Lecture 11

Listen to part of a university science lecture on Island Biogeography.

Professor: Today I’d like to look at the topic of island biogeography- the study of plant and animal distributions on islands. Studies in this field ballooned soon after the publication of MacArthur and Wilson’s seminal Theory of Island Biogeography in 1967.

Their theory is a simple, elegant bit of reasoning that was a major breakthrough in modern ecological thought. The Theory of Equilibrium in Island Biogeography says that the number of kinds of plants and animals on an undisturbed island- that is, a natural island unaffected by man or other calamity- is determined by two processes, immigration and extinction. In other words, the number of species on an island is the sum of the species that arrive, breed and live there successfully, minus the number of species that arrive but fail to breed or that eventually become extinct.

If a new island starting with zero kinds of birds lies near a mainland that has 100 kinds of birds, then a certain percent of those mainland species are eventually going to find their way to the island. When the first species arrives and establishes itself, the potential number of immigrants decreases by one, since there are now only 99 potential immigrant species available from the mainland. At the same time, the potential for extinctions increases by one, because with the arrival of the first species, there is now also one species that could become extinct, where at first there was none.

You may have heard of Krakatoa, which was famously all but destroyed by a volcanic explosion, exterminating every living thing on it, back in 1883. Well, between 1883 and 1933, 34 species of birds became established there, but 5 of them also became extinct. Then, from 1933 to 1985, 14 more species established themselves, while 8 went extinct. As the theory predicts, the rate of immigration declines as the island avifauna matures. As equilibrium approaches, turnover continues, but the total number of different species levels off. When the overall bird population finally reaches a mature equilibrium- when the arrival rate of new species balances the extinction rate of unsuccessful species- the island may host anywhere from only a few to almost all of those 100 mainland species, depending on the island’s overall receptivity.

Papua New Guinea has a very rich avifauna- almost 800 species of birds- while nearby Bali only has about 300 species. Why? There are several factors that determine these numbers, and we now need to consider them.

The most obvious factor is island size. Papua New Guinea is over fifty times the area of Bali. There’s just more space available on a bigger island, so there’s more food, more places to hide, bigger territories- simply speaking, room for more birds. And the more individuals of a species there are, the bigger the gene pool, the greater the breeding opportunities, and the less danger of extinction.

But size alone is not the whole story. Just as important is the variety of habitats. A larger island is likely to have more different habitats- forest, grassland, scrub, lakes, marshes- while a small island may offer only a single habitat of sand and palm trees. Islands with multiple habitats, multiple niches, can maintain more species. With just one or two habitats available, the species list is going to be very short.

This can be seen by comparing islands that are otherwise the same size- as between coral atolls and volcanic islands, for instance. The Tuamotos Islands and the islands of Tonga both lie in the middle of the south Pacific. Both comprise many small islands with a total land area of about 800 square kilometers, but while the Tuamotos are all coral atolls with a maximum altitude of seven meters, Tonga also includes a couple of volcanic islands, one rising to 1033 meters, offering montane habitat in addition to lowlands. As we’d expect, the bird variety on Tonga- 75 species- is somewhat higher than the 57 species on the Tuamotos.

Another important factor is the island’s distance from the mainland or other species sources. Its colonists will have to come from whatever lands are nearby- and oceanic islands, islands farther away from these sources, will receive fewer species- though hardier ones- than will coastal islands closer to species sources, simply because it’s harder, even for a bird, to get to a more distant location. This is why the Hawaiian Islands, ten times the area of the Louisiade Archipelago, has fewer native birds. The Louisiades lie only 200 km from species-rich New Guinea, while the Hawaiian Islands are 3,000 km from anywhere.

So these are the main determinants of island species capacity- size, habitat diversity, and degree of isolation. And it turns out that MacArthur and Wilson’s theory can be applied to other sorts of “islands”, to other geographical areas that are isolated in some way by their surroundings. Just as an island is separated from other land by the ocean, lakes are isolated from other lakes by dry land, so that fish have as great a challenge in colonizing lakes as, say, rodents do in colonizing islands. Mountain tops are islands, separated from other mountain tops by ecologically quite different plains and valleys. And national parks are islands of original habitat isolated by the human developments around them.

Today, such areas as national parks, forest preserves and other protected natural areas are increasingly becoming isolated fragments in a clipped and cultivated world. And it with these that lessons learned from the studies of island biogeography are being applied. In our western parks, for instance, the successful re-introduction and management of large mammals like the wolf and birds like the California condor depend on research into territorial demands and ecological requirements, crucial population sizes, and individual emigration to surrounding areas, where the impact on humans can be significant.