Energy and ecosystems (A-level only)

Welcome to the flow of energy through ecosystems! In this section, you will learn:

How plants turn inorganic carbon into organic biomass, and how we measure its energy content.
The difference between gross and net primary production in plants.
How to calculate the net production of consumers (animals).
How farming practices manipulate these energy transfers to increase yields.

Where Does the Energy Come From?

In any ecosystem, the ultimate source of energy is usually the sun. Plants (and some other photosynthetic organisms) act as producers. They synthesise organic compounds, such as glucose, from atmospheric or aquatic carbon dioxide.

Once a plant has made these sugars, it splits them into two main pathways:

Respiratory substrates: Most of the sugars are broken down in respiration to release energy for the plant's immediate metabolic needs.
Biological molecules: The rest of the sugars are used to synthesise other complex biological molecules like cellulose, proteins, and lipids.

These biological molecules form the physical "stuff" of the plant.

Definition

Biomass

Biomass is the total mass of living material in a specific area at a given time. It represents the chemical energy stored within an organism.

Measuring Biomass

Because biological samples contain highly variable amounts of water, simply weighing a fresh plant (its "fresh mass") isn't a reliable way to measure its energy content. Instead, biomass is usually measured in one of two ways:

Mass of carbon per given area.
Dry mass of tissue per given area.

To find the dry mass, a sample is dried in an oven set to a low temperature (around $∘C100\text{ }^\circ\text{C}$ to evaporate the water without burning the tissue). The sample is weighed at regular intervals until the mass becomes constant. A constant mass proves that all the water has been driven off.

Using Calorimetry to Find Energy

Once you have dry biomass, you can estimate the chemical energy stored inside it using a technique called calorimetry.

The standard apparatus used is a bomb calorimeter. A known mass of the dry sample is placed in a sealed combustion chamber (the "bomb") and ignited electrically. The chamber is surrounded by a known volume of water. As the biomass burns, the chemical energy is released as heat, which transfers to the water.

Diagram of a bomb calorimeter

By measuring the temperature rise of the water, you can calculate the energy released from the biomass. The sealed chamber and the insulated outer container ensure that heat loss to the environment is minimised, making the estimate highly accurate.

Primary Production: Energy in Plants

When we talk about the energy plants capture, we use specific terms to separate the total energy they trap from the energy they actually keep.

Gross primary production ( $GP P$ ): This is the total chemical energy store in plant biomass, in a given area or volume, over a given time. It represents all the chemical energy the plant successfully captured from photosynthesis.
Respiratory losses ( $R$ ): The plant must use some of this energy to stay alive, so a portion is lost to the environment as heat during respiration.
Net primary production ( $N P P$ ): This is the chemical energy store left over in the plant biomass after respiratory losses have been taken into account.

Analogy

Gross vs Net

Think of Gross Primary Production ( $GP P$ ) like a gross salary — it's the total amount of money you earn. Respiratory losses ( $R$ ) are like the taxes you pay. Net Primary Production ( $N P P$ ) is your net salary — the actual take-home pay you get to keep and spend.

The relationship is written as:

N P P = GP P - R

The $N P P$ is incredibly important because it is the energy available for the plant's own growth and reproduction. More importantly for the ecosystem, it is the energy available to organisms in the next trophic level — such as the herbivores that eat the plants, or the decomposers that break them down.

Example

Calculating NPP and energy efficiency

A field of wheat captures a Gross Primary Production ( $GP P$ ) of $year−124\,500 \text{ kJ m}^{-2} \text{ year}^{-1}$ . The wheat loses $year−111\,200 \text{ kJ m}^{-2} \text{ year}^{-1}$ as heat from respiration. Calculate the Net Primary Production ( $N P P$ ) and determine what percentage of the $GP P$ is successfully converted into $N P P$ .

State the formula relating the variables:

N P P = GP P - R

Substitute the given values to find $N P P$ :

24\,500 - 11\,200 = 13\,300 \text{ kJ m}^{-2} \text{ year}^{-1}

Set up the percentage efficiency calculation (part over whole, multiplied by 100):

\text{Percentage efficiency} = \left( \frac{NPP}{GPP} \right) \times 100

Substitute the values and evaluate:

\text{Percentage efficiency} = \left( \frac{13\,300}{24\,500} \right) \times 100 = 54.3\%

Secondary Production: Energy in Consumers

When a consumer (an animal) eats a plant, it does not absorb all the $N P P$ . Animals are not perfectly efficient machines.

Flow diagram of energy transfer in an animal consumer

When an animal ingests food ( $I$ ), a large amount of the chemical energy passes straight through the digestive system and is lost to the environment in faeces and urine ( $F$ ). The energy that is successfully absorbed is then subjected to the animal's own high respiratory losses ( $R$ ), which are lost to the environment as heat.

The net production of consumers ( $N$ ) — sometimes called secondary production — is the energy left over to form new animal biomass. It is calculated as:

N = I - (F + R)

$N$ : Net production
$I$ : Chemical energy store in ingested food
$F$ : Chemical energy lost to the environment in faeces and urine
$R$ : Respiratory losses to the environment

Example

Calculating consumer net production

A cow ingests $kJ140\,000 \text{ kJ}$ of energy from grass per day. It loses $kJ65\,000 \text{ kJ}$ in faeces and urine, and $kJ55\,000 \text{ kJ}$ through respiration. Calculate the net production of the cow and find the percentage of ingested energy converted to biomass.

State the formula for consumer net production:

N = I - (F + R)

Substitute the values for faeces/urine and respiration into the brackets to find total losses:

65\,000 + 55\,000 = 120\,000 \text{ kJ}

Subtract the total losses from the ingested energy to find $N$ :

140\,000 - 120\,000 = 20\,000 \text{ kJ}

Calculate the percentage of ingested energy converted into biomass:

\text{Percentage} = \left( \frac{N}{I} \right) \times 100 = \left( \frac{20\,000}{140\,000} \right) \times 100 = 14.3\%

Productivity Rates and Units

When we look at production over time, we call it productivity. Productivity is measured as a rate. The standard units for primary or secondary productivity are often $year−1\text{kJ ha}^{-1} \text{ year}^{-1}$ (kilojoules per hectare per year).

Tip

Decoding the units

You need to understand why these specific units are used:

per hectare ( $ha−1\text{ha}^{-1}$ ): Ecosystems come in different sizes. Standardising by a specific area allows us to fairly compare the productivity of different environments (e.g., a huge forest vs a small meadow).
per year ( $year−1\text{year}^{-1}$ ): Taking an annual average smooths out the effects of seasons. A plant photosynthesises much faster in summer than in winter, so a yearly rate gives a more accurate overall picture.

How Farming Practices Increase Efficiency

Human agriculture is essentially the deliberate manipulation of food webs to maximise the net production ( $N P P$ of crops or $N$ of livestock) that ends up on our plates. Farmers use two main strategies to increase the efficiency of energy transfer:

1. Simplifying Food Webs

In a natural ecosystem, energy flows to many different organisms. Pests and weeds act as rival consumers or rival producers. By simplifying the food web, farmers reduce energy losses to these non-human food chains:

Herbicides kill weeds, removing competition for light, water, and soil minerals, allowing the crop to achieve a higher $GP P$ .
Pesticides / Insecticides kill insects that eat the crop, preventing the crop's biomass from being transferred to pests.

2. Reducing Respiratory Losses

If you look at the equation $N = I - (F + R)$ , decreasing $R$ will automatically increase $N$ . Intensive farming practices reduce the respiratory losses of livestock:

Restricting movement: Keeping animals in small pens means less muscle contraction, which requires less ATP, meaning a lower rate of respiration.
Keeping animals warm: Housing animals indoors in heated environments means the animals don't have to respire as much to generate body heat.

Because less energy is lost as heat to the environment, more of the ingested energy goes towards growth (increasing biomass).

Key Idea

The goal of agriculture

Farming mathematically aims to maximise $N P P$ in crops (by reducing competition) and maximise $N$ in livestock (by maximising $I$ with high-nutrition feed, and minimising $R$ through restricted movement and heating).

Exam technique

In the exam

Watch out for units: In calculation questions, you may be given a daily rate but asked for an annual rate. Multiply by $365$ before stating your final answer.
Be specific about heat loss: When defining respiratory losses ( $R$ ), do not just say "energy used for respiration". State that it is energy lost to the environment as heat during respiration.
Link to the equations: If an exam question asks how a farming practice increases yield, write out the terms in the formula. For example, "Keeping cows indoors reduces heat loss, which decreases respiratory losses ( $R$ ). Since $N = I - (F + R)$ , a lower $R$ means a higher net production ( $N$ )."

Self review

Check yourself

What is the difference between biological fresh mass and dry mass, and why is dry mass used to measure biomass?
How does gross primary production ( $GP P$ ) differ from net primary production ( $N P P$ )?
If a caterpillar ingests $\text{ J}$ of plant material, excretes $\text{ J}$ in faeces, and uses $\text{ J}$ in respiration, what is its net production?
Explain why expressing productivity in $year−1\text{kJ ha}^{-1} \text{ year}^{-1}$ is better than just $kJ\text{kJ}$ .
How does keeping chickens in enclosed, temperature-controlled barns increase their net production?