- Define and explain Food Webs in your own words
- Use key terms such as trophic level accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
No organism lives alone: every meadow and ocean is a web of feeding relationships and recycled materials. In this chapter you will build food webs, explain pyramids of biomass, follow carbon around its cycle, and evaluate how human activity is reshaping the living world.
This lesson focuses on Food Webs: build food webs and identify producers, consumers, predators and decomposers.
Build food webs and identify producers, consumers, predators and decomposers.
Key ideas
Food webs and interdependence
A food web links many food chains, showing that most organisms eat, and are eaten by, several species. Removing one species ripples through the web: if rabbits decline, foxes may starve while grass flourishes. Decomposers complete the picture by breaking down dead organisms and waste, recycling nutrients back to producers.
Pyramids of biomass and energy loss
A pyramid of biomass shows the dry mass at each trophic level, and it is almost always pyramid-shaped because biomass is lost between levels. Only about 10% of energy is transferred: the rest is lost through respiration and movement, in uneaten parts, and in waste such as faeces and urine. This is why food chains rarely have more than five trophic levels — there is too little energy left.
Key term — trophic level: A feeding level in a food chain or web, such as producer, primary consumer or secondary consumer.
In the food chain grass → rabbit → fox → flea, identify the producer, a primary consumer and a secondary consumer.
Producer: grass. Primary consumer: rabbit. Secondary consumer: fox (the flea is a tertiary consumer).
Answer: Producer: grass. Primary consumer: rabbit. Secondary consumer: fox (the flea is a tertiary consumer).
- Saying decomposers are not needed Without decomposers, nutrients would stay locked in dead bodies and waste. Decomposers recycle them into the soil for producers to reuse.
- Drawing a pyramid of biomass with the widest bar at the top Biomass decreases up the chain because energy is lost at each transfer, so the producer bar at the bottom is always the widest.
Practice
Most organisms eat several different species and are eaten by several predators, so a web shows the true feeding relationships and how species depend on one another.
Only about 10% of energy transfers at each level, so after several transfers too little energy remains to support another trophic level.
Any two of: respiration by plants, animals and decomposers; combustion of fossil fuels or wood; decay releasing carbon dioxide.
Any three of: respiration (released as heat); movement; uneaten parts such as roots or bones; waste products such as faeces and urine.
Quick check
Which of these best defines "trophic level"?
Give two human activities that increase atmospheric carbon dioxide and one way to reduce them.
- Food Webs: build food webs and identify producers, consumers, predators and decomposers.
- Food webs and interdependence: A food web links many food chains, showing that most organisms eat, and are eaten by, several species.
- decomposer: An organism, such as a bacterium or fungus, that breaks down dead material and waste, returning nutrients to the soil.
- Watch out for: saying decomposers are not needed