Ecosystems
Overview
An ecosystem is a living community together with the physical surroundings it depends on. This module explains how ecosystems are built, how energy and nutrients move through them, how communities change over time, and the main types you will meet in the field, from forests to estuaries.
Learning objectives
By the end of this module, you’ll be able to:
- 1Describe the structure and function of an ecosystem, including producers, consumers and decomposers
- 2Explain energy flow, food chains, food webs and ecological pyramids
- 3Describe ecological succession with examples
- 4Compare forest, grassland, desert and aquatic ecosystems
3.1 Concept, Structure and Function
Three topics — click to expand each one.
Concept of an ecosystem
Ecosystems can be as small as a pond or a rotting log, or as large as a forest or an ocean. They are open systems: energy and matter enter and leave.
Structure: producers, consumers and decomposers
Abiotic (non-living) components
Sunlight, temperature, water, air, soil and nutrients.
Biotic (living) components
- Producers (autotrophs): green plants, algae and some bacteria that make food by photosynthesis
- Consumers (heterotrophs): herbivores (primary consumers), carnivores (secondary and tertiary consumers), omnivores and detritivores such as earthworms
- Decomposers: bacteria and fungi that break down dead matter and return nutrients to the soil and water
Function: productivity, nutrient cycling and ecosystem services
An ecosystem works through three linked processes: energy flow (from the sun through food), nutrient cycling (carbon, nitrogen, phosphorus and water move round and round) and regulation (feedbacks that keep the system relatively stable). Primary productivity is the rate at which producers capture energy and build biomass.
Ecosystem services
- Provisioning: food, water, timber, medicines
- Regulating: climate, floods, water purification, pollination
- Cultural: recreation, spiritual and aesthetic value
- Supporting: soil formation, nutrient cycling
3.2 Energy Flow, Food Chains and Succession
Four topics — click to expand each one.
Energy flow in the ecosystem
Energy flows in one direction: sun to producers to consumers, with heat lost at every step. Only about 10% of the energy at one trophic level is passed to the next (the “10% rule”); the rest is used for life processes or lost as heat. This is why food chains are short, and why there is far less energy, and fewer animals, at the top.
Food chains and food webs
A food chain shows who eats whom in a single line, for example: grass → grasshopper → frog → snake → eagle. Real ecosystems are tangled food webs, where most organisms eat, and are eaten by, several others. Grazing chains begin with living plants; detritus chains begin with dead matter and are very important in forests and wetlands.
Ecological pyramids
| Pyramid | Shows | Shape |
|---|---|---|
| Of numbers | Number of organisms at each level | Usually upright; inverted for one tree feeding many insects |
| Of biomass | Total dry weight at each level | Upright on land; can be inverted in oceans where tiny phytoplankton are eaten as fast as they grow |
| Of energy | Energy flowing through each level | Always upright |
Ecological succession
Succession is the gradual, orderly replacement of one community by another until a stable climax community develops.
- Primary succession begins on bare surfaces with no soil, such as new lava, rock or sand (lichens and mosses first, then grasses, shrubs, trees). The island of Krakatau, sterilised by a huge eruption in 1883, was recolonised in this way.
- Secondary succession follows disturbance of an area that already has soil, such as an abandoned farm or a burnt forest, and is faster.
- Succession in water is called a hydrosere (a pond slowly filling up and becoming land); on dry bare rock it is a xerosere.
3.3 Types of Ecosystems
Four topics — click to expand each one.
Forest ecosystem
Forests are dominated by trees and are the most structurally layered ecosystems (canopy, understorey, shrub and ground layers). India has tropical evergreen forests (Western Ghats, North-East), tropical deciduous forests (the largest share), thorn forests, mountain forests and mangroves such as Pichavaram in Tamil Nadu and the Sundarbans. Producers are the trees; consumers range from insects and birds to elephants and tigers; decomposition on the forest floor is rapid in warm, wet climates.
Grassland ecosystem
Grasslands occur where rainfall is too low for forests but higher than in deserts. Grasses are the main producers, grazed by herbivores such as antelope, blackbuck and wild asses, and preyed on by wolves, foxes and big cats; many ground-nesting birds, including the Great Indian Bustard, depend on them. Grasslands are often wrongly labelled “wasteland”, which has led to their conversion for plantations and farming.
Desert ecosystem
Deserts get very little rain and have extreme temperatures. Plants have adaptations such as thick waxy skins, water storage (cacti, succulents), reduced leaves, and deep roots; animals are often nocturnal, burrow, or conserve water (for example the camel). India’s Thar Desert and the cold desert of Ladakh and Spiti are examples.
Aquatic ecosystems
| Type | Features | Examples |
|---|---|---|
| Ponds and lakes (still fresh water) | Zones by depth and light; plankton at base of food web; prone to eutrophication | Village tanks, Wular Lake |
| Streams and rivers (flowing fresh water) | Oxygen-rich, changing along the course from source to mouth | Ganga, Cauvery, Adyar |
| Oceans (salt water) | Largest ecosystem; coral reefs, deep sea, open water | Gulf of Mannar |
| Estuaries | Where river meets sea; brackish water; very productive nurseries for fish and prawns | Pichavaram, Adyar estuary |
Readings
- Bharucha, Erach. Textbook for Environmental Studies for Undergraduate Courses. University Grants Commission, 2004. (Unit 3: Ecosystems.)
- Odum, Eugene P. Fundamentals of Ecology. W.B. Saunders, 1971.
- Townsend, C., J. Harper, and M. Begon. Essentials of Ecology. Blackwell Science.
- Ramsar Convention on Wetlands: ramsar.org
- Ministry of Environment, Forest and Climate Change: moef.gov.in
Discussion
Think of an ecosystem near you (a pond, park, hillside or estuary). Name one producer, one consumer and one decomposer, and say what would happen if one of them disappeared. Post your example, then respond to a classmate.
Practice quiz
Ungraded — check your understanding before the assignment.
1. The term “ecosystem” was introduced by:
2. Which organisms break down dead matter and return nutrients to the environment?
3. About how much energy passes from one trophic level to the next?
4. Which ecological pyramid is always upright?
5. Succession that starts on bare rock with no soil is called:
Graded assignment
Local food web — 300–400 words plus a sketch
Choose a small local ecosystem (campus garden, pond, park, roadside tree). Identify at least 8 organisms, draw a food web showing who eats whom, name the producers, consumers and decomposers, and explain what might disturb it.
| Criterion | Weight |
|---|---|
| Accurate identification of organisms and roles | 30% |
| Correct, clear food web | 30% |
| Discussion of disturbance and consequences | 25% |
| Clarity and structure | 15% |