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I’m Jonathan Tomkin from the University of Illinois. This is the image of St. Matthews’ Island, which is in the Bering Sea, uh, which is off the coast of Alaska. In this lecture we’re going to be considering if human beings are more like reindeer or cells. We better hope we’re more like cells. In 1944, 29 reindeer were introduced to the island [popping noises] which previously only had foxes and voles [crowd saying “awwww”] as resident mammals. Can you guess what happened to the number of reindeer on the island? At first the population exploded. The grey numbers are the data on which the red line is based. In 1963-1964 there was a very harsh winter. Coincident with that, there was an enormous decline in the number of reindeer. The reindeer had eaten all of the fodder available on the island, so mass starvation followed. The population plummeted from around 6,000 to just 42. By the 1980s, no reindeer were left. They never recovered from this overexploitation era. Clearly, we don’t want to be like the reindeer, but not everything has this reindeer growth curve. Cells growing in isolation on a petri dish, with a finite food source, don’t grow exponentially, because as their numbers increase, uh, the amount of food available, uh, decreases relative to their numbers, and so their growth rate slows. These cell cultures both grew to a maximum size at a slowing rate as they approached this maximum population, the sustainable path. And so, using these two as examples, ecologists would talk about there being different types of population growth curves. One would be a J-shaped growth curve like we saw for the reindeer. The other shape would be more like an S shape, and as you can see they’re called J shape and S shape because they’re similar to the, uh, the way the letters are written. S-shape growth curves are for systems that match the resources available, so they might grow quickly in the beginning, but growth slows down as the carrying capacity — the amount of food available in the case of, say, the reindeer on the island, or the amount of f-f, uh, food for the cells in the petri dish — are exhausted. Imagine grass on a field, for example. There’s only a finite amount of sunlight, so even if there is no predators for grass, you know, we have no grazing, there are no reindeer in this case, if we plant some grass in one corner it will spread and grow, but it won’t grow beyond a certain amount. It won’t grow beyond the area that can be supported by the sunlight. Th-this maximum population is known as the carrying capacity. So although the grass might grow quickly in the beginning, its growth will slow as it runs out of fresh, open land to colonize. Other organisms, like the reindeer on the island, can grow past this carrying capacity, at least for a short while. They do this by unsustainably using up resources, so they don’t save some fodder for the next season; they eat everything that is available. Because this system is not in balance, it eventually crashes. You cannot have a population that lives beyond the carrying capacity indefinitely. Eventually it has to go back down or below the carrying capacity. Given that a system has a carrying capacity, and for the purposes of our discussion, S-shaped growth might be considered sustainable, whereas J-shaped growth is clearly unsustainable. The problem for us, if we’re thinking about “Is our growth curve sustainable?” is that the two curves can look very similar for a very long time.