Bahamas islands were giant labs for lizard experiment

Wrapping entire islands in the Bahamas with netting, introducing snakes to two other islands and measuring the fitness of hundreds of lizards using treadmills: one of the most ambitious ecological field experiments ever conducted has resolved a long-standing question about the evolution of lizards.

Lizards of the genus Anolis are found throughout the American tropics, where they vary widely in size and shape depending on ecological conditions. It has long been thought that predation is the most important evolutionary force for continental lizard populations, whereas on islands competition between lizards themselves is more important. Until now, though, no one had tested this experimentally.

Ryan Calsbeek and Robert Cox of Dartmouth College in Hanover, New Hampshire, cut no corners in their experiment. They excluded predators from two small, uninhabited islands in the Bahamas by wrapping the islands – about 1000 square metres each – with netting to keep out predatory birds. Meanwhile, they enhanced predation on two other islands by introducing lizard-eating snakes.

To vary the amount of competition, they seeded one of each pair of islands with high densities of Anolis sagrei lizards, and the other with lower densities of the animals.