Ecosystem – Class 12 Biology Chapter 12 Notes & NCERT PDF

Introduction

An ecosystem is a functional unit of nature where living organisms interact among themselves and with their physical environment. Ecosystems vary from a small pond to a large forest or sea; the entire biosphere can be seen as a global ecosystem. Ecosystems are broadly terrestrial (forest, grassland, desert) or aquatic (pond, lake, wetland, river, estuary), and there are man-made ones like crop fields and aquaria. This chapter studies structure, productivity, decomposition, energy flow and ecological pyramids. It is heavily tested through diagrams (pyramids, decomposition cycle) and "distinguish between" questions.

Key Concepts

12.1 Ecosystem — Structure and Function

Biotic and abiotic components interact to form a physical structure. The identification of plant species and their vertical distribution at different levels is called stratification — e.g. trees occupy the top layer, shrubs the second, herbs and grasses the bottom layers.

Pond as an example. Abiotic components: water, dissolved inorganic and organic substances, and the rich soil deposit at the bottom. Solar input, temperature cycles, day length and other climatic conditions regulate the whole pond. Autotrophs: phytoplankton, some algae and floating, submerged and marginal plants. Consumers: zooplankton, free-swimming and bottom-dwelling forms. Decomposers: fungi, bacteria and flagellates, especially abundant at the bottom.

The pond performs all the functions of an ecosystem: (i) conversion of inorganic into organic material using radiant energy (autotrophs); (ii) consumption of autotrophs by heterotrophs; (iii) decomposition and mineralisation of dead matter to release minerals for reuse; (iv) unidirectional movement of energy towards higher trophic levels and its dissipation to the environment as heat.

The four basic components of functioning are: productivity, decomposition, energy flow and nutrient cycling.

12.2 Productivity

The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis is primary production, expressed as weight (g m⁻²) or energy (kcal m⁻²). Its rate is productivity, expressed in g m⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹, so ecosystems can be compared.

Primary productivity depends on the plant species, the environment, nutrient availability and photosynthetic capacity. The annual NPP of the whole biosphere is about 170 billion tons (dry weight) of organic matter. Despite occupying about 70% of the surface, the oceans produce only 55 billion tons; the rest is on land.

12.3 Decomposition

Decomposers break down complex organic matter into inorganic substances (CO₂, water, nutrients). Dead plant remains (leaves, bark, flowers) and dead animal remains including faecal matter form detritus, the raw material. The important steps are:

Fragmentation, leaching and catabolism operate simultaneously.

Decomposition is largely oxygen-requiring. Its rate is controlled by chemical composition of detritus and climatic factors: slower if detritus is rich in lignin and chitin, quicker if rich in nitrogen and water-soluble substances like sugars. Warm and moist environments favour decomposition; low temperature and anaerobiosis inhibit it, causing build-up of organic material.

12.4 Energy Flow

Except for deep-sea hydrothermal ecosystems, the sun is the only source of energy. Of the incident solar radiation, less than 50 per cent is photosynthetically active radiation (PAR). Plants and photosynthetic bacteria fix solar energy; plants capture only 2–10 per cent of the PAR, and this small amount sustains the entire living world. Energy flow is unidirectional — sun → producers → consumers — obeying the second law of thermodynamics; ecosystems need a constant energy supply to synthesise molecules and counteract the tendency towards disorderliness.

Producers are autotrophs; consumers are heterotrophs — primary consumers (herbivores), secondary consumers (primary carnivores) and tertiary consumers (secondary carnivores). A simple grazing food chain (GFC): grass → goat → man.

The detritus food chain (DFC) begins with dead organic matter and is made of decomposers (heterotrophic organisms, mainly fungi and bacteria, also called saprotrophs) which secrete digestive enzymes on dead matter and absorb the products. In an aquatic ecosystem, GFC is the major conduit of energy flow; in a terrestrial ecosystem, a much larger fraction flows through the DFC. The DFC may be connected with the GFC at some levels; the natural interconnection of food chains makes a food web.

Organisms occupy a place in the food chain called their trophic level. Each trophic level has a certain mass of living material at a particular time called the standing crop, measured as biomass or number per unit area. Biomass measured as dry weight is more accurate. The number of trophic levels is restricted because energy decreases at successive levels — the 10 per cent law: only 10 per cent of energy is transferred from the lower trophic level to each next level.

12.5 Ecological Pyramids

The base is the producers (first trophic level), the apex the top consumers. Three types: pyramid of number, pyramid of biomass, pyramid of energy. They are generally upright. Exceptions: the pyramid of numbers in a tree ecosystem (one tree supports many insects) is inverted; the pyramid of biomass in the sea is inverted because the biomass of fishes far exceeds that of phytoplankton. The pyramid of energy is always upright — it can never be inverted, because some energy is always lost as heat at each step. In Figure 12.5 producers convert only 1% of incident sunlight into NPP.

Limitations of ecological pyramids: they do not take into account the same species belonging to two or more trophic levels (e.g. a sparrow eats seeds, fruits, peas as a primary consumer and worms and insects as a secondary consumer); they assume a simple food chain, which never exists in nature, and do not accommodate a food web; and saprophytes are not given any place although they play a vital role.

The chapter summary also notes nutrient cycling (gaseous cycles like carbon; sedimentary cycles like phosphorus) and ecosystem services — the products of ecosystem processes.

Diagrams to Know

Key-Facts Sheet

Board Exam Focus

Key Terms

Common Mistakes