Organisation of an Ecosystem (B7.2)

Food chains and food webs

A food chain shows feeding relationships. Energy flows from left to right:

• Producer (autotroph — photosynthesises) → Primary consumer → Secondary consumer → Tertiary consumer
• Each level is a trophic level.

Biomass and energy decrease at each trophic level — typically only 10% is transferred (90% lost as heat from respiration, used for movement, undigested waste).

Food webs show multiple interconnected food chains and better represent real ecosystems.

Producers and decomposers

• Producers: plants and algae — convert light energy into chemical energy by photosynthesis.
• Consumers: animals — eat other organisms.
• Decomposers: bacteria and fungi — break down dead organic material; return nutrients to soil/water.

Sampling techniques

To estimate population size:

• Random quadrats: count organisms in randomly placed quadrats; calculate mean; scale up to whole area.
• Transects: sample along a line to show change in species distribution across a habitat (e.g. zonation on a rocky shore).
• Mark-release-recapture (capture-recapture):

  Population estimate = (First capture × Second capture) / Number of recaptures
  

  Assumptions: population closed (no births/deaths/migration); marks don't affect survival; random remixing.

The carbon cycle

Carbon moves between organisms and the environment:

1. Photosynthesis: CO₂ from atmosphere → organic carbon in plants
2. Feeding: carbon passes along food chain
3. Respiration: organic carbon → CO₂ returned to atmosphere
4. Decomposition: decomposers break down dead matter → CO₂ released
5. Combustion: burning fossil fuels → CO₂
6. Fossilisation: organic matter compressed over millions of years → coal, oil

The water cycle

1. Evaporation from seas, lakes, rivers → water vapour in atmosphere
2. Transpiration from plants → water vapour
3. Condensation → clouds form
4. Precipitation (rain/snow) → water returns to ground
5. Runoff → streams → rivers → sea
6. Percolation → groundwater

Common exam errors

1. Saying energy is "lost" at each trophic level — energy is transferred/dissipated as heat, it is not destroyed.
2. Forgetting decomposers in nutrient recycling.
3. Not knowing the mark-release-recapture formula.