Ecosystem

Ecosystem (coined by A.G. Tansley, 1935): • A structural and functional unit of nature comprising the living (biotic) community and the non-living (abiotic) environment interacting with each other through the flow of energy and cycling of materials • Examples: Pond, forest, grassland, desert, coral reef, agricultural field Main components: Abiotic (Non-living) components: 1. Physical factors: Light, temperature, humidity, rainfall, wind 2. Inorganic substances: N₂, CO₂, H₂O, O₂, minerals (soil) 3. Organic compounds: Carbohydrates, proteins, lipids, humus — link biotic and abiotic 4. Climate: The general atmospheric conditions over a long time Biotic (Living) components: 1. Producers (Autotrophs): • Photosynthetic organisms: green plants, algae, cyanobacteria • Convert solar energy into chemical energy (food) • Form the base of all food chains • Also called primary producers 2. Consumers (Heterotrophs): • Primary consumers (herbivores): eat producers — grasshoppers, deer, rabbits • Secondary consumers (primary carnivores): eat herbivores — frogs, small birds • Tertiary consumers (secondary carnivores): eat primary carnivores — snakes, large birds • Top carnivores (apex predators): lions, tigers, sharks 3. Decomposers (Saprotrophs / Detritivores): • Bacteria and fungi that break down dead organic matter • Return nutrients to the abiotic environment (mineralisation) • Essential for nutrient cycling • Examples: Bacillus, Pseudomonas, Aspergillus, Penicillium, earthworms (detritivores) 4. Detritivores: • Organisms that feed on detritus (dead organic matter) • Examples: earthworms, millipedes, woodlice

Food Chain: • A linear sequence of organisms where each organism is eaten by the next • Represents the transfer of energy and matter through feeding relationships • Each step in the food chain is a trophic level Types of food chains: 1. Grazing food chain (GFC): Starts from living green plants Producer → Primary Consumer → Secondary Consumer → Tertiary Consumer Example: Grass → Grasshopper → Frog → Snake → Hawk Example: Phytoplankton → Zooplankton → Small fish → Large fish 2. Detritus food chain (DFC): Starts from dead organic matter (detritus) Dead organic matter → Detritivores → Predators of detritivores Example: Dead leaves → Earthworm → Robin → Hawk Food Web: • A complex network of multiple interconnected food chains in an ecosystem • More realistic representation of feeding relationships — most organisms eat more than one type of food • Provides stability to the ecosystem: if one species declines, predators can switch to other prey • The more complex the food web, the more stable the ecosystem Example of a forest food web: • Grass is eaten by grasshopper, rabbit, deer • Grasshopper is eaten by frog, birds • Frog is eaten by snake, birds • Rabbit is eaten by fox, hawk • All are eaten by top predators (tiger, lion) • All organisms eventually become detritus eaten by decomposers Importance of food chains and webs: • Show energy flow through ecosystem • Explain the concept of trophic levels • Basis for understanding biomagnification • Used in ecological management

Ecological Pyramid: • A graphical representation showing the relationship between different trophic levels of a food chain with respect to the number, biomass, or energy of organisms at each level • Coined by Charles Elton (Eltonian pyramids) • Usually pyramid-shaped — broad at base (producers), narrow at top (top consumers) Types of Ecological Pyramids: 1. Pyramid of Numbers: • Represents the number of individuals at each trophic level • Usually upright (broad base) – Example (grassland): Grass (millions) → Grasshoppers → Frogs → Snakes → Hawks • Inverted pyramid possible: – Example (tree ecosystem): One tree (1) → Many insects → Fewer birds → Even fewer hawks (1 tree supports many insects — inverted pyramid) • Limitation: Does not account for size differences between organisms 2. Pyramid of Biomass: • Represents the total dry weight (biomass) of organisms at each trophic level • Usually upright (terrestrial ecosystems): More plant biomass than herbivore biomass, etc. – Example: Forest: Plants (200 t/ha) → Herbivores (67 t/ha) → Carnivores (4 t/ha) • Inverted pyramid (aquatic ecosystems): – Example: Marine/pond ecosystem: Phytoplankton biomass < Zooplankton biomass at a given time – Phytoplankton reproduce so rapidly that their standing crop is low – But production over time is actually greater than zooplankton 3. Pyramid of Energy: • Represents the total amount of energy at each trophic level per unit area per unit time • ALWAYS UPRIGHT — energy is always lost at each step (10% law) • Most accurate and informative pyramid – Example: Producers (10,000 kcal) → Primary consumers (1,000 kcal) → Secondary consumers (100 kcal) → Tertiary consumers (10 kcal) • Never inverted because: – Energy is lost as heat at each trophic level (2nd law of thermodynamics) – Energy cannot be recycled (unlike matter) • Accounts for energy loss via respiration, excretion, heat at each level

Carbon Cycle: • The movement of carbon atoms through the biosphere, atmosphere, hydrosphere, and lithosphere • Carbon exists as: CO₂ in atmosphere, carbonate in oceans, organic compounds in living organisms, fossil fuels, limestone Main processes: 1. Carbon fixation (Atmosphere → Biosphere): • Photosynthesis: Plants and algae absorb CO₂ from atmosphere 6CO₂ + 6H₂O → (light energy) → C₆H₁₂O₆ + 6O₂ • CO₂ dissolved in ocean water → carbonic acid → bicarbonates (absorbed by marine organisms for shells) 2. Carbon release by organisms (Biosphere → Atmosphere): • Respiration (all living organisms): Organic compounds → CO₂ + H₂O + Energy C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP • CO₂ released into atmosphere 3. Decomposition: • Decomposers break down dead organic matter → release CO₂ and water • Partial decomposition in anaerobic conditions → formation of peat, coal, petroleum (over millions of years) 4. Combustion (fossil fuel burning): • Burning coal, oil, natural gas releases carbon stored over millions of years back into atmosphere • Major anthropogenic source of CO₂ → climate change, greenhouse effect • Forest fires also release large amounts of CO₂ 5. Ocean carbon exchange: • Oceans absorb ~30% of anthropogenic CO₂ • Atmospheric CO₂ dissolves in seawater → carbonic acid → bicarbonate • Marine organisms (corals, molluscs) use carbonate for shells • When they die, carbonate settles → limestone formation (geological time scales) 6. Geological processes: • Weathering of limestone releases CO₂ • Volcanic activity releases CO₂ from Earth's interior Human impact on carbon cycle: • Fossil fuel combustion: 7–8 billion tonnes of carbon per year → CO₂ increase (280 ppm pre-industrial → >420 ppm today) • Deforestation: Reduces CO₂ fixation; releases stored carbon • Cement production: CaCO₃ → CaO + CO₂ • Result: Enhanced greenhouse effect → global warming

Biomagnification (Biological Magnification / Bioaccumulation): • The progressive increase in the concentration of a toxic, non-biodegradable chemical substance as it moves up from one trophic level to the next in a food chain • Occurs because the substance cannot be broken down (non-biodegradable) or excreted efficiently → accumulates in body fat • Higher trophic level organisms consume many organisms from the level below → concentrate more of the substance Conditions for biomagnification: 1. Substance must be soluble in fat (lipophilic) 2. Non-biodegradable (persists in environment) 3. Absorbed from environment or food into organisms 4. Organisms cannot excrete it efficiently Classic example — DDT (Dichlorodiphenyltrichloroethane): • DDT is a persistent organochlorine pesticide (now banned in most countries; still used for malaria control) • Highly lipophilic, non-biodegradable — persists in environment for decades Biomagnification in an aquatic ecosystem: Level | Organism | DDT Concentration Water | — | 0.003 ppm Producers | Phytoplankton | 0.04 ppm (13× water) Primary consumer | Zooplankton | 0.23 ppm (8× phytoplankton) Secondary consumer | Small fish | 2.07 ppm (9×) Tertiary consumer | Large fish | 13.8 ppm (7×) Top predator | Fish-eating birds (osprey, pelican) | up to 25–76 ppm (5×) Effects on top predators: • Fish-eating birds: DDT → metabolised to DDE → interferes with calcium metabolism → thinning of eggshells → eggs crack under parent's weight → reproductive failure • Bald eagle, Peregrine falcon, Brown pelican populations crashed due to DDT in 1950s–1970s • After DDT ban (USA, 1972): populations recovered Other examples of biomagnification: • Mercury in marine food chains (Minamata disease, Japan — methylmercury from factory effluent → fish → humans → neurological damage) • PCBs (polychlorinated biphenyls) in Arctic food chains • Lead, cadmium in terrestrial food chains

Productivity: • The rate of biomass production (energy fixation) in an ecosystem per unit area per unit time • Measured in g/m²/year (dry weight) or kcal/m²/year (energy) Primary Productivity: • The rate of organic matter (biomass) synthesis by producers (autotrophs) per unit area per unit time • ONLY producers are considered for primary productivity Gross Primary Productivity (GPP): • The TOTAL rate of organic matter (biomass) produced by photosynthesis per unit area per unit time, INCLUDING what will be used in plant respiration • Represents the total photosynthetic output • GPP = total photosynthesis rate Net Primary Productivity (NPP): • The rate of organic matter accumulation that is available for consumers (herbivores) after accounting for the energy used by producers in their own respiration • NPP = GPP − Respiration (R) • NPP represents the biomass actually available for higher trophic levels • This is what ecologists usually measure as 'standing crop' Relationship: • NPP = GPP − R • GPP = NPP + R Secondary Productivity: • Rate of energy stored by consumers (herbivores, carnivores) at various trophic levels • Energy assimilated by consumers from consumed food − their respiration Factors affecting primary productivity: • Light intensity (limiting in deep water) • Temperature (affects enzyme activity) • Water availability (terrestrial ecosystems) • Nutrient availability (especially N and P) Examples of NPP (approximate): • Tropical rainforest: 2200 g/m²/year (highest) • Open ocean: 125 g/m²/year • Desert: 90 g/m²/year (lowest) • Total global NPP: ~170 billion tonnes dry weight/year