 But I am behind on a couple of things. Now my semester is really flying as well. And I promised you some practice questions. So by tomorrow morning, I'll have some practice questions for the exam on B-space, promise. I am now starting to get more questions about what will be on the exam in terms of my strategy for what I will cover. I'll address that one more time now. And it's the same as I said in the beginning really with regard to the fact that the most important topics are the ones I cover in the lecture. And the book is most important where it overlaps the subjects in the lecture. But the reading is required. And anything I don't cover in the lecture that's in the book, I expect you to know about it. But it's going to be heavily de-emphasized, if addressed at all, OK? So that's the story I'm sticking to. I'll focus on conceptual understanding. You don't need to memorize species names, dates of so and so's birth, things like that. It's about concepts and it's about understanding. All right. I'm talking about moving into ecosystem ecology today. And then Wednesdays, what's today? I have no idea what today is. Today is Wednesday, right? Friday's lecture will be an extension of this focusing on bigger scales, moving up to the whole global ecosystem. So really, we're going to have an ecosystem ecology one here and an ecosystem ecology two on Friday. And hopefully, you can see how we're working in the framework of the levels of organization concept. But first today, I will want to continue with our look at food webs and finish that material up. Consider the length of food chains within food webs. And the constraints on the lengths of food chains. And then we'll get into ecosystem structure and metabolism. And finish, hopefully, with some discussion of experimental ways to study whole ecosystems in the field. And I won't talk about ecosystem studies in the lab very much, but those are extremely important as well. So I used a different diagram at the end of the last lecture in talking about the Antarctic marine web. And I stressed the importance of krill, these tiny crustaceans, at the base of this web and how they sustain a trophic web of incredible complexity and diversity with some of the largest organisms on Earth occupying this web. It's not only the krill that sustain this ecosystem. The krill have to eat, and they feed on phytoplankton. There are other herbivorous plankton that are also consuming these primary producer phytoplankton along with the krill. And those plankton also feed things like squid and fish. This fish compartment, as you might imagine, it has a bunch of species in it. It's a huge compartment represented just by a single circle, just like the birds are. There's a penguin, but there are a bunch of kinds of birds in this compartment. So we summarize a community and an ecosystem by compartmentalizing it like this and then linking the compartments with these arrows that represent flows of energy and materials, energy and nutrients, in the form of food. So this is a trophic web, if that is our focus on food. Trophic just really refers, the root refers to nutrients or food or something like that. So you can basically think of it as synonymous with food. It's a trophic web. It's a food web. You can take various perspectives on this web. An ecosystem ecologist will be more focused on the physical and chemical foundations of this system. But a community ecologist or an ecologist emphasizing the biological perspective might focus more on the natural history of the component organisms and their interrelationships in these dynamics. And both perspectives are important. In the history of ecology, they've been somewhat divergent and bridges between the ecosystem ecology component and the community ecology component are very important and still need to be formed in many ways. So we can look at these things as functional groups in terms of their natural history. And I just want to give you one term because it's an important one in community ecology that I haven't mentioned before. And just make sure we haven't missed it. The term guild, you may have heard that word guild outside of science. It's been adopted into ecology from human society, human communities. Guilds in a medieval context are these semi-cooperative groups of workers that focus on a particular craft in common. And they come to terms of relationship that ideally help each individual in their practice of their craft, whether it's whatever that craft may be. It's a guild. And so in ecology, we've borrowed that term and we can define it as a group of species that use a common resource in a similar way. Now, you sometimes see that the definition of the guild would stop here. But this final component in a similar way is really important to the definition of the term. So let's think about that bird compartment in the food web diagram. You're going to have your penguins that aren't flying at all, right? They're just waddling around and diving into the water and swimming after fishes. But then you're going to have a large number of birds aerially foraging and diving for fishes. So you're going to have birds diving from the sky and you're going to have birds on the water surface swimming beneath the water. They're all eating fishes, right? They're all eating fish. Let me say my ichthyology friend tells me I must say fish when I mean a bunch of individual fish, but I must say fishes when I mean multiple species of fish. So I try to follow that so I don't get in trouble with him. A guild, if we define these organisms that occupy these groups as foraging in a similar way, those would be two different guilds, the birds that are diving from the sky versus the birds that are swimming from the surface below water. They're foraging in a different way. And in that sense, occupy different guilds. They're all eating fish, so we can put them all into one ecosystem compartment potentially with one arrow from fish to all those birds. But if we're thinking biologically and about the natural history dynamics of these systems, they belong to separate guilds. And that suggests different relationships among them within each guild. It's a really interesting focus of study. And I'll come back to it a little bit in a later lecture. But I didn't want to leave it out entirely. So when we see a diagram like this, we recognize that there are various ways to look at it and interpret it. Many perspectives on the same system. So let's talk about food chains. And my daughter saw this one this morning and said, oh, it's a big fishy, eating a middle fishy, eating a smaller fishy. And that's exactly what it is. That's the classic, rather toothy fishies, I guess. But that's the classic idea of a food chain. Big things eat little things that eat littler things. And often, that's the way it does work with those types of size relationships. But not always. We could think of a group hunting type of fish, piranhas for a very popular example, may school and eat something larger than themselves. So it's not always a simple size relationship like that. But some of the time it is. So these food chains, we need to develop a few new terms. And here is an example of a terrestrial chain and a marine chain. At the base, we have the primary producers. In the terrestrial environment, it's usually plants. And in a marine environment, we speak of the phytoplankton, algae, plants, bacteria, various kinds. These are the autotrophs. I won't go to the board just not to interrupt, but that's A-U-T-O-T-R-O-P-H, autotroph. We said trophic meant nutrient or food. Auto means self. So an autotroph is something that makes food itself. Of course, it's relying on an external energy source in the form of the sun to make its food. But these are the organisms that don't eat other organisms. They eat solar energy, if you want. Then we have the organisms that eat them, that eat the autotrophs. And these are all heterotrophs. So hetero meaning other. These things all eat other forms of life. They all need biological forms of energy, biochemicals. And these are synthesized by the autotrophs and then taken in by the heterotrophs. And we have these different levels that form within a community and ecosystem. And they're more or less discreet, these levels. But we can distinguish this first order of consumers, the primary consumers, the herbivores or the zooplankton, depending on which type of ecosystem you're in, and then various carnivores, secondary consumers, tertiary consumers, quaternary consumers, secondary carnivores, tertiary carnivores, if you want. And then sometimes for these topmost carnivores, if they really don't have any consumers themselves, if there's nothing really that eats them, we sometimes talk about apex predators at the apex of this hierarchy or this chain. You know, killer whale. I don't think there are many things that eat an adult killer whale, maybe other killer whales, I don't know. Or a grizzly bear excluding humans. There's not much that kills and eats an adult grizzly bear. But things are not as simple as these chains. If you think about our food web diagram, it's a whole bunch of chains linked together. Thus the web idea, a bunch of chains linked together into a web. But one of the consistent characteristics of food webs is that many organisms are omnivorous or multi-trophic. They eat from not just a single compartment, not just along a single chain. They eat from multiple compartments and eat across levels. So a snake may indeed eat a shrew, but it might also occasionally eat insects, like grasshoppers. I'm not sure a rattlesnake could be bothered to eat grasshoppers, but many snakes do. Both mammals and first-order insects. So that's omnivory and that's very characteristic of food webs. So food webs have incredible complexity as a result of this richness of interrelationships. Let's think about causality in food chains and we can distinguish between bottom-up forces and these are forces that act from the autotrophic base or indeed from the energy source itself, like the sun, via perhaps the nutrients and minerals in the ecosystem that the autotrophs themselves are using. And they act up the chain in a bottom-up sense in terms of the cause and effect relationships of these dynamics. Against that we can note top-down dynamics where changes in the populations of these higher-order consumers influence the lower-order consumers and may act all the way to the autotrophic base and indeed to the physical ecosystem. I doubt the activities of these hawks are ever going to influence the sun itself by their dynamics, but they might indeed, in striking ways, influence the chemistry of the soils in these local systems. In those types of top-down dynamics, we sometimes speak of a trophic cascade and these two direction, I mean, of course these types of forces can coexist in the same system. You have bottom-up dynamics and top-down dynamics and ecologists try to tease them apart. In the trophic cascade, something interesting to note about them, trophic cascade working top-down, is that the effects on the subsequent levels are reversed in the influence that they have on population numbers and biomass at those subsequently lower levels. So you might imagine that something that increases the hawk population, like, say, more nesting sites. If more trees are available as a result of people not cutting down the dead old trees but leaving them as a result of some decision to do so, the hawks might have more places to nest in these old snags, these old dead trees, and hawk numbers might increase. But as a result, the rattlesnake numbers will drop because there are more hawks to eat them. And with the drop in rattlesnake numbers, the shrews and field mice go crazy and there are lots of them. And now we have memory of Darwin and Darwin's famous example of the food chain and the sense in which even that chain articulated then had hints of this type of cascading effect of alternating forces along the chain. Here's a really dramatic example based on data from a study in Utah, a rather stunning example. I hope it holds up because it's just in terms of the validity of the data because it's really incredible. I've modified the figures a little bit, but what we're talking about is the fact that in Zion, up until the 1920s, you had lots of mountain lions. With more and more visitors, tourists and human activity in the park, mountain lion numbers dropped. And so here we have two columns that we can look at and we have a series of levels that you can think of in basically in terms of our trophic levels with the physical ecosystem here at the base. And we have two columns, one which shows areas or times in which cougars are common and the other when cougars are rare. So when human visitors in millions per kilometer are high, the cougars are rare. When the cougars are rare, of course, as evidenced here by the amount of scat, the amount of poo of cougars, there's very little cougar poo when the cougars are rare, of course, and there's a lot when they're common. And the deer numbers are high when they're not around, the cougars are not around, because there's nothing eating them and they don't have that check on their population growth. They're lower absent low when the cougars are present. Deer certainly can coexist with cougars, they just exist in lower numbers. Where the deer are numerous, there's very little recruitment of new cottonwood trees, which are a favored plant for these herbivores. And where there's little recruitment of the cottonwoods, there's very high levels of erosion along the banks because these cottonwoods, which are stream loving trees, don't hold the soils together and during high water levels, during storms, there will be runoff of the soils and loss of minerals and nutrients into the waters and they're carried away. Look at the effect that the cougars and perhaps the humans are having on the physical structure of the ecosystem all the way down here at the base. And the consequences are not just those for these organisms, but in an area that has cougars, mountain lions, cougars, pumas, catamounts, American lions, they're all the same name for the same cat, the same kind that was found here in Berkeley. There's a shrine to that mountain lion I just saw and shout at conceder if you want to go leave a memorial or your remembrances, you may, leave a flower. Yeah, same type of cat, this is the cat spread all over this country and indeed into South America, a very widespread cat that goes by many names, mountain lion, pumacan collar. Where they're present and common in Zion now, you get stream systems that look like this as a result of these food web dynamics. And it's not only that you have more cottonwoods, but you have more stream side vegetation, cat tails, rushes, you have more amphibians, frogs and toads that live here. So it affects in a striking way the whole system. If you want, what are the cougars, what role are the cougars playing in this system? They're keystones, certainly they're not common, cougars are never common. As you, these higher levels, along these higher levels as we see, organisms tend to be less and less numerous as you go up levels. And cougars being an apex predator are never common, but look at the cascading effect they have on this whole system, just incredible. Suggesting that they're a keystone species here. Slide I was playing with, I spent a little too much time on. I wanted to think about how many linkages there are in these systems. This is a freshwater ecosystem, it could be in Europe or North America, where the phytoplankton are eaten by zooplankton, zooplankton by minnows, the minnows by perch, the perch by pike, and the various larger fish by osprey, say. Think about the number of linkages here. You have your autotrophs, you have your first order, consumers, second order, third order, fourth order. The pike, that's a, pike get very big and they'll eat a perch once they get big enough, but they'll also feed here. The little, little ones, you know, pikes don't start big, they start tiny, and they get big. The tiny ones might even eat from this compartment. And that's important to think about. As organisms grow, they shift in their trophic relationships. Think about a crocodile. Starts as a little baby eating insects or, you know, tiny little things that it can catch on the, right on the edge of the waters where it's hiding in the grasses so that it doesn't get eaten by a big bird. But when that crocodile is an adult, it's going to be eating antelope or something like that. So it's going to change in its trophic dynamics as it grows during its lifetime. Heterotroph is just a consumer that relies on other organisms to eat. So it doesn't make its own food. So it relies on pre-synthesized organic compounds. It doesn't synthesize those organic compounds itself. I was told that I missed a couple of hands last lecture that were up and that I just didn't see them, so please wave if a hand does go up. So how many links do we have here? Well, one, two, three, four. Or one, two, three, four, five, maybe? Now that would be four. One, two, three, four. Or if the osprey is so big that it eats a small pike that's eaten a perch, maybe you could add a fifth. But four or five links, that's usually, that's usually all you see in a community, in an ecosystem. Food, these trophic chains tend to be pretty short. You can get exceptional circumstances where you see a couple more, but not too many. Why so short? What limits them? I'll just put a, I'll just focus on one classical hypothesis and in your advanced classes you can learn about other hypotheses for this and the debates about them. But I'll just give you the classic one, sometimes called the energetic hypothesis for what constrains the length of food chains. So food chain length is limited by the inefficiency of energy transfer along the chain. It's a very old idea. It goes back at least to Charles Elton, animal ecologist, Charles Elton. And there's a lot of support for it. So I'm just going to focus on that one for you guys to understand. And I'll start to develop the basis for an understanding of it now. In talking about ecosystem metabolism. So we can take one of our more complex food webs and simplify it further according to the new terms we have. Driven by the sun, most systems, most ecosystems on earth are powered by the sun. There are other ecosystems powered by other energy sources. Someone give me an example of one? Hydrothermal vents in the deep ocean based on some chemical cocktail and heat that's coming from within the earth. Yeah, that's a good example of one. No sun shines in those depths and it's driven by a different source of energy. But we focus on this type because it's by far the most common. So solar energy is what's used by the primary producers in generating their mass. In the process, much is lost to heat, radiated away as heat. And that's not recoverable by the system. That's dissipated by the system and lost. But the mass and the materials synthesized in a compartment like this are recycled in the system through trophic relationships. Much of it going directly to the herbivores. But if not directly eaten by the herbivores, if this tissue dies, it goes into this very large compartment that we call detritus. And the detritus, this is just your hummus layer in a forest or your, yeah, it's a massive component of these systems that in a food web diagram often will just be left out. But this detritus, this detrital base, this base of dead organic matter is itself consumed by a huge number of microorganisms that themselves might be eaten by primary producers. So in a way, a new food chain is starting here in a real way on the basis of dead organic matter, on the basis of detritus. And a zebra that dies from this herbivore compartment would go into this detrital box. Say it was just, it died naturally, it drowned, it would, and was missed by the crocodiles, it would end up in this detrital box. And then maybe the vultures find it. And so the detrital load can enter this system in various ways, it's an extremely important component. So one thing we wanna highlight here is the recycling of matter and the dissipation of energy in the form of heat. Yeah, so if you want to measure, if you want to measure this system and its dynamics, you need, you can choose a currency, you can choose a common currency that works across levels and energy works quite well in calories or jewels, jewels of energy, that could be your currency because all of these things are reducible to the jewels of energy that they hold and can potentially provide. Or you might focus on carbon, some element that forms a basis for the growth and existence of the elements of these boxes. You might use another element, but carbon is so abundant in all organisms and in each of these boxes, calcium might be used in some systems where it's very important. Carbon and energy measured in jewels are often your currencies. So from the sun, contributing this many kilocalories of energy per meter squared per year, less than 1% of it is synthesized by autotrophs. The amount synthesized on land is roughly equal to the amount synthesized in the oceans, even though the oceans occupy more than twice the area on the earth's surface as the land. So your terrestrial environments are more productive per unit area than the oceans, but less than 1% of this energy is being captured. The rest is bouncing around is radiant heat energy. So of that 1% that's captured, fully 55% is respired and lost from the system by the autotrophs. 45% roughly is assimilated and available for along the chain or available for the processes of growth and reproduction of the organism itself, what I just said, huh? And there's your energy that enters the next level, broken down by the amount entering, the grazing component and the amount entering the decomposer component, highlighting what I just suggested about the very great importance of the detrital compartment here, relatively speaking. Yeah, so sorry, but net primary production, we can define your familiar with your gross wages and your net wages if you've had a job. Your gross primary production is the amount is the amount of this energy synthesized by the autotrophs, but the net primary production is that gross amount minus that lost in respiration. So we can distinguish gross and net primary production and now I have to click through my builds and realize that these studies of metabolism go back to the individual organisms. We're summing across individual organisms when we look at these whole system dynamics. So an individual organism like a caterpillar, if it takes in 200 joules of energy in the form of the leafy material that it eats, most of it's going to be lost to e-gestion to just pass through the system and unassimilated. And a bunch more lost to cellular respiration. The chemical energy needed to drive the biology of the organism and only a small components left for growth and reproduction. See these inefficiencies of transfer of energy across into when an organism eats and then when that organism is eaten, how much energy is lost along the chain and someone who wants to use the metaphor of organisms. Organisms are like leaky buckets and I often think of that when I watch my kids eat. It's very dramatic examples of they're leaky buckets. Not only is it in terms of e-gestion but should we just drop stuff? Crumbs get stuck in our flavor savers and it's just an inefficient process. This, I hope I have, yeah. This is what can account for one of the major aspects of the structure of communities in the form of these pyramids and also our energetic hypothesis for why food chain lengths are relatively short. They're short because of the inefficiency of energy transfer. There's not enough energy to sustain the higher and higher trophic levels. There's only so much energy in the system as assimilated by the autotrophs and it's pretty much lost after four or five lengths. That's that classical hypothesis that explains much of these data pretty well. There are alternatives. So one of the things you see in these systems characteristically are these types of pyramids where if you gathered up all the autotrophs and checked how much energy they contained, it would be a small fraction of what was available in the sunlight but as a result of that inefficiency of transfer, you have this rough amount, 10,000 joules say. And then according to these next levels, there are orders of magnitude less, 10-fold decreases in available energy as you move along the system as a result of a roughly 10% efficiency of energy exchange. This gives you a pyramid of productivity, a pyramid of production, but also a pyramid of numbers as measured by numbers of individuals or biomass. Here's a bog system in Florida where there's 809 grams of autotrophic material per square meter, but much less at these higher levels. It's not exactly 10%, but it's certainly the shape of a pyramid, I think. Yeah, well, it's the shape of a hierarchy like that. Yeah, you can see why the pyramid metaphor is used. So your book provides a nice example of one test from Australia of the energetic hypothesis based on a direct manipulation of tree holes. Trees often have holes in them. Those are fully functioning ecosystems in there as a result of the detrital matter that enters. You have food chains that develop in these tree holes of microorganisms and small insects and things, occasionally an amphibian or something like that. So what they did, you can read about the details in the book. I've presented a somewhat different diagram from the one in the book. It's a little richer than the one given in the book in Campbell. This is from Crabbs textbook, textbook by Charles Crabbs. I should give credit there based on this study. So the productivity of these tree holes was manipulated by excluding the amount of litter that could fall into those holes. The natural baseline litter fall is called high here and then it's manipulated to a low point at one one hundredth of the natural level of litter input. Litter just being another term for detritus, right? The number of species, the species richness in the tree holes decreased relative to that amount of material input. And the trophic lengths of the chains, the longest chains or the average number of links in the chains also was seen to go down from somewhere between four and five on average to down to somewhere between one and two with low input. So if you reduce, if you limit the size of that basal compartment, you have this bottom up effect on the structure of the chains and web. No, sorry, the litter's being eaten. The detritus is being consumed by the microorganisms. Just like in that one food web diagram we had where the detrital box was linked to the microorganism box. They're feeding on that dead organic matter and then other organisms feeding on them and other organisms feeding on them. So here's your typical pyramid of biomass. I'll give you this slide and it's an important thing to think about. Won't go into the details here. But note that in some aquatic systems, you can have an inverted pyramid of numbers, biomass here, where your zooplankton, if you gathered up all that zooplankton and weighed it, it would weigh a lot more than your autotrophs if you gathered them all up and weighed them. And isn't that a violation of what I've just been talking about about your pyramid of numbers? This type of inverted system in an aquatic environment. I can let you think about that and this gives you a clue as to why that might exist. So the study of these whole ecosystems, as you might imagine, is really challenging. For one thing, they might exist over huge areas. For another, they involve a huge diversity of organisms, a bunch of different kinds of organisms. We tend to specialize, right? If you're good at bacteria, you might not be good at elephants. There's just not time to be good at all these things. But a community ecologist and an ecosystem ecologist does need to be something of a jack of all trades when it comes to understanding the natural history of organisms. It makes these fields very integrative in that sense because one has to be general in their approach. And hopefully not so diffuse that specific problems can't be addressed. It's a very challenging field of study and it's often whole teams of investigators that are involved. And that's the case in this experimental forest area, very famous in New England, New Hampshire, Northern New Hampshire for its studies of whole ecosystems. You have teams of workers there from several universities that have been studying these things since the 60s where they have pretty much full rein to do their work along with the forestry service. And they can do these massive studies in conjunction with forestry. For example, they can... This area was important in our understanding of acid rain and the effects of clear cutting forests on ecosystem dynamics. What they could do is watch after a clear cut where all the trees are removed from a section and compare that with an area where the vegetation wasn't clear cut completely but selectively logged, where you go in and you take your desirable trees out but you don't just level everything down to the naked ground. And compare that with a water shed, one of these areas of capture of precipitation and water that's distinct from the neighboring system as a result of the geological underpinnings. You can see how this is a cavity in the slope here. This is going to be its own water shed with its own stream system, separate from this system. So you can compare these systems and compare it back to the originally vegetated system. And if you have an apparatus like this at the bottom of the water shed where you can capture all of the water leaving that system as flowing water because they've all run as little creaklets and rivulets into this larger stream that are now exiting the base of the water shed at this point. You can imagine, you can monitor the chemistry, for example, of this water and compare a clear cut water shed to a selectively logged water shed to one that's unmanipulated. And look at the influence very, in a very quantitative way of the influence of these logging practices on water chemistry. Or if you have this apparatus set up and you haven't manipulated this community at all, what about changes in climate patterns or the phenomenon of acid rain? You don't hear that much about it anymore, but it was a big, big deal in the 70s and 80s. Still a big problem where pollution gets into the atmosphere and mixes and is deposited through rainfall into local ecosystems. Those chemicals, those pollutants make their way into the systems and part of the metabolism or at least part of the circulation and the dynamics of exchange in these systems and then they exit the system here. So one can study inputs in the form of the precipitation. At the top of the water shed, you can imagine there would be a collector collecting precipitation to analyze the chemistry of the rainfall. And here we have our collector at the base to analyze the chemistry of the effluent. And then you can study the various components themselves. You can sample from the trees and from the organisms in here and start to study the ecosystem as a whole. All right, I'll let you go.