Big picture
An ecosystem is a functional unit of nature in which living organisms interact with each other and with their physical environment. A pond, a forest and a grassland are natural ecosystems; a crop field and an aquarium are man-made ones.
Every ecosystem does two basic things: energy flows through it in one direction (from the sun, through producers and consumers, lost as heat), and nutrients cycle within it. This chapter looks at how much energy producers capture (productivity), how dead matter is broken down (decomposition), how energy passes along food chains, and how ecosystems can be pictured as ecological pyramids.
Learning goals
Explain the structure and functions of an ecosystem, using a pond as an example.
Define primary productivity, GPP, NPP and secondary productivity.
Describe the steps of decomposition and the factors that control it.
Explain energy flow, the 10 per cent law, and grazing and detritus food chains.
Explain trophic levels and food webs.
Explain pyramids of number, biomass and energy, with upright and inverted examples, and their limitations.
Core ideas
1. Structure and function
Structure: the biotic components (producers, consumers, decomposers) and abiotic components (light, temperature, water, soil, nutrients). The vertical distribution of species at different levels is called stratification (for example, trees on top, shrubs in the middle, herbs and grasses at the bottom of a forest).
Functions: productivity, decomposition, energy flow and nutrient cycling.
Pond as an ecosystem. Abiotic components: water with dissolved inorganic and organic substances, and the rich soil deposit at the bottom. Producers: phytoplankton, some algae, and floating, submerged and marginal plants. Consumers: zooplankton, free-swimming and bottom-dwelling animals. Decomposers: fungi, bacteria and flagellates, especially abundant at the bottom. The solar input, cycling of temperature, day length and other climatic conditions regulate the rate of function of the whole pond.
2. Productivity
Primary production is the amount of biomass or organic matter produced per unit area over a period by plants during photosynthesis, expressed as g/m² or kcal/m². The rate of biomass production is productivity, expressed as g/m²/year or kcal/m²/year.
Gross primary productivity (GPP): the rate of production of organic matter during photosynthesis.
Respiration losses (R): part of GPP is used by plants in respiration.
Net primary productivity (NPP) = GPP − R. NPP is the biomass available for consumption by heterotrophs (herbivores and decomposers).
Secondary productivity: the rate of formation of new organic matter by consumers.
Example. If GPP is 1,000 kcal/m²/year and plants use 400 kcal/m²/year in respiration, NPP = 1,000 − 400 = 600 kcal/m²/year.
Primary productivity depends on the plant species in an area, environmental factors, availability of nutrients and the photosynthetic capacity of plants. The annual NPP of the whole biosphere is about 170 billion tons (dry weight) of organic matter, of which the oceans contribute only about 55 billion tons, despite covering about 70% of the Earth's surface.
3. Decomposition
Decomposers break down complex organic matter into inorganic substances like CO₂, water and nutrients. The raw material is detritus: dead plant remains (leaves, bark, flowers) and dead remains of animals, including faecal matter.
Steps of decomposition
Step | What happens |
|---|---|
Fragmentation | Detritivores such as earthworms break detritus into smaller particles |
Leaching | Water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts |
Catabolism | Bacterial and fungal enzymes degrade detritus into simpler inorganic substances |
Humification | Accumulation of a dark-coloured amorphous substance called humus, highly resistant to microbial action, which decomposes extremely slowly; it is colloidal and serves as a reservoir of nutrients |
Mineralisation | Humus is further degraded by some microbes, releasing inorganic nutrients |
Decomposition is largely an oxygen-requiring process.
Factors controlling the rate of decomposition
Chemical composition of detritus: decomposition is slower if detritus is rich in lignin and chitin, and quicker if rich in nitrogen and water-soluble substances like sugars.
Climate: warm and moist environments favour decomposition; low temperature and anaerobiosis inhibit it, leading to accumulation of organic material.
4. Energy flow
Except for deep-sea hydrothermal ecosystems, the sun is the only source of energy for all ecosystems on Earth.
Of the incident solar radiation, less than 50% is photosynthetically active radiation (PAR).
Plants capture only 2–10% of the PAR, and this small amount sustains the entire living world.
Energy flow is unidirectional: from the sun to producers and then to consumers. It is not recycled; at each step, much is lost as heat.
10 per cent law (Lindeman). Only about 10% of the energy at one trophic level is transferred to the next.
Example. If producers fix 10,000 J of energy, herbivores get about 1,000 J, primary carnivores about 100 J, and secondary carnivores about 10 J. This is why food chains rarely have more than 4–5 trophic levels.
5. Food chains, trophic levels and food webs
Food chain | Starts with | Example |
|---|---|---|
Grazing food chain (GFC) | Producers (living plants) | Grass → goat → man |
Detritus food chain (DFC) | Dead organic matter (detritus), consumed by decomposers (saprotrophs: fungi and bacteria) | Dead leaves → fungi and bacteria → detritivores |
In an aquatic ecosystem, the GFC is the major conduit for energy flow; in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain. The DFC may be connected to the GFC at some levels (some organisms of the DFC are prey to GFC animals).
Trophic levels. Organisms occupy a specific place in a food chain according to their feeding relationship:
Trophic level | Organisms |
|---|---|
First | Producers (plants) |
Second | Primary consumers (herbivores) |
Third | Secondary consumers (carnivores) |
Fourth | Tertiary consumers (top carnivores) |
Each trophic level has a certain mass of living material at a given time, called the standing crop, measured as biomass or number per unit area.
Food web: natural interconnection of many food chains. One organism may occupy different trophic levels in different food chains.
6. Ecological pyramids
The relationship between trophic levels can be shown as a pyramid, with producers at the base and top carnivores at the apex.
Pyramid | What it shows | Shape |
|---|---|---|
Pyramid of numbers | Number of individuals at each level | Usually upright (grassland). Inverted in a tree ecosystem: one tree supports many insects, which support fewer birds |
Pyramid of biomass | Dry weight of organisms at each level | Usually upright (forests, grasslands). Inverted in the sea: the biomass of fish far exceeds that of phytoplankton at a given time, because phytoplankton multiply rapidly |
Pyramid of energy | Energy flowing through each level over a period | Always upright, because energy is lost as heat at each step and can never be greater at a higher level |
Limitations of ecological pyramids:
They don't take into account the same species belonging to two or more trophic levels.
They assume a simple food chain, which almost never exists in nature; they don't accommodate a food web.
Saprophytes are not given any place, even though they play a vital role in the ecosystem.
7. Ecosystem services
Healthy ecosystems provide ecosystem services: forests purify air and water, mitigate droughts and floods, cycle nutrients, generate fertile soils, provide wildlife habitat, maintain biodiversity, pollinate flowering plants, provide storage sites for carbon, and offer aesthetic, cultural and spiritual values. Robert Constanza and colleagues estimated the price tag of nature's life-support services at about US $33 trillion a year, nearly twice the global GDP at the time; of this, soil formation accounts for about 50%, and recreation and nutrient cycling for less than 10% each.
A reliable approach to questions
For productivity, use NPP = GPP − R and state units.
For decomposition, list the five steps in order, and explain the effect of lignin, chitin, nitrogen, sugars, temperature and moisture.
For energy, apply the 10 per cent law level by level.
For pyramids, give the example and reason whenever a pyramid is inverted, and remember that the energy pyramid is always upright.
Connections that matter
Organisms and populations covers interactions such as predation and competition within ecosystems.
Biodiversity and conservation explains the importance of protecting ecosystems.
Microbes in human welfare covers decomposers in sewage treatment and biogas.
Photosynthesis (Class 11) explains how producers capture solar energy.
Common mistakes
Saying energy is recycled in an ecosystem. Energy flows one way; nutrients cycle.
Mixing up GPP and NPP. NPP = GPP − respiration.
Saying the energy pyramid can be inverted. It is always upright.
Writing that lignin and chitin speed up decomposition. They slow it down.
Saying most energy in terrestrial ecosystems flows through the grazing food chain. More flows through the detritus food chain.
Revision checklist
I can describe the structure and functions of a pond ecosystem.
I can define and calculate GPP, NPP and secondary productivity.
I can explain the steps and controlling factors of decomposition.
I can explain energy flow and the 10 per cent law.
I can distinguish grazing and detritus food chains and explain food webs.
I can explain the three pyramids with upright and inverted examples, and their limitations.