting, there was a nice mix of students, postdocs, research scientists and professors present. Oh, and I was there too -- but my professional status is somewhat ambiguous at this time.

Before we began, we discussed logistics -- should we meet every week to discuss one chapter, or meet on alternate weeks to discuss two chapters? We decided to meet alternate weeks to discuss two chapters to ensure we have something to talk about if one of the chapters turns out to be a dud.

This discussion prompted me to ponder what I plan to do here: I’ve decided to summarise the group’s discussions of one successive chapter on Wednesday each week. Based upon the ever-present risk of a dud-chapter, you might suspect that I am either an optimist or a gambler -- time alone will reveal which notion is correct.

That said, my one-chapter-per-week schedule will give you a week to read and ponder each chapter and to add your thoughts to the discussion before the comments section here closes. It also gives me the opportunity to do some background work (time permitting), which might include linking to relevant primary literature.

The first chapter, “Island Biogeography in the 1960s: Theory and Experiment”, was written by Edward O. Wilson who, along with his friend and colleague Robert MacArthur, co-discovered the Theory of Island Biogeography. (Chapter one of this book is available as a free PDF, courtesy of Princeton University Press). This chapter is a narrative that tells of the social, intellectual and scientific contexts that gave rise to island biogeography theory. It tells of MacArthur and Wilson’s desire to remedy the “growing decrepitude of our specialties (as we saw it), in dismaying contrast to the newly triumphant emergence of molecular biology.” [p. 4] To this end, MacArthur and Wilson agreed that ecology and evolutionary biology needed foundations upon which explanations can be developed “bottom-up”, from lower to higher levels of biological organisation.

Wilson was an entomologist specialising in ants, and his extensive field work with Melanesian ants was the source of many of his scientific insights. Studies indicated that most species of Melanesian ants originate in tropical Asia (with fewer originating in Australia), and they entered the Melanesian chain of archipelagoes primarily through New Guinea. Wilson showed that, as expected, the newest arrivals are relatively ubiquitous, with little geographic variation, and are specialised on marginal habitats where competition is low. These marginal habitats serve as staging grounds for further expansion throughout the archipelago. But when a species colonises an island with unoccupied central rainforest habitats, they experience “ecological release”: they move in, adapt to the new conditions and diverge, or “speciate”. The ant colonists may also generate new species that are adapted to local marginal habitats, which then puts them in a position to leap-frog outward to colonise yet more islands, allowing the cycle to begin again (Figure 1.2):

Although observations provide the raw material and the inspiration for science, they themselves are not hypotheses until the patterns that they suggest are applied more generally and their predictive value tested. Only after the validity of a hypothesis has been repeatedly demonstrated is it referred to as a scientific theory.

So were Wilson’s Melanesian ants somehow special, or did they suggest a pattern followed by other island-dwelling ant species elsewhere? By other organisms?

Wilson writes: “One day, in a eureka [sic] moment consuming only a few minutes, I saw a relation between the spread of species between islands and archipelagoes, on the one hand, to within-island speciation and shifts in habitat preference during evolution, on the other. This was in 1958. I believe I was the first to see such a connection; at least I was not guided by any other work I knew at the time.” [p. 3]

Wilson collected observations from birds and ants that described the relationships between species competition, distance from the mainland, and whether there was ecological space for more species to colonise a particular island -- a phenomenon known as “equilibrium” or “saturation”. Wilson used these observations to draw island area-species curves (summarised in Figure 1.4).

Using these curves, MacArthur mathematically described the relationship between immigration and extinction rates of species on an island as a function of the numbers of species already present -- and identified the point where these curves cross as equilibrium, that point where immigration and extinction rates are equal (Figure 1.5):

To test this hypothesis and to identify whether species equilibria even exist, Wilson and several colleagues artificially created “miniature Krakatoas” -- islands swept clean of terrestrial animal life just as the Indonesian volcano, Krakatoa, had been after its major explosion in 1883. They did this by fumigating small mangrove islets to exterminate all arthropods (Wilson & Simberloff, 1969) -- a process that resulted in one peculiar photograph making it into this chapter. Wilson and his colleagues then routinely surveyed the arthropod species that recolonised these islets.

They found that the numbers of species on these “miniature Krakatoas” returned to pre-extermination levels within two years, where they remained stable thereafter, which demonstrated that species equilibria do exist. The predicted distance effect was also confirmed: the farther an islet was from the mainland, the fewer species it held.

Perhaps most interesting were the details of the colonisation process, which depended upon the natural history of the individual species as well as species groups. For example, studies found that “ballooning” spiders wind-borne upon silken threads arrived early but suffered rapid turnover. In contrast, mites tended to arrive later but persisted with less turnover.

Some surprises were in store for MacArthur and Wilson and their colleagues, too. For example, they discovered that species compositions differed from island to island -- and even on the same island before and after defaunation (Simberloff and Wilson, 1971). One could interpret these surprises as the results of plain old luck -- an errant wind? favourable weather? an accidental fire? an explosive volcano? disease? -- playing a defining role in the success of the initial colonisation.

As an aside, I was amused to learn a new word, mentioned by Wilson, and probably coined by him. Nesiophilia: the inordinate fondness and hungering for islands -- a condition that Wilson speculates may be genetic. My own experience suggests he’s right.

Questions we discussed at length:

• Would it be possible to do “island fumigation” experiments today?
  
• How could the researchers be certain all life had been eradicated before conducting island recolonisation studies? (Fresh lava flows and the 1883 Krakatoa explosion notwithstanding.)
  
• Why are there so few published island recolonisation studies for plants? (We did note that there are a few published plant recolonisation studies for South American flora and some flora & fauna surveys of Krakatoa following its massive explosion in 1883 -- but are there many more?)

Next week, I will share the discussions provoked by (much longer!) chapter two; “Island Biogeography Theory: Reticulations and Reintegration of ‘a Biogeography of the Species’”. (If you wish to participate in this experimental online book club, you can purchase The Theory of Island Biogeography Revisited from Princeton University Press or from Amazon [UK: hardcover/paperback/Kindle UK; US:hardcover/paperback/Kindle eTextbook].)

Background reading:

What sort of experiment is this?

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