The Rule of the Islands

The Rule of the Islands
July 3, 2015 

Islands have their own special mystique. It seems silly until you start thinking about it. Islands should be land just like any other, but they are not. Cut off from the rest of the world by long leagues of water, they are free from the influence of the mainland world. They can develop on their own: wild and unpredictable.

And that is fascinating to us, as a quick scroll down the Netflix page will show you. Where is the buried treasure? Island. Where did the villain hide his secret base? Island. Where do the monsters live? Island.

In a lot of ways, it is fitting that movies like King Kong and Jurassic World are set on islands because those are the only places that could support life so vastly different from the mainland. While not quite as dramatic as dinosaurs or stories-tall gorillas, new research from Duke University adds to a growing body of evidence that animals on islands generally grow to be much larger or much smaller than their mainland relatives.

Biologists Paul Durst and Louise Roth showed specifically that island rodents are 17 times more likely than their mainland counterparts to be among the largest or smallest in their species.

Biologically speaking, islands often present unique ecosystems with different ecological pressures and roles to fill than the mainland. Within these unique and isolated ecosystems, plants and animals evolve in very different ways thanCapybara they do on the mainland to best take advantage of their surroundings.

Charles Darwin saw this in the multitude of finches in the Galapagos: each specialized to live well on its own island. Those finches were a major contributor to the theory of evolution by natural selection.

The finches were all different species, but in the last 50 years, biologists have found evidence that even within the same species, island dwellers tend to look much different than mainland dwellers, generally much larger or much smaller.

Animals like deer, elephants and rabbits tend to be smaller while marsupials, some birds and now rodents tend to get bigger. The collection of hypotheses that attempt to explain this phenomenon is called the “Foster Rule” — after the scientist who first studied it — or the “Island Rule.”

Smaller versions of large animals are well documented. On the mainland, animals like elephants and deer roam far and wide to graze. On an island, they often do not have as much grazing land, and many scientists believe that because resources on islands are more limited, the smaller versions of these animals have an advantage because they don’t need to eat as much.

As for the tiny animals, some scientists believe that because there are fewer animals around, the small island critters also do not need to compete as much for shelter and resources, so they can grow larger. Also, islands generally have less biodiversity than the mainland, so animals can take on more roles — or niches — in the ecosystem. So the small animals can take on the niches of medium-sized animals.

Durst and Roth were searching for evidence that rodents are, in fact, larger on islands, and they studied data from 1000 populations of 60 rodent species.

They found that more than half of the island rodents ranked in the largest or smallest 2.5% of their species. Some of the rodents were even twice as large as their average mainland cousins.

The research appeared in the Proceedings of the Royal Society B.

— Daniel Lane

Daniel Lane covers science, engineering, medicine and the environment in North Carolina.

GSK Credit