The teacher's online companion to Science World, providing your middle school and high school students with science news and rich informational texts that connect STEM to the Common Core
Welcome Ms. SimpkinMy Dashboard
Science World HomeNewsFood Chains and Webs
A grizzly bear catches a fish on the Brooks River in Alaska.
Plotz/iStockphoto
Food Chains and Webs
From Grolier's New Book of Knowledge
Plotz/iStockphoto
A food chain describes who eats whom. An oak tree grows leaves. A caterpillar eats some leaves. A bird called a towhee eats the caterpillar. A hawk eats the towhee. This is an example of a food chain. Every form of life is food for another.
PRODUCERS, CONSUMERS, AND DECOMPOSERS
Every food chain begins with a food producer. Oak trees and other green plants are food producers. They use energy in sunlight to make food.
Animals are consumers. They cannot make their own energy. They get energy by consuming (eating) other organisms. Some consumers eat plants. They are called herbivores (plant eaters). Caterpillars are herbivores. Some consumers eat herbivores. They are called carnivores (meat eaters). Towhees are carnivores. Some carnivores, such as hawks, eat other carnivores.
One more group needs to be added to food chains. These are the decomposers. They feed on wastes from other organisms. They also eat the remains of dead plants and animals. Decomposers include insects, worms, bacteria, and fungi. They play a very important role in a community. As they feed on dead matter, they break it down into simple chemicals. These chemicals are returned to the soil or the water. The chemicals become available to plants and algae, which use the chemicals to produce new growth.
THE WEB OF LIFE
A food chain usually has about six links. But most animals eat more than one kind of food and therefore are in more than one food chain. The food chains within a community are often connected and related. This network, or combination of food chains, is called a food web.
Scavengers and Predators. Sometimes there are special relationships between the members of a community. Some of the consumers are predators—they kill prey for food. Eagles, owls, wolves, and humans are predators. Predators are usually well suited for catching, killing, and eating their prey. They have keen senses of sight, smell, or vision. They can detect other animals from far away. Many predators can move quickly to catch a fleeing creature.
Some animals are scavengers. Scavengers eat the bodies of animals that have died or have been killed. In this way they return the materials of the dead animals to the food cycle. Vultures and jackals are scavengers.
Symbiosis. Different species in a community may live together in special relationships, called symbiosis. There are different kinds of relationships. It depends on whether organisms win, lose, or "draw."
One form of symbiosis is commensalism. In this relationship one organism gains and the other seems to neither gain nor suffer. For example, a woodpecker pecks a hole in a tree while searching for insects to eat. Then it abandons the hole. A bluebird may make a nest in the hole. The bluebird has a new home. The woodpecker has lost nothing.
Another kind of symbiosis is parasitism. It occurs when one organism—the parasite—takes nourishment from another organism—the host—and harms the host in some way. Mistletoe is a parasite plant. It grows on trees by absorbing the tree's nutrients. The tree is weakened as a result.
Mutualism is a type of symbiosis in which both organisms gain from each other. The Nile crocodile opens its mouth wide and allows a small bird—the Egyptian plover—to pick pieces of food from between its teeth. This helps both organisms. The crocodile gets its teeth cleaned while the bird gets a meal.
The opposite of symbiosis is competition. Competition can occur within one species or between different species. Individuals fight each other for food, water, light, or living space. There are many examples of competition between species. Both hawks and owls eat mice. Every mouse that a hawk eats is one less for an owl, and, of course, the reverse is also true. Cattails and duckweed plants often grow next to each other. They compete for space, light, and an adequate supply of water. Deer are examples of competition within a species. Many deer live in woodlands where humans are building houses. The deer must compete with other deer for the dwindling food supply. In general, the ones who win the competition for food and space are better equipped to find food and to fight off other hungry animals.