Productivity = rate of biomass production or energy storage per unit area per unit time. Units: g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹. GPP (Gross Primary Productivity): total rate of photosynthesis including what is used in respiration. Also called total assimilation. NPP (Net Primary Productivity) = GPP − Respiration (R). This is what's available for herbivores and decomposers. Typical: GPP of tropical forest = 2000-3000 g m⁻² yr⁻¹. R = ~50% of GPP. NPP = ~50% of GPP. Secondary Productivity: rate of energy storage at consumer level (herbivores, carnivores). Always lower than primary productivity due to energy losses at each trophic level.

Energy flows unidirectionally: Sun → Producers → Primary consumers → Secondary consumers → Tertiary consumers → Decomposers. At each trophic level: ~10% energy transferred to next level (10% law of energy transfer, Lindemann 1942). Losses: heat (respiration), uneaten material, undigested material (faeces). Energy pyramid is always upright. Biomass pyramid: usually upright (terrestrial); inverted in aquatic (plankton biomass < fish biomass at a given time). Numbers pyramid: sometimes inverted (tree → insects → birds).

Grazing food chain: starts with living plants (producers). Producer → Herbivore → Carnivore. Detritus food chain: starts with dead organic matter (detritus). Dead matter → Decomposers (fungi, bacteria) → Detritivores. In most ecosystems, detritus food chain transfers MORE energy than grazing food chain (especially in forests). Food web: interconnected food chains. More complex, more stable (if one species is lost, others compensate). Keystone species: disproportionately large effect on ecosystem relative to biomass (e.g., sea otters control sea urchin population → maintains kelp forest).

Carbon cycle: CO₂ → photosynthesis → organic C → respiration/decomposition → CO₂. Atmospheric CO₂ = 0.04%. Residence time: atmosphere ~5 years, ocean ~1500 years. Nitrogen cycle: N₂ → fixation (Rhizobium, Azotobacter, lightning) → NH₃ → nitrification (NH₃→NO₂→NO₃, by Nitrosomonas and Nitrobacter) → plant uptake → protein → decomposition → ammonification → nitrification or denitrification (NO₃→N₂, by Pseudomonas). Phosphorus cycle: sedimentary cycle (no atmospheric phase). Weathering → phosphate → plants → animals → decomposition → soil → sediment. No gaseous phase = slow cycle.

Detritus (dead organic matter) → Fragmentation (detritivores: earthworms, millipedes, woodlice) → Leaching (water-soluble compounds move to lower soil layers) → Catabolism (enzymes break down complex to simple inorganic) → Humification (dark amorphous humus formation) → Mineralisation (humus → inorganic nutrients: Ca²⁺, PO₄³⁻, SO₄²⁻). Rate of decomposition affected by: temperature (warm = faster), moisture (optimal = faster), C:N ratio of detritus (low C:N = faster — nitrogen-rich = faster decomposition), O₂ availability (aerobic = faster). Tundra: decomposition very slow (cold) → peat accumulation.

Primary succession: on bare substrate (no soil). Pioneer species → climax community. Takes thousands of years. Examples: rock → lichens → mosses → herbs → shrubs → trees. Secondary succession: on previously vegetated area after disturbance (fire, flood, deforestation). Faster than primary (soil present). Stages in hydrosere (aquatic → terrestrial): Open water → Phytoplankton → Rooted aquatic plants → Reed marsh → Sedge meadow → Shrub → Forest. Climax community: final stable community in equilibrium with climate. Xerosere: succession on dry bare rock. Lithosere: rock → lichen → moss → herbs → shrubs.

Three types: Numbers: number of organisms at each trophic level. Usually upright, but inverted for tree-insect-bird. Biomass: total dry weight at each level. Upright in terrestrial. Inverted in aquatic (phytoplankton biomass < zooplankton at given time). Energy: total energy at each trophic level. ALWAYS upright (energy always decreases up the trophic levels — can never be inverted). Energy pyramid gives most accurate picture of energy flow. 10% law: only 10% of energy passes from one trophic level to the next. Rest lost as heat.

GPP measurement: O₂ evolution method (light and dark bottles). NPP = amount of organic matter accumulated by producers. Measured as: dry weight of biomass, calorific value (kcal), carbon content, or O₂ production. Chlorophyll content correlates with GPP. Remote sensing: NDVI (Normalized Difference Vegetation Index) measures vegetation greenness → estimates NPP globally. Global NPP: terrestrial = ~120 Pg C yr⁻¹, ocean = ~50 Pg C yr⁻¹. Most productive ecosystems: tropical rainforests, estuaries, coral reefs, swamps. Least productive: open ocean, deserts, tundra.