Ecology and Ecosystem Dynamics

Biotic and Abiotic Interactions

Let’s learn about how living and nonliving things work together in nature.

Biotic means living things. Plants, animals, and fungi are biotic things.

Abiotic means nonliving things. Water, air, sunlight, soil, and rocks are abiotic things.

An ecosystem is a place where living things and nonliving things are together. A pond, a garden, a forest, and even a schoolyard can be ecosystems.

In an ecosystem, biotic and abiotic things interact. That means they affect each other and work together.

For example, a plant is biotic. The plant needs sunlight, water, air, and soil. Sunlight, water, air, and soil are abiotic. The plant grows because of these nonliving things.

Animals are biotic too. Many animals need water to drink, air to breathe, and places like trees or rocks for shelter. Those are abiotic and biotic parts of their home.

Fungi, like mushrooms, are also biotic. They grow in places with water, air, and soil. They are part of the living world.

Main Idea: Living things need nonliving things to live.

• Plants need sunlight, water, air, and soil.
• Animals need water, air, and food.
• Fungi need water, air, and a place to grow.

Biotic things can help other biotic things too.

• A bird may build a nest in a tree.
• A rabbit may eat grass.
• A bee may visit a flower.

Abiotic things can change what happens in an ecosystem.

• If there is lots of sunlight, plants can grow well.
• If there is not enough water, plants may dry out.
• If the air is clean, animals and plants can live better.

When we map an ecosystem, we look for the living and nonliving parts.

You can ask:

• What living things are here?
• What nonliving things are here?
• How do they work together?

Let’s look at some places and name the biotic and abiotic parts.

Example 1: A Garden

In a garden, you might see flowers, worms, birds, and mushrooms. These are biotic because they are living.

You might also see sunlight, water, soil, air, and rocks. These are abiotic because they are nonliving.

How do they interact? The flowers need sunlight and water. The birds may use plants for shelter. Worms live in the soil.

Worked Example 1

Question: Is a flower biotic or abiotic?

Step 1: Ask, “Is it living?”

Step 2: A flower is living.

Answer: A flower is biotic.

Worked Example 2

Question: Is sunlight biotic or abiotic?

Step 1: Ask, “Is it living?”

Step 2: Sunlight is not living.

Answer: Sunlight is abiotic.

Example 2: A Pond

In a pond, you may find fish, frogs, water plants, ducks, and fungi. These are biotic.

You may also find water, rocks, mud, air, and sunlight. These are abiotic.

How do they interact? Fish live in water. Frogs need water and air. Plants in the pond need sunlight and water to grow.

Worked Example 3

Question: In a pond, a frog sits on a rock near the water. What is biotic, and what is abiotic?

Step 1: Find the living thing. The frog is living.

Step 2: Find the nonliving things. The rock and water are not living.

Answer: The frog is biotic. The rock and water are abiotic.

Example 3: A Forest

In a forest, there are trees, deer, birds, insects, and mushrooms. These are biotic.

There is also soil, sunlight, air, water, and rocks. These are abiotic.

How do they interact? Trees need sunlight, water, air, and soil. Deer may eat plants. Birds may live in trees.

Worked Example 4

Question: A tree grows in soil and uses sunlight and water. What does this show?

Step 1: Name the living thing. The tree is living, so it is biotic.

Step 2: Name the nonliving things. Soil, sunlight, and water are abiotic.

Step 3: Tell how they work together. The tree needs these abiotic things to grow.

Answer: This shows a biotic and abiotic interaction.

How to Map an Ecosystem

1. Look around the place.
2. Find the living things.
3. Find the nonliving things.
4. Think about how they help each other.

Here is a simple way to sort what you see:

• Biotic: tree, bird, flower, frog, mushroom
• Abiotic: water, rock, sunlight, air, soil

Try It in Your Mind

If you see a bee on a flower on a sunny day, what can you name?

• Bee = biotic
• Flower = biotic
• Sunlight = abiotic

They are all part of one ecosystem.

Important Things to Remember

• Biotic means living.
• Abiotic means nonliving.
• Ecosystems have both biotic and abiotic parts.
• Living things need nonliving things.
• Living things also interact with other living things.

Brief Summary

Nature is full of living and nonliving things. Living things are biotic. Nonliving things are abiotic.

In an ecosystem, plants, animals, and fungi live with water, air, sunlight, soil, and rocks. They interact every day. When we look closely, we can see how everything is connected.

Biomes of the World

Have you ever noticed that different places on Earth have different kinds of weather, plants, and animals? Some places are hot and rainy. Some are dry and sandy. Some are very cold and snowy.

Scientists group these large regions into biomes. A biome is a very big area of Earth that has a certain kind of climate, plants, and animals.

Climate means the usual weather in a place over a long time. Climate includes things like:

• How hot or cold a place is
• How much rain or snow it gets
• What kinds of seasons it has

The climate of a place helps decide which living things can survive there. For example, a cactus can live in a dry desert, but it would not do well in a freezing tundra.

In this lesson, we will learn about some major biomes of the world:

• Tundra
• Taiga
• Rainforest
• Desert
• Grassland

Why are biomes important?

Biomes help us understand how living things fit their environment. Plants and animals in each biome have special features that help them live there.

For example:

• A polar bear has thick fur to help it stay warm in cold places.
• A camel can go a long time without water in the desert.
• Tall trees in rainforests grow well where there is lots of rain.

1. Tundra

The tundra is a very cold biome. It is often found far north, near the top of the world.

The tundra has:

• Very cold temperatures
• Little rainfall
• Long winters and short summers
• Few trees

Because it is so cold, plants in the tundra are usually small. You may find mosses, grasses, and tiny flowers.

Animals in the tundra include:

• Arctic foxes
• Caribou
• Snowy owls
• Polar bears

These animals have ways to stay warm, like thick fur or layers of fat.

2. Taiga

The taiga is also called the boreal forest. It is a cold biome with many evergreen trees.

The taiga has:

• Cold, snowy winters
• Short, cool summers
• More trees than the tundra

Many trees in the taiga are cone-shaped evergreens, like pine and fir trees. These trees keep their needles all year long.

Animals in the taiga include:

• Moose
• Bears
• Wolves
• Lynx

The taiga is colder than many forests, so the animals there also need ways to stay warm.

3. Rainforest

The rainforest is a warm, wet biome. It gets lots and lots of rain.

Rainforests have:

• Warm temperatures
• A great deal of rain
• Many tall trees and thick plant growth
• Many kinds of living things

Because rainforests have so much water and warmth, plants grow very well there. Many animals can also find food and shelter.

Animals in the rainforest include:

• Monkeys
• Parrots
• Frogs
• Jaguars

Rainforests are some of the most crowded biomes with life. Many plants and animals share this space.

4. Desert

The desert is a very dry biome. Deserts get very little rain.

Deserts have:

• Very little rainfall
• Dry land
• Hot days in many deserts
• Cool or cold nights in some deserts

Plants in the desert must save water. Cacti and other desert plants can store water in their stems or roots.

Animals in the desert include:

• Camels
• Lizards
• Snakes
• Scorpions

Many desert animals rest during the hottest part of the day. This helps them stay cool and keep water in their bodies.

5. Grassland

The grassland is a biome where grasses grow well, but there are not many trees.

Grasslands have:

• Open land
• Lots of grass
• Some rain, but not enough for many forests
• Warm or cool seasons, depending on the place

Grasslands can be home to large groups of animals that move across the land to find food.

Animals in grasslands include:

• Bison
• Zebras
• Lions
• Prairie dogs

Grasses are important in this biome because many animals eat them or use them for shelter.

How climate shapes a biome

The two biggest clues for a biome are often temperature and rainfall.

If a place is:

• Cold and dry, it may be tundra.
• Cold with many evergreen trees, it may be taiga.
• Warm and very wet, it may be rainforest.
• Very dry, it may be desert.
• Open and grassy, it may be grassland.

Plants and animals depend on these conditions. If the climate changes, the living things in that biome may have a hard time surviving.

Plants and animals are adapted to their biome

Adapted means a living thing has features or behaviors that help it survive in its home.

Here are some examples of adaptation:

• Thick fur helps animals stay warm in the tundra and taiga.
• Waxy leaves help some desert plants keep water.
• Long roots help plants reach water underground.
• Strong tree trunks help rainforest trees grow tall toward sunlight.

Comparing biomes

Let’s compare the five biomes we learned.

• Tundra: very cold, few trees, small plants
• Taiga: cold forest, many evergreen trees
• Rainforest: warm and wet, many plants and animals
• Desert: very dry, plants and animals that save water
• Grassland: open land with lots of grasses

Worked Example 1: Finding the biome

Question: A place is hot, dry, and gets very little rain. Which biome is it?

Step 1: Think about the climate. It is hot and dry.

Step 2: Ask which biome has very little rain.

Answer: It is a desert.

Worked Example 2: Matching animals to a biome

Question: Which biome is the best home for a polar bear?

Step 1: Think about what a polar bear needs. It has thick fur and lives in very cold places.

Step 2: Choose the biome that is very cold and near the north.

Answer: The best biome is the tundra.

Worked Example 3: Comparing two biomes

Question: How is a rainforest different from a grassland?

Step 1: Think about rainfall. A rainforest gets lots of rain. A grassland gets some rain, but less.

Step 2: Think about plants. A rainforest has many tall trees. A grassland has mostly grasses and fewer trees.

Answer: A rainforest is wetter and has many tall trees. A grassland is more open and has lots of grass.

Worked Example 4: Using clues

Question: A place has long, cold winters and many evergreen trees. Which biome is it?

Step 1: Look at the weather clue: long, cold winters.

Step 2: Look at the plant clue: many evergreen trees.

Step 3: Match both clues to a biome.

Answer: It is the taiga.

Tips for remembering biomes

• Tundra = treeless, freezing land
• Taiga = cold forest of evergreen trees
• Rainforest = rainy, warm, full of life
• Desert = dry, dry, dry
• Grassland = wide open grassy space

Let’s review

A biome is a huge region with its own climate, plants, and animals. Climate helps decide what can live in a biome.

The tundra is very cold with few trees. The taiga is a cold forest with evergreen trees. The rainforest is warm and wet with many living things. The desert is very dry. The grassland is open and grassy.

When you study a biome, look for clues about temperature, rainfall, plants, and animals. These clues help you figure out which biome you are learning about.

Abiotic vs. Biotic Factors

Everything in an ecosystem works together. An ecosystem is a place where living things and nonliving things interact.

For example, a pond ecosystem has fish, frogs, plants, water, rocks, mud, and sunlight. Some of these are alive, and some are not. Both kinds are important.

Scientists use two special words to talk about these parts of an ecosystem: biotic and abiotic.

Biotic factors are the living parts of an ecosystem, or things that were once living. Plants, animals, mushrooms, and tiny living things like germs are biotic factors.

Abiotic factors are the nonliving parts of an ecosystem. Sunlight, water, air, rocks, and soil are abiotic factors.

A simple way to remember this is:

• Biotic = living
• Abiotic = nonliving

Both biotic and abiotic factors help shape an ecosystem. Living things need nonliving things to survive. For example, plants need sunlight, water, air, and soil. Animals need water, air, and shelter.

Living things also affect each other. A rabbit eats plants. A fox may hunt a rabbit. A tree gives shade to a bird. These are all ways biotic factors interact.

Nonliving things can change what happens in an ecosystem too. If there is not enough rain, plants may not grow well. If there is too little sunlight, some plants may not survive. If the soil is poor, roots may not grow well.

This means ecosystems are shaped by both living interactions and nonliving conditions.

Main Teaching Points

• Biotic factors are living things.
  • Examples: trees, grass, birds, insects, fish, deer, fungi
• Abiotic factors are nonliving things.
  • Examples: sunlight, water, air, temperature, rocks, soil
• Living things depend on nonliving things.
  • A plant needs water and sunlight.
  • A fish needs water with enough air in it.
• Living things also affect other living things.
  • An owl catches a mouse.
  • Bees help flowers by moving pollen.
  • Disease can make animals sick.
• When one factor changes, the ecosystem can change.
  • Less water can mean fewer plants.
  • Fewer plants can mean less food for animals.

How to Tell the Difference

You can ask yourself these questions:

1. Is it alive now?
2. Was it once alive?
3. If yes, it is biotic.
4. If no, it is abiotic.

For example, a tree is alive, so it is biotic. A fallen leaf was once part of a living plant, so it is also biotic. A rock was never alive, so it is abiotic.

Examples in Different Ecosystems

Forest

• Biotic: deer, squirrels, trees, worms, mushrooms
• Abiotic: rain, sunlight, soil, air, temperature

Desert

• Biotic: cactus, lizard, snake, hawk
• Abiotic: sand, heat, sunlight, little water, wind

Pond

• Biotic: fish, frogs, algae, insects, ducks
• Abiotic: water, mud, sunlight, rocks, temperature

Worked Example 1: Sort the items

Items: frog, sunlight, water, grass

Step 1: Ask if each item is living or nonliving.

• Frog = living
• Sunlight = nonliving
• Water = nonliving
• Grass = living

Step 2: Put them into groups.

• Biotic: frog, grass
• Abiotic: sunlight, water

Worked Example 2: What does a plant need?

A flower is growing in a garden. It needs sunlight, water, soil, and air.

Question: Are these needs biotic or abiotic?

Step 1: Think about each one.

• Sunlight = nonliving
• Water = nonliving
• Soil = nonliving
• Air = nonliving

Answer: These are all abiotic factors.

Why it matters: The flower is biotic, but it needs abiotic factors to live.

Worked Example 3: What happens when one factor changes?

Imagine there is very little rain for a long time.

Step 1: Rain is a nonliving part of the ecosystem, so it is an abiotic factor.

Step 2: With less rain, plants may dry out and not grow well.

Step 3: If there are fewer plants, animals that eat plants may have less food.

Answer: One abiotic change, less rain, can cause changes in biotic factors like plants and animals.

Worked Example 4: Living things affecting living things

In a field, rabbits eat grass, and foxes hunt rabbits.

Question: Are these interactions biotic or abiotic?

Step 1: Rabbits, grass, and foxes are all living things.

Step 2: When living things affect other living things, that is a biotic interaction.

Answer: Rabbits eating grass and foxes hunting rabbits are both biotic factors at work.

Things to Watch Out For

• Do not think only animals are biotic. Plants, fungi, and tiny living things are biotic too.
• Do not think abiotic means unimportant. Abiotic factors are very important because living things need them.
• Remember that something that was once living, like a fallen leaf, is counted as biotic.

Quick Check

Try these on your own:

• Is a rock biotic or abiotic? Abiotic
• Is a bird biotic or abiotic? Biotic
• Is soil biotic or abiotic? Abiotic
• Is a mushroom biotic or abiotic? Biotic
• Is sunlight biotic or abiotic? Abiotic

Brief Summary

An ecosystem has biotic factors and abiotic factors. Biotic factors are living things, or things that were once living. Abiotic factors are nonliving things like sunlight, water, air, rocks, and soil.

Both kinds of factors matter. Living things depend on nonliving things, and living things also affect one another. When biotic or abiotic factors change, the whole ecosystem can change too.

Ecological Niches vs. Habitats

Plants and animals live in many different places on Earth. A fish may live in a pond. A fox may live in a forest. A cactus may live in a desert. In science, we use special words to talk about where an organism lives and what it does there.

Two important words are habitat and niche. These words are not the same. Learning the difference helps us understand how living things survive in nature.

A habitat is an organism's home, or address. It is the place where a plant or animal lives. A habitat gives the organism what it needs, like food, water, air, and shelter.

A niche is an organism's job, or role. It tells how the organism lives in its habitat. It includes what it eats, how it gets food, how it stays safe, and how it helps or affects other living things around it.

A simple way to remember this is:

• Habitat = where it lives
• Niche = what it does

Think about a person. A person's address tells where they live. A person's job tells what they do. In nature, a habitat is like an address, and a niche is like a job.

Main Teaching Point 1: What is a habitat?

A habitat is the natural place where a living thing is found. Different habitats have different features. Some are wet, some are dry, some are hot, and some are cold.

Here are some examples of habitats:

• Forest
• Pond
• Ocean
• Desert
• Grassland
• Arctic

Each habitat gives living things what they need to survive. For example, a pond has water for frogs and fish. A forest has trees for birds and squirrels. A desert has dry land where cacti and lizards can live.

Main Teaching Point 2: What is a niche?

A niche is the special part an organism plays in its habitat. It is the way the organism lives and works in that place.

An organism's niche may include:

• What it eats
• What eats it
• Where it hides or rests
• When it is active, such as day or night
• How it helps the habitat

For example, a bee's niche is to drink nectar from flowers and help pollinate plants. A rabbit's niche is to eat plants and serve as food for animals like foxes. A worm's niche is to break down dead plants in the soil.

Main Teaching Point 3: Habitat and niche work together

Every organism lives in a habitat and has a niche. These two ideas fit together.

For example, a frog's habitat may be a pond. Its niche may be eating insects, hiding near water plants, and being food for bigger animals. The pond is where it lives. Eating insects and being part of the food chain is what it does.

If we only say the habitat, we know the place. If we only say the niche, we know the role. To understand an organism fully, we often need both.

Main Teaching Point 4: Different organisms can share a habitat

Many living things can live in the same habitat. For example, a pond can be home to frogs, fish, turtles, insects, and water plants.

Even if they share one habitat, they may have different niches. One animal may eat plants. Another may eat insects. Another may hide in mud. Another may swim near the top of the water.

This is helpful because not every organism is doing the exact same job. Different niches help living things share space and resources.

Main Teaching Point 5: Why niches are important

In nature, every living thing has an important role. Some animals spread seeds. Some insects pollinate flowers. Some animals hunt. Some organisms clean up dead matter.

When each organism does its niche, the habitat stays healthier. Living things depend on one another in many ways.

If one organism disappears, other living things may be affected. For example, if bees are gone, many flowers may not be pollinated as well. That shows why a niche matters.

Worked Example 1: Bird in a tree

A robin lives in a forest. It builds a nest in a tree and eats worms and insects.

Let's sort the information:

• Habitat: forest
• Niche: builds nests in trees and eats worms and insects

Why? The forest is the robin's place. Eating worms and living as a small bird in the forest is its role.

Worked Example 2: Frog in a pond

A frog lives in a pond. It eats insects and is eaten by snakes and birds.

Let's sort it:

• Habitat: pond
• Niche: eats insects and becomes food for other animals

This example shows that a niche can include more than one job. The frog is both a hunter of insects and food for larger animals.

Worked Example 3: Bee in a garden

A bee lives in a garden area with many flowers. It drinks nectar and carries pollen from flower to flower.

Let's sort it:

• Habitat: garden
• Niche: drinks nectar and helps pollinate flowers

This is a very important niche. The bee helps plants make seeds so more plants can grow.

Worked Example 4: Fish in the ocean

A clownfish lives in the ocean near sea anemones. It eats small bits of food and stays close to the anemone for safety.

Let's sort it carefully:

• Habitat: ocean
• Niche: eats small food pieces and uses sea anemones for protection

Notice that the habitat answer is still the place, the ocean. The niche answer tells how the clownfish lives in that place.

How to tell the difference

If you are not sure whether something is a habitat or a niche, ask these questions:

1. Is this telling where the organism lives? If yes, it is probably the habitat.
2. Is this telling what the organism does? If yes, it is probably the niche.

Here are some quick practice ideas:

• Desert → habitat
• Eats mice at night → niche
• Pond → habitat
• Pollinates flowers → niche

Common mix-up

Sometimes students say a habitat and a niche are the same because both describe an organism. But they describe different parts of its life.

• The habitat tells the location.
• The niche tells the role.

For example, saying “a rabbit lives in a grassland” gives the habitat. Saying “a rabbit eats grass and is food for foxes” gives the niche.

Let’s compare them side by side

• Habitat: home, place, address
• Niche: job, role, way of living

Both are important. A habitat helps us know where an organism belongs. A niche helps us know how it fits into the living world.

Brief Summary

A habitat is the place where an organism lives. A niche is the job or role the organism has in that place. Many organisms can share one habitat, but they may have different niches. Remember: habitat = address and niche = job.

Producers and Primary Productivity

Every living thing needs energy to live, grow, and move. In an ecosystem, energy starts with special living things called producers.

Producers are living things that make their own food. Most producers are plants, but some algae also do this. They use sunlight, water, and air to make food.

This is why producers are the base of a food chain. Animals cannot make their own food like plants can, so they depend on producers directly or indirectly.

Primary productivity means how much food or energy producers make in a place over time. A place with many healthy plants usually has high primary productivity. A place with very few plants usually has low primary productivity.

Introduction: Where does food energy begin?

Think about a rabbit eating grass. Then think about a fox eating the rabbit. Where did the energy begin? It began with the grass, because the grass used sunlight to make food.

This means the energy for many living things starts with producers. Without producers, there would not be enough food energy for the rest of the ecosystem.

Main Teaching Point 1: What are producers?

Producers are living things that can make their own food. They do not need to eat other living things to get started.

• Grass is a producer.
• Trees are producers.
• Flowers are producers.
• Algae in water can be producers too.

Producers use:

• sunlight
• water
• air

With these, they make food for themselves. That food gives them energy to grow. It also becomes energy for animals that eat them.

Main Teaching Point 2: Why are producers so important?

Producers are important because they start the flow of energy in a food chain.

Here is a simple food chain:

sunlight → grass → rabbit → fox

The grass is the producer. The rabbit gets energy by eating the grass. The fox gets energy by eating the rabbit. So even the fox depends on the producer in the beginning.

If there were no grass, the rabbit would have no food. If there were no rabbits, the fox would have less food. This shows why producers support other life in an ecosystem.

Main Teaching Point 3: What is primary productivity?

Primary productivity tells us how much food producers make in an area. It is about how much plant food and energy is being made.

If a forest has many plants growing well, it has high primary productivity. If a dry, rocky place has only a few small plants, it has low primary productivity.

We can think about it like this:

more healthy producers = more food energy made

fewer healthy producers = less food energy made

Main Teaching Point 4: What helps producers grow well?

Producers make more food when they have what they need.

• enough sunlight
• enough water
• good air
• space to grow

When producers get enough of these things, they can grow strong and make lots of food. Then there is more energy available in the ecosystem.

When producers do not get enough sunlight or water, they may not grow well. Then they make less food, and there is less energy for other living things.

Main Teaching Point 5: Producers on land and in water

On land, producers are usually plants like grass, bushes, and trees.

In water, producers can be water plants and algae. Tiny algae may be small, but they are still very important. They can make food and help support fish and other water animals.

So producers can live in many places:

• gardens
• forests
• fields
• ponds
• lakes
• oceans

Worked Example 1: Finding the producer

Question: In the food chain sunlight → plant → mouse → owl, which living thing is the producer?

Step 1: Look for the living thing that makes its own food.

Step 2: The plant uses sunlight to make food.

Answer: The plant is the producer.

Worked Example 2: Comparing two places

Question: Which place has higher primary productivity?

• Place A: a green field with lots of grass and flowers
• Place B: a dry place with only a few small plants

Step 1: Remember that primary productivity means how much food producers make.

Step 2: A place with more healthy plants usually makes more food.

Answer: Place A has higher primary productivity.

Worked Example 3: What happens if producers decrease?

Question: A pond has less algae than before. What might happen to the animals in the pond?

Step 1: Algae are producers in the pond.

Step 2: If there is less algae, less food energy is being made.

Step 3: Animals that depend on that food energy may have less to eat.

Answer: Some pond animals may have less food because there are fewer producers.

Worked Example 4: Using a simple number idea

We can use a simple idea to compare productivity.

If Garden 1 has 8 healthy plants and Garden 2 has 3 healthy plants, the garden with more healthy plants will usually make more food energy.

We can compare with numbers:

\(8 > 3\)

So Garden 1 likely has higher primary productivity.

Important idea to remember

Producers do not just feed themselves. They also help feed the whole ecosystem. Even animals that never eat plants still depend on producers, because their food gets energy from producers too.

Let’s review

1. Producers are living things that make their own food.
2. Most producers are plants, and some algae are producers too.
3. Producers use sunlight, water, and air to make food.
4. They are the start of energy flow in a food chain.
5. Primary productivity means how much food producers make in a place over time.
6. More healthy producers usually means more energy for the ecosystem.

Brief Summary

Producers are the living things that make food and begin the flow of energy in ecosystems. Plants and algae are producers because they use sunlight, water, and air to make food. Primary productivity tells us how much food producers make in a place. When producers grow well, they provide more energy for all the other living things around them.

Food Chains and Food Webs

Living things need energy to grow, move, and stay alive. Most energy in nature starts with the Sun. Plants use sunlight to make their own food. Then animals get energy by eating plants or by eating other animals.

A food chain shows one simple path of energy moving from one living thing to another. A food web shows many food chains connected together. Food webs help us see how plants and animals depend on each other in an ecosystem.

Let’s learn how food chains and food webs work.

1. Energy starts with the Sun

The Sun gives light and warmth to Earth. Plants use sunlight, air, and water to make their own food. Because plants make their own food, they are called producers.

Examples of producers are:

• grass
• trees
• flowers
• bushes

Animals cannot make their own food like plants can. Animals must eat plants or other animals. Animals are called consumers.

2. What is a food chain?

A food chain is a line that shows who eats whom. It also shows how energy moves. The arrow points to the living thing that gets the energy.

For example:

grass → rabbit → fox

This means:

• The grass gets energy from the Sun.
• The rabbit eats the grass.
• The fox eats the rabbit.

So the energy moves from the grass to the rabbit to the fox.

3. Parts of a food chain

Most food chains begin with the Sun and then a producer. After that come consumers.

• Producer: a living thing that makes its own food, like grass
• Primary consumer: an animal that eats plants, like a rabbit
• Secondary consumer: an animal that eats other animals, like a snake or fox
• Top consumer: an animal at the top of a chain that is not usually eaten by other animals in that chain

You do not need to memorize all of these names right away. The big idea is simple: energy moves from plants to animals.

4. What is a food web?

In real life, animals do not eat just one kind of food. A fox may eat rabbits, mice, or birds. A bird may eat seeds and insects. Because living things have many food choices, nature is not just one straight chain.

A food web is made of many connected food chains. It shows the many paths energy can take in an ecosystem.

Here is a simple idea of a food web in a field:

• grass → rabbit → fox
• grass → mouse → fox
• seeds → bird → hawk
• grass → grasshopper → bird → hawk

These chains connect because some animals eat more than one thing.

5. Why food webs are important

Food webs help us understand how living things depend on one another. If one part of the web changes, other parts may change too.

For example, if there is less grass:

• rabbits may have less food
• mice may have less food
• foxes may have less to eat because rabbits and mice may become fewer

This shows that all parts of an ecosystem are connected.

6. Food chains and food webs in different places

Food chains and food webs can be found in many habitats.

Forest example:

• Sun → tree leaves → caterpillar → bird

Pond example:

• Sun → water plants → small fish → big fish

Ocean example:

• Sun → sea plants → small shrimp → fish → seal

Even though the plants and animals are different, the pattern is the same. Energy starts with the Sun, then moves to producers, then to consumers.

7. Worked Examples

Example 1: A simple food chain

Suppose we have: Sun → grass → deer

Question: Where does the deer get its energy?

Step 1: The grass gets energy from the Sun.

Step 2: The deer eats the grass.

Answer: The deer gets its energy from the grass.

Example 2: Add one more animal

Now look at: Sun → grass → mouse → owl

Question: What does the owl eat?

Step 1: Find the arrow pointing to the owl.

Step 2: The arrow comes from the mouse.

Answer: The owl eats the mouse.

Example 3: Find the producer

Look at this chain: Sun → berry bush → bird → hawk

Question: Which living thing is the producer?

Step 1: A producer is a plant that makes its own food.

Step 2: The berry bush is a plant.

Answer: The berry bush is the producer.

Example 4: A small food web

• grass → rabbit → fox
• grass → mouse → fox
• grass → grasshopper → bird → fox

Question: What are two things the fox can eat?

Step 1: Look for arrows pointing to the fox.

Step 2: The arrows come from rabbit, mouse, and bird.

Answer: The fox can eat rabbit and mouse. It can also eat bird.

8. How to read arrows the right way

This part is very important. In food chains and food webs, the arrow shows where the energy goes.

In grass → rabbit, the rabbit eats the grass. The energy moves to the rabbit.

A good way to remember is: the arrow means “is eaten by” or “gives energy to”.

9. Quick tips to remember

• Energy starts with the Sun.
• Plants are producers because they make their own food.
• Animals are consumers because they eat plants or other animals.
• A food chain is one path of energy.
• A food web is many connected food chains.
• The arrow points to the eater.

10. Let’s review

Food chains and food webs show how living things get energy. Plants use sunlight to make food, and animals get energy by eating plants or other animals.

A food chain is one simple line, like grass → rabbit → fox. A food web is a group of connected food chains. Food webs show that ecosystems are full of connections, and a change in one part can affect many other parts.

When you look at a food chain or food web, remember to ask:

• Where does the energy start?
• Which living thing is the producer?
• Who eats whom?
• Where do the arrows point?

If you can answer those questions, you are doing a great job understanding food chains and food webs.

Symbiosis is a special kind of relationship between two different living things. These living things live very close together, and they affect each other.

Sometimes both living things are helped. Sometimes only one is helped. Sometimes one is helped and the other is harmed. Scientists group these close relationships into three main types: mutualism, commensalism, and parasitism.

Learning these three types can help us understand how plants and animals survive in nature.

What is symbiosis?

Symbiosis means a close relationship between two different kinds of living things. The two living things are called organisms.

When we study symbiosis, we ask one big question: Who is helped, and who is harmed?

• Mutualism: both organisms benefit.
• Commensalism: one organism benefits, and the other is not helped or harmed.
• Parasitism: one organism benefits, and the other is harmed.

A simple way to remember this is:

• Mutualism = +/+
• Commensalism = +/0
• Parasitism = +/-

The plus sign means helped, the zero means no change, and the minus sign means harmed.

1. Mutualism

In mutualism, both organisms get something good from the relationship. Each one benefits.

Sometimes one organism gets food while the other gets help. Sometimes both get protection. Sometimes one spreads seeds or pollen for the other.

Example: Bees and flowers

A bee lands on a flower to drink nectar. Nectar is food for the bee.

As the bee moves from flower to flower, it carries pollen. This helps flowers make seeds and grow more plants.

• The bee benefits because it gets food.
• The flower benefits because it gets help with pollen.

This is mutualism because both are helped.

Example: Clownfish and sea anemones

A clownfish can hide safely among the sea anemone's stinging arms. Many other fish stay away.

The clownfish may also help the sea anemone by cleaning it or bringing bits of food.

• The clownfish benefits by getting protection.
• The sea anemone benefits by getting help.

This is also mutualism.

2. Commensalism

In commensalism, one organism benefits, and the other is not helped or harmed. One gets something useful, and the other is mostly unaffected.

Example: A bird nesting in a tree

A bird may build its nest in the branches of a tree. The bird gets a safe place to live and raise its babies.

The tree is usually not helped, but it is not harmed either.

• The bird benefits by getting shelter.
• The tree is unaffected.

This is commensalism.

Example: Barnacles on a whale

Barnacles are small animals that can attach to a whale's skin. As the whale swims, the barnacles travel to new places and can find food in the water.

The whale is usually not helped or harmed very much by the barnacles.

• The barnacles benefit by getting a ride and better access to food.
• The whale is unaffected.

This is commensalism.

3. Parasitism

In parasitism, one organism benefits, but the other is harmed. The organism that benefits is called a parasite. The organism that is harmed is called the host.

The parasite gets food or a place to live from the host. The host loses energy, health, or comfort.

Example: A tick on a dog

A tick attaches to a dog's skin and drinks the dog's blood.

• The tick benefits because it gets food.
• The dog is harmed because it loses blood and may feel sick or itchy.

This is parasitism.

Example: Lice on a person

Lice live in hair and feed from the scalp.

• The lice benefit because they get food and a place to live.
• The person is harmed because the scalp becomes itchy and uncomfortable.

This is parasitism.

How to tell the three types apart

When you see two organisms together, ask these questions:

1. Are both helped? If yes, it is mutualism.
2. Is one helped and the other unchanged? If yes, it is commensalism.
3. Is one helped and the other harmed? If yes, it is parasitism.

You can think of it like this:

• Mutualism: both smile.
• Commensalism: one smiles, one shrugs.
• Parasitism: one smiles, one frowns.

Worked Example 1

Situation: A bee drinks nectar from a flower. Pollen sticks to the bee and is carried to another flower.

Step 1: Is the bee helped? Yes. It gets food.

Step 2: Is the flower helped? Yes. It gets help moving pollen.

Answer: This is mutualism because both organisms benefit.

Worked Example 2

Situation: A bird builds a nest in a tree. The bird gets a safe home. The tree is not helped or harmed.

Step 1: Is the bird helped? Yes.

Step 2: Is the tree helped or harmed? No.

Answer: This is commensalism because one benefits and the other is unaffected.

Worked Example 3

Situation: A tick drinks blood from a dog.

Step 1: Is the tick helped? Yes. It gets food.

Step 2: Is the dog helped or harmed? Harmed.

Answer: This is parasitism because one benefits and the other is harmed.

Worked Example 4

Situation: Barnacles attach to a whale. The barnacles get a ride through the ocean. The whale is not really helped or hurt.

Step 1: Are the barnacles helped? Yes.

Step 2: Is the whale helped? No.

Step 3: Is the whale harmed? No.

Answer: This is commensalism.

Important ideas to remember

• Symbiosis means a close relationship between two different organisms.
• In mutualism, both organisms benefit.
• In commensalism, one benefits and the other is not affected.
• In parasitism, one benefits and the other is harmed.

These relationships help shape ecosystems. Organisms depend on one another in many ways. Some relationships are helpful for both, some help only one without causing trouble, and some hurt one organism.

Quick Check

• If both organisms are helped, think mutualism.
• If one is helped and the other feels no change, think commensalism.
• If one is helped and the other is hurt, think parasitism.

Lesson Summary

Symbiosis is a close relationship between two different living things. There are three main types. Mutualism means both benefit, commensalism means one benefits while the other is unchanged, and parasitism means one benefits while the other is harmed.

To identify the type of symbiosis, always ask: Who benefits? Who is harmed? Is anyone unchanged? Those questions will help you sort the relationship correctly.

Invasive Species and Ecosystem Disruption

An ecosystem is a place where living things and nonliving things work together. Plants, animals, water, soil, air, and sunlight are all part of an ecosystem.

In a healthy ecosystem, organisms depend on one another. Some animals eat plants. Some animals eat other animals. Plants need sunlight, water, and soil. Everything is connected.

Sometimes a new living thing is brought into an ecosystem where it did not live before. This can cause big changes. Today, we will learn about invasive species and how they can disrupt, or upset, an ecosystem.

What is an invasive species?

An invasive species is a plant or animal that comes from a different place and spreads in a new area. It can cause harm to the plants, animals, and habitats already there.

Not every new species is invasive. A species becomes invasive when it spreads quickly and causes problems.

What does non-native mean?

A non-native species is a plant or animal that does not naturally live in a place. It may arrive by accident or be brought by people.

• A seed might stick to a shoe or a truck.
• A fish might be released into a pond.
• An animal might escape from a home or farm.

If that non-native species grows too much and hurts the ecosystem, it is called invasive.

Why can invasive species be a problem?

In their new home, invasive species often do not have the same natural predators. A natural predator is an animal that hunts and eats another animal.

When there are no predators to keep their numbers under control, invasive species can grow quickly. Then they may take too much food, space, sunlight, or water.

This causes ecosystem disruption. That means the normal balance of the ecosystem gets disturbed.

How invasive species disrupt ecosystems

1. They use up resources.
   Invasive species may take food, water, sunlight, or space that native species need.
2. They crowd out native species.
   Native species are the plants and animals that naturally belong in an area. Invasive species may grow or spread so fast that native species cannot keep up.
3. They change habitats.
   Some invasive plants cover ponds, fields, or forests. This can change where animals live and find food.
4. They upset food chains.
   If one species grows too much or pushes out another species, animals higher and lower in the food chain can be affected.

Native species and invasive species

Native species have lived in an ecosystem for a long time. They are part of the natural balance there.

Plants, animals, and other living things in that place have learned to live together. They compete for resources, but usually there is a balance.

An invasive species can change that balance very fast. Native species may not know how to protect themselves from the new species.

Example 1: A fast-growing plant in a pond

Imagine a pond with fish, frogs, insects, and water plants. The pond plants help give shelter to small animals.

Now imagine a new plant is brought to the pond. It grows very fast and spreads across the top of the water.

This plant blocks sunlight. The other pond plants cannot get enough light, so they begin to die.

If the pond plants die, small pond animals may lose their homes. Fish and frogs may have less food and less space. One new plant has changed the whole pond ecosystem.

Example 2: A new animal that eats too many insects

Imagine a new animal enters a forest. It eats many insects every day.

At first, this may not seem like a big problem. But if it eats too many insects, there may not be enough insects left for birds, frogs, and other animals that also need them for food.

Then those native animals may have trouble finding enough to eat. The food chain gets out of balance.

Worked Example 1: Finding the invasive species problem

A lake has native fish and native water plants. Then a non-native plant arrives and spreads quickly. Soon it covers much of the lake surface.

Question: What problem might happen?

Step 1: Think about what plants need. Plants need sunlight.

Step 2: If one plant covers the lake surface, sunlight may not reach plants below.

Step 3: If native plants die, fish may lose food or shelter.

Answer: The invasive plant may block sunlight and harm native plants and fish.

Worked Example 2: Looking at predators

A non-native rabbit is brought to a grassy area. In that new place, there are no animals that hunt the rabbit.

Question: Why might the rabbit population grow very fast?

Step 1: Natural predators help control animal populations.

Step 2: If there are no predators, more rabbits stay alive.

Step 3: More rabbits can have more baby rabbits.

Answer: The rabbit population may grow fast because nothing is keeping its numbers under control.

Worked Example 3: Counting resource use

In a garden, native caterpillars eat 10 leaves each day. Then an invasive insect arrives and eats 30 leaves each day.

Question: How many more leaves does the invasive insect eat each day than the native caterpillars?

We subtract:

$$30 - 10 = 20$$

Answer: The invasive insect eats 20 more leaves each day.

This means the invasive insect is using more of the garden's plant resources.

Worked Example 4: What happens next?

A new fish enters a pond and eats many tiny animals. The tiny animals used to be food for native fish.

Question: What may happen to the native fish?

Step 1: Native fish need the tiny animals for food.

Step 2: If the new fish eats too many tiny animals, there is less food left.

Step 3: Native fish may not get enough to eat.

Answer: The native fish population may get smaller because there is less food.

Signs that an invasive species is disrupting an ecosystem

• One species spreads very quickly.
• Native plants or animals begin to disappear.
• Food becomes harder for native animals to find.
• The habitat starts to change.
• The ecosystem becomes less balanced.

How people can help

People can help protect ecosystems from invasive species.

• Do not release pets into the wild.
• Do not move plants or animals from one place to another.
• Clean shoes, boats, and gear so seeds or small organisms do not spread.
• Learn about native plants and animals in your area.

Even small actions can help keep ecosystems healthy and balanced.

Why this matters

Plants and animals need safe places to live. When invasive species spread, native species can lose food, space, and shelter.

A healthy ecosystem has balance. Protecting native species helps protect that balance.

Summary

An invasive species is a non-native plant or animal that spreads and causes harm. It can grow quickly because it may not have natural predators in its new home.

Invasive species can take resources, crowd out native species, change habitats, and disrupt food chains. When we understand this, we can make better choices to help keep ecosystems balanced and healthy.