Food Chains and Webs

by: Tanya Travis

A food chain shows how organisms get food and energy.

A food web is all the food chains in an ecosystem.

What is a pond ecosystem?

A pond seems quiet but if you look more closely you will see a lot of activity.  Many different living things, or organisms, use or live in ponds.  The pond is their habitat.

Many species, or kinds of plants and animals, can be seen in a pond.  Deer drink the water.  Cattails, grasses, and other plants grow along the shore.  Racoons search for food, and turtles sun themselves.  Lily pads and beetles float on the water.  Schools of fish swim in the water.

Many different types of organisms are in a ponds population.  A population is all the members of one species that live in the same area.  For example, all the cattails in a pond form a population of cattails.  Together all the populations make up the pond community.

A pond gets heat and light energy from the sun.  These are nonliving things.  Acting with each other, all the nonliving and living things form a pond ecosystem.  Other kinds of ecosystems are forests, deserts, saltwater, and grasslands.

Living Things Interact

Organisms in ecosystems interact in many ways.  A snake interacts with a mouse when it chases and catches it.  A vine interacts with a tree when it grows and winds around it.

All animals need food to stay alive.  Some animals eat other animals.  Snakes eat mice.  The snake is the predator.  A predator is an animal that kills and eats other animals.  The mouse is the snake's prey.  The prey is the animal that is eaten. 

Some animals have special body parts or behaviors that help them survive in their environments.  A rabbit's long ears help them hear and their long legs help them run fast.  These adaptations help it to survive.  Owls have big eyes that let in a lot of light to help them see in the dark and find food at night.

Another adaptation that keeps animals safe from predators is camouflage.  Animals with camouflage blend into their environment.  The animal's skin might have a pattern that looks like bark, leaves, or rocks.  The spots on a fawn's brown back are a kind of camouflage.  They look like spots of sunlight between leaves and branches.  These spots help the fawn hide in the forest.

Mimicry is another adaptation that can help prey.  Their shape and color markings make them look a lot like a different organism or object.  One kind of fly looks like a wasp and this tricks birds who don't like to eat wasps. 

Organisms also interact as parasites.  A parasite lives in or on another living thing, called a host.  A tick is a parasite.  A tick lives on an animal and feeds on the host's blood.

Energy in an Ecosystem

All living things need energy to live.  The sun is the source of energy for most life on Earth.  Green plants are producers.  They take in energy from the sun and use it to make their own food.  Grasses, elm trees, and string bean plants are producers.  The food producers make contains nutrients.  All living things need nutrients to stay alive.

Animals can't make their own food so they get their energy from eating plants or other animals.  Animals are consumers.  Some consumers eat only plants, they are called herbivores.  Some eat only other animals, they are called carnivores.  Some eat both, they are called omnivores.

Decomposers are living things that get their energy from plants and animals that have died.  Earthworms, bacteria, and mushrooms are decomposers.  They break down the matter in dead organisms into smaller and smaller bits.  Many of the nutrients return to the soil.

Food Chains

A food chain shows how living things get food and energy.  The energy source for almost all food chains is the sun.  The first organism in a food chain is a producer.  In a meadow, grass is a producer.  Crickets eat the grass, snakes eat the crickets, and the hawks eat the snakes.

When plants and animals die, they are broken down by decomposers.  The nutrients return to the soil and can be used by the producers.

Food Webs

Often ecosystems have many food chains.  Grass and mice live in a meadow, but so do birds, insects, and many other organisms.  All the food chains in an ecosystem connect to form a food web. 

The difference between a food chain and a food web is that a food chain only follows one path as animals find food.  A food web is several food chains joined together.  A hawk might eat a mouse, a squirrel, a frog, or another animal.  A snake may eat a beetle, mouse, or another animal.  Every other animal may eat many different types of animals in a food chain.

Energy Pyramids

An energy pyramid shows the amount of energy that moves through a food web.  The producers are at the bottom of the pyramid and have the most energy.  It also has the most organisms.  Going up the pyramid, some energy is used up at each level.  At the top of the pyramid, not much energy is left.  Only a few animals are at the top.  Energy flows through all ecosystems in similar ways. 

A small fish eats algae and gets energy from the nutrients in the algae.  The fish then uses the energy to move and grow.  If an eel eats the fish, the eel will not get all the energy that first came from the algae because the fish has used up some of that energy.  The eel would get its energy from the nutrients in the fish. 

How do ecosystems change?

Sometimes places change.  This may happen slowly.  A pond may fill in with dirt and dead leaves.  After many years, the pond is gone and the area becomes a meadow. 

Change can also happen quickly.  Lightning strikes and starts a forest fire.  In just a few days, trees and wildlife are gone.

Plant and animal populations change too.  Sometimes many members of a species die or are killed.  The species can become endangered.  This means very few are left.  If all the members of a species die, the species becomes extinct.

Diversity is the number of species in an area.  A rain forest can have thousands of species.  Diversity makes the ecosystem strong because small changes are less likely to affect the whole ecosystem.  If a disease attacks one kind of tree in the rainforest, that species would be the only one to die.  The disease might not harm other trees that are different.  The animals that depend on the trees can move to trees that are still living. 

Man can change an ecosystem.  Forests can be cut down, marshes filled in for houses, pollution dumped into rivers that kill the organisms living their, etc.  If man affects the environment in ways that cause damage ecosystems can be destroyed or greatly weakened.