Unit 8 · Ecosystems and the Flow of Energy

Look at a tree in a park. It is huge. Its trunk is heavier than a car. Where did all that wood come from? For hundreds of years, most people gave the same answer. The tree ate the soil. Roots pull food up from the dirt, they said, and the dirt turns into wood.

It sounds right. Roots do go down into the soil. Plants die in bad soil. But a good idea is not proof. Science asks for a test. A test is a way to check an idea against what really happens. Jan van Helmont ran one of the first careful tests on plants.

His plan was simple. Weigh the soil. Weigh the tree. Wait. Weigh them both again. If the tree ate soil, the soil would lose as much weight as the tree gained. He needed a balance, a tool that compares weights, and a lot of patience.

Here are the numbers. The young willow weighed 5 pounds. After five years it weighed about 169 pounds. So the tree gained about 164 pounds. The dry soil started at 200 pounds. After five years it was lighter by only about two ounces. Two ounces is about the weight of a golf ball.

Compare the two numbers. The tree gained more than 2,600 ounces. The soil lost 2 ounces. Those numbers do not match. If the tree had eaten the soil, the pot would be almost empty. It was still full. The old idea failed the test.

Van Helmont then made a claim. A claim is what you say the evidence shows. His claim was that the tree was made of water. Water was the only thing he had added. It was a reasonable claim. But he had not tested the air. That was the gap in his thinking, the thing he did not check.

Air seems like nothing. You cannot see it. You walk through it without feeling it. But blow up a balloon. Now the balloon is bigger and a little heavier. Something went inside. Air is made of tiny bits called gases. A gas spreads out to fill any space. Air has weight, just not much.

One of the gases in air is carbon dioxide. You breathe some out with every breath. A candle makes some as it burns. There is only a little of it in air, less than one part in a thousand. But plants need it. They pull it in through tiny openings in their leaves.

This is the missing piece in van Helmont’s story. His willow was pulling carbon dioxide from the air, day after day, for five years. Carbon is what wood is mostly made of. Burn a log and you get black charcoal. That black stuff is carbon. It came from the air, one invisible bit at a time.

Put a plant in a closet for two weeks. The new leaves come out pale and thin. The plant leans toward the crack of light under the door. Now put it back in a sunny window. In a few days the leaves are green again. Light is not just nice for a plant. Light is fuel.

Leaves are green because they are full of chlorophyll. Chlorophyll is a green substance that catches light. Think of a leaf as a flat solar panel. It is wide and thin so it can soak up as much light as it can. A big tree has thousands of these panels.

Light carries energy. Energy is what makes things happen. It can move things, heat things and change things. Sunlight can warm a sidewalk. In a leaf, sunlight does something better. It powers a change that builds food.

Here is the recipe that van Helmont could not see. A leaf takes in carbon dioxide from the air. Roots bring water up from the soil. Chlorophyll catches sunlight. Then the leaf uses that light energy to join the water and carbon dioxide into sugar. This process is called photosynthesis, which means making with light.

Sugar is food. It is a small package of stored energy. The plant uses some right away and stores the rest. Wood, leaves, roots and seeds are all built from that sugar. So a tree really is built from air and water, with sunlight as the power.

Photosynthesis makes something extra. When the leaf joins water and carbon dioxide, it has leftover oxygen. Oxygen is the gas you and every animal need to breathe. The leaf releases it into the air. Every breath you take has oxygen that came from a plant.

A plant makes sugar all day when the sun is out. At night it makes none. So the plant stores extra sugar for later. It packs many sugars together into starch. Starch is like a stack of energy blocks. A potato is a starch store. So is a grain of rice, a kernel of corn and a bean.

You can find starch with a drop of iodine. Iodine turns a dark blue-black when it touches starch. Put a drop on a slice of potato and it goes dark. Put a drop on a leaf that sat in the light and it goes dark too. Cover half a leaf with foil for a day and only the lit half turns dark. That test was worked out in the 1860s by a German scientist, Julius Sachs.

The stored sugar is the whole reason a farm works. Corn stores energy in its kernels. Wheat stores it in seeds that become flour. Sugar beets store it in their roots. When you eat bread or corn, you are eating sunlight that a plant caught and put away.

A cow stands in the sun all day, but it does not turn green. It cannot make sugar from light. No animal can. So a cow eats grass. The grass made sugar and starch from sunlight, and the cow takes that stored energy. Every bite of grass is a bite of stored sunshine.

A hawk does not eat grass. It eats a mouse. But the mouse ate seeds, and the seeds came from a plant that used the sun. So the hawk’s energy still came from a plant. Trace any animal’s food back far enough and you always reach a plant, and behind the plant, the sun.

Food is a fuel, something that stores energy for later use. Different foods store different amounts. Fats store the most energy for their size. Sugars and starches store less, but they are quick to use. A running dog, a flying bird and a thinking student all burn food for energy.

You bite an apple. Your teeth crush it. Your spit starts to break it down. You swallow, and the food slides down a tube into your stomach. The stomach squeezes and mixes it with strong juices. After a few hours, the apple is a soupy mush. This whole breaking-down is called digestion.

The mush moves into a long, coiled tube called the small intestine. Here the pieces get very small. Now they are small enough to pass through the tube’s wall and into your blood. Blood is the delivery truck of the body. It carries the tiny food bits to your muscles, your brain and your skin.

The parts your body cannot use keep going and leave as waste. So food does not stay in your stomach. It is taken apart, sorted and delivered. A sugar bit from your apple can reach a muscle in your leg and be burned for energy within a few hours.

A scraped knee heals in about a week. New skin fills in. Where did that new skin come from? Your body built it from food. Food is not only fuel. It is also building material. The protein in eggs, beans, fish and meat is taken apart and rebuilt as muscle, skin and hair.

Kids grow. A ten-year-old is taller than she was at eight. That extra bone and muscle came from food, the same way a tree’s extra wood came from air and water. Nothing in your body appeared from nowhere. Every bit was eaten first.

Some foods bring small helpers. A vitamin is one kind. A mineral is another. Your body needs only a little of them, but it cannot do without them. Calcium, a mineral in milk and greens, helps build bone. Vitamin C, found in oranges and peppers, helps skin heal. A body that gets only sugar has energy but nothing to build with.

The United States government draws a picture of a healthy meal. It is a plate cut into four parts, with a small cup beside it. One part is fruits. One part is vegetables. Together those two fill half the plate. One part is grain foods, such as bread, rice, corn or oats. One part is protein. The cup is dairy, such as milk or yogurt.

Each group does a job. Grains bring starch for energy. Protein foods bring building material. Fruits and vegetables bring vitamins, minerals and fiber. Dairy brings calcium for bones. No single food does everything. That is why the plate has parts.

A plate like this is a guide, not a rule for every bite. A taco can cover all five groups. So can a bowl of rice with beans, vegetables and a glass of milk. Foods from many countries fit. The point is the mix, not one right meal.

Think of your body as a wood stove. Food goes in. Energy comes out as movement, heat and thinking. If you put in a little more than you burn, the body stores the extra as fat. That is useful. It is a reserve for a cold day or a long hike. Too much stored for too long can strain the heart and bones.

Moving uses energy. Sitting still uses a little. Running uses a lot. A student who plays outside after school burns more than one who sits. Neither is bad. The trick is to balance what goes in with what goes out over days and weeks, not every single meal.

Now step back and see the whole path. Sunlight hit a corn leaf in an Illinois field. The leaf made sugar and stored it as starch in a kernel. The kernel became cornbread. You ate it, digested it, and your blood carried it to your legs. Then you ran across a playground. That run was sunlight, released.

Every living thing needs energy. Plants make their own food from sunlight, water and carbon dioxide. Because they make food, plants are called producers. Each plant is a producer. Grass, oak trees, cattails and the green algae on a pond are all producers. Almost every bit of food on Earth starts with one of them.

Animals cannot make food. They must take it. So animals are called consumers. A vole that eats grass is a consumer. An owl that eats the vole is a consumer too. Some consumers eat only plants. Some eat only animals. Some, like people and raccoons, eat both.

There is a third group that is easy to forget. Mushrooms, molds, bacteria and earthworms break down dead things. Each of these is a decomposer. Without them, dead leaves and dead animals would pile up forever. With them, the pieces go back into the soil, water and air, ready for producers to use again.

A food chain is a line that shows who eats whom. Start with the sun. Then grass. Then a vole that eats the grass. Then a barn owl that eats the vole. Each arrow means energy moves this way. Sun to grass to vole to owl.

Here is the surprise. Most of the energy does not make it up the chain. The vole uses most of the grass energy just to stay warm, run and breathe. Only a small share becomes vole body. A rough rule is that about one tenth of the energy passes to the next eater. The rest is used up or given off as heat.

That rule explains a pattern you can see. A field has tons of grass, thousands of voles, and only a few owls. There is not enough energy left at the top to feed many. It also explains why food chains are short. After four or five steps, almost nothing is left to pass on.

A single food chain is too simple. The barn owl does not eat only voles. It eats mice, shrews and small birds too. The vole is eaten by owls, but also by foxes, hawks and snakes. Draw all those arrows and you get a tangle. That tangle is a food web.

A food web is stronger than a chain. If voles have a bad year, the owl can hunt more mice. If one plant dies off, most eaters have another. The more links in the web, the harder it is to break. That is one reason a habitat with many kinds of living things is a healthy one.

Picture a creek bank covered with plants, with birds in the bushes, insects in the air and worms in the damp soil. Every living thing there depends on others. Pull one thread and others move. A food web is a map of those threads.

Stand at the edge of a pond in June. The water looks green. That green is algae, tiny plants too small to see one at a time. Algae float in the sunlight and make food. Cattails and water lilies grow at the edge. Those are the pond’s producers.

Now look closer. A snail scrapes algae off a rock. A tadpole nibbles on it too. A dragonfly nymph, the young form of a dragonfly, hunts the tadpole under the water. A bluegill eats the nymph. A great blue heron stands still on one leg, then stabs the bluegill.

That is one chain, but the web is much larger. Ducks eat the water plants. Frogs eat flies. Turtles eat almost anything. Under the mud, bacteria and worms break down whatever dies and sinks. The pond feeds itself, as long as the sun keeps shining on the algae.

Illinois is called the Prairie State. Before farms, most of it was prairie, a sea of grass with almost no trees. Some grasses grew taller than a person. Their roots went down deeper than the plants stood tall. Those roots built the deep black soil that makes Illinois farmland so rich today.

The producers are the grasses and wildflowers. Grasshoppers, caterpillars and beetles eat them. Meadowlarks and sparrows eat the insects. Voles and mice eat seeds. Foxes, coyotes, hawks and owls eat the voles. Long ago, bison grazed here too, eating tons of grass and leaving droppings that fed the soil.

Fire is part of this web. Lightning fires swept the prairie every few years. Grasses survived because their growing parts are safe underground. Trees did not. Without fire, trees creep in and the prairie turns to forest. Today, people set careful fires to keep the last prairies alive.

Walk into an oak woods in October. Acorns crunch under your feet. An acorn is a seed packed with starch. Squirrels, deer, wild turkeys, blue jays and white-footed mice all eat acorns. In a year with many acorns, the mouse numbers climb. The next year, the owls and foxes that eat mice do well too.

Above your head the oak leaves are turning brown. Soon they fall. A forest drops a thick blanket of leaves every autumn. If nothing broke them down, the forest would be buried in a few decades. Instead, fungi, bacteria, worms and tiny insects eat the leaves. By next summer, the old leaves are crumbly dark humus.

That humus is rich soil. The oak’s roots take up water and minerals from it. So the oak feeds the mice. The mice feed the owl. The owl’s pellet falls to the ground, and decomposers turn it back into soil. On the forest floor the web closes into a cycle. A cycle is a loop that goes around and around.

Leave a slice of bread on the counter for two weeks. Fuzzy blue-green spots appear. That is mold, a kind of fungus. It is eating the bread. Molds, mushrooms and bacteria are the great recyclers. They take dead things apart, bit by bit, and use the pieces.

Earthworms help. They pull dead leaves into the soil and eat them. What comes out the other end is rich, crumbly soil. Over many years, worms slowly move the whole top layer of soil through their bodies.

As decomposers work, they breathe out carbon dioxide, the same gas you breathe out. They also leave behind minerals in the soil. So the carbon in a dead leaf goes back to the air. The minerals go back to the ground. A plant can then use both to grow a new leaf.

Follow one bit of carbon. It starts in the air as carbon dioxide. An oak leaf takes it in and makes sugar. The sugar becomes an acorn. A mouse eats the acorn. Now the carbon is mouse muscle. An owl eats the mouse. Now it is owl. The owl breathes out, and the carbon goes back to the air as carbon dioxide.

Or the owl dies. Decomposers eat the body and breathe the carbon out. Either way, the carbon ends up in the air again. Then another leaf takes it in. The same bit of carbon can go around this loop for thousands of years. It is never used up. It just changes what it is part of.

Water goes around too. Rain soaks the soil, roots pull it up, leaves let it out into the air, and it falls again as rain. Minerals go from soil to plant to animal to decomposer to soil. This is the cycle of matter. Energy flows through a habitat and is lost as heat, but matter stays and goes round.

Put the pieces together and you get an ecosystem. That means all the living things in a place. It also means the nonliving things they need: sunlight, water, air and soil. A pond is an ecosystem. So is a prairie, a forest, or even the strip of weeds in a cracked parking lot.

A healthy ecosystem is in balance. Producers make enough food. Consumers eat but do not wipe out their food. Decomposers keep up with what dies. Numbers go up and down from year to year, but they stay within limits. No one kind of living thing takes over.

Healthy also means many kinds. A prairie with a hundred kinds of plants can lose a few in a drought. It keeps going. A field with one kind of grass can be wiped out by one disease. Scientists count the kinds of living things in a place. That count is one sign of health.

Pull one thread and the web moves. In Yellowstone National Park, people killed off the wolves by the 1920s. With no wolves, elk numbers grew. The elk ate young willow and aspen trees down to stubs along the streams. Beavers, which need willows, became scarce.

In 1995, wolves were brought back. Elk became fewer and more careful. In some places young willows grew tall again, and beavers returned. Scientists are still studying how much of the change came from the wolves and how much from weather and other causes. But the lesson is clear. Removing one link changed many.

It works the other way too. Add a new living thing and the web shifts. Zebra mussels arrived in the Great Lakes in the late 1980s on ships. They now cover rocks and pipes in Lake Michigan by the billions. They filter out tiny algae that young fish once ate. A new link can be as big a shock as a missing one.

In the 1960s the bald eagle was almost gone from the lower 48 states. Only about 400 nesting pairs were left. Scientists traced the cause to a bug-killing chemical called DDT. It washed into water, built up in fish, and built up even more in the eagles that ate the fish. It made their eggshells so thin that they cracked in the nest.

The United States banned DDT in 1972 and protected the eagle by law. Slowly the numbers climbed. By 2007 there were nearly 10,000 pairs. That year the eagle was taken off the endangered list. Today thousands of eagles spend the winter along the Mississippi and Illinois rivers. People gather on the bluffs to watch them fish.

People protect habitats in many ways. In Illinois, volunteers plant prairie seeds and set careful fires to keep prairies alive. Bison graze again at Nachusa Grasslands and at Midewin, near Joliet. Peregrine falcons nest on Chicago skyscrapers after nearly vanishing. Each is a web being repaired one link at a time.