Food Webs and Human Impact on Ecosystems — Answers
Answer sheet with model answers and success criteria for the 7th Grade ecosystems & food chains worksheet.
Success TutoringWorksheetsAnswer sheet7th Grade · Science · Ecosystems & Food ChainsFood Webs and Human Impact on Ecosystems — Answers
Part A: Reading a food web
Use this grassland food web. Each arrow means is eaten by.
Grass → grasshopper · Grass → rabbit · Grass → mouse
Grasshopper → spider · Grasshopper → frog · Spider → frog
Mouse → snake · Mouse → hawk · Frog → snake · Snake → hawk
Rabbit → fox
A food chain shows one path of energy from a producer to a top consumer. A food web shows all the linked food chains in an ecosystem. Bacteria and fungi act as decomposers for every organism in the web.
- Producer
- Consumer
- Herbivore
- Carnivore
- Decomposer
- Trophic level
- Makes its own food by photosynthesis, using energy from sunlight
- Gets its energy by eating other organisms
- A consumer that eats only animals
- The feeding position of an organism in a food chain
- Breaks down dead organisms and wastes, returning nutrients to the soil
- A consumer that eats only plants
Answer: Producer → Makes its own food by photosynthesis, using energy from sunlight; Consumer → Gets its energy by eating other organisms; Herbivore → A consumer that eats only plants; Carnivore → A consumer that eats only animals; Decomposer → Breaks down dead organisms and wastes, returning nutrients to the soil; Trophic level → The feeding position of an organism in a food chain
Answer: Grass.
Answer: The grasshopper, the rabbit and the mouse.
Answer: The frog. Grass → grasshopper → frog (secondary consumer); grass → grasshopper → spider → frog (tertiary consumer). (The snake and the hawk also feed at more than one level, so a correct answer naming either with two matching chains is accepted.)
Answer: Grass → grasshopper → spider → frog → snake → hawk (6 organisms).
Answer: Four: grass → mouse → hawk; grass → mouse → snake → hawk; grass → grasshopper → frog → snake → hawk; grass → grasshopper → spider → frog → snake → hawk.
- AThe direction that energy (food) flows
- BWhich animal is bigger
- CWhich animal hunts at night
- DThe direction the animals move
Answer: The direction that energy (food) flows
Arrows point from the organism being eaten to the organism that eats it, showing the flow of energy.
Part B: What if a population changes?
Use the food web in Part A. Explain each answer using the feeding links.
Answer: The number of frogs would most likely increase, because the snake is the only animal in this web that eats frogs.
Answer: The number of spiders would most likely decrease, because grasshoppers are the spider's only food in this web.
Answer: Grasshoppers (eaten directly) and spiders (which also depend on grasshoppers).
Answer: The rabbit population would most likely increase, because nothing else in this web eats rabbits. More rabbits would eat more grass, so the amount of grass would likely decrease, which could then reduce food for the grasshoppers and mice.
Answer: True
If one food source declines, consumers with other food sources can switch, so the whole web is less likely to collapse.
Part C: Energy in food chains
Only a small part of the energy in one trophic level is passed on to the next, often about 10%. The rest is used for living (moving, keeping warm, growing) and is released to the surroundings as thermal energy, or stays in parts that are not eaten.
Answer: About 1000 kJ
- 10% of 10,000 kJ = 0.1 × 10,000 = 1000 kJ
Answer: Spiders about 100 kJ; frogs about 10 kJ
- Spiders: 10% of 1000 kJ = 100 kJ
- Frogs: 10% of 100 kJ = 10 kJ
Answer: Most is used by the organisms for life processes (released as thermal energy during respiration and used for movement), and some is in parts that are not eaten or are lost as waste, which pass to decomposers.
Answer: So little energy is left after each step (only about 10% passes on) that there is not enough energy to support another population at the top.
Part D: Human impact on ecosystems
- Clearing land for farms and cities
- Fertilizer washing into rivers and lakes
- Introducing a species to a new area
- Overfishing
- Plastic waste in the ocean
- Removes fish faster than they can reproduce, affecting the animals that eat them
- Animals can become tangled in it or swallow it
- It may compete with or prey on native species that have not evolved ways to defend themselves against it
- Destroys habitats, so organisms lose shelter and food
- Causes rapid algae growth that can use up oxygen in the water when the algae die and decompose
Answer: Clearing land for farms and cities → Destroys habitats, so organisms lose shelter and food; Fertilizer washing into rivers and lakes → Causes rapid algae growth that can use up oxygen in the water when the algae die and decompose; Introducing a species to a new area → It may compete with or prey on native species that have not evolved ways to defend themselves against it; Overfishing → Removes fish faster than they can reproduce, affecting the animals that eat them; Plastic waste in the ocean → Animals can become tangled in it or swallow it
Answer: True
Each predator eats many prey and stores the chemical in its body, so top predators such as the hawk can end up with the highest concentration. This is called biomagnification.
- AReplanting native trees to reconnect patches of forest
- BAdding fertilizer to nearby rivers
- CIntroducing a new predator to the area
- DRemoving all the insects from the area
Answer: Replanting native trees to reconnect patches of forest
Replanting (revegetation) restores habitat and food sources and creates corridors so animals can move between patches.
Answer: Any two, for example: protecting areas as national parks or reserves; replanting native vegetation; controlling invasive species; reducing plastic and chemical pollution; fishing within sustainable limits; using fertilizers and pesticides carefully.
Part E: Explain it
Model answer: The fertilizer contains nutrients that help algae grow. The algae grow very quickly (an algal bloom) and cover the surface, blocking light from water plants below, which die. When the algae and plants die, decomposers such as bacteria break them down. The bacteria use up large amounts of oxygen from the water for respiration. Fish cannot get enough oxygen, so they die. Farmers could prevent this by using less fertilizer, not spreading it before heavy rain, or keeping a strip of plants along the edge of the water to soak up runoff.
Success criteria:
- Nutrients cause rapid growth of algae
- Algae and plants die and are decomposed
- Decomposers use up the oxygen in the water
- Fish die from lack of oxygen
- A sensible prevention strategy
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.