Ecosystems, Energy, and Matter
Trace energy flow through food webs and matter cycling through carbon, nitrogen, and water cycles. Explain why energy transfer is inefficient across trophic levels.
A lesson of Biology, in Homeostasis, Body Systems, and Ecology, about 10 minutes.
By the end of this lesson, you will be able to trace energy flow through food webs, calculate trophic efficiency, describe nutrient cycles, and explain why energy decreases at higher trophic levels.
Energy Flow Through Ecosystems
Autotrophs (producers): capture energy from sunlight (photosynthesis) or chemicals (chemosynthesis). The base of all food webs.
Heterotrophs (consumers): obtain energy by eating other organisms.
- Herbivores (primary consumers): eat plants
- Carnivores (secondary consumers): eat herbivores
- Tertiary consumers: eat secondary consumers
- Decomposers/detritivores: break down dead organic matter
Food chain: linear sequence of feeding relationships.
Food web: network of interconnected food chains.
Energy Transfer and Trophic Efficiency
The 10% rule: only about 10% of energy from one trophic level is transferred to the next. The rest is lost as heat (metabolic heat, cellular respiration).
Example:
- 1000 J stored in plants
- → 100 J available to herbivores
- → 10 J available to carnivores
- → 1 J available to top predators
Why this matters: this is why food chains rarely have more than 4–5 trophic levels — not enough energy remains.
Biomass pyramids: the energy pyramid corresponds to a biomass pyramid (less biomass at higher trophic levels).
Nutrient Cycles
Unlike energy, matter cycles through ecosystems. Carbon, nitrogen, phosphorus, and water cycle between biotic (living) and abiotic (nonliving) components.
Carbon Cycle
- Photosynthesis: CO₂ → organic carbon (removed from atmosphere)
- Respiration: organic carbon → CO₂ (returned to atmosphere)
- Decomposition: returns carbon from dead matter to atmosphere
- Combustion: burns fossil fuels → CO₂ released
- Ocean: CO₂ dissolves; carbonate chemistry; marine organisms
Current issue: fossil fuel combustion is increasing atmospheric CO₂ faster than natural sinks can absorb it → climate change.
Nitrogen Cycle
- Atmosphere is 78% N₂, which most organisms cannot use directly
- Nitrogen fixation: bacteria (Rhizobium in legume roots, Azotobacter free-living) convert N₂ → NH₃ (ammonia)