Ecosystems And Food Webs

Study revision notes for Ecosystems And Food Webs

Ecosystems and Food Webs Study Pack

1. Introduction / Essential Question

Essential Question

How do living things interact with each other and with their environment, and what happens when one part of an ecosystem changes?

Introduction / Hook

Imagine walking through a park, forest, desert, beach, or even a school garden. You might notice plants, insects, birds, soil, water, sunlight, rocks, and maybe signs of animals you cannot see. At first, these may look like separate things. Scientists see them as parts of a connected system.

An ecosystem includes all the living and nonliving parts of an area and the interactions among them. A pond ecosystem might include fish, algae, insects, frogs, bacteria, sunlight, water, oxygen, mud, and temperature. A city park ecosystem might include grass, squirrels, worms, trees, fungi, birds, humans, rainfall, air, and soil.

In this study pack, you will investigate how energy moves through ecosystems, how matter cycles through living and nonliving parts, how food chains and food webs show feeding relationships, and why biodiversity helps ecosystems stay stable. You will also practice interpreting diagrams, data tables, graphs, and experiment scenarios.

As you read, keep asking:

What do you notice?
What patterns do you see?
What evidence supports this idea?
How could a scientist test this?
How might this affect people and the environment?

2. Key Vocabulary / Definitions

Core Science Vocabulary

Term	Student-Friendly Definition	Example
Ecosystem	A system made of living and nonliving parts interacting in an area	A pond with fish, plants, water, mud, sunlight, and bacteria
System	A group of parts that interact and work together	A forest ecosystem, a human body, or a weather system
Biotic factor	A living or once-living part of an ecosystem	Trees, insects, dead leaves, bacteria
Abiotic factor	A nonliving part of an ecosystem	Sunlight, water, air, rocks, temperature
Organism	One individual living thing	One rabbit, one oak tree, one mushroom
Species	A group of similar organisms that can reproduce with each other	Gray wolves, red maple trees, honeybees
Population	All members of one species living in the same area	All the frogs in one pond
Community	All the different populations living and interacting in an area	Frogs, fish, algae, insects, and bacteria in a pond
Habitat	The place where an organism lives and gets what it needs	A cactus habitat might be a desert
Niche	The role an organism has in its ecosystem	A bee pollinates flowers and uses nectar for food
Producer	An organism that makes its own food, usually by photosynthesis	Grass, algae, trees
Consumer	An organism that gets energy by eating other organisms	Deer, wolves, grasshoppers, humans
Herbivore	A consumer that eats plants or algae	Rabbit, deer, caterpillar
Carnivore	A consumer that eats animals	Hawk, shark, snake
Omnivore	A consumer that eats both plants and animals	Bear, raccoon, human
Decomposer	An organism that breaks down dead organisms and waste	Fungi, bacteria
Detritivore	An organism that eats dead material or waste pieces	Earthworm, millipede, some crabs
Food chain	A simple model showing one path of energy flow	Grass → rabbit → fox
Food web	A model showing many connected food chains in an ecosystem	A web linking grass, insects, mice, birds, snakes, and hawks
Energy	The ability to cause change or do work	Sunlight energy helps plants make sugar
Matter	Anything that has mass and takes up space	Water, carbon dioxide, oxygen, sugar, minerals
Predator	An organism that hunts and eats another organism	Owl hunting a mouse
Prey	An organism that is hunted and eaten	Mouse eaten by an owl
Competition	A struggle between organisms for the same limited resource	Plants competing for sunlight
Mutualism	A relationship where both organisms benefit	Bees get nectar; flowers get pollinated
Commensalism	A relationship where one organism benefits and the other is not helped or harmed much	A bird nesting in a tree
Parasitism	A relationship where one organism benefits and the other is harmed	A tick feeding on a deer
Biodiversity	The variety of living things in an ecosystem	A coral reef has high biodiversity
Stability	The ability of a system to keep functioning when conditions change	A diverse prairie may recover after a drought
Hypothesis	A testable explanation or prediction	If fertilizer increases plant growth, then fertilized plants will grow taller
Variable	A factor that can change in an investigation	Amount of water, light, temperature
Evidence	Data or observations used to support a claim	A graph showing plant growth over time

Vocabulary Connections

The words ecosystem, system, energy, and matter are closely connected. An ecosystem is a system because it has interacting parts. Energy flows through the system, usually starting with sunlight. Matter cycles through the system as atoms move between air, soil, water, and living things.

3. Core Science Concepts

3.1 Ecosystems Are Made of Living and Nonliving Parts

Every ecosystem has biotic and abiotic factors.

Biotic factors include:

plants
animals
fungi
bacteria
dead leaves
animal waste

Abiotic factors include:

sunlight
water
air
temperature
soil minerals
rocks
wind

These parts interact. For example, a plant needs sunlight, water, carbon dioxide, and minerals to grow. A rabbit may eat the plant. A fox may eat the rabbit. Bacteria and fungi may break down dead plants, dead animals, and waste, returning matter to the soil.

scientificDiagram: Levels of Organization in an Ecosystem

Organism → Population → Community → Ecosystem

Example:

One frog → all frogs in the pond → frogs, fish, insects, algae, bacteria → the pond community plus water, sunlight, mud, oxygen, temperature

What do you notice? Each level includes more parts than the level before it.

3.2 Energy Flows Through Ecosystems

Most ecosystems get energy from the Sun. Producers, such as plants and algae, capture sunlight energy through photosynthesis. During photosynthesis, producers use sunlight, carbon dioxide, and water to make sugar and oxygen.

Simple photosynthesis model:

carbon dioxide + water + sunlight energy → sugar + oxygen

The sugar stores energy. When consumers eat producers, some of that energy moves into the consumer. When predators eat prey, energy moves again. However, energy transfer is not perfect. At each step, organisms use energy for movement, growth, body heat, repair, reproduction, and life processes. Much energy is released as heat.

Because energy is lost at each transfer, food chains usually have only a few levels.

flowDiagram: Energy Flow

Sun ↓ Producers ↓ Primary consumers ↓ Secondary consumers ↓ Tertiary consumers ↓ Decomposers help recycle matter from all levels

Energy flows in one main direction. Matter cycles.

3.3 Matter Cycles Through Ecosystems

Energy flows, but matter cycles. Matter includes atoms and molecules such as carbon, oxygen, nitrogen, water, and minerals. The atoms in a plant may later become part of an animal, then part of soil, air, or another plant.

Decomposers are very important because they break down dead organisms and waste. This returns nutrients to soil and water, where producers can use them again.

Example:

A tree takes in carbon dioxide from the air.
The tree uses carbon atoms to build sugars and wood.
A caterpillar eats a leaf.
A bird eats the caterpillar.
Waste and dead organisms are broken down by decomposers.
Matter returns to soil, water, or air.
Plants use some of the matter again.

3.4 Food Chains Show One Path of Energy

A food chain is a simple model. It shows one possible path of energy from one organism to another.

Example:

Grass → grasshopper → frog → snake → hawk

The arrows mean "energy moves to." The grasshopper gets energy from grass. The frog gets energy from the grasshopper.

Students sometimes think the arrow means "eats." A better way to read it is:

"Grass is eaten by the grasshopper, so energy moves from grass to grasshopper."

3.5 Food Webs Show Many Connected Paths

Real ecosystems are more complex than one food chain. Most organisms eat more than one kind of food, and many organisms are eaten by more than one predator. A food web shows many connected food chains.

foodWeb: Meadow Food Web

Sun ↓ Grass, clover, wildflowers

Grass → rabbit → fox Grass → grasshopper → frog → snake → hawk Clover → mouse → snake → hawk Wildflowers → butterfly → bird → hawk Dead plants and animals → fungi and bacteria → nutrients in soil → plants

ASCII food web:

                Hawk
              ↑  ↑  ↑
            Snake Bird Fox
             ↑     ↑    ↑
           Frog  Butterfly Rabbit
             ↑       ↑      ↑
        Grasshopper Wildflowers Grass
             ↑              ↑
          Grass           Clover

Decomposers act on dead material from every level.

Food webs help scientists predict what might happen if one population changes. For example, if a disease reduces the rabbit population, foxes may have less food and may hunt more mice or birds.

3.6 Producers, Consumers, and Decomposers Have Different Roles

Ecosystems need different roles to keep functioning.

Producers capture energy and build food molecules. They form the base of most food webs.

Consumers get energy by eating producers or other consumers.

Decomposers break down dead organisms and waste, helping recycle matter.

Without producers, most consumers would not have food. Without decomposers, nutrients would stay locked in dead material and waste.

comparisonGrid: Ecosystem Roles

Role	What It Does	Examples	Why It Matters
Producer	Makes food using sunlight or chemical energy	Grass, algae, trees	Brings energy into the food web
Primary consumer	Eats producers	Rabbit, deer, grasshopper	Moves energy from producers to animals
Secondary consumer	Eats primary consumers	Frog, small fish, spider	Helps control herbivore populations
Tertiary consumer	Eats secondary consumers	Hawk, shark, wolf	Often a top predator
Decomposer	Breaks down dead organisms and waste	Bacteria, fungi	Recycles matter and nutrients

3.7 Biodiversity Supports Ecosystem Stability

Biodiversity means variety of life. It can include:

variety of species
variety of genes within a species
variety of habitats and ecosystems

An ecosystem with high biodiversity often has more possible food sources and more ways to recover after change. For example, if one plant species decreases during a drought, herbivores may be able to eat other plants. If a food web has only a few species, the loss of one species can cause a bigger disruption.

High biodiversity does not mean an ecosystem cannot be damaged. Pollution, habitat destruction, invasive species, climate change, and overharvesting can still cause serious harm. But biodiversity can improve resilience, which is the ability to recover after disturbance.

3.8 Populations Change Over Time

Populations can increase, decrease, or stay about the same. Population size depends on resources and conditions.

Factors that can increase a population:

more food
more water
more shelter
fewer predators
successful reproduction
favorable weather

Factors that can decrease a population:

less food
disease
drought
more predators
habitat loss
competition
pollution

3.9 Predator-Prey Relationships Create Patterns

Predator and prey populations often affect each other.

If the prey population increases, predators may have more food, so predator numbers may later increase. If predator numbers increase too much, they may eat more prey, causing prey numbers to decrease. Then predators may have less food, and predator numbers may decrease.

This does not happen perfectly like a machine. Weather, disease, migration, human activity, and other food sources can also affect populations. Still, predator-prey graphs often show connected patterns.

graph: Predator and Prey Population Pattern

Example data for a small island ecosystem:

Year	Rabbits	Foxes
1	80	8
2	120	10
3	160	15
4	130	22
5	90	20
6	70	14
7	110	11
8	150	16

Text graph:

Rabbits: 80 ── 120 ── 160 ── 130 ── 90 ── 70 ── 110 ── 150 Foxes: 8 ─── 10 ─── 15 ─── 22 ─── 20 ── 14 ─── 11 ─── 16

What pattern do you notice? The fox population tends to rise after the rabbit population rises. This makes sense because foxes depend partly on rabbits for food.

3.10 Ecosystems Can Be Changed by Natural Events and Human Activities

Natural changes include:

floods
droughts
wildfires
storms
volcanic eruptions
seasonal temperature changes
disease outbreaks

Human activities include:

building roads and cities
farming
pollution
removing forests
introducing invasive species
restoring habitats
protecting endangered species
reducing waste and runoff

Some changes are harmful, some are helpful, and some depend on the situation. A wildfire may destroy some habitats in the short term but can also return nutrients to soil and help some ecosystems renew. Pollution can reduce oxygen in water and harm aquatic life. A habitat restoration project can increase biodiversity and improve ecosystem health.

4. Examples, Case Studies, and Real-World Applications

Case Study 1: Kelp Forest Food Web

Kelp forests are underwater ecosystems found in cool ocean water. Kelp is a type of large algae. It acts like a producer and provides habitat for many organisms.

Simplified kelp forest food web:

Kelp → sea urchin → sea otter Kelp → small fish → larger fish → seal Dead organisms → decomposers and scavengers

Sea otters eat sea urchins. Sea urchins eat kelp. If sea otter numbers decrease, sea urchin numbers may increase. If sea urchins increase too much, they may eat large amounts of kelp. This can reduce habitat for fish and other organisms.

This example shows a trophic cascade, where a change at one feeding level affects other levels in the food web.

Case Study 2: School Garden Ecosystem

A school garden might include:

producers: lettuce, beans, flowers, grass
consumers: bees, butterflies, aphids, birds, squirrels
decomposers: fungi, bacteria, earthworms
abiotic factors: sunlight, water, soil, temperature, rocks

Investigation question:

How does the amount of sunlight affect plant growth in a school garden?

Possible hypothesis:

If bean plants receive more hours of sunlight each day, then they will grow taller because they can photosynthesize for more time.

Variables:

independent variable: hours of sunlight
dependent variable: plant height
controlled variables: plant species, soil type, water amount, container size

Evidence:

Students could measure plant height every three days and record the data in a table.

Case Study 3: Lake Pollution and Algal Blooms

Fertilizers from lawns and farms can wash into lakes after rain. Fertilizers often contain nutrients such as nitrogen and phosphorus. These nutrients can cause algae to grow quickly. This is called an algal bloom.

At first, more algae may seem helpful because algae are producers. But when large amounts of algae die, decomposers break them down. Decomposers use oxygen in the water. If oxygen levels drop too low, fish and other aquatic animals may die.

Cause-and-effect chain:

Fertilizer runoff → extra nutrients in lake → rapid algae growth → algae die → decomposers increase → dissolved oxygen decreases → fish may die

This example shows why ecosystems must be studied as systems. One change can affect many parts.

Real-World Applications

Scientists, engineers, farmers, city planners, and conservationists use ecosystem knowledge to:

protect endangered species
design wildlife crossings over roads
restore wetlands
improve water quality
manage fisheries
reduce invasive species
protect pollinators
plan sustainable farms
understand climate change effects

STEM connection:

An environmental engineer might design a wetland area to filter polluted runoff before it reaches a river. To do this well, the engineer must understand plants, soil, water flow, microorganisms, and animal habitats.

5. Tables and Data

dataTable: Pond Ecosystem Survey

A class surveys a pond ecosystem and counts organisms seen in one hour.

Organism	Role	Number Observed
Algae	Producer	200 patches
Duckweed	Producer	75 plants
Water snails	Primary consumer	48
Tadpoles	Primary consumer	36
Dragonfly nymphs	Secondary consumer	22
Small fish	Secondary consumer	18
Frogs	Secondary consumer	9
Herons	Tertiary consumer	2
Fungi and bacteria	Decomposer	Not counted directly

Data questions:

Which organisms are producers?
Which organism was the top predator in this survey?
Why are fungi and bacteria difficult to count directly?
What might happen to the heron population if small fish and frogs decreased?
What evidence shows that this pond has more than one feeding level?

dataTable: Effect of Light on Algae Growth

Students grow algae samples under different hours of light each day.

Light per Day	Average Algae Growth After 10 Days
0 hours	1 mm
4 hours	9 mm
8 hours	18 mm
12 hours	24 mm
16 hours	25 mm

What pattern do you see?

Algae growth increased as light increased from 0 to 12 hours. Growth changed very little from 12 to 16 hours. This suggests that light helps algae grow, but another factor may become limiting after a certain point.

graph: Text Bar Graph of Algae Growth

0 hours: # 4 hours: ######### 8 hours: ################## 12 hours: ######################## 16 hours: #########################

Each # represents about 1 mm of growth.

Data Interpretation Tip

When reading a graph or table:

Read the title.
Identify the variables.
Look for the highest and lowest values.
Look for patterns, increases, decreases, or leveling off.
Use numbers as evidence in your explanation.

Example claim:

More light increased algae growth up to about 12 hours per day.

Evidence:

Algae grew 1 mm with 0 hours of light, 18 mm with 8 hours, and 24 mm with 12 hours.

Reasoning:

Algae use light energy for photosynthesis, so more light can increase growth until another resource limits growth.

6. Text / ASCII Diagrams and Visual Aids

scientificDiagram: Energy Pyramid

Energy available usually decreases at higher feeding levels.

         Tertiary consumers
              Hawk
         Energy: lowest
        ------------------
        Secondary consumers
          Snake, frog
        ------------------
        Primary consumers
       Rabbit, grasshopper
        ------------------
            Producers
          Grass, clover
         Energy: highest

Why is the base largest? Producers capture energy from sunlight and support the rest of the food web.

flowDiagram: Matter Cycling

Air, water, soil nutrients ↓ Producers build body materials ↓ Consumers eat producers or other consumers ↓ Waste and dead organisms form ↓ Decomposers break materials down ↓ Matter returns to air, water, and soil

experimentSetup: Testing Compost and Plant Growth

Question:

Does compost affect plant growth?

Setup:

Group A: bean plants in regular soil Group B: bean plants in soil with compost

Controlled variables:

same plant species
same number of seeds
same container size
same amount of water
same light exposure
same measuring schedule

Measurement:

Measure plant height every three days for three weeks.

Possible data table:

Day	Group A Average Height	Group B Average Height
0	0 cm	0 cm
3	2 cm	3 cm
6	5 cm	7 cm
9	8 cm	12 cm
12	11 cm	16 cm
15	14 cm	20 cm
18	17 cm	24 cm
21	20 cm	28 cm

Scientific reasoning:

If Group B grows taller, the evidence may support the claim that compost improves plant growth. However, students should also consider sample size, measurement accuracy, and whether all variables were controlled.

infographic: Ecosystem Change Chain

Change: drought reduces plant growth

↓

Less food for herbivores

↓

Herbivore population may decrease or move away

↓

Predators may have less food

↓

Predators may decrease, move, or switch prey

↓

Food web balance changes

scenarioCard: Invasive Species

Scenario:

A new fish species is introduced into a lake. It eats the same insects as native fish and has no major predators in the lake.

Think about:

What resources might the new fish compete for?
How might native fish populations change?
What data should scientists collect?
How could this change affect birds that eat fish?
What evidence would show that the new fish is causing a problem?

Possible investigation:

Scientists could compare insect populations, native fish numbers, and bird nesting success before and after the new fish arrives.

7. Common Misconceptions

Misconception 1: Food chains show everything in an ecosystem.

Correction:

A food chain shows only one pathway of energy. A food web is more realistic because organisms usually have multiple food sources and predators.

Misconception 2: Arrows in a food chain mean "eats."

Correction:

Arrows show the direction energy moves. Grass → rabbit means energy moves from grass to rabbit when the rabbit eats the grass.

Misconception 3: Decomposers are not important because they are small.

Correction:

Decomposers are essential. They recycle matter from dead organisms and waste. Without decomposers, nutrients would not return to soil and water efficiently.

Misconception 4: Energy cycles like matter.

Correction:

Matter cycles, but energy flows. Energy enters most ecosystems as sunlight and eventually leaves as heat.

Misconception 5: Predators are bad for ecosystems.

Correction:

Predators help control prey populations and can support ecosystem balance. Removing a predator can cause major changes in a food web.

Misconception 6: Biodiversity only means having many animals.

Correction:

Biodiversity includes plants, animals, fungi, bacteria, genetic variety, and habitat variety.

Misconception 7: All ecosystem changes are harmful.

Correction:

Some changes are harmful, some are helpful, and some are part of natural cycles. Scientists study evidence to understand effects.

Misconception 8: Plants get their food from soil.

Correction:

Plants need minerals and water from soil, but they make sugar using sunlight, carbon dioxide, and water. The sugar is their food.

Misconception 9: A larger animal is always higher in the food web.

Correction:

Feeding level depends on what an organism eats, not its size. A tiny parasite may feed on a large animal. A large elephant is a primary consumer because it eats plants.

Misconception 10: If one species disappears, the rest of the ecosystem will always be fine.

Correction:

The effect depends on the species and the food web. Some species have many connections, so their loss can affect many other populations.

8. Science Thinking Tips

Tip 1: Use Claim-Evidence-Reasoning

When explaining ecosystem changes, use CER.

Claim: Answer the question clearly.

Evidence: Use observations, data, or examples.

Reasoning: Explain why the evidence supports the claim using science ideas.

Example:

Question:

How might fewer plants affect rabbits?

Claim:

Fewer plants would likely decrease the rabbit population.

Evidence:

Rabbits are primary consumers that eat plants.

Reasoning:

If plants decrease, rabbits have less food energy available. With less food, fewer rabbits may survive and reproduce.

Tip 2: Compare and Contrast Carefully

When comparing food chains and food webs:

Both show feeding relationships.
Both show energy movement.
Food chains are simple and linear.
Food webs are more complex and realistic.

Use words like:

both
however
similar
different
unlike
in contrast

Tip 3: Identify Variables in Investigations

Ask:

What is the scientist changing? This is the independent variable.
What is the scientist measuring? This is the dependent variable.
What should stay the same? These are controlled variables.

Example:

If students test how water amount affects plant growth:

independent variable: amount of water
dependent variable: plant height or mass
controlled variables: plant type, soil, light, container size, temperature

Tip 4: Interpret Food Webs with Arrows

To read a food web:

Start with producers.
Follow arrows to consumers.
Identify organisms with many arrows connected to them.
Ask what might happen if one population increases or decreases.
Remember that decomposers recycle matter from all organisms.

Tip 5: Watch for Limiting Factors

A limiting factor is something that restricts population growth. Examples include food, water, space, shelter, disease, and predators.

If a population has plenty of food but little water, water may be the limiting factor. If algae have plenty of light but not enough nutrients, nutrients may limit growth.

Tip 6: Use Precise Vocabulary

Instead of saying:

"The hawk gets food from the grass."

Say:

"Energy originally captured by grass moves through the food web to the hawk when the hawk eats consumers that depend on producers."

Precise vocabulary helps your explanation sound scientific and clear.

9. Interactive Thinking Tasks and Discussion Prompts

Task 1: Build a Food Chain

Use these organisms:

grass, owl, mouse, snake

Put them in an order that shows energy flow.

Then answer:

Which organism is the producer?
Which organism is the primary consumer?
Which organism is the top predator in this chain?
What would the arrows mean?

Task 2: Food Web Prediction

In a meadow, hawks eat snakes and mice. Snakes eat frogs and mice. Frogs eat grasshoppers. Grasshoppers eat grass.

Predict what might happen if a disease reduces the frog population.

Use these sentence starters:

I predict that...
My evidence is...
This happens because...

Task 3: Biodiversity Discussion

Two fields are compared.

Field A has one crop species and very few insects.

Field B has grasses, wildflowers, shrubs, bees, beetles, birds, spiders, worms, and fungi.

Discuss:

Which field has greater biodiversity?
Which field may be more stable after a pest outbreak?
What evidence supports your answer?
What information would you still want to collect?

Task 4: Design a Mini Investigation

Question:

Do earthworms affect how quickly dead leaves break down?

Plan an investigation:

What is your hypothesis?
What is the independent variable?
What is the dependent variable?
What variables should be controlled?
What data would you collect?
How would you make the test fair?

Task 5: Human Impact Scenario

A wetland near a town is drained to build a shopping center.

Discuss:

Which organisms might lose habitat?
How could the food web change?
How might flooding or water quality be affected?
What design choices could reduce environmental harm?

10. Practice Questions

A. Quick Recall Questions

What is an ecosystem?
What is the difference between biotic and abiotic factors?
Give two examples of producers.
What does a consumer do?
What role do decomposers play?
What does biodiversity mean?
What is a food chain?
What is a food web?
In a food chain, what do arrows show?
What is a population?
What is a community?
What is a habitat?
What is a niche?
What is a hypothesis?
What is a variable?
What is evidence?
Why do most ecosystems depend on sunlight?
What is a limiting factor?
Name one predator-prey relationship.
Why is matter recycled in ecosystems?

B. Multiple Choice Questions

Choose the best answer.

Which is an abiotic factor in a forest ecosystem? A. Deer B. Mushroom C. Sunlight D. Oak tree
Which organism is usually a producer? A. Hawk B. Grass C. Rabbit D. Fox
What do arrows in a food chain show? A. The direction energy moves B. The size of each animal C. The order organisms were discovered D. The direction animals walk
Which food chain is written correctly for energy flow? A. Hawk → snake → mouse → grass B. Grass → mouse → snake → hawk C. Snake → grass → hawk → mouse D. Mouse → grass → snake → hawk
Which organism breaks down dead material and waste? A. Producer B. Decomposer C. Predator D. Prey
Which is the best definition of biodiversity? A. The number of rocks in an ecosystem B. The variety of living things in an ecosystem C. The total amount of sunlight in an area D. The size of one animal population
A rabbit eats grass. The rabbit is a: A. Producer B. Primary consumer C. Decomposer D. Tertiary consumer
A hawk eats a snake that ate a mouse. The hawk is most likely a: A. Producer B. Herbivore C. Higher-level consumer D. Decomposer
Which statement about energy in ecosystems is most accurate? A. Energy cycles forever without loss. B. Energy usually flows from the Sun to producers to consumers. C. Energy begins with decomposers in all ecosystems. D. Energy moves only from animals to plants.
Which statement about matter in ecosystems is most accurate? A. Matter cycles through living and nonliving parts. B. Matter disappears when organisms die. C. Matter only moves through animals. D. Matter cannot be reused.
Which is a population? A. All organisms in a pond B. All frogs in one pond C. Water, rocks, and sunlight D. One frog on a lily pad
Which is a community? A. All fish, frogs, insects, algae, and bacteria in a pond B. One fish in a pond C. The water temperature of a pond D. All ponds in a state
Which relationship is predator-prey? A. A bee pollinates a flower while getting nectar B. A fox hunts a rabbit C. A bird builds a nest in a tree D. Two plants grow near each other
Which is an example of mutualism? A. A tick feeds on a deer B. A hawk eats a mouse C. A bee gets nectar and pollinates a flower D. A mushroom breaks down a dead log
Which is an example of competition? A. Two plants need the same sunlight and water B. A fungus breaks down dead leaves C. A frog eats a fly D. A bird lays eggs
What might happen if producers disappear from a food web? A. Consumers would have more food. B. The food web would likely collapse. C. Decomposers would stop needing matter. D. Predators would become producers.
Why are food webs more realistic than food chains? A. They show only one feeding path. B. They show many connected feeding relationships. C. They do not include producers. D. They only show predators.
What is an invasive species? A. A native species that always helps biodiversity B. A species introduced to a new area that may cause harm C. A species that has no effect on ecosystems D. A species that lives only in water
Which is a limiting factor for a population? A. Unlimited food B. Unlimited space C. Lack of water D. Extra sunlight at night
Which variable is changed by the scientist in an experiment? A. Independent variable B. Dependent variable C. Controlled variable D. Random error
Which variable is measured in an experiment? A. Independent variable B. Dependent variable C. Controlled variable D. Constant container
A student tests whether fertilizer affects plant height. What is the dependent variable? A. Type of fertilizer B. Plant height C. Amount of light kept the same D. Type of pot kept the same
Which is evidence? A. A guess with no data B. A measurement table showing plant growth C. A random opinion D. A question that cannot be tested
What is a hypothesis? A. A testable explanation or prediction B. A final answer that never changes C. A list of materials D. A food web diagram
Why can algal blooms harm fish? A. Decomposers breaking down dead algae can use up oxygen. B. Algae always eat fish. C. Algae remove all sunlight from Earth. D. Fish cannot live near producers.
Which is the best example of a decomposer? A. Bacteria B. Hawk C. Deer D. Grasshopper
What happens to much of the energy at each feeding level? A. It is released as heat or used for life processes. B. It becomes new sunlight. C. It disappears as matter. D. It all transfers to the next level.
Which ecosystem would likely have higher biodiversity? A. A field with one crop species only B. A forest with many plant, animal, fungus, and bacteria species C. A parking lot with no plants D. A jar with one species of algae
A food web has many arrows leading to and from one species. What does this suggest? A. The species has no role. B. The species is connected to many feeding relationships. C. The species cannot affect the ecosystem. D. The species is abiotic.
Which question is most scientific and testable? A. Are frogs the best animals? B. Do frogs look interesting? C. How does water temperature affect tadpole growth? D. Should everyone like ponds?
Which organism is an omnivore? A. A deer that eats leaves only B. A bear that eats berries and fish C. A mushroom breaking down a log D. Grass making sugar
Which statement best explains ecosystem stability? A. Nothing in the ecosystem ever changes. B. The ecosystem can keep functioning or recover after some changes. C. Only predators survive. D. All populations must be exactly equal.

C. Short Answer Questions

Explain why producers are important in most ecosystems.
Describe how decomposers help recycle matter.
Compare a food chain and a food web.
Explain how a drought could affect a meadow food web.
Why might a predator population decrease after a prey population decreases?
How can biodiversity help an ecosystem recover after a disturbance?
Explain the difference between a habitat and a niche.
A student says, "Plants eat soil." Correct this statement using science vocabulary.
Explain why food chains usually do not have many levels.
Describe one way humans can help restore an ecosystem.

D. Data and Graph Interpretation Questions

Use the algae growth data:

Light per Day	Average Algae Growth After 10 Days
0 hours	1 mm
4 hours	9 mm
8 hours	18 mm
12 hours	24 mm
16 hours	25 mm

What is the independent variable?
What is the dependent variable?
How much did algae grow with 8 hours of light?
Between which two light levels did growth change the least?
Write a claim about how light affects algae growth.
Use two numbers from the table as evidence.
Suggest one controlled variable for this investigation.
What other factor might limit algae growth after 12 hours of light?

E. Longer Written / Reasoning Questions

A farmer notices that insects are eating crop plants. The farmer wants to remove all spiders from the field because spiders seem scary. Explain why removing spiders could make the insect problem worse. Use food web reasoning.
A lake receives fertilizer runoff after heavy rain. Over several weeks, algae increase, then many fish die. Explain this sequence using cause and effect.
Compare a low-biodiversity ecosystem and a high-biodiversity ecosystem. Which might be more stable after a disease affects one plant species? Explain your reasoning.
Design an investigation to test how different amounts of water affect plant growth. Include a hypothesis, independent variable, dependent variable, controlled variables, and data you would collect.
In a forest food web, oak trees produce acorns. Mice, deer, and squirrels eat acorns. Foxes eat mice and squirrels. Hawks eat mice. Predict what might happen if a late frost kills many oak flowers, causing fewer acorns to form. Include at least three effects.

11. Answer Key

A. Quick Recall Answers

An ecosystem is a system of living and nonliving parts interacting in an area.
Biotic factors are living or once-living; abiotic factors are nonliving.
Examples include grass, algae, trees, and wildflowers.
A consumer gets energy by eating other organisms.
Decomposers break down dead organisms and waste and recycle matter.
Biodiversity is the variety of living things in an ecosystem.
A food chain shows one path of energy flow.
A food web shows many connected food chains.
Arrows show the direction energy moves.
A population is all members of one species in an area.
A community is all the populations interacting in an area.
A habitat is where an organism lives.
A niche is an organism's role in its ecosystem.
A hypothesis is a testable explanation or prediction.
A variable is a factor that can change in an investigation.
Evidence is data or observations used to support a claim.
Producers use sunlight to make food, supporting most food webs.
A limiting factor is something that restricts population growth.
Examples include fox-rabbit, owl-mouse, snake-frog, or shark-fish.
Matter is reused as atoms move through organisms, air, water, and soil.

B. Multiple Choice Answers

C. Short Answer Sample Answers

Producers are important because they capture energy, usually from sunlight, and make food molecules. Consumers depend directly or indirectly on producers for energy.
Decomposers break down dead organisms and waste. This returns nutrients and other matter to soil, water, and air so producers can use them again.
A food chain shows one path of energy flow, such as grass → rabbit → fox. A food web shows many connected feeding relationships and is more realistic.
A drought could reduce plant growth. Herbivores may have less food, so their populations may decrease or move. Predators that eat those herbivores may also be affected.
If prey decreases, predators may have less food. With less energy available, fewer predators may survive and reproduce.
Biodiversity can help because organisms may have alternative food sources or roles. A more diverse ecosystem may have more ways to keep functioning after change.
A habitat is where an organism lives. A niche is the organism's role, including what it eats, what eats it, and how it interacts with the ecosystem.
Plants do not eat soil. Plants make sugar using sunlight, carbon dioxide, and water through photosynthesis, while soil provides water and minerals.
Food chains usually do not have many levels because energy is used and released as heat at each transfer. Less energy is available at higher levels.
Humans can restore ecosystems by planting native species, reducing pollution, removing invasive species, protecting habitats, or restoring wetlands.

D. Data and Graph Interpretation Answers

The independent variable is hours of light per day.
The dependent variable is average algae growth after 10 days.
The algae grew 18 mm with 8 hours of light.
Growth changed the least between 12 hours and 16 hours.
Example claim: More light increases algae growth up to about 12 hours per day.
Example evidence: Algae grew 1 mm with 0 hours of light and 24 mm with 12 hours of light.
Controlled variables could include algae species, container size, water amount, nutrient amount, temperature, or starting algae amount.
Nutrients, space, carbon dioxide, temperature, or starting population size might limit growth.

12. Model Answers / Suggested Responses

Longer Written Question 1 Model Answer

Removing spiders could make the insect problem worse because spiders are predators that eat insects. In a crop field food web, crop plants are producers and many insects are primary consumers because they eat the plants. Spiders are consumers that can help control insect populations. If the farmer removes spiders, fewer insects may be eaten. The insect population could increase, causing more crop damage. A better plan would be to collect evidence about which organisms are eating the crops and which predators help control them.

Key points:

Spiders are predators.
Many crop-eating insects are primary consumers.
Removing predators can increase prey populations.
Food webs help predict indirect effects.
Evidence should guide decisions.

Longer Written Question 2 Model Answer

Fertilizer runoff can add extra nutrients, such as nitrogen and phosphorus, to a lake. These nutrients can cause algae to grow rapidly, forming an algal bloom. When large amounts of algae die, decomposers break them down. Decomposers use dissolved oxygen during this process. If oxygen levels drop too low, fish and other aquatic animals may not get enough oxygen to survive. This sequence shows that adding nutrients to one part of an ecosystem can affect producers, decomposers, oxygen levels, and consumers.

Key points:

Fertilizer adds nutrients.
Algae increase because they use nutrients and light.
Dead algae are broken down by decomposers.
Decomposition uses oxygen.
Low oxygen can kill fish.

Longer Written Question 3 Model Answer

A high-biodiversity ecosystem might be more stable after a disease affects one plant species. In a low-biodiversity ecosystem, many consumers may depend on the same plant. If that plant decreases, herbivores may lose a major food source, and predators may later be affected. In a high-biodiversity ecosystem, herbivores may have other plant species to eat, and the food web may have more alternative pathways for energy flow. However, scientists would still need evidence, such as population data and food web connections, to predict the exact effects.

Key points:

High biodiversity means more variety of life.
More variety can provide alternative food sources.
Low biodiversity can make a system more vulnerable.
Stability means continued function or recovery after change.
Evidence is needed for a strong prediction.

Longer Written Question 4 Model Answer

Hypothesis:

If bean plants receive a moderate amount of water each day, then they will grow taller than plants that receive too little or too much water because plants need water for life processes but overly wet soil can reduce root health.

Independent variable:

Amount of water given each day.

Dependent variable:

Plant growth, measured as height in centimeters or mass after a set time.

Controlled variables:

Use the same plant species, same soil type, same container size, same light exposure, same temperature, same number of seeds, and same investigation length.

Data collection:

Measure plant height every three days for three weeks. Record observations such as leaf color, number of leaves, and whether the soil is dry or soggy.

Fair test:

Test several plants in each water group and calculate an average. Keep all variables the same except the amount of water.

Longer Written Question 5 Model Answer

If a late frost kills many oak flowers, fewer acorns may form. Mice, deer, and squirrels would have less food because they eat acorns. Some of these animals might decrease, move to another area, or switch to other food sources. Foxes and hawks could also be affected because foxes eat mice and squirrels, and hawks eat mice. If mouse numbers decrease, hawks may have less food or may hunt other prey more often. This shows how a change to a producer can affect several consumers in a food web.

Key points:

Oak trees are producers.
Fewer acorns means less food for acorn-eating consumers.
Mouse, deer, and squirrel populations may decrease or change behavior.
Foxes and hawks may have less prey.
Effects can spread through the food web.

13. Final Revision Checklist

Use this checklist before a quiz, discussion, or assessment.

□ I can define ecosystem, population, community, habitat, and niche.

□ I can identify biotic and abiotic factors in an ecosystem.

□ I can explain how energy flows from the Sun to producers and consumers.

□ I can explain how matter cycles through living and nonliving parts of ecosystems.

□ I can read arrows in food chains and food webs correctly.

□ I can identify producers, consumers, decomposers, predators, and prey.

□ I can explain why decomposers are important.

□ I can describe how biodiversity can support ecosystem stability.

□ I can interpret data tables and simple graphs about ecosystems.

□ I can identify independent variables, dependent variables, and controlled variables.

□ I can use evidence to support a scientific claim.

□ I can explain common misconceptions, such as "plants eat soil" or "energy cycles."

□ I can predict how a change in one population may affect other parts of a food web.

□ I can describe real-world ecosystem issues such as invasive species, fertilizer runoff, habitat loss, and restoration.

□ I have attempted the practice questions.

□ I have reviewed the answer key and model answers.

□ I can write a Claim-Evidence-Reasoning explanation about an ecosystem change.

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