The sun rises over a woodland, flooding the vast cathedral of the forest with light. It's spring, and you can almost see the bare branches stiffening and reaching hungrily for the sun's warmth. As the northern hemisphere tilts sunward and the angle of light falling through the branches to the forest floor becomes more direct each day, the forest stirs and wakes from its long sleep. A fantastic chain of events is about to begin, and it all starts with the phenomenal energy of the sun.
That energy, flung outward into space from a nuclear furnace of unimaginable size and intensity, has travelled more than ninety million miles to reach Earth in the form of heat, light and ultraviolet radiation. Although most of it is either deflected by the Earth's magnetic field, reflected back into space by high-altitude clouds, or converted into ambient heat when it hits the upper atmosphere, enough of it reaches the earth's surface to warm it directly. A seed that has spent years slumbering under the leaf litter now uses that heat energy to begin tapping into its own tiny powerhouse of stored protein and carbohydrate energy. A small shoot sprints to the surface, where it unfolds green leaves mottled with red.
Now the young plant can tap into the energy source it truly desires: sunlight. Most of the sun's energy that reaches the plant reflects off its leaves as coloured light (green mottled with red) or is converted into ambient heat as the plant works to combine solar energy with carbon dioxide, water, and soil in order to make a small store of protein and carbohydrate energy for itself; nevertheless it gathers enough to send up a slender stem and unfold a dainty yellow flower. It is a trout lily, so called for the reddish speckling of its leaves, and it belongs to the class of wildflowers known as spring ephemerals. This means that after a few short weeks it completes its life cycle and drops tiny seeds to the forest floor.
One seed is dragged away by foraging ants, who eat a fleshy bit attached to one end and leave the rest to slumber and await another spring. The ants, like all insects, run on such a lean energy budget that they're sensitive to changes in the amount of ambient heat energy in their environment, and on this warm day they're moving faster than they did earlier in spring. As they scurry this way and that, foraging above ground and distributing food to hatching larvae underground, they convert most of the energy they receive from the seed casing into more ambient heat. But by eating enough seed casings they're able to build up and maintain their bodies with energy-rich proteins and carbohydrates.
Other forest dwellers are well aware of this fact. One ant, venturing up a nearby tree and onto a branch during a subsequent foraging mission, is picked off and eaten by a black-capped chickadee. Although she's many times bigger than the ant, and enjoys the benefits of a circulatory system that distributes heat and nutrient energy from the insects she consumes throughout her body, she too lives close to the edge in terms of energy requirements. If she were to be startled from her roost on a midwinter's night, there's a good chance she wouldn't live to see the morning. But on this spring day she's busily converting most of the energy from her meal into ambient heat as she flutters back and forth between feeding and checking on her clutch of eggs, each of which contains a substantial chunk of her own energy-rich proteins and carbohydrates.
Those proteins and carbohydrates make her eggs tempting targets for a passing blue jay, who spies the chickadee pair bustling in and out of the tree cavity where their eggs are hidden. He perches nearby and performs his best imitation of a red-tailed hawk's harsh keeeerr. As other forest birds dive for cover, the jay swoops in, nabs a brown-speckled egg, and gulps down the energy-rich meal with gusto. Most of that energy will be converted to ambient heat by his muscles, which are strong enough to lift him into the air and power him from one perch to another as he patrols his family's territory.
That is, that's what the egg's energy would have gone toward, but as the jay finishes eating, just out of sight of his protective family group, he fails to notice the red-tailed hawk that has appeared on the scene to investigate what sounded like a hostile male intruding on his territory. The red-tail nabs the jay in turn, and carries off to his nest the energy of both egg and egg-thief in one neat package. Most of it will be converted into ambient heat by his hardworking muscles, of course, but for now he enjoys the fullness of his belly as he rests, gazing out over his territory. The late afternoon sunlight is slanting through the trees and across the meadow that is his kingdom. He's an old widower, and he knows that this meal may well be one of his last. A younger and more vigorous male has been pushing into his territory lately and claiming a greater and greater share of the energy the meadow can provide.
Not long afterward the younger hawk succeeds in pushing the older male out. The deposed monarch lives a few more hungry days before the supply of protein and carbohydrate energy in his body runs too short to sustain him. He dies on the forest floor, where the decomposers- insects, worms, fungi, and bacteria too numerous to name- get to work on him almost right away. From their perspective there's still a lot of energy in the dead body of the hawk, though most of it will be converted into ambient heat as the decomposers gassily digest the hawk's tissues and engage in their own minute struggles for survival.
At long last the light energy that began this convoluted story has been converted almost entirely into heat energy, too diffuse to do anything but raise the temperature of the woods by a tiny fraction of a degree, while the matter that has carried that energy has undergone transformation after transformation before coming full circle as soil, water, and carbon dioxide. That, my friends, is life: one of the greatest stories ever told, and one far more rich and complex than my simple retelling can do justice to. What's more, it's playing out all around us as I write these words and as you read them.
There are two morals I'd like to draw your attention to before wrapping up this post. The first is that matter moves in circles but energy moves in straight lines. That is, matter can be re-arranged and transformed again and again (from soil to living tissue and back to soil indefinitely), while energy moves in one direction only: from highly concentrated and usable forms, through various transformations, toward its ultimate end as
diffuse heat. You can picture it as an energy cascade, splashing downward from the sun at the top through to the hawk and the decomposers near the bottom. Just as waterfalls never, ever flow uphill, diffuse energy can never, ever be made useful again without downgrading even more useful energy in the conversion process. Science knows this principle as the second law of thermodynamics.
You can picture the way the second law works a different way by thinking back to Jonathan Erb and his mill. The stream flowing downhill toward the mill has lots of useful energy. The waterwheel that turns the millstone or electric generator is tapping into the stream's energetic flow before it slows down, broadens, and reaches the sea, which has loads of energy but practically none that is useful. Luckily for the Erbs, the stream is replenished by energetic weather systems that do the work of carrying water and useful energy uphill in the form of rain. If you imagine Jonathan trying to do that work himself, say with a pair of buckets, you can easily see why it's laughable to try to work against the second law: he would spend more energy carrying water uphill than he would gain from its flowing back downhill. Even vast, highly energetic weather systems are subject to the second law, since they receive heat energy from the sun, whirl it about the planet, and slowly exhaust themselves as the concentrated heat that fuels them diffuses or leaks back into outer space. Using the metaphor of running water we can phrase the first moral of the story succinctly, if somewhat fancifully: energy flows downhill.
The second moral is that each conversion step in energy's downward journey has a cost to be paid in diffuse heat. You can't get more energy out of less, only less out of more. You can't even get out an equivalent amount, because, as I tried to make clear in my story, acquiring useful energy always
costs useful energy, even for plants standing still and soaking in
sunlight. That's why a forest ecosystem always contains more plants than insects, more insects than small birds, and more small birds than predators. The species we typically think of as being at the top of the food chain are in one sense bottom feeders; that is, in terms of the energy cascade. Being a carnivore is tough because there's not enough useful energy at the bottom of the cascade to allow you to make mistakes without serious consequences, i.e. a net energy loss after hunting, i.e. starvation. The second moral of the story is as harsh as it is true: nothing comes for free.
Energy flows downhill and it's never free. Got that? Keeping these two principles in mind, we can move on to looking at energy from a very different perspective next week. This time the story's going to involve humans, lots of humans, doing some very interesting things with energy.