EEB 122: Principles of Evolution, Ecology and Behavior?

Lecture?30. Energy and Matter in Ecosystems

https://oyc.yale.edu/ecology-and-evolutionary-biology/eeb-122/lecture-30

Now we're going to talk about a different kind of ecology,?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.?

• In ecosystem ecology, one is primary concerned about the flow of matter and energy.?The paradigm here is thermodynamics. So the mass balance equations, the second law, entropy increases, things like that.?The kinds of measurements that get made are physical, chemical and geological here.?So you don't see Latin names and?you don't see species names in ecosystem ecology. They worry about?ecosystem compartments and stuff that moves between them.?So the connection here is primarily to the biosphere?and?geology.?Here people are looking upward at larger, more complicated, bigger kinds of things;

• And in community ecology primarily with?inter-specific interactions.?Here the issues are primarily competition, predation and history, and space and they're biological here.?Latin names?and?species names?are definitely present and they're important in community ecology.?They worry mostly about?species abundances and how it changes in time and space.?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.?

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.?

Now what is an ecosystem? 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. 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.

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,?dark water at mid-ocean ridges, and which is occurring in the subterranean part of the biosphere that goes down?five or ten kilometers, where there are bacteria that are living deep in the ground. And?the pervasive influx of life, into the subterranean environment, is an important part in the biogeochemistry of the planet; that's not covered here.

• 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,?such as?open ocean, continental shelf, desert and rainforest are the main biomes on the planet.

• If you look at net primary production per square meter, you get a totally different view.?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.

• If you're going to make an ecosystem productive, you?need to get?nutrients?from upwelling. Upwelling occurs on?continental margins. There's?a tremendous amount of fertilizer in the middle of the ocean?but it's?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.

• 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. You 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?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?the earth?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.?

So this is an overall view of energy flow on the planet, at least for the photosynthetically driven part of the planet. So?how much of the sunlight that comes into the planet is actually captured by life? How efficient has the planet become at capturing photons??I don't know the precise number, but it's down between 1/10 and 1%.?

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.?In the red arrows it is?shit and corpses.?It's pretty big. So we are deeply indebted to dung beetles and?to fungi.

Now how does that look in space??

• 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.?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?

• When?a tree?dies,?it releases a lot of carbon.?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.

• 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.

• If?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. 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.?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. These are the kinds of broad-scale biological differences that are important to pay attention to if you're doing ecosystem ecology.

If you look across the world at grasslands, forests and the open ocean, you see a nice food pyramid. The green is the producers, the yellow is the herbivores, the red is the carnivores.

• 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.

• 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.?

• Out in the?open ocean, 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 much biomass of the algae, in the open ocean.?

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??

• They are more efficient, but they're more efficient in a particular sense that makes a big difference to rates. This is the difference--this is a still photo and this is a movie. Okay? So there's.?It's a difference in rate.

• Algae reproduce a lot faster.?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.?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.

That's a bit of the overall description of the world's ecosystems. Now let's take a look at cycles of matter. 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.

• 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.

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.?

• 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.

• 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.?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.?

So let's run through the nitrogen?cycle.?

• Almost all the nitrogen that's on the surface of the planet is biologically inaccessible; it's in the form of N2. N2?is a molecule that is extremely difficult to react with. And there are--it can be converted into something that's biologically accessible, primarily by bacteria and by cyanobacteria and by lightening.

• Much more important are the bacteria and cyanobacteria that can convert nitrogen into nitrate. And most of that's going on in the soil.?So this is biological fixation. It's bringing it down into the soil. It gets processed and de-nitrified and goes back up as N2. There's some industrial fixation; the Bosch-Haber process is doing that. This runs off, gets into the ocean and fertilizes the ocean.

• If we look at the impact of nitrate and sulfate on human problems and on terrestrial ecosystems, one of the big ones is acid precipitation.?Because of the?Hadley cells and the jet stream,?the air flowing from west to east across the continent,?all of that emission of human industry and?cars?is getting picked up and getting dropped on lakes in Canada and the Northeastern United States.

• Acid precipitation?was a much bigger problem for a lake that was sitting on granite than for a lake that was sitting on limestone. Because limestone's basic and abuffer. So limestone will basically put a lot of carbonate into the water and buffer this. So this was a big problem, and in fact some lakes were losing all of their fish populations.?

It still remains a serious problem, and it's a situation that also causes international tensions because basically Canada is being treated as an externality by the United States, and Scandinavia is being treated as an externality by Germany.?

The hydrological cycle is critical because you can't grow plants without water, and you can't grow humans without water either. And as the human population has gone up over five, and now through six billion, fresh water on the planet is becoming very scarce.

• The standard issue with water is that most of it's in the ocean, and it evaporates from the ocean; of course, it evaporates more when the oceans are warmer.

• Remember the El Ni?o effect,?when that warm water from the Western Pacific flows back over towards the Eastern Pacific, then the evaporation of the oceans increases, you get a lot more water in the atmosphere, and rainfall goes up, from the Galapagos to Arizona, and right through to Connecticut.?So the oceans are a very important source of evaporation.

• The?evaporation off of freshwater lakes is also significant, and anyone who lives in Rochester or Buffalo will tell you just exactly how significant it is, after they've had a three-foot snowfall from lake effect snow,?which is basically been driven by this process.

• So the water goes up and it cycles through the rivers, back into the ocean, and long-term basically the amount going in equals the amount going out, and as long as the West Antarctic ice sheet doesn't collapse, or Greenland melts, the level of the ocean stays about the same.

If we go to the Mediterranean, and we look at the impact of what the dairying culture did, on the Mediterranean--so people started keeping sheep and goats, and goats are incredibly efficient at removing brush and grass from the landscape--basically what the goats did is that they desertified the periphery of the Mediterranean, all the way around. They did it between about 5000 and 2000 years ago.?

• We can actually, by looking the Amazon rainforest, we can get a pretty clear idea of what was going on in the periphery of the Mediterranean. So the transpiration from the trees, in the Amazon, is taking a huge amount of water out of the soil and putting it up into the atmosphere every day.

• So wherever you are in the Amazon Basin, usually by noon or about 2:00 in the afternoon, you've got a cloud sitting over your head, and that's the water that came out of the ground that day. And there's also moisture, of course, that's coming in from the Atlantic, and it's blown west, up towards the Andes, in the clouds.

• And because of this transpiration, if you look at a molecule of water that's coming in off the South Atlantic, by the time it hits the Andes it's gone in and out four times; it's rained four times by the time it gets to the Andes. So having the forest there is making very efficient use of that water.

• It is a positive feedback loop whereby the presence of the forest is maintaining the presence of the forest. And if you cut down the forest, the rainfall will decrease and the total plant growth will diminish, and that will accelerate the conversion of forest into savanna. So this is an extreme case of that process.?You're going to have planetary forces generating rainfall, whether there are trees there or not, but you move just 30 degrees north, to the Mediterranean, and you don't have that.

• At the Mediterranean, you have got, in your Hadley cell circulation, you have got cold, dry air falling, and you don't get the planetary forces regenerating and replenishing the rainfall and the forests that had been there had been an important local source of transpiration into the atmosphere.?

Now atmospheric carbon dioxide and greenhouse gasses and global warming. And it is the immediate source of carbon for terrestrial organisms, but it's really only a tiny part of the global carbon cycle. So if we look at the carbon cycle, and we look at storage in gigatons of carbon--the storage here is in black and the flux in gigatons of carbon is in purple.?

• So there's about 750 gigatons of carbon in the atmosphere. In the surface of the ocean there's about 1000 gigatons. In the deep ocean there's 38,000 gigatons of carbon, and so forth. All the vegetation of the globe has got a bit less carbon in it than there is in the atmosphere, and only about 1/50th?as much as there is in the deep ocean. And there is carbon which is moving between all these compartments.

• The fossil fuel and cement production of the world has got about 4000 gigatons stored, and it's putting about 5 gigatons per year into the atmosphere.

• So if we look at that flux, overall there's a big exchange between the oceans and the atmosphere. There's a pretty big impact of photosynthesis.?

• Plant respiration is putting just about everything back into the atmosphere that it's taking out. So this is pretty much a wash right here. The increment from fossil fuel and land use looks pretty small compared to the overall process. But the critical thing is whether or not at equilibrium you're just pushing that equilibrium a little bit, because these are rates, and rates accumulate.

This is the Mauna Loa direct measurement signal over here, and these are inferences of past levels of carbon dioxide concentration in the atmosphere. This is parts per million by volume in the atmosphere.?And you can see that there's a signal that human industrial activity has been increasing carbon dioxide level arguably since the late nineteenth century?and it's accelerating upward. These are from ice core measurements that are done mostly in Greenland by Danes and Swiss who go to Greenland and bore down through the icecap.?

Now let's put that in perspective,?and basically what it shows is that most of the time the earth has had a lot more carbon dioxide in its atmosphere than it currently has; much more than anything that has been contributed by human activity and industry in the last 150 years.

The important message is that at the scale of life on the planet, global warming is trivial. Life has dealt with it and will deal with it just fine. There will be extinctions, but life will not go extinct because of global warming. There's been plenty of species that could deal with much warmer conditions on the planet, and those kinds of things will increase in abundance as things warm.?

However that doesn't mean that global warming is not important. Global warming is especially important because of sea level rise, and because of the increase in variation in weather patterns, which means that both periods of drought and floods will become more frequent at intermediate latitudes, and the intensity and the number of major storms will probably increase.?

Now the fate of the carbon that was in the original planetary atmosphere can be sketched here. And basically what you see is that it's mostly in limestone and in sediment, and there's a huge chunk of it sitting in the ocean as bicarbonate.?Fossil fuel, organic sediment and so forth, you find that there's dissolved CO2, which actually is the molecule CO2, and not as the bicarbonate ion, in the ocean. Living biomass, fairly small; methane in the atmosphere pretty small.?

So where is the biggest source of carbon that might get mobilized into the atmosphere?And in fact it's in methane hydrate.?

• Methane hydrate will be a solid in cold water but it will melt and release methane with just a little increase in temperature.?It turns out there's about 100 trillion cubic meters of methane hydrate stockpiled around the planet, sitting there in sediments, ready to be mobilized.

• So if the world's oceans warm up by a few degrees, there will be a very dramatic positive feedback effect as this methane comes bubbling out. And methane is a more efficient greenhouse gas than carbon dioxide, by quite a bit. So its contribution to global warming could triple and really accelerate, if things warm up.

• And this is just a picture showing you that methane hydrate is stored in places like the sediments underneath deep water. So you have, say in the Arctic, it will be fairly shallow; off Louisiana it will be fairly deep. But there's a lot of it.?

The phosphorous cycle is?different from the carbon and nitrogen cycles because phosphorous doesn't have a gaseous phase. It's a solid or a liquid. And it's the scarcest essential element. Of course, we need it for ATP, we need it to build the phosphate sugar backbone of DNA, and we need it for energy transmission and so forth; all life needs it for that. But it is really pretty scarce in the crust. So it is usually limiting.?

Now if you just go out there and you pour a bunch of phosphate into the landscape, this is what you get.?You get lakes filled with algal blooms, and that is showing you the dramatic response of. The algal population and plant population in lakes is showing you that phosphorous really is the limiting factor. So phosphate fertilizer is very important in agro-ecosystems, and phosphate fertilizer gets washed into lakes and fish die.?

Why do fish die in an eutrophic lake that has a lot of algae in it? What's killing them? When does the oxygen disappear from the lake??It happens at night.?At night the algae are all there. They're not making oxygen because there's no photons coming in, but they still have to breathe themselves, and they just suck all the oxygen out of the water and the fish are asphyxiated. They're just naturally living their lives as normal healthy algae and they suck the oxygen out of the water at night.?