Ecosystem

2.1 Core concepts

• An ecosystem is a functional unit of nature where living organisms interact among themselves and with the surrounding physical environment; sizes range from a small pond to a forest or sea, and the entire biosphere is the global ecosystem (NCERT §12 intro, p. 205).
• Ecosystems are broadly divided into terrestrial (forest, grassland, desert) and aquatic (pond, lake, wetland, river, estuary); crop fields and aquaria are man-made ecosystems (NCERT §12 intro, p. 205).
• The interaction of biotic and abiotic components yields a characteristic physical structure for every ecosystem; the two main structural features are species composition (list/enumeration of species) and stratification (vertical distribution of species, e.g., trees at top, shrubs middle, herbs/grasses at bottom of a forest) (NCERT §12.1, p. 206).
• The four functional aspects of an ecosystem are: (i) Productivity, (ii) Decomposition, (iii) Energy flow, (iv) Nutrient cycling (NCERT §12.1, p. 206).
• In a pond, the abiotic component is water with dissolved substances and bottom soil; autotrophs are phytoplankton, algae and floating/submerged/marginal plants; consumers are zooplankton and free-swimming/bottom-dwelling forms; decomposers are fungi, bacteria and flagellates abundant at the bottom (NCERT §12.1, p. 206).
• Primary production = biomass or organic matter produced per unit area over a time period by plants during photosynthesis; expressed in g m⁻² (weight) or kcal m⁻² (energy). Rate of biomass production = productivity, in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹ (NCERT §12.2, p. 207).
• GPP = rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration (R). NPP = GPP − R. NPP is the biomass available for consumption by heterotrophs (herbivores + decomposers). Secondary productivity = rate of formation of new organic matter by consumers (NCERT §12.2, p. 207).
• Annual NPP of the whole biosphere ≈ 170 billion tons (dry weight) of organic matter; oceans contribute only 55 billion tons despite occupying ~70% of Earth's surface (NCERT §12.2, p. 207).
• Decomposition = breakdown of complex organic matter into inorganic substances (CO₂, water, nutrients) by decomposers; detritus (dead leaves, bark, flowers, animal remains and faecal matter) is the raw material (NCERT §12.3, p. 207).
• Steps of decomposition (operate simultaneously): fragmentation (detritivores like earthworms break detritus into smaller particles), leaching (water-soluble inorganic nutrients percolate into soil horizon and get precipitated as unavailable salts), catabolism (bacterial and fungal enzymes degrade detritus into simpler inorganic substances), humification (forms dark amorphous humus, colloidal, resistant to microbial action, reservoir of nutrients), mineralisation (humus is further degraded to release inorganic nutrients) (NCERT §12.3, pp. 207–208).
• Decomposition is largely oxygen-requiring; slower if detritus is rich in lignin and chitin, quicker if rich in nitrogen and water-soluble sugars; warm and moist conditions favour decomposition while low temperature and anaerobiosis inhibit it (NCERT §12.3, p. 208).
• Sun is the only energy source for ecosystems (except deep-sea hydrothermal vents). PAR (photosynthetically active radiation) = less than 50% of incident solar radiation; plants capture only 2–10% of PAR, and this small fraction sustains the entire living world (NCERT §12.4, p. 209).
• Energy flow obeys the first and second laws of thermodynamics; it is unidirectional — sun → producers → consumers → decomposers (NCERT §12.4, p. 209).
• A simple grazing food chain (GFC) is: Grass (Producer) → Goat (Primary Consumer) → Man (Secondary Consumer); the detritus food chain (DFC) begins with dead organic matter and is dominated by saprotrophic fungi and bacteria (NCERT §12.4, pp. 209–210).
• In aquatic ecosystems, GFC is the major energy conduit; in terrestrial ecosystems, a much larger fraction of energy flows through the DFC. The interconnection of GFC and DFC forms a food web (NCERT §12.4, p. 210).
• Trophic levels: producers (1st), primary consumer/herbivore (2nd), secondary consumer/carnivore (3rd), tertiary consumer/top carnivore (4th, e.g., man, lion) (NCERT §12.4 Fig. 12.2, p. 210).
• Standing crop = mass of living material (biomass) or number per unit area at a particular time at each trophic level; biomass is measured in fresh or dry weight, with dry weight being more accurate (NCERT §12.4, p. 211).
• 10 per cent law (Lindemann) — only 10% of the energy is transferred to each trophic level from the lower trophic level; this restricts the number of trophic levels in a grazing food chain (NCERT §12.4, p. 211).
• Ecological pyramids — base = producers (1st trophic level), apex = top consumer; expressed as pyramid of (a) number, (b) biomass or (c) energy (NCERT §12.5, p. 212).
• Most pyramids are upright (producers > herbivores > carnivores). Exceptions: pyramid of number for a big tree-insect-bird system can be inverted (or spindle-shaped); the pyramid of biomass in the sea is inverted because biomass of fishes far exceeds the standing crop of phytoplankton (NCERT §12.5, pp. 212–213).
• The pyramid of energy is always upright and can never be inverted, because some energy is always lost as heat at each transfer (NCERT §12.5, p. 213).
• An ideal energy pyramid: 1,000,000 J sunlight → 10,000 J producers → 1,000 J primary consumer → 100 J secondary consumer → 10 J tertiary consumer; primary producers convert only 1% of incident sunlight into NPP (NCERT §12.5 Fig. 12.4d, p. 213).
• Limitations of ecological pyramids: they do not account for the same species at two or more trophic levels, assume a simple linear food chain (ignoring food webs), and exclude saprophytes despite their vital role (NCERT §12.5, p. 214).
• Nutrient cycling is of two types — gaseous (reservoir = atmosphere/hydrosphere; e.g., carbon cycle) and sedimentary (reservoir = Earth's crust; e.g., phosphorus cycle). Outputs of ecosystem processes are termed ecosystem services, e.g., purification of air and water by forests (NCERT §12 Summary, p. 214).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 12.1 (p. 208) — Decomposition cycle in a terrestrial ecosystem: tree → leaf fall → detritivores/fungi/bacteria → leaching → further decomposition by earthworms/mites → organic-rich soil → tree.
• Figure 12.2 (p. 210) — Trophic levels: Producer (phytoplankton, grass, trees) → Primary Consumer/Herbivore (zooplankton, grasshopper, cow) → Secondary Consumer/Carnivore (birds, fishes, wolf) → Tertiary Consumer/Top Carnivore (man, lion).
• Figure 12.3 (p. 211) — Energy flow through trophic levels with heat loss at each step; producer → herbivore → carnivore → top carnivore + parallel detritus channel.
• Figure 12.4a (p. 212) — Pyramid of numbers (grassland): 5,842,000 producers → 7,08,000 primary consumers → 3,54,000 secondary consumers → 3 tertiary consumers.
• Figure 12.4b (p. 212) — Upright pyramid of biomass (dry weight kg m⁻²): P=809, PC=37, SC=11, TC=1.5.
• Figure 12.4c (p. 212) — Inverted pyramid of biomass: P=4, PC=21 (small phytoplankton standing crop supports large zooplankton standing crop).
• Figure 12.4d (p. 213) — Ideal pyramid of energy: 1,000,000 J sunlight → 10,000 J producers → 1,000 J PC → 100 J SC → 10 J TC; producers fix only 1% of incident sunlight as NPP.

2.4 Common confusions / NTA trap points

• PAR vs incident solar radiation vs PAR captured. PAR is less than 50% of incident radiation; of this PAR, only 2–10% is captured by plants. Don't confuse "50%" (PAR fraction) with "2–10%" (capture efficiency).
• GPP − R = NPP. NPP is what is left after respiration losses; it is NOT the same as GPP. NPP is the biomass available to heterotrophs.
• Pyramid of energy is ALWAYS upright — even in the sea. Only pyramid of number and pyramid of biomass can be inverted.
• Inverted biomass pyramid in the sea — phytoplankton (producers) have small standing crop because of high turnover; fishes have higher biomass.
• Carbon = gaseous cycle (reservoir: atmosphere/hydrosphere). Phosphorus = sedimentary cycle (reservoir: Earth's crust). NTA loves to swap these.
• Decomposition rate — slower with lignin/chitin; faster with nitrogen and sugars. Warm + moist favours decomposition; low temperature + anaerobiosis inhibit it.
• Detritivores ≠ decomposers. Detritivores (e.g., earthworms) do fragmentation; decomposers (fungi, bacteria) do catabolism, humification and mineralisation.
• Aquatic vs terrestrial energy flow — in aquatic ecosystems GFC dominates; in terrestrial ecosystems a much larger fraction goes through DFC.