10% law (Lindemann): Only 10% of energy at each trophic level is transferred to the next. Energy is lost as heat, respiration, excretion at each step.

Pyramids: Pyramid of energy — always upright (energy always decreases). Pyramid of numbers — can be inverted (single large tree → many insects). Pyramid of biomass — usually upright; inverted in sea (phytoplankton < zooplankton at any instant due to rapid turnover).

Productivity: GPP = total photosynthesis. NPP = GPP - respiration. Secondary productivity = rate of energy assimilation by consumers.

Carbon cycle: CO₂ ← Photosynthesis → Organic carbon → Decomposition/Respiration/Combustion → CO₂. Human impact: fossil fuel burning increases atmospheric CO₂.

Nitrogen cycle: N₂ → NH₃ (nitrogen fixation by Rhizobium, Azotobacter, cyanobacteria) → NO₂⁻ (nitrification by Nitrosomonas) → NO₃⁻ (nitrification by Nitrobacter) → plant uptake → organic N → NH₄⁺ (ammonification) → N₂ (denitrification by Pseudomonas).

Phosphorus cycle: No gaseous phase. Rock weathering releases PO₄³⁻. No atmospheric reservoir. Slow cycle.

Types: Genetic diversity (within species), species diversity (between species), ecosystem diversity (across habitats).

Hotspots (mega-diverse countries): 34 biodiversity hotspots globally. India's hotspots: Western Ghats + Sri Lanka, Himalaya, Indo-Burma, Sundaland. Hotspot = endemic species-rich + severely threatened (>70% original habitat lost).

IUCN categories: Extinct → Extinct in Wild → Critically Endangered → Endangered → Vulnerable → Near Threatened → Least Concern.

India: 8.1% of world's biodiversity. 45,000 plant species, 89,000 animal species. 12th mega-diversity country.