Go forth and multiply

Despite the autumn rains settling in, we spent one of our last classes island hopping. From Myasia to McIsland, Spiceland to Big Isi we wondered at the vast variety of life. And yet stopping off at Elbario, Demonstrator Island and Little Joe, we found them barren, desolate, in some cases with no life at all. Why could this be?

As you may have guessed, this week we were looking into a classic theory in ecology: Island Biogeography. Back in the 1960s, heart-throb scientists Robert MacArthur and Edward Owen Wilson were puzzling over the differences in diversity between islands – differences that seemed to show predictable patterns. They discovered that larger islands tended to have a much larger variety of species than smaller ones, but also that isolated islands had fewer species than those close to the mainland. The resulting trade-off has been a cornerstone of ecological theory ever since.

islandbiogeography

Island biogeography in graphical form.

One of the main challenges to testing this theory is the challenge of conducting experiments. Ethics boards are less than keen on allowing researchers to wipe out all island life nowadays (not that it hasn’t happened in the past – see Simberloff & Wilson, 1969 – https://www.jstor.org/stable/1934856), while waiting for recolonisation is not always within the realms of a typical three year PhD project. Of course, these issues become less problematic if you replace animals with cereal and natural dispersal mechanisms with a class-full of students.

The experiment conducted by our Edinburgh Conservation Science class was remarkably simple. Each student chose a tupperware container (henceforth ‘island’) and placed it in the grassy ocean outside the classroom. From a single point of dispersal on the ‘mainland’ we all threw cereal at the ocean, subjecting its chance of survival to fate and the reasonably stiff October breeze. Everyone then claimed their island, counted the number of colonising ‘species’, and measured both island size and distance from the dispersal point.

As a class we now had data to test the theory of island biogeography. We found that:

  1. Species richness decreases with distance from the mainland (linear model slope=-0.77, p<0.05)
  2. Species richness increases with larger island area (linear model slope=0.016, p<0.001)

In other words, a success! MacArthur and Wilson can breathe a sigh of relief.

ibfig

The relationships between ‘island’ species richness, distance from the ‘mainland’ (left), and size of island (right).

We also discussed some of the problems with this dataset. Many of our islands were ambitiously distant in their grassy ocean, resulting in a highly zero-inflated dataset. This might give us the appearance of more certainty in our results that we actually have. In fact, if we remove islands without any species at all, the relationship between richness and distance is no longer significant (p=0.46). Another problem is the shape of the relationship. Just as expected by theory, cereal animal species increased non-linearly with proximity and area; fitting the relationship linearly therefore might not be appropriate. This is extremely important if we want to predict change or extrapolate from our data.

Finally, not all creatures are created equal. Some can swim, some can fly, some leave all the travelling to their offspring. While our cereal showed little of this diversity, we still had a go at seeing if there were differences between chocolate ovoids (Cereala cocoa) and pale corn-moons (Cereala lupina). There’s really too few data for anything to be statistically significant (and they’re in the same genus anyway so probably have similar dispersal mechanisms), but lupina seems to do better on large islands and have a greater dispersal capability. But we’ll be making no conservation management decisions based on this fact alone.

ibfig2

Difference in dispersal and colonisation ability for the two cereal ‘species’.

So all and all, what did we find? Whether it is breakfast cereal and tupperware or real species and real islands, the theory of Island Biogeography holds up!

By Haydn

Data: islandbiogeog2016

Code: island_bio

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One thought on “Go forth and multiply

  1. Pingback: Getting quantitative & testing Island Biogeography Theory | Cons. Sci.

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