What this chapter is about
An ecosystem is a functional unit of nature where living organisms interact with each other and with their physical environment. This chapter introduces you to how ecosystems are structured and how they function through the flow of energy and cycling of nutrients. You will learn about the different components that make up an ecosystem, from producers to decomposers, and understand how energy moves through food chains and food webs.
At the Class 12 level, this chapter builds on your earlier understanding of ecology from organisms and populations. Here you move from individual organisms to understanding how entire communities function together with their abiotic environment. You will study important ecological processes like primary productivity, decomposition, energy flow, and biogeochemical cycles that keep ecosystems running.
After studying this chapter, you should be able to describe the structure of any ecosystem, trace the path of energy through trophic levels, explain why only about ten per cent of energy passes from one level to the next, understand how carbon and nutrients cycle through ecosystems, and appreciate the concept of ecological succession.
Key ideas
- An ecosystem has two components: abiotic (non-living factors like temperature, water, soil, light) and biotic (living organisms including producers, consumers, and decomposers).
- Producers (autotrophs) are organisms like green plants and cyanobacteria that fix solar energy through photosynthesis; they form the base of every food chain.
- Primary productivity is the rate at which biomass is produced per unit area per unit time; gross primary productivity (GPP) is the total rate of photosynthesis, while net primary productivity (NPP) = GPP minus respiration.
- Energy flow through an ecosystem is unidirectional — energy enters as sunlight, passes through trophic levels, and is lost as heat at each step; it cannot be recycled.
- The ten per cent law states that on average only about ten per cent of the energy at one trophic level is available to the next trophic level; the rest is lost in respiration, undigested matter, and heat.
- Decomposition is the breakdown of dead organic matter by decomposers (bacteria and fungi) into simpler inorganic substances; this process releases nutrients back into the ecosystem.
- Biogeochemical cycles (carbon cycle, nitrogen cycle, phosphorus cycle) describe the movement of chemical elements between the biotic and abiotic components of ecosystems.
- Ecological succession is the gradual and predictable change in species composition of a community over time, leading eventually to a stable climax community.
Formulas and facts to remember
- Net Primary Productivity (NPP) = Gross Primary Productivity (GPP) − Respiration (R). NPP represents the biomass available to heterotrophs.
- Ten per cent law: Only about 10% of energy is transferred from one trophic level to the next; 90% is lost as heat or used in metabolism.
- Trophic levels: Producers → Primary consumers (herbivores) → Secondary consumers (carnivores) → Tertiary consumers (top carnivores) → Decomposers act on all levels.
- Standing crop: The mass of living material (biomass) present in an ecosystem at a given time, measured in g/m² or kg/m².
- Detritus: Dead organic matter (fallen leaves, animal remains, faeces) that serves as food for decomposers.
- Humification: Formation of humus, a dark, amorphous, colloidal organic matter resistant to microbial action, during decomposition.
- Mineralisation: Release of inorganic nutrients (like nitrates, phosphates) from humus by decomposer activity.
- Ecological pyramids: Graphical representations of trophic structure; pyramid of numbers, pyramid of biomass, and pyramid of energy. The pyramid of energy is always upright.
Worked examples
Example 1: Calculating Net Primary Productivity
A grassland ecosystem has a gross primary productivity of 4200 g/m²/year. The plants use 1680 g/m²/year for their own respiration. Calculate the net primary productivity and state what this value represents.
Solution: NPP = GPP − Respiration NPP = 4200 − 1680 NPP = 2520 g/m²/year
This means 2520 grams of biomass per square metre per year is available to herbivores and other consumers in this grassland. This is the actual organic matter stored by plants after meeting their own energy needs.
Example 2: Energy transfer through trophic levels
In a pond ecosystem, the producers (phytoplankton) receive 10,000 kJ of solar energy and fix 1000 kJ as GPP. Assuming the ten per cent law applies at each transfer, calculate the energy available at the level of secondary consumers (small fish that eat zooplankton).
Solution: Energy fixed by producers (available to primary consumers) = 1000 kJ Energy at primary consumer level (zooplankton) = 10% of 1000 = 100 kJ Energy at secondary consumer level (small fish) = 10% of 100 = 10 kJ
Thus, only 10 kJ of energy reaches the secondary consumers from an initial fixation of 1000 kJ by producers. This explains why food chains rarely have more than four or five trophic levels.
Example 3: Identifying trophic levels
In a forest ecosystem near the Western Ghats, the following organisms are found: banyan tree, caterpillar, sparrow, snake, and fungi on dead wood. Assign each to its trophic level and identify one food chain.
Solution:
- Banyan tree: Producer (T1) — fixes solar energy through photosynthesis
- Caterpillar: Primary consumer (T2) — eats leaves of the tree
- Sparrow: Secondary consumer (T3) — eats caterpillars
- Snake: Tertiary consumer (T4) — eats sparrows
- Fungi: Decomposers — break down dead matter at all levels
One possible food chain: Banyan tree → Caterpillar → Sparrow → Snake
Common mistakes
- Thinking energy can be recycled in an ecosystem → Energy flows in one direction only and is lost as heat; only nutrients are recycled.
- Confusing GPP with NPP → GPP is total photosynthesis; NPP is what remains after plant respiration and is available to consumers.
- Believing all ecological pyramids are upright → Pyramids of numbers and biomass can be inverted in certain ecosystems (like a tree ecosystem or an ocean), but the pyramid of energy is always upright.
- Assuming decomposers occupy a single trophic level → Decomposers obtain energy from dead organisms of all trophic levels; they are not part of the linear food chain.
- Confusing primary succession with secondary succession → Primary succession occurs on bare, lifeless substrate (like bare rock); secondary succession occurs on areas where life previously existed but was disturbed (like abandoned farmland).
Quick revision
- Ecosystem = biotic community + abiotic environment functioning together.
- NPP = GPP − Respiration; this is the biomass available for consumers.
- Energy flow is unidirectional; about 90% is lost at each trophic level.
- Decomposition involves fragmentation, leaching, catabolism, humification, and mineralisation.
- Carbon cycle involves photosynthesis (uptake) and respiration/combustion (release) of CO₂.
- Succession ends in a climax community that is relatively stable.