Learning focus
Use precise biological vocabulary, interpret diagrams and data, explain mechanisms as linked sequences, and evaluate biological applications and environmental decisions.
Interconnected chains
A food web is a network of interconnected food chains. Most organisms have more than one food source and more than one predator, so a web is more realistic than a single chain.
Direct effects
If a prey population decreases, predators relying on it may have less food. If a predator decreases, some prey populations may rise. These are direct effects based on one feeding link.
Indirect effects
Food webs also produce indirect effects. Increasing one herbivore may reduce plants, which then affects another herbivore. A predator may switch prey, spreading effects through the web. Predictions should use named links rather than general statements.
Stability
Alternative food sources can make a web more resilient, but they do not guarantee stability. Disease, habitat change or loss of a highly connected species can still cause large changes.
Practical or data skill
Remove one species from a printed food web and predict two direct and two indirect consequences, stating each pathway.
Examination tip
Use “may” for food-web predictions because alternative interactions can alter the outcome.
Review questions and suggested answers
Question 1
Define a food web.
Suggested answer
A network of interconnected food chains.
Question 2
What may happen to predators if their prey declines?
Suggested answer
Their population may decline or they may switch prey.
Question 3
Why are indirect effects harder to predict?
Suggested answer
Changes pass through several interacting feeding relationships.
