Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Endotherms heat themselves with metabolic heat and need much more food. Ectotherms follow the surroundings but regulate by behaviour (basking, shade, clustering).
- Energy budget: net gain → stored, growth, more offspring; net loss → mass loss, fewer offspring, eventually death.
- Some organisms (Daphnia, aphids) reproduce asexually when food is plentiful and switch to sexual reproduction when it becomes scarce or conditions worsen.
- Levels of organisation: population → community → ecosystem → biome.
- Trophic levels: producers; primary, secondary, tertiary, quaternary consumers; decomposers.
- Autotrophs: photosynthetic (light) or chemosynthetic (small inorganic molecules, even without oxygen). Heterotrophs (herbivores, carnivores, omnivores, scavengers, decomposers) break down carbohydrates, lipids and proteins from autotroph-derived matter.
- Less sunlight or fewer producers → less energy for every level above → smaller populations and fewer levels.
Key relationships
| Cycle | Main reservoirs | Key processes |
|---|---|---|
| Water | oceans, surface water, atmosphere, organisms | evaporation, condensation, precipitation, transpiration |
| Carbon | atmosphere (CO₂), organisms, fossil fuels | photosynthesis (removes CO₂); respiration, decomposition, combustion (return CO₂) |
| Nitrogen | atmosphere (N₂, largest), soil, organisms | fixation (N₂ → NH₃ → NH₄⁺), assimilation, ammonification, nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻), denitrification (NO₃⁻ → N₂) |
| Phosphorus | rocks, soil, water, organisms (no major gas phase) | weathering releases PO₄³⁻; uptake by producers; feeding; excretion and decomposition |
| Equation | Use |
|---|---|
| NPP = GPP − producer respiration | energy available to consumers |
| efficiency = next level ÷ this level × 100 | usually about 10% |
| assimilated = ingested − egested; growth = assimilated − respired | an organism’s energy budget |
Assumptions behind the numbers
- The 10% figure is an average; real values vary.
- Energy budgets assume no other losses (such as urine) unless stated.
- A change in producers is assumed to pass up the chain with the same efficiencies.
Mistakes to avoid
- Saying energy is recycled.
- Saying plants take N₂ from the air.
- Mixing up nitrification (makes nitrate) and denitrification (makes N₂).
- Forgetting combustion as a source of CO₂.
- Saying ectotherms cannot regulate temperature at all.
- Multiplying by 10% once too many or too few times.
Quick self-check
- Producers store 8000 kJ. About how much reaches secondary consumers? (8000 × 0.1 × 0.1 = 80 kJ)
- Which organisms can capture energy at a dark hydrothermal vent? (Chemosynthetic autotrophs, using inorganic molecules such as hydrogen sulfide.)
- Why does a mouse need more food than a lizard of the same mass? (It uses metabolic heat to keep its body warm.)
Next: practice questions.