Key facts & rules

• Producer: photosynthetic organism (plant/algae) — first trophic level.
• Consumer: organism that eats other organisms.
• Decomposer: breaks down dead organic matter (bacteria, fungi) — recycles nutrients.
• Food chain: linear energy flow. Food web: interconnected food chains.
• Carbon cycle: photosynthesis removes CO₂; respiration, decomposition, combustion release CO₂.
• Nitrogen cycle: nitrogen fixation (bacteria) → nitrification → assimilation → denitrification.
• Limiting factors for population: food, water, space, predation, disease.
• Biodiversity: variety of species in an ecosystem — higher = more stable.