C4.2 Transfers of energy and matter

Ecosystems run on a one-way flow of energy and an endless recycling of matter — two ideas that look similar but behave very differently. Sunlight enters, is captured by producers, passes up the food chain and is eventually lost as heat, never to be reused. Carbon, nitrogen and other nutrients, by contrast, cycle round and round between organisms and their environment. C4.2 asks you to follow energy as it dwindles at each step (the reason food chains are short) and to follow matter as it is continually reused. Keep energy flows, matter cycles at the front of your mind.

Trophic levels and feeding relationships

A trophic level is the position an organism occupies in a feeding sequence. Energy enters almost all ecosystems through producers (autotrophs) — mainly plants and algae — which convert light energy into chemical energy in organic molecules by photosynthesis. Organisms that obtain energy by eating others are consumers (heterotrophs):

• Primary consumers (herbivores) eat producers.
• Secondary consumers eat primary consumers.
• Tertiary consumers eat secondary consumers, and so on.

A food chain shows a single linear feeding sequence, with arrows pointing in the direction of energy flow (from the eaten to the eater). In reality organisms eat and are eaten by many species, so feeding relationships are better represented as an interconnected food web. Decomposers (such as bacteria and fungi) and detritivores (such as earthworms) feed on dead organic matter and wastes, releasing nutrients back into the environment.

Energy flow and the loss at each transfer

Energy enters food chains as light and leaves as heat. The crucial point is that the transfer between trophic levels is inefficient: on average only about 10% of the energy in one trophic level is passed on to the next, with the rest lost. Energy is lost because:

• Much of the energy in food is released as heat during cell respiration and lost to the surroundings.
• Not all of an organism is eaten or digestible; energy remains in faeces, bones, fur and other unconsumed parts.
• Energy is lost in excretory products such as urea.

Because this heat energy cannot be recaptured by organisms, energy flow through an ecosystem is one-way; it is not recycled. This loss at every step explains two patterns: food chains are usually short (typically four or five links) because too little energy remains to support further levels, and pyramids of energy are always upright, narrowing sharply towards the top. Each bar of a pyramid of energy shows the energy (per unit area per unit time) at that trophic level.

The carbon cycle and recycling of matter

Unlike energy, the chemical elements that make up living things are finite and must be recycled. The carbon cycle is the standard example. Carbon moves between the atmosphere, organisms and the environment through balanced processes:

• Photosynthesis removes CO₂ from the atmosphere and fixes carbon into organic compounds in producers.
• Cell respiration by all organisms returns CO₂ to the atmosphere.
• Feeding passes carbon compounds from one organism to the next.
• Decomposition of dead matter by decomposers releases CO₂ through their respiration.
• Combustion of biomass and fossil fuels releases stored carbon as CO₂.

Carbon can be locked away for long periods in carbon sinks such as fossil fuels (coal, oil and gas), limestone and peat. Because decomposers continually release nutrients from dead organisms, the same atoms of carbon, nitrogen and other elements are used again and again — matter is recycled while energy is not.

Pyramids and the impact of human activity

Ecological pyramids summarise an ecosystem level by level. A pyramid of energy is always upright because of the loss at each transfer, and its units (energy per area per time, for example kJ m⁻² yr⁻¹) make it the most reliable type. The continual recycling of nutrients keeps producers supplied with raw materials, so the same carbon released by a decomposing leaf today may be fixed by a plant tomorrow.

Human activities disturb these balanced cycles. Burning fossil fuels and large-scale deforestation release CO₂ faster than photosynthesis and the oceans can absorb it, raising atmospheric CO₂ concentration. Because CO₂ is a greenhouse gas, this contributes to the enhanced greenhouse effect and global warming. Understanding the carbon cycle therefore underpins the climate-change material elsewhere in the course.