Ecology & Ecosystems
In short: Food chains, trophic levels, the 10% law and the biogeochemical cycles.
Environment & Ecology › Ecology & Biodiversity · 40 min read · Free to read · English & Telugu
Bilingual study notes for the Andhra Pradesh Police SI & Constable exams (SLPRB AP) by AP Police Hub, a product of Anvil Academy.
How nature works as one connected system — the levels of life, the living and non-living parts of an ecosystem, food chains and the 10% energy law, the pyramids and cycles, how bare rock turns into a forest, and the wetlands, mangroves and grasslands of India and Andhra Pradesh.
Forest ecosystem beside the Bhadra reservoir, Western Ghats, India — a living web of producers, consumers and water · Wikimedia Commons (CC BY 2.0, Kandukuru Nagarjun)
1. What is Ecology — and the Levels of Life
Ecology is the study of how living things interact with one another and with their non-living surroundings. The word comes from the Greek oikos ('house') + logos ('study') — literally 'the study of nature's household'. The German zoologist Ernst Haeckel coined the term (as *oekologie*) in 1869. Nothing in nature lives alone: a plant needs sunlight and soil, a deer needs the plant, a tiger needs the deer — ecology is the science of these threads that bind life together.
Levels of ecological organisation (small → big)
From a single organism up to the whole living Earth| Level | What it means |
|---|
| Organism | A single individual — one tiger, one neem tree |
| Population | All individuals of one species in an area — all tigers of a forest |
| Community | All the different populations living together — tigers + deer + trees + birds |
| Ecosystem | The community plus its non-living surroundings (soil, water, air, sunlight) |
| Biome | A very large ecosystem set by climate — desert, tropical forest, tundra |
| Biosphere | All biomes together — the entire life-supporting layer of the Earth |
2. The Ecosystem — Its Living & Non-living Parts
An ecosystem is a working unit of nature where living organisms (biotic) interact with their physical surroundings (abiotic) through a flow of energy and a cycling of nutrients. A pond, a forest, a grassland — even a fish tank — is an ecosystem. The British botanist Arthur G. Tansley coined the word 'ecosystem' in 1935. Ecosystems come in two broad kinds: natural (forest, ocean) and man-made / artificial (crop field, aquarium, an Andhra Pradesh village tank).
The two building blocks of every ecosystem| Component | Means | Examples |
|---|
| Abiotic (non-living) | Physical & chemical surroundings | Sunlight, temperature, rainfall, humidity, soil, water, air, CO₂, minerals |
| Biotic (living) | All living organisms, grouped by how they get food | Producers, consumers, decomposers |
- Producers (autotrophs) — Green plants and algae that make their own food from sunlight, water and CO₂ by photosynthesis. They are the base of every food chain — all other life ultimately eats what producers build. In the sea, tiny floating plants (phytoplankton) are the chief producers.
- Consumers (heterotrophs) — Animals that cannot make food and must eat others. Primary consumers = herbivores (deer, goat, grasshopper); secondary consumers = small carnivores (frog, snake); tertiary / top consumers = big carnivores (tiger, eagle). Omnivores (humans, bear) eat both plants and animals.
- Decomposers (detritivores) — Bacteria and fungi (also called saprotrophs) that break down dead bodies and waste, returning nutrients to the soil and closing the loop. Without them, dead matter would pile up and nutrients would never recycle. They are nature's cleaners and recyclers.
3. Food Chain, Food Web & the 10% Energy Law
A food chain is the sequence of who-eats-whom, along which energy flows from one step to the next: grass → grasshopper → frog → snake → eagle. Each feeding step is a trophic level ('trophic' = feeding). In real life chains cross and interlock into a food web — a more realistic network, because most animals eat several kinds of food and are eaten by several predators.
The trophic levels of a grassland food chain| Trophic level | Role | Example |
|---|
| T₁ — First | Producer | Grass |
| T₂ — Second | Primary consumer (herbivore) | Grasshopper |
| T₃ — Third | Secondary consumer (carnivore) | Frog |
| T₄ — Fourth | Tertiary consumer | Snake |
| T₅ — Fifth | Top carnivore | Eagle |
A food web is all the food chains of one ecosystem interlocked — producers at the base, consumers rising to the top predator, and decomposers returning nutrients to the soil. Forest food web diagram · Wikimedia Commons (CC BY-SA 4.0, Roberta Rosina)
- The 10% law (Lindeman, 1942) — Energy flows in one direction and shrinks at every step — only about 10% of the energy at one trophic level passes up to the next; the other 90% is lost as heat in respiration and life activity. This 'ten per cent law' was given by Raymond Lindeman. Because so little energy survives upward, food chains rarely have more than 4–5 links.
- Two types of food chain — Grazing food chain starts with living green plants (grass → deer → tiger). Detritus food chain starts with dead organic matter (leaf litter → earthworm/fungi → birds). On land most energy flows through the detritus chain; in the open ocean the grazing chain dominates.
4. Ecological Pyramids — Number, Biomass & Energy
If we stack the trophic levels one above another and draw each as a bar, we get an ecological pyramid — the idea of Charles Elton. There are three kinds, by what we count: the number of organisms, their biomass (total living weight), or the energy they hold. Usually the base (producers) is widest and the apex (top carnivores) narrowest — an upright pyramid. But two of the three can sometimes turn upside-down.
The pyramid of energy — of the 100% captured by producers, only about 10% reaches each higher level (the rest is lost as heat), so it is always upright and never inverts. Energy pyramid diagram · Wikimedia Commons (CC BY-SA 4.0, Swiggity.Swag.YOLO.Bro)
Which pyramid can be inverted — a favourite exam trap| Pyramid of… | Usually | Can it invert? |
|---|
| Number | Upright | Yes — one big tree feeds many insects & birds (inverted) |
| Biomass | Upright | Yes — in a pond, tiny phytoplankton weigh less than the zooplankton eating them (inverted) |
| Energy | Upright | NEVER — always upright (10% law makes each level hold less energy) |
5. Productivity — How Much Food Nature Makes
Productivity is the rate at which producers build new organic matter (food) from sunlight. Think of a green plant as a small factory: the total food it makes is the Gross Primary Productivity (GPP); but the plant burns some of that for its own breathing (respiration, R); what is left over for animals to eat is the Net Primary Productivity (NPP).
- Primary vs secondary — Primary productivity = food made by producers (plants). Secondary productivity = new body-mass built by consumers (animals) from the food they eat.
- Standing crop & biomass — The standing crop is the total living matter (biomass) present at a given moment. Productivity is the *rate* of making it; standing crop is the *amount* present right now.
6. Biogeochemical Cycles I — Carbon, Oxygen & Water
While energy flows through and is lost, matter cycles round and round. The movement of a chemical element between living things and the environment (bio = life, geo = rock/soil/air/water) is a biogeochemical cycle. Cycles whose main store is the air or ocean are gaseous (carbon, nitrogen, oxygen); those stored mainly in the Earth's crust are sedimentary (phosphorus, sulphur).
- The carbon cycle — Plants take CO₂ from the air during photosynthesis and lock it into their tissues; respiration, decay and burning return it to the air. Carbon is stored for ages in fossil fuels and limestone. Burning coal, oil and gas releases extra CO₂ — the chief driver of global warming.
- The oxygen cycle — Oxygen is released as the main by-product of photosynthesis and is used up in respiration and combustion. High in the atmosphere, oxygen forms the ozone (O₃) layer that shields life from the Sun's harmful UV rays.
- The water (hydrologic) cycle — Water circulates between ocean, air and land through evaporation → condensation → precipitation → runoff. The Sun powers it; forests and wetlands slow the runoff and recharge groundwater, which is why they are vital in drought-prone regions like Rayalaseema in Andhra Pradesh.
7. Biogeochemical Cycles II — Nitrogen & Phosphorus
Air is 78% nitrogen, yet plants cannot breathe it in — free N₂ gas is unusable. It must first be fixed into nitrates. Phosphorus, by contrast, has no gaseous phase at all — it moves only through rock, soil and water, which makes it the classic sedimentary cycle.
How nitrogen is made usable
The steps of the nitrogen cycle| Step | What happens | Done by |
|---|
| Nitrogen fixation | Free N₂ → usable nitrates/ammonia | Rhizobium (legume roots), Azotobacter, cyanobacteria, lightning |
| Nitrification | Ammonia → nitrites → nitrates | Nitrifying bacteria (*Nitrosomonas, Nitrobacter*) |
| Denitrification | Nitrates → free N₂ back to air | Denitrifying bacteria (*Pseudomonas*) |
- The phosphorus cycle — Phosphorus is released by the weathering of rocks, taken up by plants as phosphate, passed along food chains, and returned to the soil by decomposers. It has no atmospheric (gaseous) form — its main reservoir is rock. It is often the limiting nutrient for plant growth.
- When nutrients overload water — Excess nitrogen and phosphorus from fertilisers and sewage cause eutrophication — a bloom of algae that uses up oxygen and suffocates fish. It is a real threat to India's lakes, including Kolleru and many urban lakes and tanks.
8. Ecological Succession — Bare Rock to Forest
Ecological succession is the slow, orderly change of the community of an area over time — one set of species replacing another until a stable, self-sustaining community is reached. The first species to arrive are the pioneer community; the final, stable stage is the climax community (often a forest). Each in-between stage is a seral stage (sere).
Primary vs secondary succession| Feature | Primary succession | Secondary succession |
|---|
| Starts on | A brand-new, lifeless surface — bare rock, cooled lava, new sand dune | Land that already had life & soil but was cleared — after fire, flood, abandoned farm |
| Soil at start | No soil — must form first | Soil already present |
| Pioneers | Lichens & mosses (they break rock into soil) | Grasses & weeds (fast-growing) |
| Speed | Very slow (soil must build) | Much faster (soil ready) |
9. Types of Ecosystems — Land & Water
Ecosystems split into two great families: terrestrial (on land) and aquatic (in water). Terrestrial ones are shaped mainly by climate; aquatic ones by water depth, salt content and flow.
The main ecosystem types| Family | Type | Example |
|---|
| Terrestrial | Forest | Western Ghats rainforest; dry deciduous forests of Andhra Pradesh's Eastern Ghats (e.g. Nallamala) |
| Terrestrial | Grassland | Savanna; Deccan thorn-scrub grassland (Great Indian Bustard habitat) |
| Terrestrial | Desert | Thar Desert (Rajasthan) |
| Aquatic | Freshwater | Lakes, rivers, tanks — Kolleru, the Krishna and Godavari, village cheruvu |
| Aquatic | Marine (sea) | Oceans, coral reefs — Gulf of Mannar |
- Lentic vs lotic (freshwater) — Lentic = still water (ponds, lakes, tanks). Lotic = flowing water (rivers, streams). Villages across Andhra Pradesh depend on thousands of man-made irrigation tanks (cheruvu) — a lentic ecosystem of huge importance to farming.
10. Wetlands, Mangroves & Estuaries
Where land and water meet, some of the richest and most useful ecosystems form. Wetlands are lands soaked or covered by shallow water — marshes, lakes, tanks. They filter pollutants, recharge groundwater, tame floods and shelter birds, earning the name 'kidneys of the landscape'.
- Ramsar Convention & sites — The Ramsar Convention (1971, Ramsar, Iran) protects wetlands of international importance. India now has over 90 Ramsar sites (2025) — the most in Asia — with Tamil Nadu leading among the states. Kolleru Lake (between the Krishna and Godavari deltas) is Andhra Pradesh's Ramsar wetland and India's largest shallow freshwater lake.
- Mangroves — Salt-tolerant forests of the inter-tidal coast, with special stilt roots and breathing roots (pneumatophores). They are the coast's first wall against cyclones and tsunamis. The Sundarbans (West Bengal) is the world's largest mangrove forest; Coringa (Godavari estuary, Andhra Pradesh) is among India's largest.
- Estuaries — An estuary is where a river meets the sea and freshwater mixes with salt water — highly productive nurseries for fish and prawns. The Godavari and Krishna form large estuaries on the Andhra Pradesh coast — the Godavari estuary shelters the Coringa mangroves.
11. Habitat, Niche & Keystone Species
A few ideas explain how species share a place without wiping each other out. A habitat is *where* an organism lives — its address. A niche is *how* it lives — its profession: what it eats, when it is active, its role in the ecosystem. Two species can share a habitat, but if they occupy the same niche they compete until one is pushed out (Gause's competitive exclusion principle).
Four kinds of special species| Type | Meaning |
|---|
| Keystone species | One species that holds the whole ecosystem together — remove it and the system collapses (e.g. tiger, sea otter, fig tree, elephant) |
| Flagship species | A charismatic species used to rally public support for conservation (e.g. tiger, panda) |
| Umbrella species | Protecting it (and its large range) automatically protects many others (e.g. tiger, elephant) |
| Indicator species | Its presence/health signals the state of the environment (e.g. lichens = clean air; frogs = clean water) |
12. Biomagnification (DDT) & Ecosystem Services
Some poisons do not break down and do not leave the body — they build up along the food chain. Bioaccumulation is the build-up of a toxin inside one organism over its life. Biomagnification is the increase in that toxin's concentration as we move up the trophic levels — the top predator carries the heaviest dose.
Ecosystem services — what nature does for us free
Ecosystem services are the benefits a healthy ecosystem gives humanity — usually for free. A landmark 1997 study (Robert Costanza and team) valued the world's ecosystem services at tens of trillions of dollars a year, more than the entire global economy.
- Provisioning — Goods we take directly — food, water, wood, medicine, fibre.
- Regulating — Nature keeping the planet livable — climate control, clean air, pollination, flood & disease control, water purification.
- Supporting & cultural — The base processes (soil formation, nutrient cycling, photosynthesis) plus recreation, tourism and spiritual value.
13. At a Glance — Exam Capsule
The whole lesson boiled down to the facts most likely to be asked. Revise this table the night before the exam.
Ecology & ecosystems — quick-revision facts| Point | Fact to remember |
|---|
| Term 'Ecology' coined by | Ernst Haeckel, 1869 |
| Term 'Ecosystem' coined by | A. G. Tansley, 1935 |
| 10% energy law by | Raymond Lindeman (1942) |
| Pyramid always upright | Pyramid of ENERGY |
| NPP formula | NPP = GPP − Respiration |
| Cycle with NO gaseous phase | Phosphorus (sedimentary) |
| N-fixation done by | Rhizobium (legume roots) + lightning |
| Pioneers on bare rock | Lichens & mosses |
| Habitat vs Niche | Address vs Profession |
| Biomagnification example | DDT (& mercury – Minamata) |
| Ramsar Convention year | 1971 (India 90+ sites, 2025) |
| World's largest mangrove | Sundarbans (West Bengal) |
| India's largest freshwater lake | Kolleru (shallow; Krishna–Godavari) |