Ecosystems and energy flow
How ecosystems work: food chains and webs, GPP and NPP, the 10% law, ecological pyramids, biomagnification and what it means for agroecosystems.
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A paddy field, a village pond, a forest and a grazing pasture each contain plants, animals and microbes interacting with soil, water, air and sunlight. Each of these units is an ecosystem, a term coined by the British ecologist A. G. Tansley in 1935. Two processes hold every ecosystem together: a one-way flow of energy, which enters as sunlight and leaves as heat, and the cycling of nutrients, such as carbon, nitrogen and phosphorus, which are used again and again.
Understanding energy flow explains why food chains are short, why there are far fewer tigers than deer, why animal products cost more land than grain, and how pesticides such as DDT concentrate in top predators.
Components of an ecosystem
- Abiotic components: sunlight, temperature, water, air, soil minerals and nutrients.
- Producers (autotrophs): green plants, algae and some bacteria, which fix solar energy by photosynthesis.
- Consumers (heterotrophs):
- primary consumers (herbivores), such as grasshoppers, cattle and aphids;
- secondary consumers (carnivores that eat herbivores), such as frogs, spiders and ladybird beetles;
- tertiary consumers (top predators), such as snakes, hawks and tigers.
- Decomposers and detritivores: bacteria, fungi, earthworms and termites, which break down dead matter and release nutrients.
Each feeding step is a trophic level.
Food chains and webs
A food chain is a sequence of who eats whom. For example: rice plant → stem borer → spider → frog → snake.
There are two main kinds:
- Grazing food chains start with living plants.
- Detritus food chains start with dead organic matter, such as leaf litter and crop residues, and pass through decomposers and detritivores. In forests and soils, most energy flows through detritus chains.
In reality, chains interconnect into food webs. Most animals eat several foods and are eaten by several predators. Complex webs are generally more stable: if one prey becomes scarce, predators switch to others. This is the ecological basis of natural pest control.
Productivity
NPP is the energy available to herbivores and decomposers, and to us as harvested crops. Globally, land and oceans together produce about 105 billion tonnes of carbon in NPP each year, split roughly equally between them.
Productivity is limited by light, water, temperature and nutrients. Tropical rainforests, swamps and marshes, and well-managed irrigated crops are among the most productive systems per unit area. Deserts and the open ocean are among the least.
Only a small fraction of sunlight becomes plant biomass. Photosynthesis usually captures about 1–2% of incident solar energy over a season, rising to about 3–4% in very productive C4 crops such as sugarcane and maize at their peak.
The 10% law
Raymond Lindeman (1942) showed that energy transfer between trophic levels is inefficient. On average, only about 10% of the energy at one level is incorporated into the next. Ecological efficiency actually ranges from about 5% to 20%. The rest is lost because:
- much food is not eaten, or not digested, and becomes detritus;
- most assimilated energy is used in respiration and lost as heat, in accordance with the second law of thermodynamics;
- only the remainder becomes new body tissue (secondary production).
This is why food chains rarely have more than four or five levels: there is not enough energy left to support another level.
Feeding people from a hectare
Suppose a hectare of maize gives 10,000 units of food energy. Eaten directly as grain, all 10,000 feed people. Fed to livestock at a 10% transfer efficiency, it yields about 1,000 units as meat or milk, enough for a tenth as many people. Real conversion efficiencies vary: poultry and milk are relatively efficient, beef much less so. The calculation shows why diets rich in animal products need far more cropland, and why animals that eat grass or crop residues people cannot eat add food without competing for it.
Ecological pyramids
Charles Elton (1927) described ecological pyramids:
| Pyramid | What it shows | Shape |
|---|---|---|
| Numbers | Individuals at each level | Usually upright; inverted in a tree ecosystem (one tree supports many insects) or a parasite chain |
| Biomass | Dry mass at each level | Usually upright; inverted in some aquatic systems, where small, fast-reproducing phytoplankton support a larger standing mass of zooplankton |
| Energy | Energy flow per unit time at each level | Always upright, because energy is lost at each transfer |
Biomagnification
Some persistent, fat-soluble chemicals, such as DDT, other organochlorines, mercury and some PCBs, are not broken down or excreted easily. They accumulate in organisms (bioaccumulation) and become more concentrated at each trophic level (biomagnification), because each predator eats many contaminated prey. Rachel Carson's Silent Spring (1962) drew attention to DDT thinning the eggshells of birds of prey. Most uses of DDT are now banned, and modern pesticides are tested for persistence and bioaccumulation before approval.
Agroecosystems
Farms are ecosystems managed by people. Compared with natural ecosystems, agroecosystems:
- have low diversity, often one crop and a few associated species;
- rely on external inputs of fertiliser, energy, water and pesticides;
- export much of their production as harvest, removing nutrients that must be replaced;
- have simplified food webs, which makes pest outbreaks more likely.
Practices that restore ecological functions include crop diversification, intercropping, rotations, residue recycling, integrated pest management, and habitat for natural enemies and pollinators. These are central to agroecology and sustainable intensification.
Key points
- An ecosystem is a community of organisms plus its abiotic environment (Tansley, 1935). Energy flows one way, while nutrients cycle.
- Producers, consumers (primary, secondary, tertiary) and decomposers occupy trophic levels linked in grazing and detritus food chains and webs.
- NPP = GPP − plant respiration; photosynthesis captures about 1–2% of sunlight.
- Lindeman's 10% law: about a tenth of energy passes to the next level. Food chains are short as a result.
- Pyramids of numbers and biomass can be inverted; the pyramid of energy is always upright.
- Persistent chemicals biomagnify up food chains. Agroecosystems are simplified, input-dependent and export nutrients in harvest.
What the exams ask
- Define ecosystem and list its components.
- Distinguish grazing and detritus food chains, and food chains and food webs.
- Define GPP and NPP, and explain the 10% law.
- Describe ecological pyramids and explain why some are inverted.
- Explain biomagnification with an example.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why is the pyramid of energy never inverted?
Energy is lost as heat at every transfer, so each level always has less energy flowing through it than the level below. Numbers and biomass are snapshots of what is present at one time, so they can be inverted when small organisms reproduce quickly or one large producer supports many consumers.
Do decomposers get energy too?
Yes. All dead matter, wastes and uneaten food pass to decomposers, which use the energy for their own growth and respiration. In many ecosystems, especially forests and soils, more energy flows through decomposers than through grazing herbivores.
How does energy flow relate to soil fertility?
Crop residues, roots and manure feed the soil food web of bacteria, fungi, earthworms and others. As these organisms respire and decompose organic matter, they release nutrients for the next crop and build soil organic matter. Removing or burning residues starves this food web.
Sources
- Odum, E. P. and Barrett, G. W. Fundamentals of Ecology, 5th edition. Thomson Brooks/Cole, 2005.
- Lindeman, R. L. "The trophic-dynamic aspect of ecology." Ecology 23, 399–417 (1942).
- Field, C. B., Behrenfeld, M. J., Randerson, J. T. and Falkowski, P. "Primary production of the biosphere: integrating terrestrial and oceanic components." Science 281, 237–240 (1998).
- Tansley, A. G. "The use and abuse of vegetational concepts and terms." Ecology 16, 284–307 (1935).
- Gliessman, S. R. Agroecology: The Ecology of Sustainable Food Systems, 3rd edition. CRC Press, 2015.
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