EEB 122: Principles of Evolution, Ecology and Behavior

Lecture?25. Interactions with the Physical Environment

https://oyc.yale.edu/ecology-and-evolutionary-biology/eeb-122/lecture-25

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.?

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?in his book?A Study of Experimental Medicine.

• L.J. Henderson?introduced the idea of?the fitness of the environment?in his book.?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."

• 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.?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.

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.?

The outline of the lecture is going to be a bit about temperature and about thermal regulation.?

• 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.?

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.?

• 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, it can do things like having its back facing east or its back facing west, depending upon whether it's morning or afternoon. 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.

• 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?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.?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? This is the body temperature of a model endotherm, and this is its heat production here.

• 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 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 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.

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.?

• Weddell seal or a Leopard seal,?a humpback whale or a blue whale,?Bears and people?have subcutaneous fat that serves as an insulator.

• Shivering and huddling; well you know all about shivering. Huddling is something, for example, emperor penguins?huddle and get into a big circle to?form?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.

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.?

• In concurrent flow you would have venous blood going in this direction; and then running right next to the vein you've got an artery, going in the same?direction.?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.

• 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 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.

• 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.?

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?you look into the?body of a?chipmunk, you will?find that there are specific places where it has deposits of brown fat. And this is what brown fat looks like under a microscope, as opposed to?white fat.

• 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.

• 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. When chipmuck is?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.

• What's going on in one bout of hibernation is that?they drop their metabolic rate and??body temperature, it can get down to maybe 3 or 4 degrees. And then they will arouse, eat and then do it again.?This is regulated actually both by physiology,?behavior, and?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.?

Why do really small things not hibernate? What's their problem with surface area?and volume?

• 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.

• But 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.

• What about something that's big? Bears don't hibernate, they sleep. Hibernation is a condition where you really drop your body temperature a lot. 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. 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.?

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. 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.

So that's another illustration of physiological ecology. Let's now go to plants, and think about water in the soil.?

• Plants need sunlight, they need water, they need carbon dioxide.?They're going to get their water out of the soil, and they're going to do it with roots.?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.

• 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?in terms of bars. 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.

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.?

• 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.

• The transpirational pull is being caused by the water that evaporates inside the leaves. 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.?

• This is back to L.J. Henderson; water has these amazing properties, in this case,?amazing surface tension. Water can climb up the edge of a glass. And that's caused by the hydrogen bonds between the water molecules. They have this?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. The combined force?generated by billions of these things is strong enough?to lift water from the roots up 100 meters.?

You have to build a heck of a straw to accomplish that,?and that's what xylem is. 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.?

• 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.

• In the long-grass prairie,?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.?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. 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.?

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.?

• 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.

• So mangroves have these morphological adaptations. Their roots stick up little siphons or?snorkels?so that the roots can suck oxygen down?from above, and get a flow of oxygen coming down that will help them out. 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.

Physiological and morphological adaptations?are determining the range of conditions and resources under which they can survive and reproduce. We can summarize it in the form of an ecological niche.?

• 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.?

Here's a two-dimensional niche. 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.?

So the niche is an N-dimensional hyper-volume, which?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.?

• You can think of those dimensions both as abiotic and as biotic. 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. So the niche of one species is going to be co-evolving with the niche of another species.

• All the biological evolution in the world isn't going to do very much to the distribution of temperature on the planet. So biotic variables are not going to be causing a co-evolutionary response in the abiotic ones.?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.

What that means is that niches aren't pre-existing molds?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. They're really dynamic things.?