Trace human effects through food webs
Changing a food chain or web can affect organisms beyond the species directly targeted by a human activity.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Know that organisms interact with other organisms and their environment.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why can changing one population affect other organisms in a food web?
What you will learn
- Explain that altering food chains and webs can negatively affect habitats
- Trace a supported effect from a changed feeding relationship
Follow links beyond the immediate target
A food web links feeding relationships among organisms in a habitat. If humans remove, add or greatly reduce one population, organisms that feed on it may have less food, while organisms it used to eat may increase. These linked changes can affect the habitat even when the original action targeted only one species.
The syllabus asks students to state that altering food chains and food webs can have a negative impact on habitats. A clear explanation follows the arrows or feeding links and avoids claiming that every population must change in the same direction.
Separate a supported pathway from speculation
Start with the human change, identify the affected organism, then trace one or more feeding links. For example, overharvesting a fish can reduce food for its predators; fewer predators may then affect other prey populations. This is a possible pathway when the stated web supports it.
Food webs contain many links, so actual outcomes can depend on alternative food sources and other conditions. Use the supplied web to support each step rather than adding an unsupported chain.
Sources for this lesson
Pause and explain
People remove many prey fish from a web. Which is a supported possible effect?
Worked example
In a web, humans reduce a small fish eaten by seabirds. State one possible indirect effect.
Show the worked solution
- Follow the arrow from the fish to the seabird.
- Use “may” because the seabird could have other food.
Answer Seabirds may have less food available, so their population could be affected if the small fish is an important food source.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A human activity reduces a prey population. Use a food web to explain one possible effect on its predator.
Give me a hint
Follow the food link from prey to predator.
Compare my reasoning
- The predator obtains food from this prey.
- A smaller prey population may mean less food for the predator.
If the predator relies on this prey, reducing the prey population may reduce food availability and affect the predator population.
Look for these in your work
- Link followed in correct direction
- Effect qualified
A web shows grass → rabbit → fox. Humans reduce rabbit numbers. Give one possible change to foxes and explain it.
Give me a hint
Use the chain and allow for alternative prey.
Compare my reasoning
- Rabbits are food for foxes in this chain.
- Fewer rabbits can reduce food availability.
Foxes may have less food and their population could decrease if rabbits are an important food source and no alternative fully replaces them.
Look for these in your work
- Feeding relationship used
- Conditional explanation
Why should a student avoid saying that reducing one species always causes every other species to decline?
Give me a hint
Consider alternate food links and the evidence shown.
Compare my reasoning
- Some consumers may have alternative food sources.
- Other populations can respond differently along the web.
A food web has multiple links: some organisms may switch food sources, and some populations may increase when competition or predation changes. The supplied links and evidence determine the supported conclusion.
Look for these in your work
- Alternative links considered
- Overclaim corrected
Common mistakes
- Stopping at the directly harvested species
- Ignoring the direction of feeding arrows
- Claiming a precise population outcome without enough web or data
What can you explain now?
Why can changing one population affect other organisms in a food web?
Compare with the explanation
Feeding relationships link populations
A change can alter food availability for connected organisms.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How can changing one population affect a habitat through its food web, and what makes a prediction cautious?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.