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Lecture 3:

Listen to part of a lecture in an ecology class.

So we’re talking about biogeography. um, biogeography is the geographic distribution of plants and animals in an environment. and as you know, biogeography varies from ecosystem to ecosystem. now one, of ecosystem that we haven’t discussed yet is the island environment. now, islands have always been interesting to study because their isolation makes them it it creates a kind of laboratory for examining their ecosystems. in the um, 1960s, robert macarthur and e o wilson developed a theory they called the equilibrium theory of island biogeography. and their theory states that. at. time islands achieve equilibrium, uh, a balance in terms of of the number of species of plants, animals, insects, etc. that they can support.

And they that is macarthur and wilson developed calculations to predict the point at which equilibrium is achieved, and the number of species that a given island would be able to sustain. um, according to. to their theory. This number, the the total number of species depends on two things, the size and location of the island, for example, they predict that there will be more species on bigger islands, and there will be more species on islands that are closer to the mainland. so, how does this work? according to. to macarthur and wilson, there are. two opposing. forces constantly at work in an island environment, pushing the island toward the state of equilibrium. the first force is the immigration rate. the immigration rate tracks the addition of new species to an island. then they argue that this rate declines over time, that there are more new species early on when the islands still empty, and there’s little competition for space. however, as time progresses, there will be fewer and fewer new species that can compete with established species.

Uh, it’s not impossible for new species to be introduced at this late stage. It it might adapt better to summit it change in the evolving habitat, but it is definitely tougher.

the second force is the extinction rate. and this is the rate at which species disappear from the island. in contrast with the immigration rate. This is a force that increases over time. in the beginning, when there’s plenty of space, species don’t need to compete with each other quite so much. but as species get settled on the island and the amount of space for each species declines, conditions become much more competitive. um, and and it’s not just the weakest species that may disappear.

on a crowded island. There are likely to be smaller numbers of every species, which makes them all more vulnerable to things like, um, uh, sudden changes in the climate or or some kind of natural disaster. so there are these two opposing. and forces. And at a certain point in the island’s ecological evolution, they cancel each other out. Immigration matches extinction. at that point, equilibrium is achieved.

Okay? If you accept these assumptions, then macarthur and will says predictions about size and location makes sense. if an island is bigger, then there’s more space for a greater number of species to compete and survive. and if one island is closer or easier to get to from the mainland than another, then more species will be able to immigrate to the island from the mainland and established themselves there before before a state of equilibrium occurs. now, notice what this theory doesn’t predict.

It. It doesn’t say a thing about what type of species will be on the aisle. And it it doesn’t predict which new species will arrive or which existing species will disappear. um. um however, ever we can get some sense uh, for what those species might be by looking at the example of crack at o a you’ve probably heard of crack at, oh, a an island in indonesia. it has a volcano that erupted in 1883. well, the destruction of the ecosystem was total. Nothing survived on that island. of course, it only took a few years before the islands started to support life again, but it couldn’t just recreate its old ecosystem. the first species to take hold were some weeds, the sort of um, pioneering organisms that do well.

Uh, after the collapse of an ecosystem, beetles and spiders also returned in time. A a grassland ecosystem developed. so, there was some more established plant life.

Birds and bats were attracted to the island, and they in turn brought well dropped seeds from fruit bearing trees. as trees gained a foothold on the island. Forests eventually replaced the grasslands, and some pioneer species disappeared. The ones that couldn’t survive in a forest environment. but does crack at o approve the equilibrium theory? well, nothing about it. Disproves it so far. um, immigration and extinction rates have generally behaved. According to the theory, the number of species on the island corresponds with the author’s calculations and seems to be stable. it appears that the island has achieved the type of equilibrium state that the theory predicts. however, some recent studies suggest that other factors may also play roles in the development of an island’s biogeography.

go out for 12 hours in Quebec, Canada.