EEB 122: Principles of Evolution, Ecology and Behavior
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Principles of Evolution, Ecology and Behavior
E&EB 122 - Lecture 25 - Interactions with the Physical Environment
Chapter 1. Introduction [00:00:00]
Professor Stephen Stearns: Last time I discussed the planet as a basically physical and chemical machine, and how climate affects temperature and water and nutrient relationships on the planet, both on the continents and in the ocean. And what that does is it creates a mosaic of ecological problems for organisms. And the part of ecology that looks at how organisms individually deal with the problems posed by the environment, things like temperature and pH and water availability and stuff like that, is called physiological ecology. And I’m going to give you a brief description of that today, using a historical framework, which is up here on the first slide.
Now the purpose of this historical framework is to show you that between about 1860 and about 1960 people conceptualized both the way the organisms deal with these problems in the environment, and then how to integrate that into a comprehensive vision of where organisms can live, and why that is the case. And that vision of where organisms can live and reproduce, survive and reproduce, the ecological niche idea, is something that turned into an intellectual tool that became very useful in many different parts of ecology.
So what we’re doing here is starting off with Claude Bernard, the great French physiologist who came up with the idea that one of the basic things going on in organisms is that they’re trying to keep their inside constant, while the outside changes; the idea of homeostasis, la constance du milieu intérieur.
And well then I’ll briefly mention L.J. Henderson, who said, “You know, it’s extremely interesting that the properties of important things in the environment, like water and air and the molecules out of which organisms are built, and things like that, are in some cases extremely convenient for life.” He called that the fitness of the environment.
So he kind of turned the whole idea of evolutionary fitness around, and he said, “The environment appears to be peculiarly fit, as a place for organisms, like the ones that we know, to live in.” And, of course, that’s not too surprising, given that this is the planet on which life evolved.
Nevertheless, it’s worth considering the fact that water has some completely extraordinary properties. It has very high heat capacity. It transfers heat very rapidly, and water can put into solution probably more different kinds of atoms and molecules than almost any other solvent.
So on a planetary scale, water serves very effectively as a heat transfer mechanism; what we saw in the lecture last time. It serves very effectively as a medium in which a huge diversity of chemical reactions can occur, out of which life has selected some of them, and so forth.
And you can carry this kind of thinking on, into other parts of biology. For example, why is it that phosphorus was the element that was selected to be the medium of biological energy transfer in the form of ATP? And if you look into the shell structure of the phosphorous atom, you will find that it actually has some options for storing energy, and then forming bonds with oxygen, that help us to understand why it was phosphorous that was the one that life selected for the generalized unit of energy currency.
So L.J. Henderson’s ideas actually are kind of provocative and interesting, and the book, The Fitness of the Environment, is well written and a pleasure to read. So if you get interested in that–and by the way, Claude Bernard’s book, A Study of Experimental Medicine, is easily available in English. These people are very bright people who wrote some classics, and it’s nice to be able, as a scholar, to tap into that history of the development of ideas.
Now the culmination of this, at least for today’s purposes, was Evelyn Hutchinson’s concluding remarks at the Cold Spring Harbor Symposium on Long Island in 1959. That was the output of his graduate seminar here in this department where he had–in his graduate seminar at that time he had people like Larry Slobodkin and Bob MacArthur and Alan Kohn.
So there was kind of a dynamic group of grad students in the Yale Biology Department, studying ecology at that time, and they had come up, together with Hutchinson, with this idea of the niche as an N-dimensional hyper-volume. And that was a very powerful tool for condensing all of these ideas about how individual organisms and populations are dealing with their physical and chemical environments, and representing it as an object that then could be used further in analysis.
So that’s the framework. These are the guys, Bernard, Henderson and Hutchinson. There’s a portrait of Hutchinson in the Saybrook Dining Hall. I actually think the photo’s a little bit better. This is a bush baby; that’s a Galago. I wouldn’t mind having one of those.
And the outline of the lecture is going to be a bit about temperature and about thermal regulation. So the basic idea here is that ectotherms and endotherms have really quite different problems with temperature, and they deal with it in quite different ways. In homeotherms, we’re going to look at metabolic rate and brown fat and hibernation, and why it is that intermediate sized things hibernate. The really little ones can’t and the really big ones can’t, but the ones in the middle can.
We’ll take a bit of a look at temperature and evaporative water loss, and then I want to talk a bit about how plants deal with drought and with too little water and oxygen, and then we’ll end up with the ecological niche.
This is a very, very quick and kind of spotty summary of some major themes in physiological ecology. It’s a big field, and it contains a lot of neat experiments, and I’m only able to touch on it quickly in this course. One of the themes that I want to bring up now–and I hope I remember to come back and mention it again later in the lecture–is that we will see that organisms have lots of adaptations for dealing with the external environment. We’ll see it in the nose and the brain of the oryx, and we’ll see it in the special organs of plants to deal with oxygen problems and so forth.
And one way to think about that is that evolution has designed organisms to extend the range of environments in which they can survive and reproduce, and therefore that the definition of what is critical in the environment has been continually changed by evolution. It’s been a moving target.
You cannot think of an ecological niche as being something that pre-existed on the planet, before life started to evolve. But we now use, as a tool in ecology, the concept of the ecological nice as an artificial construct, invented by human minds, as an intellectual tool to try to make sense out of the complexity of nature. And our definition of it actually is the product of evolution, and it’s been a moving target.
So don’t think of the environment as consisting of a pre-existing chessboard on one of which- each square of which is a niche, and into which you can put an organism, and then it will all get filled up. Because the organisms themselves have been defining what these things are, as they evolve.
Chapter 2. Thermoregulation [00:08:06]
Okay, a little bit about ectotherms and endotherms. Here’s, on the X-axis, we’ve got environmental temperature, outside temperature, in Centigrade; and here we have body temperature. And a mouse is maintaining its temperature at a nice 37, and the lizard is letting its temperature fluctuate with the external environment.
We’ll see that actually lizards can control this, to some extent, behaviorally, and that things like mice, of course, do have daily temperature cycles and so forth. But just at this level–and it’s a very rough contrast–endotherms maintain constant internal temperature and ectotherms let it fluctuate.
Now lizards have a preferred temperature, and actually their preferred temperature is a bit hotter than the mouse. They like it to be oh about 30–well maybe not quite as hot as the mouse, but they like it in the mid-30s; so say around somewhere between 90 and 95 Fahrenheit, say 88 and 95. And their surroundings have a huge temperature range, and the actual temperature range–this is what you would measure in the lab.
If you made a temperature gradient, and you put your lizard in and you let it just settle down in the temperature gradient, it would wander back and forth until it found what it liked, and it would settle down right there. Its actual temperature in nature is much narrower than the range of temperatures out there in the environment.
And, for example, it can do things like having its back facing east or its back facing west, depending upon whether it’s morning or afternoon. Right at noon, when the sun’s directly overhead, it won’t orient like that. So if the lizards are basking, they bask in such a way that helps them to maintain their actual temperature above that of the environment, in this case.
They manage to be warm when they need to run fast, and they manage to be cool at night. If you’re a herpetologist and you like to catch lizards, you know that a lizard that’s been sitting on a nice warm rock is going to run away from you really quickly when you try to go up and grab it. And, of course, they have developed this because they need to get away from predators.
So they will bask in the morning, to get their temperature up, and then they will move back and forth between sun and shade during the day, to maintain their body temperature, in the high 30s, and then they go back into their burrow at night. So there is a kind of behavioral thermoregulation in this ectotherm, which is not doing its thermoregulation with internal physiology; it’s doing it by moving in and out of sun and shade.
Another very important idea for ectotherms, and particularly for small ones–the smaller you are, because of the surface area volume ratio, the more rapidly you take up or lose heat. This idea is physiological time. Okay? So physiological time is something which is really directly proportional to temperature; and you can see an illustration of it over here.
This is the percentage of development that’s going on in the course of one day. So in 24 hours this is how much development is occurring in standardized stages, depending upon temperature. And if you transform that into a rate per day and plot it against temperature, you get this nice straight line, which basically means that time is directly proportional to temperature. So the hotter it is, the faster they’ll develop.
And what that means is kind of interesting for ecology, because on the one hand you’ve got a lot of predators who are homeotherms, and who don’t have this kind of reaction at all. So they’re running around rapidly. Shrews, mice, birds; lots of things that will eat insects are fairly insensitive to the external temperature, and they can be active at all temperatures.
Whereas the insects are actually forced, by their small size and their ectotherm status, to grow more slowly when it’s cold, and they’re forced to grow more rapidly when it’s hot. And that has cascading effects on their population dynamics and on their predator/prey relations.
Now what about endotherms, how do they deal with the environmental temperature? Well this is the body temperature of a model endotherm, and this is its heat production here. Now what’s going on is that there is an upper critical temperature, and if the environmental temperature, in the long-term, goes above this upper critical temperature, the endotherm can no longer thermoregulate, and its body temperature will rise.
If I take you out into the Sahara, on a hot day, and I sit you down, you’re going to thermoregulate pretty well, until the external temperature gets above about a steady 110, or something like that, Fahrenheit, and at that point your sweating and so forth isn’t going to function anymore, and your body temperature will rise, and if that goes on very long you’ll be dead. So that’s what this critical temperature means.
Lower critical temperature. Basically if the environmental temperature drops below the lower critical temperature, then internal heat production starts to ramp up. That would be both direct burning of fat, down at the cellular level, and it would be shivering, and things like that. And that would allow you to maintain a nice steady internal temperature, until you got down to your maximum heat output, by all physiological mechanisms combined, and if the external temperature drops even further than that, and you’re no longer able to keep up, you will freeze and die down at that end. Okay?
So you can see that there is a range of environmental values that can be dealt with, and then outside that range you can’t deal with it anymore. And this is something that’s evolved, where these points are.
So what’s going on here is variations among different kinds of endotherms in their insulation blood flow, how they select microclimates, shivering and huddling. Insulation. Take a Weddell seal or a Leopard seal or something like that, freeze it, cut it in half with a band saw, look at it in cross-section. It’s about one-third fat, on the outside. It’s extremely well insulated. Same thing for a humpback whale or a blue whale. So insulation can be very important. Bears do it. We do it; we have subcutaneous fat that serves as an insulator.
Shivering and huddling; well you know all about shivering. Huddling is something, for example, that the emperor penguins do. Emperor penguins have this totally bizarre lifestyle where they have chosen to lay their eggs on part of the Antarctic continent, which is exposed during the summer, but their lifecycle is such that it takes them about six months for the eggs to hatch and then to start to feed baby.
And so at the time when they have these little chicks that need lots of warmth, and they are shuttling back and forth to try to go way out to the edge of the pack ice to get squid–because now it’s winter and the pack ice is frozen-they huddle and get into a big circle where you’ll have hundreds of these giant penguins–they’re about this big–that are all packed together.
And the Antarctic hurricanes are blowing over them with -40, -50 Fahrenheit temperatures, and the birds are basically forming a continually moving clump in which the ones on the outside are getting desperate and pushing their way into the inside, and the ones on the inside, they’re a little bit warmer and not quite so desperate, and are getting pushed out to the outside. So that’s huddling.
Chapter 3. Physiology of Internal Regulation [00:16:33]
Okay, now let’s go inside some organisms and look at some of the adaptations that evolution has produced that allow them to regulate their internal environment. So this is a classic example of something that would cause the internal environment to be held constant, despite great variation in the external environment: countercurrent heat exchangers.
And the way these things work basically is that if you have concurrent flow–this is the countercurrent case, and it’s being explained here by contrast to the concurrent case. In concurrent flow you would have say venous blood going in this direction; and then running right next to the vein you’ve got an artery, going in this direction.
So the artery perhaps is nice and warm, but it’s running next to the vein, and there’s an exchange gradient along it, and as these two things exchange heat they end up at the same temperature, coming out. That’s what happens if the flow is going in the same direction.
But if you arrange it physiologically, and morphologically, so that the flow in the artery is going in the opposite direction to the flow in the vein–so this is going into the organ and this is coming out of the organ, going back to the heart–then what goes on is that the blood that’s coming in, in the artery, is getting heated by the blood that’s going back out in the vein, and that’s going to maintain the temperature on this side.
Now which way you would want to set this up would depend upon whether you wanted the warmth to be in the core of the body or in the outside of the body. In most cases, this is in the core of the body. You guys have got this, right here. You can walk in water, and a countercurrent heat exchanger in your legs will make sure that your body core doesn’t drop temperature too much.
Yours is okay, but it’s not really nearly as impressive as the ones that are in the feet of ducks and geese and moose and things like that; that can stand around in water, which is right at the freezing point, and their core body temperature stays a nice stable 98. Okay?
Let me just mention, before I go into the oryx, that there are countercurrent exchangers that deal with ion concentrations in vertebrate kidneys–so the vertebrate kidney is actually designed using this same principle–and with oxygen concentration in fish gills. So countercurrent exchangers are something that is obviously such a good engineering idea that it has been arrived at convergently by evolution to deal with similar problems, but completely independently solved.
Let’s take a look at one. This is the desert oryx–really a beautiful animal, I’ve seen them in Namibia–and we’re going to look inside its head here. So the problem that the oryx has is that it needs to regulate both its temperature and its water supply. It’s living in the desert. And if you look into its head, it’s got a lot of exposure of its blood supply to external air coming in. It doesn’t want to lose too much water by sweating.
So what it’s done is it’s allowed its body temperature to go up to 44 degrees Centigrade. So it won’t regulate its body temperature until it hits 44 degrees Centigrade. And it’s a big animal, so overnight it can cool down, and it’ll take a long time to hit 44, during the day. But its brain would die if it ever got to 44 Centigrade.
So it has to figure out a way, how to hold its brain at a nice 39 degrees, while its body temperature, which has most of its blood supply in it, is at 44. So it’s got to drop that body temperature by about 5 degrees Centigrade going into the brain. And the way it does it is it first takes the blood and it gets cooled.
So there’s blood that’s being pumped out, into its nose–it has a great big nose–blood is getting pumped out into its nose and coming back through these veins. And this is cold because–colder–because of the evaporative processes that are going on in the nose.
It gets passed through what is called a rete mirabile, right here, and the oxygenated blood, which is going to go into the brain, passes through this, and the cold blood coming out of the nose cools the oxygenated blood off before it gets into the brain. It’s really a beautiful adaptation, and it’s something that gets repeated in other organisms to solve similar problems in other situations.
For example, tuna. Here are some yellowfin tuna, and they have a countercurrent heat exchanger, and they use it to keep cold sea water from chilling their warm venous blood that’s coming out of their hot high performance muscle. So they cannot really retain very much heat overall, because in order to get oxygen out of the water, they’re pumping–and they’re very high energy animals, so they’re pumping a lot of oxygen through their gills. And that is a big surface.
The water has, as I mentioned with Henderson, very high heat capacity and a great ability to strip heat off of the blood supply in the gills. But down the core of their body they’ve got some dark muscle that they want to keep up at 37 degrees Centigrade, so that they can do things like swim from San Francisco to Tokyo in seven days, at speeds of up to 50 kilometers an hour. These fish are amazing.
Well what they’ve done–this is sort of a perch or a largemouth bass. And you might think of that as the ancestral condition, and they’ve taken this ancestral condition, where the vein, coming back into the heart, looks like that, and the dorsal aorta going out of the heart looks like that, and that kind of circulation has been altered so that you have a rete mirabile between arteries that are running out under the surface of the skin, and veins that are running out under the surface of the skin.
And when these arteries are then pumping through the rete mirabile, into the core of the animal, they are picking up heat that’s being generated by that muscle tissue, and then in the countercurrent heat exchanger, or the rete mirabile, that heat is exchanged, going back into the veins.
So on the external part of the body the blood’s right at environmental temperature, and in the core of the body, it is probably maintained at anywhere from 10 to 20 degrees Centigrade above the environmental temperatures, so that these very efficient muscles can work.
Mammals maintain their internal temperature, particularly small mammals, maintain their internal temperature using something called brown fat. So this is now not a morphological adaptation, at the level of an organ–which is what the rete mirabile or the countercurrent heat exchanger are–this is a cellular adaptation.
So if you take a cute little eastern chipmunk and you look into its body, you find that there are specific places where it has deposits of brown fat. And this is what brown fat looks like under a microscope. This is white fat. Okay? This is what I’ve got hanging on my belly.
And in a brown squirrel, they have, above the kidney and in the back of the neck and so forth, brown fat, and the reason it’s brown is that it’s loaded with mitochondria. And so if they get a signal that the temperature’s dropping, and that comes into their brain, into their hypothalamus, they will put out a hormone that carries a hormonal signal out to their brown fat, and the mitochondria in the brown fat receiving that signal will start to simply generate energy, and that generates heat. Okay?
This is actually the mechanism that allows hibernation, because they can regulate that heat generation up or down. And hibernation is something which is done in mammals, of course, to avoid dying, in the winter.
I have, I think, three or four eastern chipmunks that live in my yard in Hamden, and they tend to disappear towards the end of September, and I’ll probably first see them again sometime during the next month. They’ve been down for several months. Okay? They’re all wrapped up in a ball underground, sleeping away. And you can only do that if you’re kind of an intermediate size.
So when it’s preparing for hibernation, it’s regulating its temperature near 37. Then when it’s down in the ground, it will drop it, down to about 10 degrees. And it’s got a temperature sensor, in its brain, which keeps it from freezing. In other words, the temperature will go down to about 10, or maybe a little bit below in some other small mammals, but it will never go to freezing. So it can tell if it’s getting dangerously cold, and it will regulate its lower temperature with the brown fat, so it doesn’t completely freeze.
And they do wake up a little bit sometimes during the winter, but they don’t really come out. They’ll wake up and roll over, and if they have stored seed underground, they will go eat their seed stores so they can keep doing this. And then right about now they’ll come out again.
So what’s going on in one bout of hibernation here–so if we just take one little bout here–they drop their metabolic rate, they drop their body temperature, and for about a week they have a very low metabolic rate and a low body temperature; it’s down, in Centigrade, it can get down to maybe 3 or 4 degrees. And then they will arouse, eat and then do it again.
So this is regulated actually both by physiology and by behavior; and by morphology. They have pouches in their cheeks where they can store the seeds, and they have a seed deposit in their burrow, and when they wake up and they need to recharge, that’s what they use, and then their physiology takes over; and that’s what gets them through the winter. Now you can imagine that this has greatly extended the geographical range in which something like a chipmunk can live.
Now I want you to think about the surface area to volume ratios for a minute. I think you all know that the surface area is proportional to the square of a body dimension, and the volume of an organism is proportional to the cube of a body dimension, so that when things get big, they have proportionally less surface area, and when they get small they have proportionally more surface area. And I want you just to take a minute and explain to your partner why it is that really small things can’t hibernate, and really big things can’t hibernate. And let’s see how you do, and I’ll give you the answer in about two minutes.
[Students confer with one another]
Student: How do bears hibernate?
Professor Stephen Stearns: They don’t.
Student: They don’t?
Professor Stephen Stearns: They sleep. Yes Myra?
Student: We learned that bears hibernate–
Professor Stephen Stearns: Bears don’t hibernate, they sleep. A very bad idea to try to take the body temperature of a sleeping brown bear. It can wake up in a hurry. A chipmunk can’t wake up.
Professor Stephen Stearns: You can pick a chipmunk up and toss it in your hand, it won’t wake up. You go in and try to take the rectal temperature of a brown bear and you better be ready to run in a hurry.
Student: What is the difference between hibernating and sleeping?
Professor Stephen Stearns: How far the body temperature will drop. A bear–your body temperature drops when you sleep; it goes from about 98.6 down to about 96 or 95, and then it warms back up when you wake up. A bear might drop down to about 90. A chipmunk will drop to 40.
Okay, who can tell me why really small things can’t hibernate? Like shrews, which I have in my garage, in the middle of the winter. Okay, why do really small things not hibernate? What’s their problem with surface area and volume?
Student: Their surface area is really big compared to their volume. So there’s a lot of surface to–
Professor Stephen Stearns: Right. So even though evolution has done a great job of developing these temperature regulating mechanisms, there comes a point at which they can no longer do it, and if you get really small, there’s just no way that you can build say a 20 gram shrew that will be able to regulate its temperature. It just has too much surface area. Okay?
What about something that’s big? I’ve heard a couple of comments about bears; bears are big. Okay? Bears don’t hibernate, they sleep. Why can the bear not hibernate? Remember, hibernation is a condition where you really drop your body temperature a lot. Yes?
Student: Maybe its volume to surface ratio is larger, and so it has [inaudible].
Professor Stephen Stearns: Yes, it can’t get rid of the heat fast enough to drop its body temperature. So just a bunch of kind of torpid bear fat, if it’s alive, is still making enough temperature. So it can’t radiate it off fast enough. So that’s basically why you get these rough limits.
Even in something which is as ectothermic as my compost heap in the backyard, whose temperature is being regulated by bacteria and fungi, I can go out there when it’s 20 degrees below 0, take the snow off, and steam will come out of my compost heap. Okay? Which is a pretty big area and it has not too much surface area for a large volume, and it maintains high temperature right through the winter.
Chapter 4. Evaporative Water Loss in Plants and Animals [00:32:17]
Okay, now what about evaporative water loss? Here is a real physiological tradeoff. If you want to maintain your internal temperature by cooling yourself through evaporation, you need a good supply of water; it can take a tremendous amount of water. As you all know, when you get really thirsty, running or working, to just maintain your proper balance of bodily fluids.
And there comes a point where the resting metabolic rate and the resting water loss really get–there’s an attempt here, with the metabolic rate starting to go up, that’s because the water loss is no longer able to cool the organism enough. So it gets up to about 42, 43 Centigrade, and this little bird is getting into serious difficulty because it can’t evaporate enough to hold its temperature down, and it’s starting to get up into dangerous territory. Okay? Really dangerous territory.
So that’s another illustration of physiological ecology. Let’s now go to plants, and think about water in the soil. Because, of course, for plants what do they need? Plants need sunlight, they need water, they need carbon dioxide; of course they need more than that, but if they’re going to make food, if they’re going to feed, they need sunlight, water and carbon dioxide. They’re going to get their water out of the soil, and they’re going to do it with roots.
And if we look into the soil, what we find basically is that at a certain pore size in the soil–the low for certain levels of water, right about here, in terms of bars. By the way, the bars would mean how much pressure do I have to exert on the soil in order to see the water come out of it?
So this would mean that I’d have to exert a pressure of 1000 atmospheres here to squeeze any water out of the soil. So this would be really dry. This would be 10 atmospheres here. And up here the water is draining away freely.
So this basically is the water which is available to the plants. It’s between about 10 atmospheres of pressure and about 1/10th of an atmosphere of pressure, right here. And that’s associated with whether you’re dealing with soils which are very fine and claylike–so they have fine particles and small pores–or whether you are dealing with soils that are gravely or sandy or things like that.
So what the plant does is it puts its roots down into the soil, and it’s going to suck that water out of the soil. Now I don’t know if any of you have ever stood on the edge of a pool and tried to suck the water from a swimming pool up a tube which is only as tall as your body; okay, less than 2 meters. It’s hard. Your cheeks really hurt. And, in fact, you can’t do it very well. Most of us can deal all right with the level of the latte or the milkshake, but the swimming pool is hard.
I now want you to think about a Redwood or a Doug fir that is going to put its roots down into the soil and suck that water up 100 meters. [Laughter] It’s not easy. There has to be tremendous negative pressure maintained, continuously over that 100 meters, to pull that water up to where a leaf can use it, to combine with carbon dioxide, using the energy from the sun, to photosynthesize 100 meters off the ground. Okay? Not simple. So how do they do it?
Well here’s a leaf, and here’s the business end of the leaf, right here, the stoma. There are some guard cells here that are regulating the diameter of the stoma. There’s carbon dioxide coming in, and the oxygen is coming out. Here’s the delivery system over here. We’ve got the xylem and the phloem. This is the vascular bundle. And the question is, how do they do it?
Well the transpirational pull is being caused by the water that evaporates inside the leaves. Okay? So if we go back to this, you should think of water that is going to evaporate and go out of the stoma–and it’s coming off of these cells right here, next to the xylem and the phloem–and it will cause, as the water is evaporating from the stoma, it will cause the water surface in the stoma to pull back into pores in the cell walls–well it’s not from the stoma, it’s actually from the cell, inside the leaf–and there it will form kind of a concave meniscus.
But it’s got very high surface tension–and this is back to L.J. Henderson; water has these amazing properties. Water has amazing surface tension. Water can climb up the edge of a glass. Okay? And that’s caused by the hydrogen bonds between the water molecules. They have this beautiful little, kind of Y-shaped structure, and they readily form hydrogen bonds. And actually liquid water is this beautiful set of sheets of these layers of molecules that have formed these bonds.
So that surface tension pulls the concavity back out. Okay? So the combined force that’s generated by billions of these things is strong enough–this should be ‘lift’ not ‘life’–to lift water from the roots up 100 meters.
Now if you’re going to do that, boy do you have to build a heck of a straw; and that’s what xylem is. Okay? The xylem vessels that will transport the water have to have very small diameters. They have to be built very strongly, because otherwise the water cone is going to be broken by cavitation, and as soon as it’s broken by cavitation, the leaves on the top dry out and die. So cavitation is a big problem; that’s the formation of a bubble, inside the xylem.
Okay, that was a little bit about the physiology of how plants drink. It’s more complicated than that, but I think that I have been able at least to illustrate the problem to you, and I think I have shown you that the physiological problem posed by the environment has been solved by the evolution of xylem and phloem; which happened about 3 to 400 million years ago, and has since been perfected to a great degree.
If you go out now and you’re doing work in the short-grass prairie, or in the long-grass prairie, for that matter–Mindy Smith works in the Kanza Prairie in Kansas, and in South Africa–and you do a section through the soil, you can see that a lot of the life of plants, and a lot of both their individual ecology and their competitive relationships with other plants, is actually being mediated by where their roots are foraging for water.
Some of them can go deep, some of them stay shallow, and they partition that soil environment into different areas that they are sucking water from. By the way, the earthworms are also partitioning it. There are some that up here and some that are down there, and some that move back and forth.
So there are some organisms, some plants, that are really extreme competitors. Eucalyptus trees from Australia, and Casuarina trees, which come from Northern Australia, New Guinea and the Solomon Islands, have been introduced around the world.
So I have been in a field station in Corsica, surrounded by Eucalyptus trees. I’ve been on the Berkeley Campus surrounded by Eucalyptus trees. I have been in Central Uganda, surrounded by Eucalyptus trees. People have just planted these things all over the world, and boy are they good at sucking water out of the soil.
And, in fact, what they’ll do is they’ll suck the water table down to where they will kill off any competitors, because they’ve just made a desert out of the upper layer of soil. Casuarina does much the same thing; and Casuarina also has the advantage that it can fix nitrogen in its root nodules, and so it can grow in places that many other things can’t. So these things spread quite well.
If you were to do this kind of section, not in a short-grass prairie, but if you were to go into the Kalahari Desert and look at how far down an Acacia tree can send its roots, it’ll go down 46 meters. Okay? Well 46 meters is over 150 feet deep, down, and it’s going to suck that water up into a tree. which is probably 20 meters high, and in the process it’s going to drive the water table down to where many other things can’t reach it anymore.
So these physiological adaptations are things that not only have consequences for the survival and the reproduction of the individual organisms, they also have consequences for everything which is living around them, and the ones that can do it better hurt the ones that can’t do it so well.
If we go into the environment of estuaries, the plants that are growing in estuaries, like these mangroves, have the problem that is basically caused by the fact that estuaries are one of the most productive ecosystems on earth. And there’s just a tremendous amount of leaf litter, and there are algae living in the water, and the leaves and the dead algae and whatnot fall down to the bottom and they start to decompose, and the bacteria that are decomposing them use up the oxygen.
And so if you take a sample down, through the mud, the soil, at the bottom of one of these mangrove estuaries, you will hit a layer that is just black. It is a very reducing environment. It’s got hydrogen sulfide, stinks like rotten eggs, and if you’re a root of a plant, living down there, you’ve got a problem, because you need oxygen.
You are a multi-cellular plant, and all of your cells have evolved in an oxygenated environment, at least in your ancestors, but now your ecology is asking you to grow in a place where in order to feed your plant, you have to probe into what is an extremely dangerous environment; it doesn’t have any oxygen in it.
And so mangroves have these morphological adaptations. Their roots stick up little siphons–okay, they have snorkels–so that the roots can suck oxygen down, from above, and get a flow of oxygen coming down that will help them out. Remember, the roots don’t have chloroplasts. They’re down in a dark environment. They can’t make their oxygen endogenously, they’ve got to get it out of the atmosphere. So this is what happens.
Chapter 5. Niches [00:43:43]
Okay, so I’ve done this fairly quickly. But the point of it is that both in plants and animals, and endotherms and ectotherms, anything you look at, any organism, from a virus and a bacterium, on up to a blue whale and a redwood, that you look at on the face of the earth, is going to be loaded with physiological and morphological adaptations, and these things are determining the range of conditions and resources under which they can survive and reproduce. So if we look at that, just as a general conceptual problem, we can summarize it in the form of an ecological niche. Okay?
So if you look at the performance of that species, with respect to some environmental variable–this could be temperature or oxygen concentration or pH–there will be a range of that environmental variable within which the organism can reproduce, there’ll be a slightly broader range within which it can grow, and there will be an even broader range within which it can survive.
So it can explore parts of the environment within which it cannot grow, and it can grow in parts of the environment within which it cannot reproduce. But there will be a core where life is easy and it can carry out its lifecycle.
For example, here’s a two-dimensional niche. This one is just measuring survival. So these are actually experimental data. This is salinity down here and temperature over here. And it’s for a sand shrimp, Crangon. And basically what this is telling you is that it has zero mortality in a salinity range of about two-thirds sea water, up to slightly over full sea water. This is full sea water right here, about 35 parts per thousand. And up here it’s showing you that it will start hitting some mortality at about 25, and some mortality at about 10. Okay?
So you could imagine carrying this process further, putting a third dimension on, putting a fourth dimension on, and having the organism tell you in what part of the potential range of conditions on the planet can it live. And there are interactions–I mean, an interaction will be any time there’s a curve in the slope here. Okay? So the range of salinities at which it has no mortality is affected, to a certain, and in this case fairly slight degree, by the range of temperatures.
So the niche is an N-dimensional hyper-volume. We just saw a two-dimensional one here, and I told you that could be extended to three, four, five, ten, however many you wanted to pack on. That is a mental tool, and it was invented by humans, actually in this building, to understand how organisms evolve to deal with environmental problems. Okay? So it’s an attempt to extract key features.
You can think of those dimensions both as abiotic and as biotic. So the abiotic ones usually are things like temperature, salinity, humidity, oxygen, carbon dioxide, pH. The biotic ones are predators, competitors, pathogens, mutualists; and the biotic ones co-evolve. Okay? So the niche of one species is going to be co-evolving with the niche of another species. So you should think of these things as changing through evolutionary time.
All the biological evolution in the world isn’t going to do very much to the distribution of temperature on the planet. So it’s not as though the biotic variables are going to be causing a co-evolutionary response in the abiotic ones; they won’t. Those things are just things that are imposed on the process.
But if you have a predator/prey interaction, or a parasite/host interaction, or two competitors dealing with each other, the area of the niche hyper-volume, within which each of them can reproduce and survive, is going to be changed by their co-evolution.
So what that means is that niches aren’t pre-existing molds, out there, into which organisms are poured. They are the products of an evolutionary play that is creating the theater while it’s writing the roles. And while the play is running, evolution is rewriting the script, it’s remodeling the actors, it’s putting in new actors, it’s redesigning the sets, and it’s renovating the theater. It’s a very long running play, it’s got a lot of characters.
So if you think of a niche as static, essentially what you’re doing is you’re just taking a snapshot out of a video, or a snapshot out of a film. Okay? They’re really dynamic things.
Okay next week, next time, we will start with population growth and the issue of what density does to population growth.
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