The Flow of Energy and Matter

Lecture: The Logic of Science

Prof. Stephen C. Stearns

OPEN YALE COURSE EEB 122: Principles of Evolution, Ecology and Behavior

Yale Univeristy

Overview

The movement of matter and energy around the planet is very important, and its study draws on geology, and meterology in addition to chemistry. Energy tends to flow upwards from plantlike producers to herbivores to carnivores before being decomposed by detritovores and cycling back into energy usable by producers, in addition to the photosynthesis or chemosynthesis used by producers to produce energy. Like energy, compounds vital to life such as carbon, nitrogen, and phosphorous flow around the planet in cycles.

Part II

(Video source: http://open.163.com)

Energy and Matter in Ecosystems

Stephen C.Stearns 

（Source：http://open.163.com）

（Chinese and English subtitle：陶子豪 ）

Now we're going to talk about a different kind of ecology. We're going to talk about the flow of energy and matter through ecosystems. Up until now we have been dealing mostly with the biological interactions of organisms with each other,with organisms of other species, and with some physiological ecology where they're dealing with the physical and chemical problems presented by the environment. But now we're going to look at the energy and flow of materials through ecosystems and biomes, and in the world as a whole, as a paradigm that's driven primarily by physics and chemistry. Now there are differences between ecosystem and community ecology, and I think that this--you know, some of this is a little bit light-hearted; some of it is dead serious. So in ecosystem ecology, one is primary concerned about the flow of matter and energy, and in community ecology primarily with inter-specific interactions. The paradigm here is thermodynamics. So the mass balance equations, the second law, entropy increases, things like that. Here the issues are primarily competition, predation and history, and space. Okay? The kinds of measurements that get made are physical, chemical and geological here, and they're biological here. So you don't see Latin names and you don't see species names in ecosystem ecology; and they are definitely present and they're important in community ecology. The way that scientists chunk reality in order to make it manageable is quite different. In ecosystem ecology, they worry about ecosystem compartments and stuff that moves between them, and in community ecology they worry mostly about species abundances and how it changes in time and space. So the connection here is primarily to the biosphere. Here people are looking upward at larger, more complicated, bigger kinds of things; and in the community ecology they're mostly looking downward at how the community interactions are driving the population dynamics of the individual species. So the connection here is to biology and the connection here is to geology. And the two things definitely connect to each other, okay? So they have strong implications for each other. But, as you know, academic specialties themselves evolve, and people develop different paradigms and different language for dealing with problems, and they have tended to remain isolated from one another, and, unfortunately, sometimes they have even tended to denigrate one another, although they're both perfectly valid ways of trying to analyze the world; they're really just trying to answer different questions. So today what I'm going to do is I'm just going to outline energy flow, cycles of materials and biogeochemical cycles through ecosystems. I'm dealing with it at a fairly descriptive level. There are methods of making this paradigm very quantitative. And if you get interested in it, really the place to go is geology. Ruth Blake does this kind of thing. She's a biogeochemist in geology. This is a part of the world that has important implications for global warming, and which is driven mostly by things that have one cell. Okay? Whether they're algae or bacteria, they are the main transducer between life and geology. Now what is an ecosystem? Well it's one of those sort of abstract terms that gets operationalized in a lot of different ways, depending on who's doing the study. But generally speaking it's the organisms in a particular place, plus the physical and chemical environment with which they're interacting. And it's often a local example of some kind of biome. Okay? So it could be a local chunk of tundra, a local chunk of rainforest, a pond. It could be an upwelling area off Peru. It could be an alpine forest. It could be a lot of different things. And people study how energy flows, and so the paradigm basically, at least for things that are in the part of the planet that are driven by the primary productivity of plants, it starts with photosynthesis, and the annual production is usually determined by temperature and moisture; certainly for terrestrial ecosystems. Left out of this description is all of the chemosynthetic activity which occurs in deep, black, dark water at mid-ocean ridges, and which is occurring in the subterranean part of the biosphere that goes down- up to say five or ten kilometers, where there are bacteria that are living deep in the ground. And actually the pervasive influx of life, into the subterranean environment, is an important part in the biogeochemistry of the planet; that's not covered here. If we look around by, just by surface area, the planet's about 65% open ocean; it's about 5% continental shelf; desert's about 5% that's extreme desert, okay, that's just about nothing on the surface; semidesert, so Sahel, that kind of area, about almost 3%, 4%; 3% tropical rainforest; and so forth. The idea here is you can take the planet and you can define different categories of the way things live on it, just by surface area, and break it down, and it looks like this. So here, if you were just to look at that, you would say, "Well,open ocean, continental shelf, desert and rainforest are the main biomes on the planet." That's just by surface area. If you look at net primary production per square meter, you get a totally different view. Okay? So this is just by surface area, and this is by primary production per square meter. Look at how lousy the open oceans are. Open oceans are deserts. Why are open oceans deserts? Student: No fresh water. Prof: What? Student: No fresh water. Prof: No, it's not that there's no fresh water. What do you need if you're going to make an ecosystem productive? Student:  Zooplankton They need nutrients. Where are they going to get it? Student: Weathering or-Prof: They could get it from weathering or they could get it from upwelling. Where does upwelling occur? Student: On a costal reef. Prof: Yeah, continental margins. Get out in the middle of the ocean and there's actually a tremendous amount of fertilizer there but it's five miles down, three to five miles down, and you just can't get it up. It's sealed off, because the top of the ocean is warm and the bottom of the ocean is cold, and there's no way that cold water can come up through warm water, unless you have Coriolis force or wind or something like that driving it. So that's why the open oceans are deserts. You'll notice that tropical rainforests are highly productive per square meter. In general forests are pretty productive. Swamps and streams are very productive. Algal beds and reefs are quite productive, and so are estuaries.So if you want to go someplace where you're you know, you're a naturalist, you like creepy-crawlies, you want frogs in your pocket, you want to see something new for Christmas, you go to these places; that's where you'll see a lot of stuff. Okay? So right there you already know where the prospecting is good, if you like to see lots of biodiversity. Now if you look at percent contribution to global primary production, the open oceans again crop up, and that is because there's just so darn much of them. If you're out there in space, looking at the world, you realize that you can fit all of the continents into the Pacific Ocean; it's bigger than all the continents put together. And most of it is open ocean; most of it is low primary productivity open ocean. But there's just so darn much of it that on the planetary scale it's making a pretty good contribution. And the tropical rainforests are big enough so that even though they're only 3 and a half or 4% of the globe, they have such high primary productivity that they're kicking in quite a bit. And the others, even though they are productive, occupy such a small portion of the globe that they're not contributing that much.Okay, so this is an overall view of energy flow on the planet, at least for the photosynthetically driven part of the planet.The sun is sending in the energy. And by the way, any idea of roughly how much of the sunlight that comes into the planet is actually captured by life? How efficient has the planet become at capturing photons? Does it capture 50%, 10%, 1%, 1/10^(10) of a percent? A guess? How many for 50% Hands up. How many for 10%? Hands up. A couple. How many for 1%? Hands up. How many for 1/10^(10) of a percent? Hands up. See the grad students think it's a 1/10^(10) of a percent. It's a small amount. I don't know the precise number, but it's down between 1/10 and 1% I think. So even after 3.5 billion years of evolutionary history, the planet has not become terribly efficient at capturing sunlight. Freeman Dyson has got this definition of different kinds of civilizations. One of the stages of civilization would be when you can put a sphere around an entire solar system and capture all of the photons coming off of the sun and harness it for running a civilization. That would capture the entire solar output. Well, you know, we are a tiny little dot on the face of the sun, and we're taking 1/10^(th) to 1% of its photons. So this isn't a very big number when you look at the solar output.What happens is that basically algae, primarily algae, but also trees and all other larger plants, are capturing this. Then the herbivores are eating the plants. The primary carnivores are eating the herbivores. The secondary carnivores are eating the primary carnivores. What do you think is in the red arrows, going off to the detritivores? In simple Anglo-Saxon, four letter words, what do you think is in the red arrow? Shit and corpses. Okay? That's the red arrow. It's pretty big. Any idea what Africa would look like if you got rid of the dung beetles?You would need hip-waders; especially in the Serengeti or any of the big national parks. A pile of elephant dung is about this big. Okay? So that's what's going off in here. We are deeply indebted to dung beetles. Believe me. And to fungi. What's going off here is respiration. So that's energy. You know, you all use it up every day. You're using up somewhere between oh 3500 and 5000 calories a day, depending on whether you're on a sports team or not. And so that's what's going off out here. And this is coming off of every level here. Okay? So you can think of this as what's left over and this is the flow of all that ATP driven stuff on the planet surface. Now how does that look in space? Well if you look at tons of carbon fixed per hectare per year, where green is a lot and yellow is a little, you can see that the forests are really important. Okay? And the closer you get to the equator and the wetter it gets, the more efficient the forests are at fixing carbon. This is for the terrestrial part of the world. If you could put the reefs in, they would be fixing carbon, and they would be withdrawing it on kind of a different timescale. Because the tropical forests, although they fix a lot of carbon, don't actually cleanse the atmosphere of CO₂, at least not at equilibrium. Why not? Student: As they're they're actually they're also respiring. Prof: They are respiring, yes. What happens to a tree when it dies? Student: It releases a lot of carbon. Prof: It releases a lot of carbon, right? So in fact you could grow up a big forest but you only get the carbon benefit the first time you grow it up; after that it goes into an equilibrium where the trees are falling down and the logs are rotting and they're releasing carbon back into the atmosphere. So yes, you can temporarily fix a lot of carbon by planting a lot of trees, but in the long run it's not a stable solution because those trees get burned up; they either get literally burned up, or they get metabolized by the detritivores, and the detritivores put the carbon back into the system. What happens when you fix carbon in a reef? You make limestone, and limestone sticks around for a long time. And if you take a big reef and you slam it into a continent with a tectonic collision, you get marble. So the marble quarries of the world are the fixed carbon of 3 to 500 million years ago. So you can actually tie up carbon for a much longer period more stably by putting it into limestone than you can by putting it into wood. However, there are also some important things about different kinds of forests and how well they can grow, and a lot depends upon whether you have a deciduous tree or a conifer. And if you look around the world at the different kinds of forests, it turns out that the coniferous forests can actually fix more carbon per year than a deciduous forest, basically because they keep on growing at times when the deciduous trees have dropped their leaves. Okay? So their primary productivity, in terms of tons of carbon per hectare per year, is about 1 and a half that of a deciduous forest. And I mention that because there are-these are the kinds of broad-scale biological differences that are important to pay attention to if you're doing ecosystem ecology. There are some that you can ignore, but this is a big difference and it's something that has to be kept track of. So I think you're getting an idea of the sort of filter that ecosystem ecology places on the details of other kinds of Biology. Ecosystem ecology, it's going to be interested in keeping track of things that make big differences to the flow of energy and materials, and it's going to say we probably want to ignore the rest, just because life is complex enough as it is. Okay? So this is really what's driving that. And that's why, if you go back and you look at those biomes, you will see that when people make biome classifications, they keep track of whether they're dealing with a coniferous forest or a deciduous forest, and things like that. Okay, if you look across the world at grasslands, forests and the open ocean, you see a nice food pyramid. And the green is the herbivores--excuse me, the green is the producers, the yellow is the herbivores, the red is the carnivores. And if you just look at biomass, you will see that in grasslands you have a few big fierce animals, that are rare, and then you've got a bunch of grazing animals that are a bit more common, and there are more of them, and then you've got a lot of plants; pretty much the same in the forest. Out in the open ocean it's really quite different. You have a few large top predators; so these are the tuna and the sharks and the whales and things like that. Then you've got a big biomass of herbivores, and then not too many-not too much biomass of the algae, in the open ocean. If you look at the energy flow for the grasslands and the forests, it's pretty similar to the standing crop.

This would be the standing crop. This is how much energy calories per square meter per day is flowing through it. But in the open ocean something is converting this kind of anomalous picture into a sort of standard food pyramid, when you look at energy flow. What's doing it? What's the difference between a grass and a single-celled alga? Student: The algae are more efficient. Prof: Well you're getting at it. They are more efficient, but they're more efficient in a particular sense that makes a big difference to rates. Okay? This is the difference--this is a still photo and this is a movie. Okay? So there's-that's the difference. It's a difference in rate. Yes? Student: Algae reproduce a lot faster. Prof: That's basically it, yes. A single celled alga can probably have two generations per day and-at least one per day--and maybe even in a warm estuary three per day, whereas a grass is probably going to be lucky to get through two or three generations per season.Okay? So there's a difference of perhaps a hundredfold in the rate. And the things that are eating them have a much, much longer lifespan. So down here what's going on is that the algae, there are not so many of them, but they're cranking over like crazy, and they're getting harvested like crazy by all of the planktivores in the ocean. So the krill, the copepods, everything that eats algae, is grazing them, and that's keeping the algae at a fairly low level. They're a long away from their own carrying capacity. They're in exponential growth rate almost all the time. So they're booming along, and it doesn't take so much standing crop to maintain a lot more biomass because they are turning over and reproducing and multiplying so quickly. So that is why you see this dramatic shift. Okay, so that's a bit of the overall description of the world's ecosystems. Now let's take a look at cycles of matter. Okay? The main compartments are oceans, fresh water, land and atmosphere, and they are exchanging materials all the time. You're already familiar with the upwelling patterns; I've mentioned that when I was discussing the Coriolis force. So this is where the nutrient-rich waters are coming to the surface. And in a place like the coast of Peru, you have millions and billions of seabirds, that are eating billions and trillions of anchovies and sardines, that are feasting on trillions and quadrillions of shrimp, that are eating algae, coming up there. Okay? So you have the cold Humboldt current coming up and bending offshore here, heading out for the Galapagos, and as it's moving out towards the west-in the Southern Hemisphere, remember, it is coming up towards the equator; the equator has a greater angular velocity than the southern part of South America, and the water is getting kind of left behind by the planet, as the planet pulls out this way. And because you have a continent here, there's no water that's left there to flow over and replace it. So the only place it can come up from is the bottom, and it comes up from the bottom and fertilizes this zone off the West Coast of South America. Well over the course of hundreds of millions of years the seabirds that nest on islands offshore, so that they can get away from the predators that would their eggs on a continent, have built up a huge deposit of guano on the Chilean Islands. And this was a matter of international significance, prior to the First World War, because nitrogen was so critical in the manufacture of arms. You remember the Oklahoma City bombing, when Timothy McVeigh simply took nitrogen fertilizer and mixed it with diesel fuel, and put it into a truck and blew up a building. A lot of energy in nitrogen; okay, nitrate, powerful stuff. And this is where the world supply was. And, by the way, that repeats at many other upwelling areas around the globe. And just prior to World War One, Haber and Bosch figured out a way of fixing nitrogen from the atmosphere, as ammonia and urea--they did it at high temperature and pressure and that was actually what kept Germany in the war between 1916 and 1918. Okay? So the reason I am mentioning this is that I'm trying to use a vivid example that shows you how the flow of materials between different compartments and ecosystems has actually influenced world history for human culture. And in this case it was basically the seabirds taking it out of the ocean and putting it on land; and putting it on in huge quantities. So now there's about 100 million tons of nitrogen fertilizer produced every year. It's about 1% of global industrial energy, and it sustains 40% of the global human population. So these kinds of processes actually are part of the substructure of our modern and post-modern culture, and it gives you some feel for an ecosystem service. Prior to the Haber-Bosch process, this fixing of nitrogen out of the atmosphere could only be done by biological organisms, and this is an estimate really of how big that ecosystem function was. Okay, so let's run through some of these cycles. Almost all the nitrogen that's on the surface of the planet is biologically inaccessible.