AQA A-Level Biology: Energy and Ecosystems

Energy enters most ecosystems as sunlight, is captured by plants, and then passes from one organism to the next along food chains. This guide explains how that energy is stored as biomass, how biologists measure it, and why so much of it is lost at each step. It also looks at how farming practices are designed to keep more of that energy in the human food chain. The recurring idea is that energy is transferred, and lost, at every stage.

How biomass is formed

Biomass is the mass of living material, and it is the store of chemical energy in an ecosystem. It begins with photosynthesis.

• During photosynthesis, plants make organic (carbon-containing) compounds from carbon dioxide taken from the air or water.
• Most of the sugars a plant makes are used straight away as respiratory substrates, releasing energy for the plant's own needs.
• The rest is used to build other biological molecules, such as carbohydrates, lipids and proteins, which make up the plant's biomass.

Measuring biomass and its energy

Biomass can be measured as the mass of carbon an organism contains, or as the dry mass of its tissue per given area.

Dry mass is used rather than fresh mass because the amount of water in a living sample varies from moment to moment and would make the measurement unreliable. To find the dry mass, a sample is dried in an oven at a temperature such as 100°C, taking care not to burn it, and is weighed repeatedly until its mass stops changing, which shows that all the water has evaporated.

The chemical energy stored in dry biomass can be estimated by calorimetry. A known mass of dry biomass is completely burned, and the heat released is used to warm a known volume of water. From the rise in the water's temperature, the energy stored in the biomass can be calculated. A calorimeter is designed to make this measurement valid: a stirrer spreads the heat evenly through the water, insulation reduces heat lost to or gained from the surroundings, and water is used because its high specific heat capacity lets it absorb a lot of energy for a measurable temperature rise.

Gross and net primary production

Not all the energy a plant captures is available to the animals that eat it, because the plant uses some for itself.

• Gross primary production (GPP) is the total chemical energy that plants transfer into biomass in a given area over a given time. It is all the energy captured from light by photosynthesis.
• Net primary production (NPP) is what is left after the plant's respiratory losses are taken away. It is the energy that actually ends up in new plant biomass.

These are linked by a simple relationship:

NPP = GPP − R, where R is the energy lost through respiration.

NPP matters because it is the energy that is genuinely available: it is used for the plant's own growth and reproduction, and it is the energy that can be passed on to other trophic levels, such as herbivores and decomposers.

Productivity

Where production is an amount of energy, productivity is the rate at which it is made. Primary productivity is the rate of primary production, and secondary productivity is the rate at which consumers store energy as biomass.

Productivity is measured in kilojoules per hectare per year (kJ ha⁻¹ yr⁻¹), an amount of energy per unit area per unit time, and each part of that unit has a reason.

• Per unit area allows for the fact that different environments cover different sizes, so measurements can be compared fairly.
• Per year allows for seasonal variation in temperature and other conditions, which changes how much biomass is made at different times, so a whole year gives a more representative and comparable figure.

Most of the light that falls on producers is never captured at all. Much of it is reflected or is the wrong wavelength to be absorbed, some misses the chlorophyll or the photosynthetic tissue, and often a factor such as carbon dioxide concentration or temperature limits the rate. As a result, only a small percentage of the available light energy ends up in biomass.

Energy transfer between trophic levels

When a consumer eats, only some of the energy in its food becomes new consumer biomass. The energy a consumer stores, its net production (N), can be written as:

N = I − (F + R)

• I is the chemical energy in the food it ingests.
• F is the energy lost in faeces and urine.
• R is the energy lost through respiration.

The efficiency of an energy transfer is the energy or biomass available after the transfer divided by the energy or biomass available before it, multiplied by 100 to give a percentage.

Energy transfer between trophic levels is inefficient, and the equation shows why. Energy is:

• lost as heat through respiration,
• locked in parts of the organism that are not eaten, such as bones or roots,
• lost in food that is not digested and leaves as faeces,
• and lost through excretion, for example as urea in urine.

Because so much is lost at each step, food chains rarely have more than four or five links, and the amount of biomass and energy falls sharply from one trophic level to the next.

Farming and energy transfer efficiency

Farming aims to keep as much of an ecosystem's energy as possible flowing into the human food chain, by cutting the losses described above.

In crop farming, the food web is simplified so that less energy is diverted to organisms humans do not eat. Herbicides kill weeds that would compete with the crop for light, pesticides kill insects that eat the crop, and fungicides reduce fungal infections, so in each case more energy goes into crop biomass. Fertilisers, such as nitrates, replace nutrients in the soil so that a lack of them does not limit growth.

In livestock farming, the aim is to reduce the animals' respiratory losses, so that more of the energy in their food is stored as biomass rather than lost as heat. Movement may be restricted and the animals kept warm, so less energy is respired to release heat; animals may be slaughtered while still young and growing, when most of their energy is going into growth; antibiotics prevent energy being lost to disease; and selective breeding produces breeds that grow faster.

These practices raise yields, but they also raise ethical and environmental questions, from animal welfare to the effects of pesticides and fertilisers on the wider ecosystem.

How this fits together

Everything in this topic follows the energy. Light is captured by plants in photosynthesis, stored as biomass, and released again through respiration at every trophic level, which is exactly why energy transfers are so inefficient. The nutrients that pass along with that energy are not lost but recycled, and how they are returned to the soil is covered in the nutrient cycles guide.