Ecology Basics

An ecosystem (term coined by A.G. Tansley, 1935) is a functional unit of biotic + abiotic components interacting via energy flow and nutrient cycling. ABIOTIC = climatic factors (light, temperature, rainfall, humidity), edaphic factors (soil), and inorganic/organic substances. BIOTIC = Producers (autotrophs), Consumers (heterotrophs), and Decomposers (saprotrophs/microconsumers). Memory aid 'P-C-D': Producers fix energy, Consumers transfer it, Decomposers recycle it. Producers are mostly green plants & chemosynthetic bacteria. Consumers: primary (herbivores), secondary, tertiary (carnivores). Decomposers (fungi, bacteria) break down dead matter — without them nutrients would be locked. Ecosystems are self-regulating, open systems requiring constant solar energy input. Largest ecosystem = biosphere; functional categories = terrestrial and aquatic.

Four key functions ('PESS' shortcut): (1) Productivity, (2) Energy flow, (3) Decomposition, (4) Nutrient cycling. Productivity: Gross Primary Productivity (GPP) is total energy fixed by producers; Net Primary Productivity (NPP) = GPP − Respiration (R). NPP is energy available to consumers. Secondary productivity is the rate of energy stored by consumers. Decomposition steps: fragmentation, leaching, catabolism, humification, mineralization. Humus is dark, amorphous, colloidal and resistant to decomposition (acts as nutrient reservoir). Decomposition is fastest when detritus is warm, moist, and rich in nitrogen/water-soluble substances; slowest when rich in lignin and chitin. Oceans, despite covering ~71% of Earth, have low NPP per unit area; most productive ecosystems are tropical rainforests, estuaries, coral reefs and swamps/marshes.

Energy flow in ecosystems is UNIDIRECTIONAL (sun → producers → consumers → decomposers); it never flows back. It obeys the laws of thermodynamics: (1st) energy is neither created nor destroyed, only transformed; (2nd) every transfer involves heat loss, increasing entropy. Lindeman's 10% Law (1942): only ~10% of energy at one trophic level passes to the next; ~90% is lost as metabolic heat. Hence food chains rarely exceed 4-5 trophic levels. PYRAMID OF ENERGY is ALWAYS UPRIGHT (energy declines upward). Memory aid: 'Energy = always upright, never inverted.' Productivity-based numbers always shrink upward. The amount of energy decreasing at successive trophic levels is the reason carnivores are fewer than herbivores.

Three types (Charles Elton): pyramids of NUMBER, BIOMASS, and ENERGY. Pyramid of ENERGY is ALWAYS upright. Pyramid of NUMBER can be inverted (e.g., a single large tree supporting many insects and birds — tree ecosystem). Pyramid of BIOMASS can be inverted in aquatic ecosystems: phytoplankton biomass at any instant is less than that of zooplankton/fish because phytoplankton reproduce and are consumed rapidly (high turnover). Shortcut: 'ENERGY always upright; NUMBER and BIOMASS may invert.' Limitations of pyramids: they assume simple food chains, ignore the same species at multiple levels, and exclude saprophytes/decomposers though they are vital. Standing crop = biomass present at a given time, measured as fresh/dry weight or energy.

A food chain is the sequence of energy transfer through eating. Two types: (1) GRAZING FOOD CHAIN (GFC) — starts with green plants/producers → herbivores → carnivores (e.g., grass → grasshopper → frog → snake → hawk). Driven by solar energy. (2) DETRITUS FOOD CHAIN (DFC) — starts with dead organic matter (detritus) → detritivores (earthworms, fungi, bacteria) → their predators. In terrestrial ecosystems much more energy flows through the DFC than the GFC; in aquatic ecosystems GFC dominates. The two are interconnected. A FOOD WEB is a network of interconnected food chains — it gives ecosystems stability and alternative feeding routes. Memory aid: 'GFC = Green start; DFC = Dead start.'

BIOACCUMULATION = build-up of a persistent toxic substance within a single organism over time (intake exceeds excretion). BIOMAGNIFICATION (biological magnification) = increase in concentration of a non-biodegradable toxicant at SUCCESSIVELY HIGHER trophic levels of a food chain. Classic examples: DDT (a chlorinated hydrocarbon) and mercury (Minamata disease, Japan). DDT biomagnification famously caused eggshell thinning in birds of prey. Substances that biomagnify are persistent, fat-soluble (lipophilic), and non-biodegradable. Top carnivores accumulate the highest concentrations — hence they are most affected. Memory aid: 'Magnify = goes UP the food chain.' Important POPs (Persistent Organic Pollutants) under the Stockholm Convention biomagnify; India is a party to this convention.

Succession = orderly, predictable, directional change in community composition over time at a site, ending in a stable CLIMAX community. PRIMARY succession occurs on a bare, lifeless area never previously colonised (new volcanic rock, bare rock, newly cooled lava, glacial moraine); it is SLOW because soil must form first. The first colonisers are PIONEER species (lichens on rock; in water, phytoplankton). SECONDARY succession occurs where a community has been removed but soil already exists (abandoned farmland, burnt forest, after floods); it is FASTER as soil and some propagules persist. Hydrarch succession starts in water and moves toward mesic (moderate moisture) conditions; xerarch starts on dry land. Both converge on a mesic climax. Memory aid: 'Primary = from scratch (no soil); Secondary = soil already there.'

Interactions classed by + (benefit), − (harm), 0 (neutral): MUTUALISM (+/+) both benefit, e.g., lichen (fungus+alga), mycorrhiza, Rhizobium-legume, pollination. COMMENSALISM (+/0) one benefits, other unaffected, e.g., orchid on a tree, barnacles on whale, cattle egret & grazing cattle. PARASITISM (+/−) parasite gains, host harmed, e.g., Cuscuta, ticks, tapeworm. PREDATION (+/−) predator kills prey. COMPETITION (−/−) both harmed (Gause's Competitive Exclusion Principle: two species competing for the same limiting resource cannot coexist indefinitely). AMENSALISM (−/0) one harmed, other unaffected, e.g., Penicillium secreting penicillin. Memory aid: 'Mutual = both win; Commensal = one wins, one neutral; Amensal = one loses, one neutral.'