Is there a reason why some islands have more wildlife species than others? The equilibrium theory of island biogeography is one attempt to explain this situation. Two ecologists, Robert MacArthur and E.O. Wilson, proposed that the number of animal species on an island reflects a balance between colonization, extinction rates, and the actual size of the island itself.
MacArthur and Wilson use the example of a new volcanic island to explain their theory. If a new island appears off of the coast of a mainland that is inhabited by 100 animal species, some would begin to immigrate to the new island and establish a population in the emptiness there. The immigration rates would decline as space is taken up, more species arrive, and can be influenced by the potential of future predators.
Extinction rates also influence island biogeography because when there are a greater number of species, there is a greater likelihood of an extinction occurring. Because island resources are naturally limited, the number of species on the island will naturally limit themselves as well to prevent an extinction from occurring.
This means an island and its closest mainland will eventually come to a point of balance, somewhere between 1-100, where the rates of immigration and extinction are equal.
Defining Extinction Under the Equilibrium Theory of Island Biogeography
When discussing extinction, the word is often defined as the “dying out of a species forever.” The dodo bird, for example, is extinct because it no longer lives on our planet.
Extinction can also occur at local, regional, national, and even continental levels as well. This is the type of extinction that is referred to in the equilibrium theory of island biogeography. When an animal species decides to leave an island environment, then it has become extinct from that environment.
It may still be thriving somewhere else, such as the closest mainland.
Equilibrium is therefore measured by the number of flora and fauna that immigrate to the island from the mainland each year and the number that choose to leave the island for another environment.
The Predictions of the Equilibrium Theory
Because there is a desire to achieve equilibrium over time, the theory proposed by MacArthur and Wilson offers some additional predictions regarding the movement of animal species. Islands that are further away from the mainland, for example, will have lower immigration rates than islands that are closer. This means islands that are further away can achieve an equilibrium with fewer animal species, even if the furthest island is larger in size when compared to the nearest island.
What this shows is that the natural isolation of an island environment can influence how flora and fauna adapt and survive. According to information published by Stanford University, a long-term study of a bird community in Surrey, England, showed that isolation affects the number of species that can be supported. The study found that 32 breeding species were found in the Surrey community, with three additions and three extinctions occurring annually.
If the Surrey community were an isolated island, it was believed that only five species would be supported over an extended period.
This means that, if all things are equal, distant larger islands will have lower immigration rates, lower extinction rates, but be able to support more species. Closer islands will have higher immigration rates, higher extinction rates, and support fewer species over time.
How Can We Use the Equilibrium Theory of Island Biogeography?
The equilibrium theory can help us to understand how island habitats have a tendency to fragment. What it does not do, unfortunately, is address what other factors may be involved in that fragmentation. There may be flora and fauna factors that may prevent one species from establishing itself after immigration. You can have a very large island, but if you don’t have any trees on that island, you won’t see many owls immigrate to it.
This theory helps us begin to think about the real world and how it operates. It shows us that when there is change that occurs, nature will do its best to take advantage of those changes. What it does not do is provide a substitute for direct observational knowledge of the patterns that nature provides.
Sometimes nature will seek to provide an equilibrium with island biogeography. At other times, no immigration may occur because the island is simply not suitable for the local animal species. By understanding isolation and its effects, we may not be able to stop localized extinction, but it does give us the tools we can use to stop global extinctions from continuing to occur.