Communities of Living Things

The part of Earth in which life exists is called the biosphere. The biosphere covers the entire surface of Earth and includes the atmosphere. An ecosystem is a smaller part of the biosphere. It is made up of all the living organisms in one area (of any size), as well as the nonliving parts of the environment, such as water and rocks.

Every living thing depends on other living things and many nonliving things. In a healthy ecosystem, all the parts are in balance, meaning the populations of plants and animals are not rising or falling in huge numbers. Even healthy ecosystems are always changing, at least in small ways. When old animals die, they make room for their young to live. When a tree falls, young seedlings begin growing in its place.

Organisms in the Environment

An environment is the living and the nonliving surroundings of an organism. All the organisms in a certain environment make a community. The community of living things where you live may include squirrels, birds, trees, and humans. A population is all the organisms of one type. Squirrels and pigeons, for example, belong to the same community, but to two different populations. Within a community, each population has its own place to live, or habitat. It also fills its own role or job, which is called a niche.

Energy Cycles

Energy from the Sun is the source of almost all energy on Earth. Green plants (the producers) trap energy during photosynthesis and convert it into glucose. Herbivores, such as cows and deer, eat the green plants. Carnivores, such as bears and falcons, eat other animals. Omnivores, such as humans and raccoons, eat both plants and animals.

The food chain is the path that energy follows as it moves through the community. It starts with green plants, the producers, and moves on through herbivores. Large carnivores are usually at the top of the food chain. When an organism dies, decomposers break apart the body, returning the nutrients to the soil. All the many food chains within a community form a food web. A change in the population of one organism of the food web will affect every other organism.



A large group of ecosystems with similar climates and communities is known as a biome. Scientists disagree about exactly where one biome ends and another begins. Most agree, however, that there are at least six distinct biomes throughout the world. These are deserts, tundras, grasslands, tropical rain forests, temperate forests, and oceans.


Lack of rainfall is the major characteristic of a desert biome. Most of the southwestern United States is a desert biome. Deserts tend to be hot during the day. At night, however, surface heat is rapidly lost. The desert at night can become quite cool.

Desert plants and animals are adapted to harsh, dry conditions. They have adapted to survive with very little water. Most desert animals are active at night when the temperature is cooler. Many of the plants, such as the cactus, store large amounts of water in their roots or stems. Some desert plants have extra deep roots. Other plants have tiny leaves or a waxy covering to reduce water loss from evaporation.


If you were to move a desert to an extremely cold area, the result would be a tundra. There are two kinds of tundras: arctic and alpine. The arctic tundra is in the far northern parts of the world. Alpine tundras are at the tops of high mountains.

In many places on the tundra, ice and snow cover the ground for long periods. The soil may never totally thaw. Polar bears, as well as migrating animals, such as birds, reindeer, and caribous, live in the tundra. Small plants, such as lichens and mosses, grow close to the ground where strong, icy winds will not damage them.


Grasslands get more rainfall than deserts but not enough to support large trees. They have long, hot summers, cold winters, and high winds. The soil is generally fertile. The dominant plants are a wide variety of grasses. Animals tend to be small, but some large grazing animals are also common.

America’s prairie is made up of grasslands that once were home to huge herds of bison. Today, there are cattle ranches on these grasslands. This area is also known as “the world’s breadbasket” because of the huge quantity of grain produced there.

Tropical Rain Forests

In a tropical rain forest, the weather is mild all the time. Both sunlight and rain are abundant. Surprisingly, the soil is not very fertile. Because of the abundance of sunlight, moisture, and minerals from decomposing plant matter, however, trees grow tall. Vines and other plants grow in the trees. Huge varieties of insects, snails, birds, snakes, frogs, and small mammals are at home in the tropical rain forest.

Temperate Forests

Temperate forests have distinct seasons. Temperature and rainfall vary with the seasons. Trees that lose their leaves in the fall are most common in southern temperate forests. Farther north, evergreens are the most common trees. Birds and mammals are abundant throughout the forest, but because so much has been cut to make room for farms and homes, the number of animals has greatly decreased. Much of the United States is now-or once was-temperate forest.


The ocean biome covers two-thirds of Earth’s surface. Climates vary, depending on location. Most of the plants and animals, such as algae, plankton, fish, and whales, live near the surface of the ocean where sunlight reaches.

Some organisms exist deep below the ocean surface where sunlight cannot penetrate. Most of these animals scour the ocean bottom for organic matter that sinks. Some, however, live near hot vents in volcanic areas. Rather than converting the Sun’s energy to produce food, these creatures produce food by converting the energy in chemicals that rise from beneath the ocean floor.

Freshwater Areas

A wide variety of organisms live in lakes, ponds, streams, and rivers. Many, such as algae, can be too tiny to see, whereas seaweed can grow into a thick mass. Animals that live in or near freshwater include insects, amphibians, and fish. Many birds build their nests along the shores and hunt for fish or other animals in the water. A few mammals, such as beavers, also live here.

Protecting Biomes

With the possible exception of deserts, all of the biomes you read about have been greatly changed by human activities. Humans have cut down vast tracts of both temperate forests and tropical rain forests. Wild grasslands have been changed into farms and ranches as well as cities and suburbs. Many freshwater lakes and rivers are being drained, at least partially, so that humans can use the water.

As a result of these and other activities, Earth is losing its wide variety of living things. This variety is called biodiversity. Biodiversity is important in every biome and ecosystem. Scientists argue that protecting biodiversity is vital to the health of life across the planet.


Ecology is the study of how organisms interact with one another and with the world around them. The predator-prey relationship is one important interaction. Predators hunt and kill other organisms-their prey. Organisms are often both predator and prey. Consider a snake, for example, being caught by an eagle. The snake may have just eaten a frog, which in turn, had just eaten a mosquito.

One community role is more important than all the others. Plants and green algae are producers, meaning they make their own food. Directly or indirectly, producers provide food for all the other organisms in an ecosystem. Animals eat either producers or other animals. They are called consumers.

Decomposers and scavengers also play an important role. They feed on dead organisms. Without them, the decaying remains of dead organisms would litter the Earth.