Transfers of energy and matter

📚The biology

Ecosystems are open systems

An open system is one in which both energy and matter can enter and exit. Natural ecosystems are open: light energy enters and heat leaves; matter also moves in and out — seeds and animals migrate in, nutrients are washed out by rivers, leaves blow away. In a closed system, only energy passes in and out, and matter is kept inside; a sealed glass mesocosm is close to this (D4.2).

Sunlight sustains most ecosystems

Sunlight is the principal source of energy for most ecosystems. Producers capture it by photosynthesis, and all other organisms depend on the chemical energy they store.

There are exceptions where no light reaches. In caves, organisms depend on organic matter washed or carried in from outside, or on chemoautotrophic bacteria. In the deep ocean, below the depth of light penetration, communities depend on dead organic matter sinking from above (“marine snow”), and around hydrothermal vents on chemoautotrophs that oxidize hydrogen sulfide.

“Sunlight is the source of energy for ecosystems” is a law-like generalization: it describes a pattern and allows predictions, but does not explain it, and it has exceptions. Unlike a theory, a law describes rather than explains. Useful generalizations are broad, based on many observations, and clear about their limits.

Energy flows through food chains

A food chain shows a sequence of feeding relationships. Chemical energy passes to each consumer as it feeds on the organism at the previous stage, stored in the carbon compounds of the food.

The arrows show the direction of transfer of energy and biomass — from the organism eaten to the organism that eats it. A food web shows many interconnected food chains in a community, reflecting the fact that most organisms eat, and are eaten by, more than one species. Construct one for a local community if you can: for example in a rice field, rice plants → rice planthoppers → spiders and frogs → snakes and egrets.

Decomposers

Decomposers (saprotrophic bacteria and fungi) obtain their energy from carbon compounds in dead organic matter: faeces, dead parts of organisms such as fallen leaves, shed skin and hair, and whole dead organisms. Detritivores such as earthworms also feed on this material.

Autotrophs

Autotrophs use an external energy source to synthesize carbon compounds from simple inorganic substances such as carbon dioxide and water. Energy is needed both for carbon fixation and for the anabolic reactions that build macromolecules.

Heterotrophs

Heterotrophs use carbon compounds obtained from other organisms to synthesize the carbon compounds they need. Complex compounds such as proteins and nucleic acids in their food are first digested — internally (animals) or externally (saprotrophs) — into monomers, and then assimilated: used to construct the particular proteins, nucleic acids and other molecules the organism requires.

Respiration in all organisms

Both autotrophs and heterotrophs release energy from carbon compounds by oxidizing them in cell respiration. Plants respire all the time, day and night, using sugars they have made; animals respire sugars and fats obtained from food. The energy is used to make ATP (C1.2).

Trophic levels

Many organisms have a varied diet and occupy different trophic levels in different food chains. A human eating rice is a primary consumer; eating chicken, a secondary consumer; eating a fish that ate smaller fish, a tertiary consumer or higher.

Pyramids of energy

A pyramid of energy shows the amount of energy passing through each trophic level of a specific ecosystem, usually in kJ m⁻² yr⁻¹. Draw it as a series of horizontal bars, producers at the bottom, with bar widths drawn to scale. It is always a pyramid shape, because energy decreases at every level.

Why energy decreases along a food chain

Only a small fraction of the energy at one trophic level — very roughly 10% — becomes available to the next. Energy is lost because:

• not all of the organism is eaten — roots, bones, shells and fur may be left, or the organism dies uneaten;
• not all of what is eaten is digested and absorbed — some, such as cellulose or hair, passes out in faeces;
• much of the energy absorbed is used in cell respiration and ultimately lost as heat, rather than being built into new biomass;
• some is lost in excretory products such as urea.

Energy in uneaten material, dead organisms and faeces is not lost from the ecosystem: it passes to decomposers and detritus feeders. They are not usually shown in food chains, but they use and release a large share of an ecosystem’s energy.

Heat loss

Energy transfers in living organisms are never 100% efficient. When chemical energy in food is converted to ATP in respiration, some is released as heat; when ATP is used for work in cells, more heat is released. Both autotrophs and heterotrophs lose heat to the environment in this way. Heat cannot be converted back into chemical energy by living organisms, so it is lost from the food chain for good. This is why energy flows through ecosystems rather than being recycled.

Why food chains are short

Because so much energy is lost at each stage, after a few trophic levels there is too little energy left to support another population. Food chains rarely have more than four or five levels. At each successive stage there are fewer organisms, or smaller organisms, so there is less biomass. But the energy content per unit mass is not reduced: a gram of eagle holds about as much energy as a gram of grasshopper. The decrease is in the total amount of biomass, not in its quality.

Primary production

Primary production is the accumulation of carbon compounds in biomass by autotrophs. It is measured as mass (of carbon) per unit area per unit time, usually g m⁻² yr⁻¹. Biomass accumulates when organisms grow or reproduce. Biomes differ greatly in their capacity to accumulate biomass: tropical rainforests, with warmth, light and water all year, have very high primary production; deserts and tundra, limited by water or temperature, have very low primary production.

Secondary production

Secondary production is the accumulation of carbon compounds in biomass by heterotrophs. It is always lower than primary production in an ecosystem, because heterotrophs lose much of the biomass they consume when carbon compounds are converted to carbon dioxide and water in cell respiration, as well as in faeces and uneaten material.

The carbon cycle

A carbon cycle diagram shows how carbon is recycled in ecosystems. The essential processes, as labelled arrows between boxes for the atmosphere, producers, consumers and decomposers:

Carbon sinks and carbon sources

An ecosystem is a carbon sink if photosynthesis exceeds respiration: there is a net uptake of CO₂, and carbon accumulates in biomass, soil or sediments (a growing forest, a peat bog). It is a carbon source if respiration exceeds photosynthesis: there is a net release of CO₂ (a drained, decomposing peatland; a forest after a fire).

Combustion

Carbon stored for long periods can be released rapidly as CO₂ by combustion of biomass, peat, coal, oil and natural gas. These carbon sinks formed at very different times: biomass over years to centuries; peat over thousands of years; coal, oil and natural gas over millions of years, from organisms that lived hundreds of millions of years ago. Combustion happens naturally — for example forest fires started by lightning — but human activities have enormously increased the rate of combustion, especially of fossil fuels.

The Keeling Curve

The Keeling Curve is the record of atmospheric CO₂ concentration measured at Mauna Loa, Hawaii, continuously since 1958. It shows two patterns:

Photosynthesis and respiration depend on each other

Aerobic respiration depends on atmospheric oxygen produced by photosynthesis, and photosynthesis depends on atmospheric carbon dioxide produced by respiration. The quantities of carbon and oxygen exchanged each year between the atmosphere and living organisms by these two processes are enormous, making this one of the major interactions between autotrophs and heterotrophs on Earth.

Recycling of all elements

Not just carbon, but all chemical elements used by living organisms — nitrogen, phosphorus, sulfur, potassium and the rest — are recycled in ecosystems. When organisms die, decomposers break down their proteins, nucleic acids and other compounds, releasing the elements as simple inorganic ions that producers can absorb again. Without decomposers, nutrients would stay locked in dead matter and ecosystems would run out of them.