Productivity in an ecosystem refers to the rate at which biomass is produced. It is not just the amount of biomass present (that would be standing crop or biomass) but the RATE at which new biomass is added per unit area per unit time. Units: gm⁻²yr⁻¹ (grams per square metre per year) or (KCal m⁻²)yr⁻¹ (kilocalories per square metre per year). The time component (yr⁻¹) is what distinguishes productivity from biomass. Biomass is expressed in gm⁻² or KCal m⁻². Productivity is expressed in gm⁻²yr⁻¹ or (KCal m⁻²)yr⁻¹. This distinction is critical for NEET — many students confuse biomass (no time) with productivity (with time).

Gross Primary Productivity (GPP): total rate of photosynthesis, including the organic matter used in plant respiration. All the energy fixed by plants. Net Primary Productivity (NPP): the biomass available to consumers. NPP = GPP - R (respiration losses). NPP is what matters for food webs. Typically: plants use about 20-40% of GPP for their own respiration. So NPP is approximately 60-80% of GPP. Secondary Productivity: rate of energy storage at consumer levels. Always less than primary productivity. About 10% of NPP is available to primary consumers, 10% of that to secondary consumers, etc. (10% law of Lindemann, 1942).

Productivity varies enormously between ecosystems. High productivity: Tropical rainforests: 2000-3000 gm⁻²yr⁻¹. High temperature + rainfall + nutrients. Estuaries and wetlands: up to 4000 gm⁻²yr⁻¹. Coral reefs: very high. Temperate forests: 600-2500 gm⁻²yr⁻¹. Low productivity: Open ocean: 2-400 gm⁻²yr⁻¹. Vast area but nutrient-limited. Deserts: 10-250 gm⁻²yr⁻¹. Water-limited. Tundra: 10-400 gm⁻²yr⁻¹. Cold-limited. Despite low productivity per unit area, the open ocean contributes significantly to global NPP because of its enormous total area (~70% of Earth surface).

Energy enters ecosystems as sunlight (photosynthetically active radiation). Flow through trophic levels: Producers (T1) → Primary consumers (T2) → Secondary consumers (T3) → Tertiary consumers (T4). At each step: only ~10% of energy transferred. ~90% lost as heat through respiration. Lindemann's 10% law (1942): approximately 10% of energy at one trophic level is available to the next level. This explains: why food chains are typically 3-5 levels long (energy runs out), why there are fewer carnivores than herbivores, why humans on a plant-based diet use less energy resources than those on meat-heavy diets. Energy flow is unidirectional and non-cyclic (unlike matter which cycles).

Standing crop: total amount of living organic matter (biomass) present at a given time. Measured as gm⁻² or KCal m⁻². Productivity: RATE at which organic matter is produced per unit time. Measured as gm⁻²yr⁻¹ or KCal m⁻²yr⁻¹. Example of the difference: Phytoplankton in the ocean have low standing crop (biomass present at any time is small) but very high productivity (they reproduce rapidly and produce biomass fast). This is why the pyramid of biomass is INVERTED in the sea (low phytoplankton biomass supports higher zooplankton/fish biomass) but the pyramid of energy is always upright (phytoplankton produce more energy per year than consumers).

Productivity is also limited by nutrient availability. Major limiting nutrients: Nitrogen (N): often limits productivity in terrestrial ecosystems. Phosphorus (P): often limits freshwater ecosystems. Iron (Fe): limits productivity in open ocean (ocean fertilisation experiments). Carbon dioxide (CO2): important for photosynthesis (rising CO2 may increase plant growth — CO2 fertilisation effect). Water: primary limiting factor in terrestrial ecosystems. Liebig's law of the minimum: plant growth is limited by the nutrient in shortest supply relative to need. Eutrophication: excess nutrient input (N and P from agriculture) → algal blooms → oxygen depletion → dead zones. Oligotrophic waters: nutrient-poor, low productivity but high biodiversity and clarity. Eutrophic waters: nutrient-rich, high productivity but lower biodiversity.

Ecological pyramids represent trophic structure. Pyramid of productivity (energy): ALWAYS upright. Energy always decreases at each trophic level. Can never be inverted. Represents the most accurate picture of energy relationships. Pyramid of numbers: can be upright (grassland), inverted (tree ecosystem), or spindle-shaped (parasitic food chain). Pyramid of biomass: usually upright in terrestrial ecosystems. INVERTED in aquatic/marine ecosystems (phytoplankton standing crop less than zooplankton, despite higher productivity). The fact that the pyramid of energy is always upright (even in marine ecosystems where biomass pyramid is inverted) shows that productivity is a better measure of the energy relationships in food webs than standing crop/biomass.

Decomposers (bacteria, fungi) play a critical role in ecosystem functioning. They break down dead organic matter (detritus) and return nutrients to the abiotic environment for reuse by producers. Types of decomposition: Fragmentation: physical breakdown by detritivores (earthworms, millipedes, fungi). Leaching: water-soluble nutrients washed into soil. Catabolism: enzymatic breakdown by microbes. Humification: formation of humus (dark, amorphous substance). Mineralisation: conversion of organic matter to inorganic minerals. Rate of decomposition depends on: Temperature (warm = faster), moisture (moist = faster), quality of detritus (lignin-rich = slow, N-rich = fast), oxygen availability (aerobic decomposition faster). Decomposition returns C, N, P, S to ecosystem. Without decomposers, nutrients would be locked in dead organic matter and unavailable to producers.