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How does energy flow through a biological community?
This section introduces you to the movement of energy throughout a biological community. You will explore the different ways organisms get their energy.
“At the junction of every pair of threads in this net of life there is a crystal bead, and each crystal bead is a living thing, shining forth with its own glow, its own radiance into space. And the glow of every crystal bead in the net of life reflects the glow of every other bead.”
Steve Van Matre
The Earth Speaks
Energy moves through biological communities, keeping the organisms alive and functioning. But keep this in mind: An energy pathway isn’t 100 percent efficient. Little bits of energy are lost along the way.
Single food chains, such as the ones described in Food Chains: How Energy Gets to You, show only one type of path that energy can take from the sun to an organism. There are many different types of paths energy can follow. Think about the food chain shown in figure below.
You eat many other things besides eggs for breakfast, lunch, and dinner. And free-range chickens eat more than just the occasional grasshopper that jumps into the barnyard. You get energy from many different sources, and so does the chicken. By putting food chains together, you create a food web. A food web shows the important links between connected food chains. Food webs show a more accurate picture of how energy is passed around in the real world than one food chain does.
What does the word community mean to you? Many people think of a community as all the people they see on a daily or weekly basis. These people might be family, neighbors, friends, teachers, mail carriers, police officers, and other people who live or work in a certain area. As a matter of fact, the human community does consist of all of the people who live around you and help you live where you do.
However, ecologists think of a community in a slightly different way. They recognize that each organism lives in a community that includes all of the other organisms with which it interacts. For example, a frog’s community includes most of the organisms that live and grow in and around the frog’s pond. The frog’s community includes the algae that grow on the bottom of the pond, because algae are eaten by the snails and insects that the frog eats. The frog’s community might include a lily pad on which the frog rests. It might even include a great blue heron, which could eat the frog if it’s not careful. The frog’s community would also include all of the other organisms that the snail, insects, lily pad, and blue heron need to stay alive.
Food Webs Can Be Complicated
A food web describes how energy flows between members of a community. Tracing the specific path of one food chain can be pretty simple. For example, the person-chicken-grasshopper-grass-sun food chain is very easy to follow. However, tracing the many paths through a food web is a little more complicated. The food web connects and interconnects all of the possible food chains in a community. For example, think carefully about the chicken in our food chain example. In a food web, me chicken will eat more than an occasional grasshopper. The chicken will eat feed corn and maybe a worm brought up by a rainstorm. A fox, which has been circling the barnyard, might break in and eat the chicken. Remember the grasshopper? What else do you think it might eat besides the grass described in our simple food chain? The grasshopper might eat some of the feed corn, also eaten by the chicken, or some of the wheat grain spread in the nesting area. So as you can see, food webs can get complicated.
Food webs in the wild are usually more complicated than our barnyard example. An ecologist named R. D. Bird described a real food web of a willow forest that he studied in central Canada. Even though this food web was fairly simple, it still included several different kinds of willow trees, six different kinds of birds, various spiders, many insects, frogs, snails, and garter snakes. The illustration below is a simplified version of the web that he described. The arrows show the direction that energy flows within the system.
Did You Know?
People who eat no meat, but do consume animal products such as milk and eggs are usually known as vegetarians. People whose diets consist only of plants with no animal products are called vegans.
The beetles and insects eat willow leaves in this willow forest. Now locate the frogs. Where do the frogs get their energy? What do they eat? Which organism gets energy from the frogs? After studying this food web for a little while, it sure looks like a “snake-eats-frog world”! One source of energy not shown in this food web is where the willow trees get their energy. You probably remember that the source of energy for almost all plants, including willow trees is-the sun!
Participants in a Food Web
In general, ecologists can divide organisms into two main groups on the basis of how they obtain energy. Ecologists call organisms that get their energy from the sun producers because they produce sugars that other organisms can eat to move, grow, and reproduce. Most producers are green plants because they capture the sun’s energy through photosynthesis.
Organisms that get their energy by eating other organisms are called consumers. They consume the energy that is produced by other living things. Consumers can also be divided into several groups. Consumers, such as cows and rabbits, which eat only plants, are called herbivores (HURB-ih-vors). Some humans choose to be herbivores. Consumers, such as bald eagles or wildcats, which eat only other animals, are called carnivores (KARN-ih-vors). Consumers, such as blue jays and most humans, which eat both plants and animals, are called omnivores (OM-nih-vors). Decomposers are the other major group of consumers. Decomposers get the energy they need by eating the remains of organisms that are already dead. Decomposers include such organisms as worms, some snails, mushrooms, dung beetles, and vultures.
As energy flows through a community, it changes form. Whenever energy changes form, some energy is lost. For example, when a light bulb changes electricity into light, the light bulb gets hot. The energy converted to heat is not converted to useful light, so it is considered lost. In the same way, a car engine changes gasoline into motion, but some of the energy in the gasoline is lost as heat. Even you lose energy. Your body changes a bowl of cereal into energy you can use to study or play. However, part of the energy in the cereal is used to keep you warm, to help you digest, and to do a lot of other chores your body does to maintain itself. Even though digesting and staying warm are important, scientists call this a loss of energy.
It’s important to scientists to observe energy flow in biological communities. So ecologists study how energy flows and is lost in biological communities. In one study, a group of ecologists counted all of the producers, herbivores, and carnivores in a field of bluegrass. They found that 5,842,424 weeds and blades of grass fed 708,624, herbivores such as grasshoppers. These herbivores fed 354,904 carnivores such as spiders, ants, and beetles. The 354,904 carnivores fed the three top carnivores, such as birds and moles. The food web these ecologists studied and described forms an ecological pyramid. What’s an ecological pyramid? First, what’s a pyramid? It’s a structure in which each level is made up of fewer stones than the level below. In a similar way, each layer in an ecological pyramid has fewer organisms than the level below it. In an ecological pyramid, many producers feed a few herbivores, which feed even fewer carnivores.