Exploring Island Biogeography Islands are unique ecosystems that are often used by ecologists to investigate basic principles. Famous naturalists R. A. McArthur and E. O. Wilson first used islands to investigate factors that determined the number of species present in a given area of land. McArthur and Wilson assumed that most islands, particularly those formed by volcanic activity were at one time empty ecological spaces with no species present. They proposed that the number of species on an island represents a balance between the rate of species colonizing the island from the mainland and the rate of extinction of species already present on the island. They developed a theory that established the subdiscipline of ecology known as island biogeography. Let's investigate how factors such as island size, distance from the mainland, and time determine the rates of species colonization and extinction. First let's investigate how island size affects the number of plant species present on an island. This map shows a number of different island groups located in the Pacific Ocean. Let's collect information about the island groups' area and the number of plant species present. These data will automatically be added to the plot of number of species versus island size. Solomon islands are forty thousand square kilometers and have six hundred and fifty four plant species. Fiji is eighteen thousand five hundred square kilometers and has four hundred and seventy six plant species. New Caledonia is fifteen thousand square kilometers and has three hundred and ninety six plant species. Samoa is three thousand one hundred square kilometers and has three hundred and two plant species. Niue is fifty square kilometers and has twenty five plant species. Cook Islands are two hundred and fifty square kilometers and have one hundred and twenty six plant species. As the area of an island increases, so does the number of different habitats found on that island. A diversity of habitats supports greater species richness. This relationship can be described by the equation: S = cAz where S represents the total number of species present, C is a constant measuring the number of species per unit of area, A is the total area of the island and Z represents the rate at which species change as the area changes. F.W. Preston first proposed this equation in 1962. Next, let's investigate how the distance from an island to the source of potential colonists (the mainland) affects the number of species that inhabit that island. The first island is hundred kilometers and has three hundred and forty four plant species. The second island is ten kilometers and has six hundred and fifty five plant species. The third island is five hundred kilometers and has two hundred and sixty three plant species. The fourth island is one thousand kilometers and has one hundred and twenty six plant species. The fifth island is two thousand five hundred kilometers and has eighty one plant species. The sixth island is five thousand kilometers and has twenty two plant species. Let's look at the graph produced from the island data we collected. Notice that Islands closer to the mainland have more species than islands far away. That means more species are capable of colonizing islands that lie closer to the mainland. This may be particularly relevant for species that are not mobile on their own. For example, the likelihood that seeds from multiple plants will be wind dispersed to an island increases with the proximity of that island to the mainland, or source pool. Islands often occur together in chains called archipelagos. In an archipelago, smaller islands tend to occur farther away from the mainland. In most natural systems, island size and distance are not independent factors. Let's now find out how the number of species on an island depends on both island size and the distance from the mainland. To learn the relationship between these multiple factors, let's first examine the island farthest from the mainland. As you can see, the number of species declines with decreasing island size and increasing distance from the mainland. Let's now understand how the processes of colonization and extinction combine to determine the number of species on an island. When an island is new or recently disturbed, many colonists can arrive because factors such as competition and predation are low. Therefore, the immigration rate of new species to an island is high and the extinction rate is low. Let's understand how the colonization rate changes over time. As more and more possible immigrants arrive on the island, there are fewer habitats available to colonize. Predation and competition play a bigger role in determining colonization success. Therefore, the colonization rate declines as the number of species increases. Now let's see how extinction rate changes as more and more species colonize the island. Initially, few species are driven to extinction. As more species arrive, the influence of predation and competition becomes stronger and makes extinction more common. The point where the curves for immigration rate and extinction rate intersect is the equilibrium number of species. Below this point colonization exceeds extinction and the number of species will continue to increase. Above this point extinction exceeds colonization and the number of species will decline. After a long time has passed since the emergence or disturbance of the island-hundreds to thousands of years -the number of species is predicted to be at this equilibrium number. Now let's see how colonization and extinction rates are influenced by island size and distance from the mainland. Islands closer to the mainland are more easily found; therefore, the colonization rates for these islands are higher. Larger islands have more resources and a greater variety of habitats; therefore, their extinction rates are lower. The highest expected species richness is predicted on large islands close to sources of potential colonists. The colonization rate is higher on near and small islands than on far and large islands. Notice that the equilibrium number of species is different for islands of different sizes and distances from the mainland. And that species richness can be predicted if the shape of the immigration and extinction curves is known. This figure encapsulates the ideas of the theory of island biogeography. MacArthur and Wilson formally developed this theory in 1967. Not only does this theory explain patterns of species richness long noticed by naturalists, but it also allows for quantitative prediction of species richness. The development of this theory allowed biogeographers to conduct more structured and more quantitative studies. Ideas from the theory of island biogeography have also been used to design nature preserves and to examine species richness in other types of isolated or highly fragmented ecosystems.