## Critical Thinking

7. Predicting Results Describe the probable effects on an ecosystem if all the plants were to die. What if all the decomposers were to die?

8. Evaluating Models A student has modeled a terrestrial ecosystem with seven trophic levels. Is this number reasonable? Explain.

9. Analyzing Concepts Explain why the same area can support a greater number of herbivores than carnivores.

Science in Action

### Testing a Theory of Biogeography

In the 1960s, mathematical ecologist Robert H. MacArthur of Princeton University and taxonomist Edward O. Wilson of Harvard University developed a theory and mathematical model of island bio-geography based on their study of species on islands. This model, and others inspired by it, is used to explain patterns of species distribution around the world.

Robert H. MacArthur

HYPOTHESIS: The Number of Species on Any Island Is Constant

Ant biologist Edward O. Wilson (1929-) and mathematical ecologist Robert H. MacArthur (1930-1972) were both interested in community patterns within nature. Shortly after they met, they decided to work together on a study of species on islands.

Wilson noticed that the number of ant species on an island correlate with the size of the island. He also noticed that when a new ant species arrives on an island, one of the species already on the island becomes extinct. However, the total number of ant species remains constant. Wilson and MacArthur hypothesized that islands have a constant number of species. They proposed that the number of species on an island reflects an equilibrium—a balance between the rate at which new species colonize the island and the rate at which established species become extinct.

METHOD: Construct and Test a Model

MacArthur and Wilson developed a mathematical model to explain their observations. The mathematics of the theory is complex, but the broad outlines center on two observable patterns: (1) large islands have more species than small islands have and (2) remote islands—those located far from the mainland or from a larger island—have fewer species than less remote ones.

MacArthur and Wilson decided to test their model on Krakatau, an island in Indonesia on which a volcano had erupted in 1883, killing most forms of life on the island. The return of plant and animal life to the island had been carefully recorded since Krakatau was first revisited in 1886.

RESULTS: Species Reached Equilibrium

After examining the records of bird life at Krakatau, they learned that the number of species had climbed to 27 before leveling off.

CONCLUSION: Prediction Was Close

Using their model, MacArthur and Wilson predicted that at the point of equilibrium, the number of bird species would be about 30. Their prediction had come close.

Edward O. Wilson

### Recent Tests of Island Biogeography

More recent studies of islands, such as those in the Sea of Cortez, suggest that many models of island bio-geography may be required to explain patterns of species distributions. Researchers have generated alternative models that account for factors such as island history, climate, and species interactions.

When the volcano that forms the island of Krakatau erupted in the 1880s, it destroyed most life on the island. ^s life returned, scientists had a unique opportunity to study ecology in action.