The Island Rule: Why Do Islands Always Have the Coolest Animals?
By Honey D. Whitney, Broadreach Instructor
If you’ve ever been to an island, you’ve probably noticed drastic differences in sizes between similar species that are found in mainland populations and on islands. Things like Key deer in the Florida Keys; they look like a normal white-tailed deer, just a whole lot tinier. Or there’s the Kakapo, the fascinating ground-dwelling, flightless parrot from New Zealand. How is that thing a parrot?! It’s the size of a small child! Whether observing modern-day species or looking through the fossil record, there are a lot of species that don’t match up, size-wise. So… why is that?
THE ISLAND RULE, or WHY ISLANDS RULE
In comes what’s known as “The Island Rule.” The Island Rule, a.k.a. “Foster’s Rule,” was first described by J. Bristol Foster in 1964. n Foster’s Nature paper, he described how some animals, like rodents, had enlarged body forms when they lived on islands while other mammals, like carnivores, even-toed ungulates (e.g. hippos and goats), and rabbits shrank down in size compared with their mainland relatives. Generally, small things became big and big things got smaller when they live on islands. Scientists like to call this phenomenon “insular dwarfism and gigantism.”
Foster was not the first scientist to observe the drastic differences between island species and their mainland counterparts, but his paper was critical in creating a shift in science. Unwittingly, Foster’s paper sparked a whole new area of research in evolutionary ecology, all focusing on the unique properties of islands and their species. A game-changing book published in 1967 called The Theory of Island Biography by E.O. Wilson and Robert MacArthur is still revered as one of the pinnacles of literature in ecological and evolutionary biology circles. (You should read it, it’s awesome!)
WHAT MAKES AN ISLAND DIFFERENT?
Sparked by Foster’s paper, another scientist named Ted Case conducted a much more extensive study on insular organisms, taking other groups like reptiles into account. His findings lead to greater understanding about species that follow the island rule and even others that do not. (It’s always important to note in science when things are the exception to the rule!) So what makes living on an island so different from living on a large continent, and how does that relate to body size? Ultimately, it all has to do with energy.
Energy is behind all biological processes, which leads to inherent motivations. To biologically function, energy is needed; energy is gained by acquiring food or sustenance. In order to get food, some energy must be spent, gathering or hunting or even making it, for some. Then there comes metabolizing or breaking it all down – it even takes energy to make energy. It’s all so exhausting! This doesn’t even include other biological processes like growing and reproducing, each of which come with their own energetic costs.
THE RULES DON’T APPLY
The interesting thing about living on islands is that some of the expenses or limitations on energy transfer found on mainland populations do not exist. There are many possibilities of what may be behind these shifts in body size, leading to the Island Rule. Some insular species may experience something called ecological release, when things like the lack of predators or competition for resources makes it easier to conserve energy and use it for other purposes, like growth. Another idea is that with the general lack of other species on islands, it allows some species to fill multiple roles in the ecosystem, also allowing for more energy consumption; this is called niche expansion. These two ideas might help explain why little things, for example the Flores giant rat, can grow so big.
BIGGER VS. SMALLER
What about the opposite? Why do normally large species become smaller? A lot of the larger species that make it to islands have big ranges and large appetites. With limited space and resources on an island, it’s much easier for smaller members to meet their energy requirements, so evolution tends to favor reduced body size for these beasties. It’s also possible that it may be easier to grow the population if sexual maturity is reached quicker in life, usually when the animal is smaller. Over time, this meant smaller and smaller individuals getting busy, leading to those insular dwarfs. These reasons may explain the existence of animals like pygmy wooly mammoths in the fossil record, as well as modern species like the Island fox or Lowland anoa.
All in all, there are a zillion different things that can influence the evolution of various species, but a lot of cool things have happened evolutionarily to make many different and special animals on islands. Their uniqueness in both features and location makes them particularly vulnerable to human activities, like habitat destruction and anthropogenic climate change. It’s up to you and me to help protect all of the amazing kinds of life we have on Planet Earth; we certainly don’t want these awesome animals to go the way of the Dodo – which, by the way, was another example of insular gigantism. Boom, SCIENCE!
Learn more about what you can do to help awesome wildlife with organizations like these:
World Wildlife Fund (WWF)
The Nature Conservancy
Honey Whitney is an ecologist, science communicator, Broadreach instructor/junkie and has an M.S. in Ecology and Conservation Biology and a B.S. in Marine Biology. When not gallivanting off to far-flung places with Broadreach, she teaches Floridians not to pollute their local waters (check out BlueLifeFL).
– Case, T. J. 1978. A General Explanation For Insular Body Size Trends in Terrestrial Vertebrates. Ecology, 59(1): pp. 1-18.
– Foster, J. B. 1964. Evolution of Mammals on Islands. Nature, 202 (4929): pp. 234-235.
– Kerridge, Emma, and Chris Rogers. The Island Rule: Hypotheses. Paleobiology Research Group. The University Of Bristol, n.d. Web. 28 Feb. 2015. <http://palaeo.gly.bris.ac.uk/macro/islandrule/hypothesis.html>.
– Lomolino, Mark V. 1985. Body Size of Mammals on Islands: The Island Rule Reexamined. The American Naturalist, 125 (2), pp. 310-316.
– MacArthur, Robert H. and Edward O. Wilson. The Theory of Island Biogeography. Princeton University Press, Princeton, N.J., 1967. 215 pp.
– Tyson, Peter. Gigantism & Dwarfism on Islands. NOVA Evolution. Nova PBS, 08 Nov. 2011. Web. 25 Feb. 2015. <http://www.pbs.org/wgbh/nova/evolution/gigantism-and-dwarfism-islands.html>.