Biology & life sciences · 03 / 12
Ecosystems: networks and recycling
Middle school · Worked example and 4 exercises with solutions, at your pace.
The principle, simply
An ecosystem links organisms and their physical environment. Food webs connect producers, consumers and decomposers. A food arrow can show matter and energy transfer from resource to consumer; state the convention. Matter can recycle while usable energy dissipates and needs continued input.
Simplified chain: arrows go from resource to consumer. Energy dissipates at each level; decomposers, not drawn, can recycle matter.
- 01Environment and species
- 02Arrow convention
- 03Network interactions
- 04Matter and energy
WORKED EXAMPLE
From principle to reasoning
Grass → rabbit → fox shows food transfer. Decomposers also process remains from each organism. Removing a species affects a network rather than just a linear chain.
Solve a problem in this subject
- Frame a question
State the relationship to explain and its scale: molecule, cell, organism, population or geological time.
- Describe evidence
Give a measurable observation, unit, comparison and conditions before explaining it. Observation and inference do different jobs.
- Connect a mechanism
Use a precise concept and label arrows as production, transfer, action or transformation. Sequence alone does not prove causation.
- Answer with a limit
Connect evidence and knowledge to the question. Distinguish the simplified model from living systems and identify a missing condition.
YOUR TURN
01 · Try before checking
Why does an arrow not necessarily point from attacker to prey?
One hint
Read its convention.
Reveal and understand the solution
Under the transfer convention, it points from consumed organism to consumer, following matter and energy.
Compare method, conditions and answer; identify the exact part to revisit.
IMPROVE THROUGH PRACTICE
Three additional challenges with solutions
Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.
What does the arrow mean in grass → rabbit → fox?
A hint to get started
Follow transfer of matter and energy.
Step-by-step solution
- The rabbit consumes grass.
- The fox can consume the rabbit.
- The arrow goes from resource to consumer, not toward the consumer’s food.
Which step did you miss? Retry without help, then explain why the method works.
Why does available energy generally fall across trophic levels?
A hint to get started
Not all ingested energy becomes transferable biomass.
Step-by-step solution
- Some is not consumed or assimilated.
- Some powers metabolism and dissipates as heat.
- Transfer efficiency is not a universal fixed value.
Which step did you miss? Retry without help, then explain why the method works.
Herbivores increase after predator loss. Can the whole ecosystem be predicted?
A hint to get started
A food web contains multiple interactions and factors.
Step-by-step solution
- Reduced predation may explain the initial rise.
- Resources, disease and other species can alter later dynamics.
- A cascade is possible, but a three-species model cannot predict everything.
Which step did you miss? Retry without help, then explain why the method works.
The trap to avoid
Avoid useful-versus-harmful species rankings; matter and energy have different balances.
Without notes, can you…
- Explain the principle in your own words?
- Rebuild the diagram and its connections?
- Solve the exercise and justify the method?
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