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Congratulations! You and your classmates have been selected to take part in an around-the-world scientific expedition. On this expedition you will collect data on the climate and typical organisms of each of Earth’s biomes. A biome is a group of land ecosystems with similar climates and organisms.

The ecologists leading your expedition have agreed to focus on six major biomes. The six major biomes that most ecologists study are the rain forest, desert, grassland, deciduous forest, boreal forest, and tundra.

Be sure to pack a variety of clothing for your expedition. You will visit places ranging from steamy tropical jungles to frozen Arctic plains. It is mostly the climate—temperature and precipitation—in an area that determines its biome. This is because climate limits the species of plants that can grow in an area. In turn, the species of plants determine the kinds of animals that live there.

Hurry up and pack—it’s almost time to go!

Rain Forest Biomes

The first stop on your expedition is a rain forest. This biome is living up to its name—it’s pouring! Fortunately, you remembered to pack a raincoat. After just a short shower, however, the sun reappears. Surprisingly, though, very little sunlight reaches you through the thick leaves above.

Plants are everywhere in the rain forest. Some plants, such as the ferns, flowers, and vines hanging from tree limbs, even grow on other plants! And animals are flying, creeping, and slithering all around you.

Temperate Rain Forests
When you hear the term rain forest, you probably think of a warm, humid, “jungle” in the tropics. But there is another type of rain forest. The northwestern coast of the United States receives more than 300 centimeters of rain a year. Huge trees grow there, including cedars, redwoods, and Douglas firs. However, it is difficult to classify this region. Many ecologists refer to this ecosystem as a temperate rain forest. The term temperate means having moderate temperatures.

Tropical Rain Forests
As you can see on the map, tropical rain forests are found in regions close to the equator. The climate is warm and humid all year long, and there is a lot of rain. Because of these climate conditions, an astounding variety of plants grow in tropical rain forests. In fact, scientists studying a 100-square-meter area of one rain forest identified 300 different kinds of trees!

Trees in the rain forest form several distinct layers. The tall trees form a leafy roof called the canopy. A few giant trees poke out above the canopy. Below the canopy, a second layer of shorter trees and vines form an understory. Understory plants grow well in the shade formed by the canopy. The forest floor is nearly dark, so only a few plants live there.

The abundant plant life in tropical rain forests provides habitats for many species of animals. Ecologists estimate that millions of species of insects live in tropical rain forests. These insects serve as a source of food for many reptiles, birds, and mammals. Many of these animals are, in turn, food sources for other animals. Although tropical rain forests cover only a small part of the planet, they probably contain more species of plants and animals than all the other biomes combined.

Desert Biomes

The next stop on your expedition is a desert. It couldn’t be more different from the tropical rain forest you just left. You step off the bus into the searing summer heat. At midday, it is too hot to walk outside in the desert.

A desert is an area that receives less than 25 centimeters of rain per year. The amount of evaporation in a desert is greater than the amount of precipitation. Some of the driest deserts may not receive any precipitation in a year! Deserts often undergo large shifts in temperature during the course of a day. A scorching hot desert like the Namib Desert in Africa cools rapidly each night when the sun goes down. Other deserts, such as the Gobi in central Asia, are cooler, and even experience freezing temperatures in the winter.

Organisms that live in the desert must be adapted to the lack of rain and extreme temperatures. For example, the stem of a saguaro cactus has folds that work like the pleats in an accordion. The stem expands to store water when it is raining. Gila monsters can spend weeks at a time in their cool underground burrows. Many other desert animals are most active at night when the temperatures are cooler.

Grassland Biomes

The next stop on the expedition is a grassy plain called a prairie. Temperatures here are more comfortable than they were in the desert. The breeze carries the scent of soil warmed by the sun. This rich soil supports grasses as tall as you. Startled by your approach, sparrows dart into hiding places among the waving grass stems.

Although this prairie receives more rain than a desert, it does not get enough rain for trees to grow. Ecologists classify prairies, which are generally found in the middle latitudes, as grasslands. A grassland is an area that is populated mostly by grasses and other nonwoody plants. Most grasslands receive 25 to 75 centimeters of rain each year. Fires and droughts are common in this biome. Grasslands that are located closer to the equator than prairies are known as savannas. A savanna receives as much as 120 centimeters of rain each year. Scattered shrubs and small trees grow on savannas along with grass.

Grasslands are home to many of the largest animals on Earth—herbivores such as elephants, bison, antelopes, zebras, rhinoceroses, giraffes, and kangaroos. Grazing by these large herbivores helps to maintain the grasslands. They keep young trees and bushes from sprouting and competing with the grass for water and sunlight.

Deciduous Forest Biomes

Your trip to the next biome takes you to another forest. It is now late summer. Cool mornings here give way to warm days. Several members of the expedition are busy recording the numerous plant species. Others are looking through their binoculars, trying to identify the songbirds. You step carefully to avoid a small salamander.

You are now visiting a deciduous forest biome. Many of the trees in this forest are deciduous trees (dee sij oo us), trees that shed their leaves and grow new ones each year. Oaks and maples are examples of deciduous trees. Deciduous forests receive enough rain to support the growth of trees and other plants, at least 50 centimeters per year. Temperatures in the deciduous forest vary greatly during the year. The growing season usually lasts five to six months.

The variety of plants in a deciduous forest creates many different habitats. Different species of birds live in different parts of the forest, eating the insects and fruits in their specific areas. Mammals such as chipmunks and skunks live in deciduous forests. In a North American deciduous forest you might also see wood thrushes, white-tailed deer, and black bears.

If you were to return to this biome in the winter, you would not see much wildlife. Many of the bird species migrate to warmer areas. Some of the mammals hibernate, or enter a state of greatly reduced body activity similar to sleep. Animals that hibernate rely on fat stored in their bodies during the winter months.

Boreal Forest Biomes

Now the expedition heads north into a colder climate. The expedition leaders claim they can identify the next biome, a boreal forest, by its smell. When you arrive, you catch a whiff of the spruce and fir trees that blanket the hillsides. Feeling the chilly early fall air, you pull a jacket and hat out of your bag.

Boreal Forest Plants
Most of the trees in the boreal forest are coniferous trees (koh nif ur us), trees that produce their seeds in cones and have leaves shaped like needles. The boreal forest is sometimes referred to by its Russian name, the taiga (ty guh). Winters in these forests are very cold. The snow can reach heights well over your head! Even so, the summers are rainy and warm enough to melt all the snow.

Tree species in the boreal forest are well-adapted to the cold climate. Since water is frozen for much of the year, trees in the boreal forest must have adaptations that prevent water loss. Fir, spruce, hemlock, and other coniferous trees all have thick, waxy needles that prevent water from evaporating.

Boreal Forest Animals
Many of the animals of the boreal forest eat the seeds produced by the coniferous trees. These animals include red squirrels, insects, and birds such as finches and chickadees. Some herbivores, such as snowshoe hares, moose, and beavers, eat tree bark and new shoots. The variety of herbivores in the boreal forest supports many large predators, including wolves, bears, great horned owls, and lynxes.

Tundra Biomes

As you arrive at your next stop, the driving wind gives you an immediate feel for this biome. The tundra is an extremely cold and dry biome. Expecting deep snow, many are surprised to learn that the tundra may receive no more precipitation than a desert.

Most of the soil in the tundra is frozen all year. This frozen soil is called permafrost. During the short summer, the top layer of soil thaws, but the underlying soil remains frozen. Because rainwater cannot soak into the permafrost, there are many shallow ponds and marshy areas on the tundra in the summer.

Tundra Plants
Plants of the tundra include mosses, grasses, shrubs, and dwarf forms of a few trees, such as willows. Most of the plant growth takes place during the long days of the short summer season. North of the Arctic Circle, the sun does not set during midsummer.

Tundra Animals
In summer, the animals you might remember most are insects. Insect-eating birds take advantage of the plentiful food and long days by eating as much as they can. But when winter approaches, these birds migrate south. Mammals of the tundra include caribou, foxes, wolves, and Arctic hares. The mammals that remain on the tundra during the winter grow thick fur coats. What can these animals find to eat on the tundra in winter? The caribou scrape snow away to find lichens. Wolves follow the caribou and look for weak members of the herd to prey upon.

Mountains and Ice

Some areas of land are not part of any major biome. These areas include mountain ranges and land that is covered with thick sheets of ice.

You read in Section 3 that the climate of a mountain changes from its base to its summit. If you were to hike all the way up a tall mountain, you would pass through a series of biomes. At the base, you might find grasslands. As you climbed, you might pass through deciduous forest and then boreal forest. As you neared the top, your surroundings would resemble the treeless tundra.

Other places are covered year-round with thick ice sheets. Most of the island of Greenland and the continent of Antarctica fall into this category. Organisms that are adapted to life on ice include emperor penguins, polar bears, and leopard seals.

Section Assessment 2.4

1. a. What are the six major biomes found on Earth?
b. How are the three forest biomes (rain forests, deciduous forests, boreal forests) alike?
How are they different?
c. What biome might you be in if you were standing on a bitterly cold, dry plain with only a few, short plants scattered around?
2. a. What are two factors are most important in determining an area's biome?
b. If deserts and tundras receive similar amounts of rainfall, why are these two biomes so different?
c. Why would hiking up a tall mountain be a good way to observe how climate determines an area's biome?
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