By Jonathan Farrow from The Thoughtful Pharaoh
Unbeknownst to the rest of us, a debate has been raging in the world of biogeography. The debate stems from a simple observation made by a young Canadian scientist in 1964: island animals are weird. Sometimes they’re way bigger than normal, like the Tenerife Giant Rat, and other times they are way smaller than normal, like the Elephas falconeri, a tiny species of elephant.
J.B. Foster published a short, two-page paper in the April 1964 edition of Nature positing that rodents get bigger and lapidomorphs (rabbits), carnivores, and artiodactyls (deer/goats) get smaller on islands. This, he thought, was because small animals found the isolation of islands to be liberating. They no longer had to worry about predators and could grow to fill their new space. Larger animals, however, might be restricted by the relative paucity of resources on islands and would have immense evolutionary pressure to become smaller.
This led to Foster’s Rule, also known as the Island Rule. It states that in general, big animals get small on islands and small animals get big. They also do so very quickly (in evolutionary terms). For instance, red deer on Jersey, an island in the English Channel, were shown to have shrunk to to 1/6th their original size in only 6000 years.
There’s a problem, though. Like pretty much every rule in biology, there are lots of exceptions. Sometimes small animals get smaller (like Brookesia micra, the world’s tiniest chameleon) and relatively big animals get bigger (like Haast’s eagles).
A 2011 article by a joint Israeli-Italian-British team of researchers calls the whole theory into question, showing that the smallest species in any given group is no more likely to be from an island than would be expected by chance. Size extremes, they say, exist everywhere. Islands don’t have some sort of monopoly. They do concede that large mammals tend to get smaller, but they think the idea that small animals get bigger only seems like common sense because they are easier to notice.
A British paper from 2008 throws even more confusion into the mix, showing that depending on the kinds of statistical tests you use, you can show that the island rule either exists or doesn’t. They suggest that the island rule should be looked at in “taxonomically restricted studies” – biologist-speak for “case-by-case basis”. That seems to kind of defeat the purpose of a nice heuristic, though.
One thing we know for sure is that islands isolate organisms. This isolation means that evolution can work differently for the island population and might lead to all sorts of interesting changes. This type of evolutionary change is also called allopatric speciation and is responsible for the variation that Darwin saw in Galapagos finches. Whether islands always create a particular kind of change is still up for discussion, but nobody can doubt that when organisms of unusual size appear, they deserve attention.