 I'd like to talk about the resiliency and fragility of ecosystems and how that depends on the interactions among organisms and how feedbacks can get amplified or dampened depending on those interactions. In particular, if an ecosystem depends on a few key species, it has a harder time responding than if there are more interconnections among the organisms and more species that play a key role in the ecosystem. So in this case, each circle represents a population of organisms, and each color represents the populations of organisms that play a similar role in the ecosystem. So like for example, here we just have one primary producer. We have primary and secondary consumers that eat the primary producers or primary consumers that eat those. And so we have like this whole interconnectedness among different organisms. So I'm going to hide the labels so that it's a little easier to see, and then we're going to go through and look at some of those interconnections in terms of the food web. So we have our primary producer here, and it's eaten by these different types of organisms. So obviously if something happens to this primary producer and say it goes extinct, the whole ecosystem will go extinct because these organisms don't have anything to eat. So this would be an ecosystem that depends on one population of organisms specifically. So these sort of primary consumers are eaten by others, and here I just show sort of a whole bunch of eras, right, where among these different organisms, like for example this red one here has three different food choices with the arrows going into it. This one has two food choices, and it feeds this one over here, which is eating from two different ecosystems and is feeding an organism over here, right. So it's all supported by this primary productivity. And then there are certain types of organisms that eat others but aren't necessarily preyed on themselves. We often call these sort of the top of the food web. So this orange population of organisms eats some from the blue and the purple, and when it dies, its resources go back into the ecosystem through degradation, but there's nothing that specifically preys on it. And I'm not showing the degradation here because it's already complicated enough, okay. And then there's this group of yellow organisms here that eat mostly from one type of organism, but this one can also eat from organisms that play a different niche role here, all right. So we can think about these as groups of organisms that have these different functions in the ecosystems. So I built my ecosystem off a model from another one here. So let me show you what that looks like. So this is a real ecosystem where each number represents a different set of organisms, and you can see that there are many, many arrows and many more populations among them. This particular ecosystem depends on a single primary producer. So it's from a lake and there's one form of algae that provides most of the biomass that these other organisms tend to eat. So if something goes wrong and this primary producer can't function in the environment, the whole ecosystem will collapse. So let's go back to our simpler food web and trace some of the dependencies within it, right. So if we can basically take some of these arrows and I'll just color them red, and this orange organism depends on a whole suite of other organisms in the ecosystem. So it colored all the arrows red that this orange organism depends on, and there are a few organisms that it doesn't immediately depend on here, and there's a network of them out here that it doesn't directly depend on, but there's a lot of interconnection between organisms in this particular food web. And so if something, for example, happened to this organism right here, and that went away, the orange organism still has a path through the food web that can supply a large part of its food. There are other organisms that are more important to the food web. So for example, this purple one right here, if this one went away, the orange organism would only have, would be very highly dependent just on this blue organism, which depends on the primary productivity, right. So this organism then becomes highly dependent on one other species or one other population of organisms, right. So different organisms within an ecosystem play different critical roles. We could, for example, say what happened if the orange organism itself went extinct, right. This organism, the purple organism was eaten by it, so maybe its population would grow, and that might increase the population of these lavender organisms over here, right. So if this one goes extinct, it can have repercussions for different parts of the food web. So we have this food web that has a lot of interconnections and the ecosystem, depending on the organism that goes extinct, but it's fairly robust to changes in populations of the organisms. We can do another one that has fewer organisms. So for example, let's get rid of a bunch of these interactions and then there are a few populations that don't have any food coming in or out, so we'll get rid of those, right. So this is a much simpler food web and one that may, for example, be more fragile to certain types of environments. So for example, in this case, if the orange population here goes extinct, there's nothing eating this purple population and it eats primary producers, these primary consumers, as well as organisms higher up. So this particular organism would be a generalist. So if there's nothing that is actually consuming this generalist, it could actually draw down the populations of, for example, this organism, maybe it makes this organism then go extinct, which then makes this organism go extinct, which then this organism would go extinct, and this one would, and then this would get less food. So now this generalist is only eating the primary producers, and that will basically extract resources, a lot of resources into one organism that doesn't have a predator and pull those resources out of the rest of the food chain. So when you have fewer connections among, within a food chain, and in particular, if you have generalists that do not have a predator, the food chain is significantly more fragile. So we can make this into a specific example, say for example, this is a sea otter, it eats more than that, but let's make the generalist a sea urchin. If the otters get hunted, for example, and go extinct, the sea urchins will eat the kelp and algae, which are the primary producers. It will also eat all sorts of other organisms, as it sort of greys, in crusts, as it, as it eats things that encross the sea floor, say for example, including mollusks and things like that. This generalist, the loss of the predator can cause an explosion in the population of generalists, which then can cause the ecosystem to collapse. So there's a, there's a big difference between an ecosystem that is fragile because of the structure of how the organisms interact versus one that's more robust. So kelp forest is fragile in terms of the interaction of the sea otters and the sea urchins, whereas something like a rainforest is more robust. A rainforest has lots of primary producers in terms of different types of plants. There are algae and cyanobacteria that grow in them as well. So there's a really large diversity of primary producers and lots of interconnections between all the different organisms. Even if we look back at this, at this pond, if you can increase the number of primary producers here, then this complex interconnection among organisms is much more resistant to change in ecosystem collapse.