Trace human impacts through a food web
Overharvesting and introducing foreign species can alter feeding links; use the web to explain possible effects without assuming every outcome.
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
- Read energy-transfer arrows and distinguish energy from matter in the 19.1 Energy flow pathway.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How can overharvesting affect a predator in a food web?
What you will learn
- Use feeding links to describe effects of overharvesting
- Explain possible effects of a newly introduced species
Overharvesting removes organisms faster than a population can replace them
If humans harvest a food species heavily, fewer individuals may remain to reproduce or serve as prey. Predators that depend on it may have less food; organisms it ate may increase if other controls do not offset the change. A web helps identify these possible direct and indirect consequences.
An effect depends on the habitat and other relationships. If a predator has alternative prey or the harvested species has other pressures, population change may differ. Use “may” unless the question supplies population evidence that supports a definite trend.
A foreign species can change existing links
An introduced species may compete with native species, eat them, become prey, or introduce disease. These altered feeding relationships can affect several populations in a web. Introduction alone does not establish that the species will become invasive or cause harm; evidence about its interactions and population is needed.
To analyse a scenario, identify the new or weakened link, trace its possible effect to connected organisms, and name evidence that could test the prediction. This keeps a plausible food-web explanation separate from a measured ecological conclusion.
Sources for this lesson
Pause and explain
What is a careful conclusion when a foreign species is introduced?
Worked example
People harvest most of a fish that eats plankton and is food for seabirds. Give one possible food-web effect and one uncertainty.
Show the worked solution
- The fish may become less available to seabirds.
- Trace what the fish eats and consider other prey or pressures.
Answer Seabirds may have less of this food source. The effect is not certain or quantified without information about alternative prey, harvest intensity and other population changes.
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.
People remove many shellfish that are eaten by shorebirds. Give one possible effect on shorebirds and one factor that could change the outcome.
Give me a hint
Trace the prey link, then check for alternatives.
Compare my reasoning
- Fewer shellfish may mean less available food.
- Other prey or habitat conditions may affect shorebirds.
Shorebirds may have less shellfish available; alternative prey and other conditions could reduce or change the effect.
Look for these in your work
- Link traced
- A limiting factor named
A newly introduced fish eats the same insect larvae as a native fish. Use competition to suggest one possible impact without claiming it must happen.
Give me a hint
Identify the shared food and phrase an evidence-bounded possibility.
Compare my reasoning
- Both fish use the same prey resource.
- The introduced fish may compete for larvae and could affect native feeding success if the resource is limited.
The introduced fish may compete with the native fish for insect larvae; an effect on the native fish depends on resource availability and other interactions.
Look for these in your work
- Shared link identified
- Conditional explanation
An introduced plant-eating animal is found in a new habitat. What two web questions would you ask before judging its impact?
Give me a hint
Ask what it eats and what organisms depend on those links.
Compare my reasoning
- Which producers does it consume?
- What native herbivores or predators share those resources or feed on it?
Check which plants it eats and whether native consumers or predators share those links. Population and habitat evidence would then be needed to judge the impact.
Look for these in your work
- Feeding link sought
- Evidence requirement included
Common mistakes
- Claiming every introduced species is harmful
- Predicting an exact population increase from one arrow
- Ignoring alternative food sources
What can you explain now?
How can overharvesting affect a predator in a food web?
Compare with the explanation
It may reduce an available prey population
The effect follows a feeding link, but its size depends on the web.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Trace one possible direct and one indirect effect of overharvesting or a newly introduced species through a food web.
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.