“Time for dinner” is a welcome sound to just about everyone. Whether it is pizza, roast chicken, a crisp apple, a cold slice of watermelon or another favorite food, most of us enjoy eating.
But eating is not just for the good tastes it brings. Eating food gives us the energy we need to live. This energy powers everything you do from playing a favorite sport to studying for a math test or goofing around with your friends. It helps you fight off illness and helps you think. It makes you grow and keeps your heart beating and the blood circulating through your body. Without energy you could not live. The same goes for every living thing on earth. Microbes, beetles, mice, hawks, flowers, salmon, crayfish, trees, elk … if it's alive, it needs energy.
So, where does all this energy come from? How does it get passed around to all living things? Energy passes from one animal to another as they eat plants or one another. This flow of energy from one living thing to another is called a “food chain or a food web”. Let's take a closer look…
To understand food chains and food webs, we must start with where the energy begins. Sunlight is energy, and plants use this energy to turn water and carbon dioxide into plant food. This process is called “photosynthesis”. Plants also need minerals and nutrients. They get these from the soil when their roots take up water. While this might not sound like the kind of food you would want to eat, this plant food allows plants to grow, flower, and produce produce things like acorns, potatoes, carrots, apples, pecans, and many other kinds of fruits.
Because plants make so much energy, they are called “producers”. Their ability to use sunlight to make food makes them a very important source of energy for other living things. Think about all the animals that eat plants. Wow, it's mind-boggling! Now, think about all the places that plants grow. From the oceans to the deserts to the mountaintops, plants can be found nearly everywhere basking in the sunlight and making their own food. And wherever plants grow, animals that depend upon them are sure to be found.
Animals that eat only plants are herbivores. In a food chain, they are called the “primary consumers”. This is because they are the first animals that eat the plants. Energy stored in the plant moves into the herbivore when it eats the plant. This energy is then used by that herbivore.
Think about you eating a fresh tomato out of your family's garden. In that case, you are the primary consumer of the tomato. Your body will use the energy from the tomato. This makes a basic food chain with two links, the tomato and you.
In the wild, a basic food chain would be plants and a bison. The bison is the primary consumer of the plants which are the producers of the energy which comes from photosynthesis which is powered by the sun. Whew! But that is often not the end of the story!
Many primary consumers are very small creatures that make great snacks for larger animals. A grasshopper eats the grass and a small mammal called a shrew eats the grasshopper. The shrew becomes the “secondary consumer” because it eats the animal that eats the plants. Other secondary consumers can enter the picture when they, in turn, eat the first secondary consumer, in this case the shrew.
These food chains can have many links. Check out this food chain:
The plant is the producer and the grasshopper is the primary consumer. All the other animals are secondary consumers. As the grasshopper eats the plant and the other animals eat one another, energy is passed along the food chain. This energy helps each animal survive until it is eaten by another animal or it dies. Both primary and secondary consumers also feed their babies, passing along energy to their growing youngsters.
At the top of the food chain is the tertiary consumer. They are the carnivores that eat the carnivores. Often, they do not have predators. Humans fall into this category, but so do many other large animals such as lions, bears, and sharks. There are times of course, where a tertiary consumer does fall victim to another predator.
As you can see from our food chain, one consumer can be eaten by another. Food chains only tend to be short if the primary consumer is a very large animal with few predators. Large herbivores such as moose, elephants, bison, or giraffes have few predators because they are so large and powerful. Their food chains are short. But what happens to the energy stored in their large bodies when they die? Enter the scavengers and decomposers!
The Clean-Up Crew: Scavengers and Decomposers
Not all plants and animals get eaten by consumers. Some are lucky enough to live out their lives and die of old age. Others get sick or suffer an accident that kills them. What is left is the dead plant or animal filled with the energy from the food it ate or made. So, what happens to that energy? Nothing in nature is wasted and that includes dead stuff.
When an animal dies, its body begins to break down. In other words, it rots, getting stinky, gross and pretty disgusting. But all that yucky stuff is just another link in the food chain as energy is passed along to others that can use it.
The first members of the clean-up crew are the scavengers. They are attracted to a dead animal by the smell, and by seeing other scavengers. A familiar scavenger is the turkey vulture. These fascinating birds are very well adapted to their jobs, using their excellent sense of smell to find their food. And smell is a big hint that something dead is lying around! Scavengers you may have seen include many familiar animals such as coyote, crow, magpie, fox, bald eagle, bears, raven, and even small songbirds such as chickadees.
As the scavengers tear apart a dead animal, the decomposers begin their work. Insects are very important decomposers. Fly maggots, ants, beetles, moths and others continue to break down the dead animal. They eat it, lay their eggs on it (which hatch into larvae that eat it), or chew through it. Bacteria and fungi continue the process of decomposition until nothing is left, and all the energy that was once in the animal is used up by other living things. This all sounds pretty disgusting, but think about a world with no scavengers or decomposers to clean up dead things. Yuck!!
The scavengers and decomposers help move energy through the food chain. Bacteria and fungi return it to the soil where plants can once again use it. (See the picture below.) Consumers use it as they eat the plants or the scavengers and decomposers. Many food chains are created and these food chains become a food web with living things all dependent upon each other for survival.
So What is a Food Web?
A food web is just another layer of the entire process. A food chain is a simplified version of what really happens in nature. Very few animals just eat one food. A food web is nature's way of making sure that there is food to go around. Let's use the simple food chain we used earlier. Imagine if a plant disease caused the one plant eaten by the grasshopper to die out. There would be no food for grasshoppers and they would eventually die.
The secondary consumers would have no food either. Spiders would die for lack of food, causing the shrews to also go hungry. You can imagine how that would affect the weasel, the red-tailed hawk and the great-horned owl. The scavengers and decomposers would be the only ones who would feast!!! But just for a short time – after that there might not be any animals left at all. A food web protects the balance of food and consumers by spreading the food sources and the eaters around.
Here is a possible food web for our sample group of living creatures. The arrows point in the direction of the energy flow; from source to consumer.
One thing that our sample does not show is that the grasshopper does not eat just one plant, but eats from many different plants. Take a look at this very simple comparison between another food chain and a food web here.
You and the Food Chain
Where do you fit into the food chain? Because you are an omnivore and eat both plants and animals, you are a primary and a secondary consumer. In most cases you are also the tertiary consumer. The carrot you ate at lunch makes you a primary consumer. But the meatloaf you ate for dinner makes you a secondary consumer. Fortunately, unless you live next door to a family of cannibals, you will probably not have to worry about another secondary consumer trying to eat you! But even though people are mostly secondary consumers, we still cannot escape the decomposers. When someone gets very old and dies their energy is eventually returned to the soil where it will be taken up by plants to begin a new food chain and continue the circle of life.
Build a Food Web
Here is a fun activity to try with your class, scout troop, or youth group. Get a large ball of yarn and have everyone make a large circle. One student holds the free end of the yarn. This student represents the sun. The “sun” tosses the ball of yarn to another student, but keeps hold of the free end. That student tells the group what part of the food chain they will be (“I am a plant or I am a hawk, etc…). They should unravel some of the yarn, hold on to it and toss the yarn ball to another student who also announces what part of the food chain they will be. Remember to hold on to part of the yarn before you toss the yarn ball. Continue tossing the yarn ball until every student is holding part of the yarn. One student tells the group what they are and begins to tug on the yarn. As other students feel the tug, they should tug back. What happens? What does this tell you about food webs?