Islands are wonderful places for the study of biology, and many important insights have been made on islands. For example, Charles Darwin's experiences in the Galapagos islands helped inspire his theory of evolution. His contemporary Alfred Russel Wallace developed a nearly identical theory of evolution around the same time, based on his observations on islands in Southeast Asia. Biologists today still use islands as living laboratories for studying the ecology and evolution of species in the wild. One of the reasons that islands are so useful for ecological research is that islands have clear boundaries, so it's easy to define what the limits are of your study. Another reason is that islands tend to have fewer species than larger landmasses, like continents. That makes it somewhat easier to figure out what's living on an island and to tease apart the different ways that island species interact with one another. An island's ecosystem is like a simplified microcosm of the larger world, but why do islands tend to have fewer species than continents? Early naturalists like Alexander Von Humboldt, Charles Darwin and Alfred Wallace observed that in general, larger areas tend to support a greater number of species than smaller areas. This is known as the species area relationship. The species area relationship is one of the most widespread and well documented patterns in ecology, and there are several reasons why it exists. For one thing, the larger an area is the more resources like food and water it's likely to have. That means that larger areas can support a greater number of individuals than smaller areas. For another thing, the larger an area is the more likely it is to contain diverse habitats, more diverse habitats can support a greater variety of species. The species area relationship became clear to biologist Edward O Wilson early in his career. Wilson had grown up in Alabama and became an expert on ants at a young age by studying the species in his own backyard. But he quickly expanded his expertise through his studies at the University of Alabama, the University of Tennessee, and later at Harvard. By the early 1950s, he had already studied ants from across much of the United States, as well as parts of Mexico and the Caribbean. He then set out to collect and study ants in New guinea and the islands of Melanesia in the South pacific. In addition to discovering many new species of ants, Wilson noticed patterns in the number of ant species on different islands. While it was true that in general, larger islands tended to have more ants, there were also some exceptions. Wilson wondered if other factors like how isolated an island is might also affect the number of ant species per island. Wilson suspected that the patterns he was observing could be described mathematically, but he didn't have a strong background in math. So he signed up for a math class at Harvard, even though by this time he was already a professor there. He also teamed up with another biologist, Robert Macarthur, who did have a strong math background and was already using it to write some influential papers in ecology. In 1962, Ed Wilson and Robert MacArthur began collaborating on a project to use mathematics to describe the patterns of species distributions on islands. The result would be one of the most influential ideas in the history of ecology, known as the Equilibrium Theory of Island Biogeography. Let's go to the board to take a look at how it works. So what Macarthur and Wilson argued is that the rate of change in the number of species present on an island will vary based on the number of species that are already present on that island. So let's imagine some hypothetical scenario, we have a mainland area and then over here is the ocean and then there's some island that pops up out of the ocean, right? Maybe there's a volcano that erupts and we get a new island. So there's no species on it to begin with, but some species from the mainland will make their way over to the island. So what Macarthur and Wilson suggested is that, the rate of immigration of new species to that island will change based on the number of species already present. So at the beginning, right, there aren't any species there. So, we're over here on the x-axis and the immigration rate will be high. But then as the number of individuals increases on the island, this immigration curve decreases, why? Because over here when there already are a lot of species present, there just aren't that many new species from the mainland that are able to make it there, right? Because they are already present. So, in other words, the species pool over here on the mainland is finite. And as they start making their way to the island, there just aren't that many new species to draw from. So the immigration curve looks like this. That's immigration. Okay, but then at the same time, Macarthur and Wilson suggested that the extinction rate of species present on the island will also vary based on the number of species present. So the extinction rate is going to vary based on things like how many resources are available, how much crowding, how much competition there is among individuals. And so what they suggested is that the extinction rate is going to look like this. Because at the beginning, when there aren't any species present, right, the extinction rate has to be low. But then the extinction rate increases based on competition and crowding as the number of species increases. So the key point here is that there's a spot here on the graph where these two lines intersect, and that represents the number of species that should be present at equilibrium, right? The immigration curve and the extinction curve overlap here, and that suggests that this is a stable point. So Macarthur and Wilson were arguing that there's an equilibrium number of species that should be present on each island. Now, they also reasoned that the immigration rate is going to be different for an island that is located closer to the mainland versus an island that is located farther away from the mainland. It's going to be more difficult for those individuals to make it to a more distant island. And so the immigration curve will actually be lower for a an island that's located farther away from the mainland, That means the point at which these curves intersect is different. So this represents the number of species present on an island that's located farther away. And this is a number of species for an island that is located near. And you can see that the number of species on the island that is farther away is less than the number of species at equilibrium on the island that is near. The other thing that Macarthur and Wilson suggested is that the extinction rate is going to vary not so much based on distance, but based on the size of the island. They reasoned that a larger island will have more resources and be able to support more individuals. So the extinction rates on average should be greater for a smaller island. So, this might represent the extinction right here for a for a small island. But if there's a larger island, then it's curve is going to look something like that. Now we have two new intersection points here and so let's label those. So this one over here, let's start with this one. This one here is where the extinction curve for a. This is for a large island. Right? So let's label these. This is small and this is large. And so the extinction rate here for a large island, that is the number of species at equilibrium for a. It's intersecting the large curve for extinction, right? And the the distant island. The island is located further away. The last point here that we need to label is the number of species at equilibrium for an island that is large and one that is located relatively close to the mainland. Right? So this is one that is large and one that is low key close to the mainland. So what we have is four different points on this graph. You can see that the one that contains the greatest number of species at equilibrium is for an island that is located closer to the mainland and that is a bit larger
