Lesson focus: disturbanceAn event that suddenly changes an ecosystem, such as a fire, flood or storm. can propagate through food webs, but ecological responses depend on interaction strength, alternative pathways and time.
Explain trophic cascadeWhen a change in one animal, such as wolves returning, has knock-on effects down the food chain., invasive-species effects and biomagnificationWhen a harmful substance becomes more concentrated at each step up a food chain, so top predators get the most. while evaluating uncertainty in food-web predictions.
Build on earlier learning
Food chains and food websThe previous lesson established web structure and arrow direction. This lesson uses that network to reason about disturbance.
You will be successful when you can…
- Distinguish direct and indirect effects.
- Explain a trophic cascade.
- Distinguish bioaccumulation from biomagnification.
- Evaluate competing ecological explanations.
Words that carry the science
- disturbance
- An event or sustained pressure that changes ecosystem conditions or populations.
- trophic cascade
- Indirect effects across trophic levels following a change in consumers or resources.
- invasive species
- A non-native species that spreads and causes ecological, economic or health harm.
- bioaccumulation
- Build-up of a substance within an organism over time.
- biomagnification
- Increase in concentration of a persistent substance at successively higher trophic levels.
- resilience
- The capacity of a system to resist change or recover after disturbance.
Trace disturbance through direct and indirect pathways
01Effects spread beyond the first feeding link
Removing a predator can release prey from predation, which may increase feeding pressure on producers. This indirect producer response is part of a trophic cascadeWhen a change in one animal, such as wolves returning, has knock-on effects down the food chain.. Adding a predator can cause the reverse pattern.
Outcomes depend on alternative prey, refuges, disease, competition and time lags. A web should therefore generate hypotheses rather than deterministic claims.
02Introduced species can reorganise interactions
A non-native species becomes invasive when its spread causes harm. It may compete, prey on unfamiliar species, introduce disease or alter habitat. Many non-native species do not become invasive.
Evidence should compare abundance, spread and ecosystem effects through time and against suitable reference sites. A simple before-and-after observation can be confounded by weather or land-use change.
03Persistent pollutants can concentrate in food webs
bioaccumulationWhen a harmful substance builds up inside a living thing over time because it cannot get rid of it. occurs within an organism when uptake exceeds loss. biomagnificationWhen a harmful substance becomes more concentrated at each step up a food chain, so top predators get the most. describes higher concentrations at higher trophic levels when consumers ingest contaminated biomass repeatedly.
The pattern is strongest for persistent, poorly excreted substances. Concentration can vary with age, diet and tissue, so trophic position alone does not determine one fixed value.
Predator and prey populations
Foxes eat rabbits. More rabbits means more food for foxes, so the fox population rises after the rabbits do. Then the rabbits fall, and the foxes follow.
Rabbit (each dot ≈ 20) Fox (each ≈ 3)
Controls
Trace a trophic cascade
A new disease sharply reduces fox numbers in a system where foxes eat rabbits and rabbits eat seedlings.
- Direct effect: fewer foxes reduces predation on rabbits.
- Rabbit survival may rise after a time lag.
- More rabbits may consume more seedlings.
- Seedling recruitment may fall, but alternative predators, food shortage or disease could weaken the chain.
How to make the reasoning visible: State the causal route and then name evidence or conditions that could change its strength.
Interpret pollutant concentrations
The values are illustrative concentrations of a persistent pollutant in wet tissue.
| Sample | Trophic context | Concentration / mg kg⁻¹ | Interpretation |
|---|---|---|---|
| Water | Abiotic environment | 0.002 | Low measured concentration |
| Algae | Producer | 0.08 | Uptake and accumulation |
| Small fish | Primary/secondary consumer | 0.42 | Higher concentration |
| Predatory fish | Higher consumer | 2.6 | Biomagnification supported |
| Fish-eating bird | Top consumer | 7.4 | Greatest measured concentration |
Why does the table support biomagnification?
Why are age and tissue type important control variables?
Check your understanding
Answer from memory, then check against the suggested answer.
What is a trophic cascade?
Distinguish bioaccumulation from biomagnification.
Does every non-native species become invasive?
A pesticide concentration is high in one old predatory fish. Why is this insufficient to prove biomagnification?
Explain how behavioural change can cause a trophic cascade even before prey numbers change.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A persistent pollutant enters a lake at low concentration. A top predator is harmed.
Explain how the pollutant enters the food chain and builds up in organisms.
(3)Explain why the top predator is harmed most.
(3)Total for this question: 6 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., keep bioaccumulation within one organism separate from biomagnification between trophic levels.