LEARNING OBJECTIVES
What you will be able to do
- trace energy through food webs
- interpret ecological pyramids
- explain trophic losses
- describe carbon and nitrogen cycles
- interpret population growth
AT A GLANCE
INTRODUCTION · THE BIG IDEA
How energy, nutrients and populations connect ecosystems.
Ecosystems link organisms through feeding relationships and recycle chemical elements through the environment. Energy behaves differently because it flows and is eventually lost as heat.
Use food webs, ecological pyramids and population curves as evidence, explaining what each representation can and cannot show.
SECTION 01
Food webs and energy
Sunlight is the main energy input. Producers make nutrients; consumers feed on organisms; decomposers feed on dead and waste material.
DETAILED EXPLANATION
- Consumers occupy primary to quaternary trophic levels; herbivores eat plants and carnivores eat animals.
- Food webs reveal effects of overharvesting or introduced species.
- Energy is lost through respiration, movement, waste and uneaten material, so chains are short and eating crops is more efficient than feeding crops to livestock.
SECTION 02
Ecological pyramids
Number pyramids count organisms and may be inverted. Biomass better represents living material but is a snapshot and may require destructive sampling.
DETAILED EXPLANATION
- A trophic level is a feeding position.
SECTION 03
Carbon and nitrogen cycles
Matter cycles although energy flows one way. Photosynthesis removes carbon dioxide; respiration, decomposition and combustion return it. Feeding transfers carbon and fossil fuels store it.
DETAILED EXPLANATION
- Nitrogen fixation by bacteria or lightning adds usable nitrogen; plants absorb nitrate to make proteins.
- Feeding transfers protein; deamination returns nitrogen compounds; denitrifying microbes release nitrogen gas.
SECTION 04
Populations
A population is one species in one area at one time; a community contains all populations; an ecosystem includes community and physical environment.
DETAILED EXPLANATION
- Sigmoid phases are lag, exponential, stationary and death.
- At carrying capacity, limiting factors make births and deaths approximately balance.
STEP-BY-STEP EXAM EXAMPLE
Estimating energy passed to the next trophic level
- Producers store 18 000 kJ of energy.
- Primary consumers receive 12% of this energy.
- Calculate 0.12 × 18 000.
- Explain that the remainder is lost through respiration, movement, waste and uneaten material.
Answer: Primary consumers receive 2160 kJ.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Energy enters as light and leaves as heat.
- Food webs connect feeding.
- Energy pyramids show transfer best.
- Nutrients cycle.
- Limiting factors shape populations.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- trace energy through food webs
- interpret ecological pyramids
- explain trophic losses
- describe carbon and nitrogen cycles
- interpret population growth