Ecosystems: Structure, Energy Flow and Nutrient Cycles

Ecology and the ecosystem concept

Ecology is the study of relationships between organisms and their environment. The term was coined by Ernst Haeckel (1866). A.G. Tansley (1935) introduced the term ecosystem: a functional unit of living (biotic) and non-living (abiotic) components interacting through energy flow and nutrient cycling.

Levels of organisation: individual → population (one species in an area) → community (all populations in an area) → ecosystem → biome (a large region with a characteristic climate and vegetation, such as tropical rainforest or desert) → biosphere.

Habitat is where an organism lives. Its niche is its functional role: what it eats, when it is active and how it interacts. Gause's competitive exclusion principle holds that two species cannot occupy the same niche indefinitely.

Structure of an ecosystem

Abiotic components: sunlight, temperature, water, soil, minerals and air.

Biotic components:

• Producers (autotrophs): green plants, algae and cyanobacteria, which convert solar energy into chemical energy by photosynthesis. In deep-sea hydrothermal vents, chemosynthetic bacteria are the producers.
• Consumers (heterotrophs):
• primary consumers: herbivores;
• secondary consumers: carnivores that eat herbivores;
• tertiary consumers: top carnivores;
• omnivores.
• Decomposers: bacteria and fungi, which break down dead matter and recycle nutrients. Detritivores such as earthworms help.

Energy flow

Energy flow is one-way: sun → producers → consumers, with energy lost as heat at each step.

The 10% law (Lindeman, 1942): only about 10% of energy passes from one trophic level to the next. This is why food chains rarely have more than 4–5 levels.

Productivity:

• Gross primary productivity (GPP): total photosynthesis.
• Net primary productivity (NPP): GPP minus respiration by plants, which is the energy available to consumers.
• Most productive ecosystems: tropical rainforests, coral reefs, estuaries and swamps.
• Least productive: deserts and the open ocean (which has a low NPP per area but a vast total because of its size).

Food chains and webs:

• the grazing food chain starts with green plants;
• the detritus food chain starts with dead organic matter, and is dominant in forests;
• food webs are interlinked food chains, which give an ecosystem stability.

Ecological pyramids:

• Pyramid of numbers: can be inverted, as in a tree supporting many insects.
• Pyramid of biomass: inverted in aquatic ecosystems, where small phytoplankton support a larger biomass of zooplankton and fish because phytoplankton reproduce fast.
• Pyramid of energy: always upright.

Biomagnification: fat-soluble, persistent toxins such as DDT and mercury concentrate up the food chain. Top predators are the most affected: DDT thinned the eggshells of birds of prey, the theme of Rachel Carson's *Silent Spring* (1962). Minamata disease (Japan) came from mercury in fish. Bioaccumulation is the build-up within one organism.

Nutrient (biogeochemical) cycles

Nutrients cycle, while energy flows.

Carbon cycle: CO₂ is fixed by photosynthesis, released by respiration, decomposition and burning of fossil fuels, and absorbed by oceans (about a quarter of human emissions), which acidifies them. Carbon sinks include forests, oceans, soils and wetlands, especially peatlands and mangroves ("blue carbon").

Nitrogen cycle:

• Nitrogen fixation: atmospheric N₂ is converted to ammonia by bacteria such as *Rhizobium* in legume root nodules, free-living *Azotobacter* and cyanobacteria (*Anabaena*, *Nostoc*). Lightning and the industrial Haber–Bosch process also fix nitrogen.
• Nitrification: ammonia → nitrite (*Nitrosomonas*) → nitrate (*Nitrobacter*).
• Assimilation by plants.
• Ammonification by decomposers.
• Denitrification: nitrate → N₂, by *Pseudomonas* in waterlogged soils.

Excess nitrogen from fertilisers causes eutrophication and releases nitrous oxide, a strong greenhouse gas.

Phosphorus cycle: a sedimentary cycle with no gaseous phase. Phosphorus is released from rocks by weathering. Its run-off also drives eutrophication.

Water cycle: evaporation, transpiration, condensation, precipitation and run-off.

Ecological succession

Succession is the gradual, predictable change in species composition over time, ending in a stable climax community.

• Primary succession: on bare rock or new land, such as lava or sand dunes. Pioneer species are lichens and mosses, which make soil.
• Secondary succession: after a disturbance (fire, abandoned farmland), where soil already exists. It is faster.
• Hydrosere (in water) and xerosere (in dry areas).

Population and community interactions

• Mutualism (+/+): lichens (algae and fungi), mycorrhiza, pollination.
• Commensalism (+/0): an orchid on a tree, barnacles on a whale.
• Parasitism (+/−).
• Predation (+/−).
• Competition (−/−).
• Amensalism (−/0): penicillium inhibiting bacteria.

Keystone species have an effect out of proportion to their abundance, such as tigers, sea otters and fig trees. Flagship species are charismatic species used to rally support, such as the tiger and the elephant. Indicator species reflect environmental health: lichens are sensitive to SO₂.

Ecotone: a transition zone between two ecosystems, such as a mangrove between land and sea. Ecotones often have greater diversity, the edge effect.

Why this matters for UPSC

These concepts underpin questions on biodiversity, pollution (biomagnification, eutrophication), climate (carbon sinks) and conservation (keystone species, protected areas). Prelims often tests definitions and examples. Mains asks how disturbances (deforestation, dams, invasive species) cascade through ecosystems.

Common prelims traps

• The pyramid of energy is never inverted; the pyramid of biomass can be inverted in water.
• Energy flows; nutrients cycle.
• The phosphorus cycle has no gaseous phase.
• *Nitrosomonas* turns ammonia into nitrite; *Nitrobacter* turns nitrite into nitrate.

Quick self-test

1. *Why are food chains short?* About 90% of energy is lost at each trophic level.
2. *What is biomagnification?* The increase in toxin concentration at higher trophic levels.
3. *What is the pioneer species on bare rock?* Lichens.

Practice: Mains question

*"Understanding energy flow and nutrient cycling is essential for sustainable management of ecosystems." Explain with examples from Indian agriculture and fisheries. (150 words)*

Major biomes and Indian examples

• Tropical rainforest: high rainfall, evergreen, the highest species diversity. In India: the Western Ghats, the North-East and the Andamans.
• Tropical deciduous forest: trees shed leaves in the dry season. India's most extensive type; sal and teak.
• Grassland and savanna: dominated by grasses with scattered trees. The Banni grasslands (Kutch), the terai grasslands, and the shola–grassland mosaics of the Nilgiris.
• Desert: low rainfall, plants with xerophytic adaptations (thick cuticles, spines, deep roots). The Thar, and the cold desert of Ladakh.
• Tundra and alpine: short growing season, mosses and lichens. Alpine meadows (bugyals) of the Himalaya.
• Aquatic: freshwater (lakes, rivers, wetlands) and marine (estuaries, reefs, open ocean).

Ecosystem stability and resilience

Resistance is the ability to withstand disturbance. Resilience is the ability to recover after it. More diverse ecosystems are generally more stable (the diversity–stability hypothesis), because different species can perform similar functions ("functional redundancy").

Tipping points: some systems shift abruptly to a degraded state when pressure crosses a threshold, such as a lake flipping from clear to algae-dominated, or a rainforest turning to savanna. Recovery can then be very hard.

Implications for management: protect keystone species and connectivity, keep diversity in agriculture and plantations, and avoid pushing systems near thresholds.

Key terms

• Trophic level: the position of an organism in a food chain.
• Carrying capacity: the maximum population an environment can sustain.
• Ecological footprint: the area needed to supply a population's resources and absorb its waste.
• Eutrophication: nutrient enrichment of water bodies, causing algal blooms and oxygen depletion.