Sustainability & Environment (Agri-Food Systems) · Foundations · also Principles of Ecology, Fundamentals of Ecology, Ecosystem Ecology, Agroecosystem Ecology (introductory)
How organisms, populations, communities and ecosystems work: energy flow, productivity, nutrient cycles, succession and population dynamics. Every sustainability argument eventually rests on this subject.
In the exams: GATE ES Section 8 (ecology and ecosystems); ICAR AICE subject 71 Unit 1; UGC NET Environmental Sciences Unit 3 (environmental biology); GATE EY core
Ecology is the scientific study of the interactions of organisms with one another and with their physical environment, and of the distribution and abundance of organisms that results. The word was coined by Ernst Haeckel in 1866 from the Greek oikos, 'house'. Ecosystem science, which grew from Arthur Tansley's introduction of the term 'ecosystem' in 1935 and Raymond Lindeman's trophic-dynamic study of 1942, follows energy and matter through living and non-living components. Together they explain how populations grow and decline, how species coexist, how communities change over time, how energy flows and nutrients cycle, how biomes are distributed across the planet, and why some systems recover from disturbance while others collapse.
Every argument in sustainability eventually rests on this subject: carrying capacity, maximum sustainable yield, biodiversity, ecosystem services, nutrient pollution, pest outbreaks and resilience are ecological ideas. This manual covers the principles at the level of university ecology courses and the examinations listed below, and points to later subjects of the track for applications: biodiversity conservation, ecosystem services and pollinators, landscape ecology, wetlands, agroecology, soil carbon, nutrient stewardship and climate.
Ecology is examined widely. GATE Ecology and Evolution (EY) and GATE Environmental Science (Section 8) test population and community ecology with numerical problems; ICAR AICE Environmental Sciences and UGC NET Environmental Sciences (environmental biology) test ecosystem structure, productivity, cycles, succession and biomes; CUET and ICAR AIEEA-PG test definitions and principles. Calculations that recur are worked here with their formulas: mark–recapture estimates, exponential and logistic growth, maximum sustainable yield, Shannon and Simpson indices, importance value index, similarity indices, the ten per cent law and the light–dark bottle method.
Ecology studies life at several levels of organisation. An individual organism responds to its environment through physiology and behaviour. A population is a group of individuals of the same species living in the same area at the same time and potentially interbreeding. A community is the assemblage of populations of different species living and interacting in an area. An ecosystem is a community together with its physical environment, linked by flows of energy and cycles of matter. A landscape is a mosaic of interacting ecosystems; a biome is a large region with characteristic climate and vegetation, such as tropical rainforest or tundra; and the biosphere is the sum of all ecosystems on Earth. Properties emerge at each level that are not present at lower levels, such as birth rates for populations, species diversity for communities and nutrient cycling for ecosystems. Autecology is the ecology of individual species or populations in relation to their environment: for example, how temperature limits the distribution of a crop pest or how a tree species responds to shade. Synecology is the ecology of communities and ecosystems: the interactions among many species and their combined relations with the environment, such as forest succession or the structure of a grassland. Ecotypes are genetically distinct populations of a species adapted to local conditions, a concept introduced by Göte Turesson (1922); they matter in breeding and restoration because seed from the wrong ecotype may fail at a restoration site.
A habitat is the place where an organism lives, its 'address'; the niche is its functional role and the full set of conditions and resources it uses, its 'profession'. Joseph Grinnell (1917) emphasised the habitat requirements of a species; Charles Elton (1927) its functional role in the community, especially its food relations; and G. Evelyn Hutchinson (1957) defined the niche as an n-dimensional hypervolume, the range of every environmental factor and resource within which a species can persist. The fundamental niche is the full range a species could occupy in the absence of competitors and enemies; the realised niche is the narrower range it actually occupies when interactions with other species are present. The competitive exclusion principle, associated with G.F. Gause's laboratory experiments with Paramecium in the 1930s and stated by Garrett Hardin (1960) as 'complete competitors cannot coexist', holds that two species competing for exactly the same limiting resources cannot coexist indefinitely; one will exclude the other. In Gause's experiments, Paramecium aurelia outcompeted P. caudatum when the two were grown together on the same food. Species coexist in nature by resource partitioning, using different parts of a resource or the same resource at different times or places: Robert MacArthur (1958) showed that several warbler species feeding in the same spruce trees forage in different parts of the canopy. Character displacement occurs when competing species evolve greater differences where they live together than where they live apart. An ecotone is a transition zone between two communities, such as a forest edge or an estuary; it often has species from both communities and some of its own, giving higher diversity, the edge effect, though edges can also expose species to predators and invaders. A keystone species has an effect on community structure disproportionate to its abundance; Robert Paine's removal of the starfish Pisaster ochraceus from rocky shores (1966) led mussels to dominate and diversity to fall. Indicator species, such as lichens sensitive to sulphur dioxide, reveal environmental conditions.
Populations have attributes that individuals do not. Population density is the number (or biomass) of individuals per unit area or volume. Crude density is per unit of total area; ecological density is per unit of habitat actually available, such as per hectare of wetland rather than per hectare of the whole district. Density is estimated by complete counts for large or conspicuous organisms, by sampling units such as quadrats for plants and sedentary animals, and by mark–recapture for mobile animals. In the Lincoln–Petersen mark–recapture method, M individuals are captured, marked and released; later a second sample of C individuals is captured, of which R are marked. Assuming marked and unmarked animals mix randomly and are equally catchable, and that the population is closed, the population estimate is N = M × C ÷ R. For example, if 50 fish are marked and a later catch of 60 contains 10 marked fish, N = 50 × 60 ÷ 10 = 300. Plant community sampling uses quadrats to estimate density (individuals per unit area), frequency (the percentage of quadrats in which a species occurs) and abundance (individuals per quadrat of occurrence, i.e. total individuals divided by the number of quadrats in which the species occurs). If a species has 40 individuals in 8 of 20 quadrats, its density is 2 per quadrat, its frequency 40 per cent and its abundance 5. Raunkiaer's frequency classes group species by frequency into classes A to E.
Population size changes through four processes: natality (births), mortality (deaths), immigration and emigration. Natality may be expressed as maximum (physiological) natality under ideal conditions or as realised (ecological) natality under actual conditions; mortality similarly. The change in a closed population is births minus deaths; per-capita birth rate (b) minus per-capita death rate (d) gives the intrinsic rate of increase r. Dispersal links populations and allows colonisation; metapopulations are networks of local populations connected by dispersal, some of which go extinct and are recolonised. Life tables record survival and reproduction by age. Survivorship curves plot the number surviving (on a logarithmic scale) against age. Type I curves show high survival until old age, then rapid mortality, typical of humans in developed countries and many large mammals. Type II curves show a roughly constant mortality rate throughout life, approximated by some birds, lizards and Hydra. Type III curves show very high early mortality followed by higher survival of the few that remain, typical of oysters, many fish, insects and plants with numerous small seeds. Age structure divides a population into pre-reproductive, reproductive and post-reproductive groups. Age pyramids with a broad base indicate a growing population; bell-shaped pyramids a stable one; and urn-shaped pyramids, with fewer young, a declining one. Sex ratio, the proportion of males to females, also affects reproduction. Fishery and wildlife managers use age structure to judge whether harvesting is removing too many young or breeding individuals.
When resources are unlimited, a population grows exponentially: dN/dt = rN, where N is population size and r the intrinsic rate of natural increase. The solution is N(t) = N₀e^(rt), giving a J-shaped curve, and the doubling time is ln 2 ÷ r, about 0.693/r. For example, with r = 0.1 per year, a population of 100 grows to 100 × e^(1) ≈ 272 in 10 years and doubles in about 6.9 years. In discrete time with non-overlapping generations, growth is described by Nt+1 = λNt, where λ is the finite rate of increase. Real populations encounter limits. The logistic model, proposed by Pierre-François Verhulst (1838) and popularised by Raymond Pearl, adds a carrying capacity K: dN/dt = rN(1 − N/K). Growth is nearly exponential at low density, slows as N approaches K, and stops at K, giving an S-shaped (sigmoid) curve. The growth rate dN/dt is greatest when N = K/2. For example, with r = 0.4 per year and K = 1000, the growth rate at N = 500 is 0.4 × 500 × (1 − 0.5) = 100 individuals per year, the maximum. This underlies the concept of maximum sustainable yield (MSY) in fisheries and forestry: harvesting at the maximum growth rate, rK/4, when the stock is held at K/2. With r = 0.4 and K = 1000, MSY = 100 a year. MSY is a useful idea but a risky management target, because r and K are uncertain and vary with environment, so harvesting at the estimated MSY has often led to collapse; modern fisheries management uses more precautionary reference points. Density-dependent factors, such as competition, predation, disease and territoriality, intensify as density rises and regulate populations; density-independent factors, such as weather extremes and fire, affect populations regardless of density.
Robert MacArthur and E.O. Wilson (1967) introduced the terms r-selection and K-selection, later developed by Eric Pianka (1970). r-selected species are favoured in unpredictable or newly available environments where rapid reproduction pays: they tend to have small body size, early maturity, many small offspring with little parental care, short lifespan and Type III survivorship; examples include many insects, annual weeds and bacteria. K-selected species are favoured in stable, crowded environments near carrying capacity: they tend to have larger body size, later maturity, few large offspring with much parental care, long lifespan and Type I survivorship; examples include elephants, whales and large trees. The scheme is a continuum and a simplification; many species combine traits, and later life-history theory considers age-specific mortality and other factors. Pest species and weeds are often r-strategists that exploit the disturbed, resource-rich conditions of farmland, which helps explain pest outbreaks after spraying kills slower-reproducing natural enemies. Conservation concern is often greatest for K-strategists, whose low reproductive rates make them slow to recover from overharvesting or habitat loss. Interacting populations can be modelled mathematically. The Lotka–Volterra competition equations extend the logistic model with competition coefficients and predict either coexistence or exclusion depending on the strength of intraspecific versus interspecific competition. The Lotka–Volterra predator–prey equations predict coupled oscillations in predator and prey numbers, with predator peaks lagging prey peaks. The fur-trading records of the Hudson's Bay Company for snowshoe hare and Canada lynx show roughly ten-year cycles often cited as an example, although food supply for hares also plays a part.
Interactions between species are classified by their effects on each partner (+ benefit, − harm, 0 no effect). Competition (−/−) occurs when species use the same limited resource; it may be interference (direct, such as aggression or territoriality) or exploitation (indirect, by depleting a shared resource), and intraspecific (within a species) or interspecific (between species). Predation (+/−) is the killing and consumption of prey; herbivory is consumption of plants, often without killing them; parasitism (+/−) is living on or in a host and drawing resources from it, usually without killing it quickly; parasitoids, such as many wasps used in biological control, lay eggs in or on hosts whose larvae eventually kill the host. Mutualism (+/+) benefits both partners: examples include rhizobia in legume root nodules (nitrogen for carbohydrate), mycorrhizal fungi and plant roots (phosphorus and water for carbohydrate), pollinators and flowers, and lichens (fungus and alga or cyanobacterium). Protocooperation is a non-obligatory mutualism. Commensalism (+/0) benefits one partner without affecting the other, such as epiphytic orchids on trees or cattle egrets feeding on insects disturbed by grazing animals. Amensalism (−/0) harms one partner while the other is unaffected, as when the mould Penicillium produces penicillin that inhibits bacteria, or when a plant's chemicals inhibit neighbours (allelopathy, as reported for black walnut). Neutralism (0/0) is the absence of interaction. Coevolution occurs when interacting species exert reciprocal selection, as in plant–pollinator specialisation and the 'arms races' between hosts and parasites.
Prey and plants have many defences. Plants use physical defences such as thorns and tough leaves, and chemical defences such as alkaloids, tannins and terpenoids; some, such as Acacia trees with ants, rely on mutualistic defenders. Animals use camouflage (crypsis), warning (aposematic) colouration that advertises toxicity, mimicry and behaviour. In Batesian mimicry, a harmless species resembles a harmful or distasteful one and gains protection, as when some harmless hoverflies resemble wasps. In Müllerian mimicry, two or more harmful species resemble each other, sharing the cost of educating predators, as in some butterflies. Predators and parasites can regulate prey populations, and top predators can cause trophic cascades, indirect effects that pass down the food chain: the reintroduction of wolves to Yellowstone National Park in 1995 is widely cited as having changed elk behaviour and numbers, with effects on vegetation, though the strength of the cascade is debated. Agriculture uses species interactions deliberately. Biological control uses predators, parasitoids and pathogens to suppress pests: classical biological control introduces natural enemies from a pest's native range, as with the vedalia beetle introduced to California against cottony cushion scale in the 1880s, while conservation biological control protects and encourages native enemies by providing habitat and reducing broad-spectrum sprays. Legume–rhizobium and mycorrhizal mutualisms reduce fertiliser needs; intercropping and push–pull systems exploit plant–insect interactions; and allelopathic cover crops can suppress weeds. Misjudged introductions, such as the cane toad in Australia, show that interactions can also have unintended consequences, which is why biological control introductions are now strictly assessed.
A community's structure includes its species composition, the relative abundance of species, physical structure such as vertical stratification (canopy, understorey, shrub, herb and ground layers in a forest), and its trophic organisation. Species richness (S) is the number of species; evenness is how equally individuals are distributed among them. Diversity indices combine both. The Shannon index (Shannon–Wiener or Shannon–Weaver) is H′ = −Σ pᵢ ln pᵢ, where pᵢ is the proportion of individuals belonging to species i. Higher values indicate greater diversity; values for real communities are typically between about 1.5 and 3.5, and the maximum for S species is ln S, reached when all species are equally abundant. Evenness can be expressed as J′ = H′ ÷ ln S. Simpson's index, D = Σ pᵢ², is the probability that two individuals drawn at random (with replacement) belong to the same species; it measures dominance, so diversity is often expressed as 1 − D (the Gini–Simpson index) or 1/D. For example, a community of three species with proportions 0.5, 0.3 and 0.2 has D = 0.25 + 0.09 + 0.04 = 0.38, Simpson's diversity 1 − D = 0.62, and H′ = −(0.5 ln 0.5 + 0.3 ln 0.3 + 0.2 ln 0.2) ≈ 1.03. Robert Whittaker distinguished alpha diversity (within a habitat or site), beta diversity (the change in species composition between habitats or along gradients) and gamma diversity (total diversity of a landscape or region). Similarity between two sites can be measured with the Jaccard index, J = c ÷ (a + b − c), or the Sørensen index, S = 2c ÷ (a + b), where a and b are the numbers of species at each site and c the number shared.
Dominant species are those that, through their numbers, size or productivity, exert the greatest influence on the community, such as sal (Shorea robusta) in sal forests or teak in teak forests. Dominance in vegetation is often measured as basal area (the cross-sectional area of stems at breast height) or cover. In Indian forest and grassland ecology, the importance value index (IVI) combines three relative measures: relative density (the density of a species as a percentage of total density), relative frequency (its frequency as a percentage of the sum of frequencies of all species) and relative dominance (its basal area as a percentage of total basal area). IVI = relative density + relative frequency + relative dominance, so it can range up to 300 for a community of one species, and the IVIs of all species sum to 300. A species with relative density 30, relative frequency 25 and relative dominance 45 has an IVI of 100, indicating that it is ecologically the most important species if others are lower. Communities vary along environmental gradients and over time. Species–area relationships show that the number of species increases with area sampled, often approximated by S = cA^z, with z commonly between about 0.15 and 0.35; this relationship underlies island biogeography theory (MacArthur and Wilson, 1967), in which species richness on islands reflects a balance between immigration (higher for islands near the mainland) and extinction (higher for small islands), and it informs the design of protected areas, as the landscape ecology subject explains. Latitudinal diversity gradients, with more species in the tropics, are among the most consistent patterns in ecology, though their causes are still debated. Dominance, diversity and structure together determine how communities respond to disturbance and how they function.
Ecological succession is the directional change in species composition of a community over time. The whole sequence of communities is a sere, each stage a seral stage, the first colonists pioneer species and the final relatively stable community the climax. Primary succession begins on newly exposed surfaces with no soil, such as bare rock, lava flows, sand dunes or newly formed ponds. Secondary succession begins where a community has been disturbed but soil remains, such as abandoned farmland, cleared forest or burned areas; it proceeds faster because soil, seeds and roots are present. Successions are named by the starting habitat. A xerosere begins in a dry habitat; on bare rock it is called a lithosere, in which crustose lichens are the pioneers, secreting acids and trapping dust that begin soil formation, followed by foliose lichens, mosses, herbs, shrubs and finally trees. A hydrosere begins in water: phytoplankton are followed by rooted submerged plants, rooted floating plants, a reed-swamp stage, a sedge meadow (marsh), woodland and eventually forest as the pond fills with sediment and organic matter. A psammosere begins on sand and a halosere in saline habitats. Mechanisms of succession were classified by Joseph Connell and Ralph Slatyer (1977) into three models: facilitation, in which early species modify conditions to make them more suitable for later species (as lichens build soil); tolerance, in which later species are simply able to tolerate lower resource levels and outcompete early ones; and inhibition, in which early species resist invasion until they are damaged or die. All three operate in different cases.
Frederic Clements (1916) proposed the monoclimax theory: each climatic region has a single climax community determined by climate, and succession is an orderly, predictable development of the community as a 'superorganism'. Henry Gleason (1926) countered with the individualistic concept: communities are coincidental assemblages of species responding individually to the environment, so succession is less predictable. Arthur Tansley's polyclimax theory recognised several climaxes in a region, determined by soil, topography, fire or grazing as well as climate, such as edaphic and fire climaxes. Robert Whittaker's climax-pattern hypothesis treats the climax as a continuum of communities varying with environmental gradients. Modern ecology sees succession as more variable than Clements proposed, influenced by disturbance regimes, chance and history, and the 'climax' as a dynamic state rather than a fixed end-point. Communities maintained short of the climax by recurring disturbance, such as grasslands kept by grazing or fire, are disclimaxes or subclimaxes. Eugene Odum (1969) described trends during succession: total biomass and organic matter increase; species diversity generally increases, though it may peak at mid-succession; food webs become more complex and detritus-based; nutrient cycles become more closed and conservative; life-history strategies shift from r- to K-selected species; and the ratio of gross production to community respiration (P/R) moves towards 1, so that net community production falls towards zero at climax, whereas early successional systems have P/R greater than 1 and accumulate biomass. These trends explain why agriculture, which keeps land in an early successional state with high net production available for harvest, requires continual inputs of energy and labour to prevent succession towards weeds, shrubs and forest. They also guide restoration: assisted natural regeneration accelerates secondary succession by protecting regenerating trees and removing barriers such as fire, grazing or invasive grasses, as the agroforestry and land restoration subjects explain.
An ecosystem has structure, its components and their arrangement, and function, the processes of energy flow, nutrient cycling, production, decomposition and regulation. The abiotic structure includes inorganic substances (water, carbon dioxide, nitrogen, phosphorus and minerals), organic compounds (humus, dead organic matter) and climatic and physical factors. The biotic structure includes producers, consumers and decomposers. Trophic levels group organisms by their position in the feeding sequence: producers (first trophic level), herbivores or primary consumers (second), carnivores feeding on herbivores (secondary consumers, third), and so on. Omnivores feed at more than one level. Decomposers and detritivores process dead matter from all levels. A food chain is a single sequence of feeding relations, such as grass → grasshopper → frog → snake → hawk. Grazing food chains begin with living plants eaten by herbivores. Detritus food chains begin with dead organic matter consumed by detritivores and decomposers; in many terrestrial ecosystems, including forests, more energy flows through detritus chains than through grazing chains, because most plant production is not eaten while alive. Parasitic food chains run from large hosts to smaller parasites and hyperparasites. Food chains seldom have more than four or five links because energy is lost at each transfer.
In real ecosystems food chains interconnect to form food webs, because most consumers eat several kinds of food and are eaten by several predators. Food webs provide alternative pathways, so the loss of one species may be buffered by others, although removal of key species can trigger cascades. Food web properties studied by ecologists include connectance (the proportion of possible feeding links that actually occur), chain length and the strength of interactions. Biological magnification (biomagnification) is the increase in concentration of persistent, fat-soluble contaminants, such as DDT, some other organochlorine pesticides, polychlorinated biphenyls and methylmercury, at successively higher trophic levels, because they are not broken down or excreted and accumulate in tissues. Top predators, such as fish-eating birds, can carry concentrations many times higher than those in water or plankton; DDT caused eggshell thinning and population declines in birds of prey such as the peregrine falcon and the bald eagle, contributing to DDT bans from the 1970s. Bioaccumulation refers to the build-up of a substance in an individual over its lifetime. The pesticides and environment subject develops these effects. The functions of ecosystems underpin the ecosystem services people rely on: provisioning (food, fibre, water), regulating (climate, flood control, pollination, pest control, water purification), cultural (recreation, spiritual value) and supporting (nutrient cycling, soil formation, primary production), in the classification of the Millennium Ecosystem Assessment (2005). The ecosystem services and pollinators subject treats these in detail, including newer classifications such as CICES and the IPBES concept of nature's contributions to people.
Energy enters most ecosystems as sunlight captured by photosynthesis and flows one way through the trophic levels, being lost as heat at each transfer, whereas nutrients cycle. This follows the laws of thermodynamics: energy is neither created nor destroyed (first law), and every transformation degrades some energy to heat, so no transfer is 100 per cent efficient (second law). Raymond Lindeman's study of Cedar Bog Lake, Minnesota (1942), established the trophic-dynamic concept of ecosystems. Only a small fraction of incoming solar radiation is fixed by plants: globally less than 1 per cent of incident sunlight, and perhaps 1–2 per cent for productive crops over a growing season, though higher for short periods. Of the energy at one trophic level, typically about 10 per cent is passed on as biomass to the next; the rest is lost in respiration and as heat, or remains unconsumed or undigested and passes to decomposers. This 'ten per cent law', associated with Lindeman, is an approximation: ecological (trophic transfer) efficiency usually ranges between about 5 and 20 per cent, higher in some aquatic systems and among ectotherms than among endotherms. For example, if producers fix 10,000 kJ m⁻² yr⁻¹, herbivores obtain about 1,000, primary carnivores about 100 and secondary carnivores about 10 kJ m⁻² yr⁻¹. Several efficiencies describe energy transfer. Consumption efficiency is the fraction of production at one level that is eaten by the next. Assimilation efficiency is the fraction of ingested energy absorbed across the gut, higher for carnivores (about 80 per cent) than herbivores (often 20–50 per cent, depending on plant fibre). Production efficiency is the fraction of assimilated energy converted to new biomass, much higher in ectotherms such as insects and fish than in endotherms such as birds and mammals, which spend most assimilated energy on maintaining body temperature. Trophic transfer efficiency is the product of these.
Ecological pyramids, introduced by Charles Elton, represent the trophic structure of an ecosystem with successive bars for each trophic level. A pyramid of numbers shows the number of individuals at each level. It is usually upright, as in a grassland with many grass plants supporting fewer herbivores and still fewer carnivores, but it can be inverted, as when a single large tree supports many insects, which in turn support many parasites; or spindle-shaped. A pyramid of biomass shows the standing mass of organisms at each level at a given time. It is upright in most terrestrial ecosystems, but can be inverted in some aquatic ecosystems such as the open ocean, where the standing biomass of phytoplankton is smaller than that of zooplankton because phytoplankton reproduce and are eaten very rapidly; their high turnover supports a larger consumer biomass. A pyramid of energy (or productivity) shows the rate of energy flow through each level per unit area per unit time. Because energy is lost at each transfer, the pyramid of energy is always upright; it can never be inverted. It is therefore the most fundamental of the three representations. Energy flow has practical implications. Feeding people directly on plants uses energy and land far more efficiently than feeding them on animals fed on crops, because at each step most energy is lost; this is why grain-fed livestock products have much larger land, water and greenhouse gas footprints per calorie or per gram of protein than plant foods, as the livestock and sustainable food systems subjects explain. Ruminants grazing on land unsuited to crops, or animals fed by-products, can however convert otherwise unusable biomass into human food.
Productivity is the rate of production of biomass or energy per unit area per unit time, such as grams of dry matter or carbon per square metre per year. Gross primary productivity (GPP) is the total rate of photosynthesis by producers. Net primary productivity (NPP) is GPP minus the respiration of the producers themselves (autotrophic respiration, Ra): NPP = GPP − Ra; it is the organic matter available to consumers and decomposers, and typically about half of GPP. Net ecosystem productivity (NEP) subtracts heterotrophic respiration (Rh) by consumers and decomposers: NEP = NPP − Rh; a positive NEP means the ecosystem is accumulating carbon. Secondary productivity is the rate of formation of new biomass by consumers. Standing crop or biomass is the amount of living matter present at a given time, a stock, not a rate. Productivity can be measured by harvesting biomass at intervals (the harvest method, used for crops and grasslands), by measuring carbon dioxide exchange with chambers or eddy-covariance towers, by remote sensing of vegetation indices, and in water by the light-and-dark bottle method. In that method, water samples are incubated in an initial bottle (measured at the start), a light bottle and a dark bottle. Photosynthesis and respiration occur in the light bottle; only respiration occurs in the dark bottle. Over the incubation, respiration = initial oxygen − dark bottle oxygen; net primary productivity = light bottle oxygen − initial oxygen; and gross primary productivity = light bottle oxygen − dark bottle oxygen. For example, with initial 8.0 mg L⁻¹, light 9.5 and dark 7.0, respiration is 1.0, NPP is 1.5 and GPP is 2.5 mg O₂ L⁻¹ over the incubation period.
Productivity varies greatly among ecosystems, controlled on land mainly by temperature, water and nutrients, and in water by light and nutrients. Among natural ecosystems, tropical rainforests, swamps and marshes, estuaries, coral reefs and algal beds have the highest NPP per unit area, while deserts, tundra and the open ocean have the lowest. The open ocean nevertheless contributes a large share of global production because of its vast area. Field and colleagues (1998) estimated global NPP at about 105 billion tonnes of carbon per year, with roughly 56 from land and 49 from the oceans: about half of the planet's primary production is marine, carried out mainly by microscopic phytoplankton. Aquatic productivity is often limited by nutrients: nitrogen tends to limit marine systems and phosphorus many freshwater lakes, and iron limits phytoplankton in some ocean regions. Upwelling zones, where nutrient-rich deep water rises to the surface, such as off Peru, support some of the world's richest fisheries. Adding nutrients to water bodies raises productivity but can lead to eutrophication, as the water quality subject explains. Agricultural systems can have high NPP, especially irrigated and fertilised crops such as sugarcane and maize (C4 plants with high photosynthetic efficiency), but much of their production is harvested and exported, and their productivity depends on inputs of fossil energy, fertilisers and water. Humans appropriate a large share of global terrestrial NPP through harvest, land-use change and fire, a measure known as human appropriation of net primary production (HANPP); estimates for the early 2000s were of the order of a quarter of potential NPP, with wide ranges depending on method. This appropriation reduces the energy available to other species and is one driver of biodiversity loss.
Biogeochemical cycles move chemical elements between living organisms and the atmosphere, hydrosphere and lithosphere. Gaseous cycles, such as those of carbon, nitrogen and water, have a large atmospheric reservoir and recover relatively quickly from disturbance; sedimentary cycles, such as that of phosphorus, have their main reservoir in rocks and sediments and cycle slowly. Nitrogen makes up about 78 per cent of the atmosphere as N₂, which most organisms cannot use. Nitrogen fixation converts N₂ to ammonia: biologically by bacteria with the enzyme nitrogenase, including symbiotic Rhizobium in legume nodules and Frankia in some non-legumes, free-living Azotobacter and Clostridium, and cyanobacteria such as Anabaena and Nostoc (including in Azolla, used as a biofertiliser in rice); industrially by the Haber–Bosch process, which now fixes roughly as much nitrogen as all natural terrestrial fixation; and in small amounts by lightning. Ammonification (mineralisation) by decomposers converts organic nitrogen to ammonium. Nitrification, an aerobic two-step process, oxidises ammonium to nitrite (by Nitrosomonas and related bacteria and archaea) and nitrite to nitrate (by Nitrobacter and related bacteria). Plants take up ammonium and nitrate (assimilation). Denitrification by bacteria such as Pseudomonas under anaerobic conditions reduces nitrate to nitrous oxide and N₂, returning nitrogen to the atmosphere; anammox bacteria convert ammonium and nitrite directly to N₂. Human activity has roughly doubled the rate of nitrogen entering the terrestrial cycle, causing eutrophication, nitrate pollution and nitrous oxide emissions; the nutrient stewardship subject treats management. The carbon cycle moves carbon between the atmosphere (as CO₂), living organisms, soils, oceans and rocks through photosynthesis, respiration, decomposition, ocean exchange, sedimentation and combustion; it is treated quantitatively in the climate system science and soil carbon subjects.
Phosphorus has no significant gaseous phase. It enters ecosystems from the weathering of phosphate rocks, cycles between soil, organisms and water, and is lost to sediments in lakes and oceans, from which it returns only over geological time through uplift. Seabird guano historically concentrated phosphorus on some islands. Because its supply is slow and soils often bind it tightly, phosphorus frequently limits productivity in freshwater and in old, weathered tropical soils. Mined phosphate rock, concentrated in a few countries, supplies fertilisers; excess phosphorus runoff is a main driver of freshwater eutrophication. Mycorrhizal fungi help plants obtain phosphorus. The sulphur cycle has both sedimentary and gaseous components. Sulphur enters from weathering of rocks, volcanic emissions and, very significantly, from burning fossil fuels, which release sulphur dioxide that forms sulphate aerosols and acid rain. In soils and water, bacteria oxidise reduced sulphur compounds to sulphate (for example Thiobacillus) and, in anaerobic sediments, reduce sulphate to hydrogen sulphide (for example Desulfovibrio). Marine phytoplankton release dimethyl sulphide, which forms sulphate aerosols that may influence cloud formation. Plants take up sulphate for amino acids such as cysteine and methionine; sulphur deficiency in crops has increased as industrial sulphur emissions and sulphur-containing fertilisers have declined. The water cycle is driven by solar energy: evaporation from oceans and land, transpiration from plants (together evapotranspiration), condensation, precipitation, infiltration, percolation to groundwater, runoff and streamflow. Vegetation affects the cycle by returning water to the atmosphere and influencing infiltration and runoff, and deforestation can reduce regional rainfall in some regions such as Amazonia. The water cycle also carries nutrients and pollutants and links all other cycles; it is treated in the hydrology and atmospheric science subjects.
Biomes are large regions characterised by their dominant vegetation and climate. Robert Whittaker's biome diagram plots biomes against mean annual temperature and precipitation. Tropical rainforests occur near the equator with high temperature and rainfall throughout the year; they have the highest species diversity and productivity of any terrestrial biome, multiple canopy layers, many epiphytes and lianas, and nutrient-poor soils because most nutrients are held in biomass and recycled quickly. Tropical deciduous (monsoon) forests, widespread in India, shed leaves in the dry season. Tropical savannas combine grasses with scattered trees in seasonally dry climates and are maintained partly by fire and grazing; they support large herbivores in East Africa. Deserts receive very little rainfall, often below about 250 mm a year, with high evaporation; their plants (xerophytes) show adaptations such as succulence, CAM photosynthesis, deep or extensive roots, reduced leaves and spines, and their animals are often nocturnal and conserve water. Temperate grasslands (prairies, steppes, pampas) have rich, deep soils with high organic matter and are among the world's most important cereal-growing regions, having been largely converted to agriculture. Temperate deciduous forests, with broad-leaved trees that shed leaves in winter, occur in moderate climates with year-round precipitation. Mediterranean shrublands (chaparral, maquis) have hot, dry summers and mild, wet winters and fire-adapted evergreen shrubs. Boreal forests (taiga), the largest terrestrial biome, are dominated by conifers such as spruce, fir and pine, with long cold winters and acidic, slowly decomposing soils, and store large amounts of carbon. Tundra, in the Arctic and on high mountains (alpine tundra), has very cold, short growing seasons, permafrost in the Arctic, low vegetation of mosses, lichens, sedges and dwarf shrubs, and low productivity. India's biogeographic classification by Rodgers and Panwar (1988) recognises ten zones: the Trans-Himalaya, Himalaya, Desert, Semi-arid, Western Ghats, Deccan Peninsula, Gangetic Plain, North-East, Coasts and Islands. The biodiversity subject covers hotspots and protected areas.
Aquatic ecosystems cover most of the Earth's surface. Freshwater ecosystems include lentic (standing water: lakes, ponds) and lotic (flowing water: rivers, streams) systems, and wetlands. In lakes, zones include the littoral zone near the shore where rooted plants grow, the limnetic (open-water) zone where light supports phytoplankton, the profundal zone below the depth of effective light penetration, and the benthic zone of the bottom sediments. In temperate lakes, summer thermal stratification forms a warm upper epilimnion, a thermocline (metalimnion) in which temperature falls rapidly with depth, and a cold, dense lower hypolimnion; because water is densest at about 4 °C, lakes overturn (mix) in spring and autumn, redistributing oxygen and nutrients. Lakes are classified by nutrient status as oligotrophic (nutrient-poor, clear, high oxygen), mesotrophic and eutrophic (nutrient-rich, productive, often with oxygen depletion in deep water). Rivers change along their length, as described by the river continuum concept (Vannote and colleagues, 1980): narrow, shaded headwaters depend on leaf litter from riparian vegetation, middle reaches have more in-stream photosynthesis, and large lower rivers carry fine organic matter. Wetlands, including marshes, swamps, bogs and floodplains, are highly productive and provide flood control, water purification and habitat, as the wetlands subject explains. Marine ecosystems include the intertidal zone between high and low tide, the neritic zone over the continental shelf (rich in nutrients and life), the oceanic (pelagic) zone of the open sea, subdivided by depth into photic and aphotic zones, and the benthic zone of the sea floor, extending to deep-sea trenches. Estuaries, where rivers meet the sea, have variable salinity, high nutrient inputs and high productivity, and serve as nurseries for fish. Coral reefs, built by corals with symbiotic algae (zooxanthellae) in warm, clear, shallow tropical waters, are among the most diverse ecosystems on Earth and are threatened by warming, acidification and pollution. Mangroves, salt-tolerant trees of tropical coasts such as the Sundarbans, protect shores, trap sediment and support fisheries.
An agroecosystem is an ecosystem modified and managed by people to produce food, fibre, fuel and other products. Compared with natural ecosystems, agroecosystems typically have lower species and genetic diversity, often a single crop variety; simplified food webs with fewer natural enemies; a deliberately maintained early successional state, kept by tillage, weeding and replanting; high net primary productivity per unit area in favourable conditions, much of which is harvested and exported; open rather than closed nutrient cycles, because nutrients leave with the harvest and must be replaced by fertilisers, manures or biological fixation; large subsidies of external energy from fuel, fertilisers, machinery and irrigation; synchronised plant phenology and uniform age structure; and dependence on human control, without which they revert through succession. These features make agroecosystems prone to pest outbreaks (large, uniform resource patches with few natural enemies), nutrient losses to water and air, soil erosion when soil is left bare, and dependence on external inputs. They also mean that agroecosystems can be redesigned to restore ecological functions: crop rotations and intercropping increase diversity and break pest cycles; legumes fix nitrogen; cover crops and residues protect soil and feed soil organisms; hedgerows and field margins host pollinators and natural enemies; integrated crop–livestock systems recycle nutrients; and agroforestry adds perennial structure. The agroecology, conservation agriculture, sustainable farming systems and agroforestry subjects develop these principles. Agroecosystems vary widely, from diverse traditional systems such as home gardens and shifting cultivation with long fallows, which retain many features of natural ecosystems, to intensive monocultures and confined livestock systems.
Ecologists distinguish several aspects of stability. Constancy is the lack of change in a variable over time. Resistance is the ability of a system to withstand a disturbance without changing much. Resilience, in the engineering sense, is the speed of return to the previous state after disturbance; in C.S. Holling's ecological sense (1973), it is the amount of disturbance a system can absorb before shifting to a different state, as the adaptation subject explains. Persistence is the length of time a system maintains its state. A grassland may have low resistance to fire, burning quickly, but high resilience, regrowing from roots within weeks; a forest may resist fire better but recover only over decades or centuries. The relationship between diversity and stability has been debated. Robert MacArthur (1955) and Charles Elton (1958) proposed that more diverse communities, with more pathways in their food webs, are more stable. Robert May (1973) showed mathematically that in randomly assembled model communities, greater complexity tends to reduce stability, stimulating research on the non-random structure of real food webs. Field experiments by David Tilman and colleagues in Minnesota grasslands (Cedar Creek) found that plots with more plant species had more stable total biomass from year to year and were more resistant to drought, although individual species populations fluctuated more. The insurance hypothesis explains this: different species respond differently to environmental changes, so diverse communities are more likely to include species that maintain function when others decline. Ecosystems can have alternative stable states. A shallow lake may be clear, dominated by rooted plants, or turbid, dominated by phytoplankton; nutrient loading can push it past a threshold into the turbid state, and reducing nutrients afterwards may not restore clarity until levels fall much lower, a property called hysteresis. Such regime shifts are also described in coral reefs shifting to algal dominance, drylands shifting to degraded states, and savannas and forests. For agriculture, the lesson is that diversity, intact soil and landscape heterogeneity buffer production against climate shocks and pests, and that pushing systems beyond thresholds can cause abrupt, persistent losses.
A student uses this subject as the scientific base for every later environmental subject and practises its calculations: Lincoln–Petersen estimates, exponential and logistic growth, MSY, Shannon and Simpson indices, Jaccard and Sørensen similarity, IVI, energy transfer through trophic levels and the light–dark bottle method. Examiners test distinctions that are easily confused: habitat versus niche, fundamental versus realised niche, Batesian versus Müllerian mimicry, GPP versus NPP versus standing crop, pyramids that can and cannot be inverted, and the bacteria of each nitrogen transformation.
A five-year practitioner, such as a forest or wildlife officer, an environmental consultant, a fisheries manager or an agronomist, uses the subject to design and analyse vegetation and wildlife surveys (quadrats, IVI, diversity indices, mark–recapture), set sustainable harvest levels with precautionary margins below MSY, interpret biomagnification risks, plan restoration that works with succession, and design farming systems that harness natural enemies, legumes and mycorrhizae.
A senior professional with 25 years' experience, such as a head of a forest department, a protected area network manager or a research institute director, uses ecological principles to set landscape and sector policy: protecting keystone species and habitat connectivity, avoiding regime shifts in lakes and rangelands, setting harvest and grazing rules that respect carrying capacity, and building agricultural landscapes whose diversity buffers climate and pest shocks.
A researcher with a PhD and 25 years' experience works on population and community dynamics, food-web structure, the diversity–stability relationship, ecosystem productivity and carbon and nutrient cycling, succession and restoration ecology, and alternative stable states, increasingly with long-term experiments, remote sensing and models. Such a researcher also links ecological theory to agricultural and conservation practice and teaches the next generation of ecologists.
This manual gives the principles taught in ecology courses but not the mathematical analysis of population models beyond the basic equations, nor evolutionary ecology, behavioural ecology or molecular ecology, which GATE EY also covers. Specific values of productivity for each biome, ecological efficiencies and diversity index ranges are typical figures that vary among studies and sites. Applications to biodiversity conservation, protected areas, wetlands, landscape ecology, agroecology, soil carbon and nutrient management are left to their own subjects. Debated topics (the strength of the Yellowstone trophic cascade, the causes of the latitudinal diversity gradient, the generality of r/K selection) are flagged rather than resolved.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
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