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Communities and Biodiversity

Biology · Communities and Biodiversity

Terrestrial Biomes

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Essential Question

How do climate and soil shape the communities on land?

A forest in Maine, a grassland in Kansas, and a desert in Arizona look very different, and the main reason is climate. In this lesson, you’ll learn how temperature and precipitation sort the land into biomes, how climate affects soil, and how plants are adapted to the conditions where they live.

Video lesson coming soon. In the meantime, read through the lesson below.

What Is a Biome?

  • Biome: a large region with a similar climate and similar kinds of plants
  • Temperature and precipitation are the main factors
  • Boundaries between biomes overlap and vary
A grid with average temperature increasing from cold on the left to warm on the right and precipitation increasing from dry at the bottom to wet at the top. Tundra is cold and dry. Boreal forest is cold with moderate precipitation. Temperate grassland is in the middle with moderate to low precipitation. Temperate forest is mild and wet. Desert is warm and dry, and deserts can also be cold. Tropical rainforest is warm and wet. A note says the figure is a schematic and regions overlap.

A biome is a large region defined by its climate and by the kinds of plants that grow there. Climate, mainly average temperature and the amount and timing of precipitation, is the main factor that decides which biome a region has. The grid shows a simple model of where six land biomes fit. Real biome boundaries are not sharp, and they overlap. Other factors, such as soil, elevation, fire, and grazing, also matter, and the model leaves out biomes such as savanna and chaparral.

Climate and Traits of Six Biomes

  • Rainforest: warm and very wet
  • Temperate forest and grassland: seasonal
  • Boreal forest and tundra: cold
  • Desert: very dry
  • Ranges are approximate and overlap
Six boxes. Tropical rainforest: 20 to 28 degrees Celsius and 200 to 450 centimeters of precipitation, tall trees in layers and very many species. Temperate forest: 6 to 20 degrees and 75 to 250 centimeters, many broadleaf trees that drop leaves in fall. Temperate grassland: 0 to 20 degrees and 25 to 75 centimeters, grasses with few trees, shaped by fire and grazing. Boreal forest: minus 5 to 5 degrees and 30 to 85 centimeters, cone-bearing trees such as spruce, fir, and pine. Tundra: below about minus 5 degrees and 15 to 50 centimeters, mosses, lichens, sedges, and low shrubs over permafrost. Desert: varies, under about 25 centimeters, widely spaced plants that store or save water.

The boxes list approximate ranges of average yearly temperature and yearly precipitation for each biome, with typical plants. Sources give slightly different ranges, and the ranges overlap, so treat them as a guide for the best fit and not as exact borders. Tropical rainforests are warm and wet all year and have very many species. Temperate forests and grasslands have distinct seasons. Boreal forest and tundra have long, cold winters. Deserts receive very little precipitation, and some are hot while others are cold.

Soil and Nutrients

  • Climate affects how fast dead matter decomposes
  • Decomposition returns nutrients to the soil
  • Rainforest: nutrients are held mostly in living plants
  • Grassland: deep roots build rich soil
  • Tundra: cold slows decomposition; permafrost below
Three boxes. Tropical rainforest, warm and wet all year: dead leaves decompose quickly and plants take up the nutrients fast, so much of the nutrient supply is in the living plants and not the soil. Temperate grassland, seasonal with moderate rain: thick deep root systems die and add organic matter, so many grassland soils are rich in nutrients. Tundra, cold with a short growing season: cold slows decomposition, so dead plant matter breaks down slowly, and a layer of frozen soil called permafrost lies below the surface.

Soil is part of what makes a biome. Decomposers break down dead plants and animals and return nutrients to the soil, and climate sets how fast this happens. In a warm, wet rainforest, decomposition is fast and plants take up the released nutrients quickly, so a large share of the nutrient supply is stored in living plants and many rainforest soils hold few nutrients. In a grassland, dense roots die and add organic matter to the soil, and many grassland soils are rich, which is one reason much of this land is farmed. In the tundra, cold slows decomposition, and a layer of permanently frozen soil called permafrost lies below the surface.

Adaptations to Climate

  • Adaptations fit organisms to their conditions
  • Desert plants save water
  • Boreal conifers shed snow
  • Grasses regrow from near the ground
Three rows, each with a condition leading to adaptations leading to a benefit. Desert: very little water leads to storing water in stems, a waxy coating, and small or no leaves, which lets plants survive long dry periods. Boreal forest: long cold winters and heavy snow lead to needle-shaped leaves, a cone shape that sheds snow, and evergreen leaves, which lets trees make food when it is warm enough and resist snow damage. Grassland: grazing and fire lead to a growing point near the ground and deep fibrous roots, so grasses regrow after being eaten or burned.

Plants and animals in a biome have adaptations, inherited traits that help them survive and reproduce in those conditions. Many desert plants, such as cacti, store water in their stems and have a waxy coating that reduces water loss, and their leaves are small or reduced to spines. Boreal conifers have needle-shaped leaves, and their cone shape lets snow slide off. Most grass leaves grow from the base, near the ground, so grasses can regrow after being grazed or burned. These traits evolved by natural selection over many generations. A trait that helps in one biome can be useless or harmful in another.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Which Biome Fits Site A?

The climograph shows sample data for Site A. Bars show precipitation each month and the line shows average temperature. Find the average yearly temperature and the yearly precipitation, then use the biome cards to decide which biome fits best. Write a claim, evidence, and reasoning.

Climograph for Site A. Monthly precipitation bars range from 1 to 8 centimeters, highest in June at 8 centimeters. The temperature line is about minus 8 degrees Celsius in January, rises to 23 in July, and falls to about minus 7 in December.
Show the solution
  1. Yearly precipitation: add the 12 bars: 2 + 1 + 3 + 5 + 7 + 8 + 6 + 5 + 4 + 3 + 2 + 2 = 48 cm.
  2. Average temperature: add the 12 monthly temperatures and divide by 12: 96 ÷ 12 = 8.0 °C.
  3. Compare with the cards: 8.0 °C is in the range for temperate forest and temperate grassland, not boreal forest. 48 cm is in the grassland range (25 to 75 cm) and below the forest range (75 to 250 cm).
  4. Claim: Site A best fits temperate grassland.
  5. Evidence: the average temperature is 8.0 °C and the yearly precipitation is 48 cm. The temperature swings from about -8 °C to 23 °C, so the site has distinct seasons.
  6. Reasoning: both values fall inside the grassland ranges, but 48 cm is too low for temperate forest. The match is based on climate alone, so scientists would also check the plants and soil.
  7. Answer: Temperate grassland: 8.0 °C average and 48 cm per year.

Example 2: A Region Gets Drier

Use the grid model. A region is a temperate forest. Over many years its climate stays mild, but precipitation falls well below the forest range. Predict which biome the climate would then fit, and explain.

The biome grid with temperature across the bottom from cold to warm and precipitation up the side from dry to wet. Temperate forest is mild and wet. Temperate grassland is directly below it, mild and with less precipitation.
Show the solution
  1. Find the starting point: temperate forest is in the mild, wet part of the grid.
  2. Change only precipitation: less precipitation moves the region down the grid, because the temperature stays the same.
  3. Read the result: directly below temperate forest is temperate grassland.
  4. Prediction: the climate would fit temperate grassland better.
  5. Limit of the model: the grid shows what climate favors. A real change in vegetation takes many years, and depends on soil, fire, and grazing too.
  6. Answer: Temperate grassland, because less precipitation at the same temperature moves the region down the grid.

Example 3: Farming Cleared Rainforest

Use the soil model. Farmers clear and burn part of a rainforest and plant crops. The first crops grow well, but yields often drop after a few years. Use the model to explain why.

Three boxes about soil. In tropical rainforest, decomposition is fast and plants take up nutrients quickly, so much of the nutrient supply is in the living plants and not the soil. In temperate grassland, deep roots add organic matter and many soils are rich. In tundra, cold slows decomposition and permafrost lies below.
Show the solution
  1. Where the nutrients are: in a rainforest, much of the nutrient supply is in living plants, and the soil holds relatively few.
  2. Clearing and burning: removing the plants removes much of that stored supply. Burning returns some nutrients as ash, which can fertilize the first crops.
  3. Heavy rain: the ash and any nutrients in the soil can wash away, and few plants remain to take them up.
  4. Explanation: the crops use up the small supply that is left, so yields often fall after a few years.
  5. Answer: Most nutrients were stored in the plants, so clearing removes them, and the nutrient-poor soil cannot support crops for long.

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Practice

Match biomes to descriptions, use the biome grid to place sites, read a climograph as evidence, and explain how climate affects soil nutrients. Try it on your own first, then check your work with the answer key.

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Lesson Slides

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Big Ideas

  • A biome is a large region with a similar climate and plants
  • Temperature and precipitation are the main factors that sort biomes
  • Biome boundaries overlap, and soil, fire, and grazing also matter
  • Climate sets how fast nutrients cycle through soil
  • Adaptations fit organisms to the conditions of their biome

Back to the essential question: climate, mainly temperature and precipitation, sorts the land into biomes, and it also sets how quickly nutrients cycle through the soil. Plants in each biome have adaptations that fit the conditions. Next, you’ll look at aquatic ecosystems, where sunlight, salt, and moving water play the role that climate plays on land.

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NGSS Alignment

This lesson builds toward these Next Generation Science Standards (NGSS) performance expectations:

  • HS-LS2-6: Evaluate the claims, evidence, and reasoning that the complex interactions in ecosystems maintain relatively consistent numbers and types of organisms in stable conditions, but changing conditions may result in a new ecosystem.

NGSS has no performance expectation written for terrestrial biomes, so this lesson uses the closest fit.

Disciplinary core ideas: LS2.C Ecosystem Dynamics, Functioning, and Resilience

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Next: Aquatic Ecosystems →