Energy and ecosystems

In a nutshell

Every ecosystem runs on energy that producers capture by photosynthesis and pass along food chains. This subtopic is about measuring that energy: how much producers store (primary production), how much consumers store (net production), and why so little of it reaches the top of a food chain.

The whole topic is bookkeeping. Energy comes in, some is lost at every step, and what is left is stored as biomass. Get the equations and the units right and most of the marks are yours.

Assumed knowledge: Photosynthesis, Respiration.

Core content

Where the energy and biomass come from

Plants synthesise organic compounds from atmospheric (or aquatic) carbon dioxide.

• Most of the sugars a plant makes are used by the plant itself as respiratory substrates.
• The rest are used to make other groups of biological molecules (proteins, lipids, nucleic acids).
• These biological molecules make up the biomass of the plant.

So biomass is the chemical energy store that is left after the plant has respired: it is what the next trophic level can eat.

Measuring biomass: dry mass and calorimetry

Biomass can be measured in two ways, both per given area:

• as the mass of carbon, or
• as the dry mass of tissue.

Dry mass is used rather than fresh mass because water content varies and carries no usable energy. A sample is dried in an oven (around 80 °C) and reweighed repeatedly until it reaches a constant mass, which confirms all the water has gone. Because drying kills the sample, a mean dry mass is taken from several individuals and scaled up to the whole area.

The chemical energy store in dry biomass is estimated using calorimetry: a known dry mass is burned completely, and the heat released raises the temperature of a known volume of water. The temperature rise gives the energy content, expressed in kJ g^-1.

Primary production: GPP and NPP

Producers do not keep all the energy they fix. Some is respired straight back to the environment, so we distinguish two quantities.

Gross primary production (GPP) is the chemical energy store in plant biomass, in a given area or volume. It is the total amount fixed by photosynthesis.

Net primary production (NPP) is the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account:

$NPP = GPP - R$

where R is the respiratory losses to the environment.

NPP is the important figure ecologically, because it is what is available for plant growth and reproduction, and what is available to other trophic levels, such as herbivores and decomposers.

The bar chart shows the split: of 25 000 kJ m^-2 year^-1 fixed (GPP), 10 000 is respired away (R), leaving 15 000 stored as new biomass (NPP).

Still don't get it? · why we subtract respiration (GPP vs NPP)

Think of a plant's energy like a monthly pay packet. GPP is your gross salary, the full amount you earn. But money leaves the account before you ever spend it: tax and deductions come straight off the top. NPP is your take-home pay, what actually lands in your account to spend or save.

For a plant, the "deduction" is respiration. The plant has to respire just to stay alive (active transport, building molecules, repair), and that energy leaves as heat and carbon dioxide. It is gone before any of the biomass can be passed on.

Now the exam version: GPP is the total chemical energy fixed by photosynthesis; R is the energy the plant loses in respiration; and NPP = GPP − R is what is left stored in biomass, available for growth, reproduction and the next trophic level. The mark is for saying the difference between GPP and NPP is biomass lost as carbon dioxide in respiration, not just "energy lost".

Net production of consumers

Consumers store energy too, but they start from food they ingest, and they lose energy two ways: some food is never absorbed, and some is respired.

The net production of consumers (N) is:

$N = I - (F + R)$

where:

• I = the chemical energy store in ingested food
• F = the chemical energy lost to the environment in faeces and urine
• R = the respiratory losses to the environment

N is the energy stored in the consumer's biomass, so it is the energy available to the next trophic level.

Of 3000 kJ m^-2 year^-1 ingested, most is lost in faeces and urine (F) and respiration (R). Only the small net production (N) slice is stored as biomass and passed on.

Productivity and its units

Productivity is the rate of production, so it is production measured per unit time as well as per unit area.

• Primary productivity = the rate of primary production (producers).
• Secondary productivity = the rate of secondary production (consumers).

Because it is a rate of energy stored over an area, the units combine energy, area and time, for example kJ m^-2 year^-1 or kJ ha^-1 year^-1. Miss the year<sup>-1</sup> and it is no longer a productivity.

Efficiency of energy transfer

Only a fraction of the energy in one trophic level ends up stored in the next, so the energy stored falls sharply at each step (a pyramid of energy):

The efficiency of transfer between two levels is:

$\text{efficiency} = \frac{\text{energy (or biomass) in a trophic level}}{\text{energy (or biomass) in the previous level}} \times 100$

Energy is lost between levels because: not all of an organism is eaten (roots, bones); some ingested energy is egested in faeces or excreted in urine without being absorbed; and much of what is absorbed is respired and lost as heat. These compounding losses are why food chains rarely have more than four or five trophic levels.

Still don't get it? · why so little energy reaches the top

Picture a fire brigade passing buckets of water down a line, but every person is allowed to drink from the bucket before handing it on, and each one gulps down most of it. By the time the bucket reaches the far end, there is barely a splash left. That is a food chain: each trophic level "drinks" most of the energy before passing the rest on.

Where does each level's share go? Some of the organism is never eaten at all. Of the food that is eaten, some is not even absorbed and leaves in faeces (egestion) or urine (excretion). Of what is absorbed, most is respired to power the animal and lost as heat. Only the leftover, the net production, is stored as biomass for the next level to eat.

Now the exam version: only a small percentage of the energy in one trophic level is transferred to the next, because energy is lost through material not being consumed, through faeces and urine, and through respiration. With roughly 90% lost at every step, there is too little energy left to support many levels, which is why food chains are short.

Farming and the efficiency of energy transfer

Farming aims to channel more of an ecosystem's energy into the human food chain, by increasing the efficiency of energy transfer in two ways set out by the specification:

• Simplifying food webs to reduce energy losses to non-human food chains. Pesticides, herbicides and removing competitors mean energy is not diverted into pests, weeds or other species. More of the crop's or livestock's biomass reaches humans instead of being lost to organisms we do not eat.
• Reducing respiratory losses within a human food chain. Keeping livestock warm and restricting their movement means they respire less, so less energy is lost as heat (R) and more is left for net production (N), i.e. growth. Warmth and shelter cut the energy the animal would otherwise spend maintaining its body temperature and moving around.

In both cases the point to make in an answer is why the practice works: it raises the proportion of available energy that is stored as biomass we can eat.

Worked examples

Worked example 1: net production of a consumer

A primary consumer ingests food containing 3000 kJ m^-2 year^-1 of chemical energy. It loses 1500 kJ m^-2 year^-1 in faeces and urine and 1200 kJ m^-2 year^-1 in respiration. Calculate its net production, N.

1. Write the equation: $N = I - (F + R)$.
2. Substitute the values: $N = 3000 - (1500 + 1200)$.
3. Work the bracket, then subtract: $N = 3000 - 2700 = 300$.
4. State the answer with units: $N = 300$ kJ m^-2 year^-1.

The units carry a mark of their own, so never drop them.

Worked example 2: efficiency of energy transfer

Using the trophic-level chart above, the producers store 15 000 kJ m^-2 year^-1 and the primary consumers store 1200 kJ m^-2 year^-1. Calculate the efficiency of energy transfer from producers to primary consumers.

1. Write the equation:

$\text{efficiency} = \frac{\text{energy in this level}}{\text{energy in previous level}} \times 100$

2. Substitute:

$\text{efficiency} = \frac{1200}{15000} \times 100$

3. Calculate: efficiency = 8.0 %.

Check it against the next step: primary to secondary consumers is $\frac{120}{1200} \times 100 = 10.0\%$, so efficiencies of this order (often quoted as roughly 10%) are what you should expect.

Common exam mistakes

• Explaining the GPP to NPP difference as just "energy is lost". The mark needs biomass lost as carbon dioxide in respiration; a vague "energy lost" does not account for the difference in biomass between GPP and NPP.
• Putting faeces (F) into the producer equation. Plants do not produce faeces. NPP = GPP − R only; F appears only in the consumer equation N = I − (F + R).
• Writing "productivity" without saying net or gross. Examiners will not credit an unspecified answer; name which one you mean.
• Giving the wrong units for productivity. It is a rate, so it needs area and time, e.g. kJ m^-2 year^-1. Mass-only units, energy-only units, or forgetting year<sup>-1</sup> all lose the mark.
• Confusing egestion (loss of undigested food in faeces, which was never absorbed) with excretion (loss of metabolic waste such as urea in urine). Use the terms correctly.
• Saying plants lose energy maintaining their body temperature. Plants do not thermoregulate; their loss is respiration. (Only mammals and birds spend energy keeping warm.)
• In efficiency calculations, dividing the wrong way round or mishandling standard form. Efficiency is always (energy at a level ÷ energy at the previous level) × 100.
• Assuming NPP always rises when there are more plants. NPP only stays constant when GPP (photosynthesis) equals respiration, and it falls when, for example, less light means less photosynthesis.

Key definitions

• Gross primary production (GPP): the chemical energy store in plant biomass, in a given area or volume.
• Net primary production (NPP): the chemical energy store in plant biomass after respiratory losses to the environment have been taken into account (NPP = GPP − R).
• Respiratory losses (R): the chemical energy lost to the environment through respiration.
• Net production of consumers (N): the chemical energy stored in a consumer's biomass, calculated as N = I − (F + R), where I is the energy in ingested food, F is the energy lost in faeces and urine, and R is the respiratory losses.
• Biomass: the mass of living material, measured as the dry mass of tissue or the mass of carbon in a given area.
• Productivity: the rate of primary or secondary production, measured as biomass (energy) in a given area in a given time (e.g. kJ ha^-1 year^-1).

Specification

• I can state that plants synthesise organic compounds from atmospheric (or aquatic) carbon dioxide, that most sugars are used as respiratory substrates, and that the rest form the biomass of the plant.
• I can state that biomass is measured as the mass of carbon or the dry mass of tissue per given area, and that the chemical energy in dry biomass can be estimated using calorimetry.
• I can define gross primary production as the chemical energy store in plant biomass in a given area or volume.
• I can define net primary production and use NPP = GPP − R, and state that NPP is available for plant growth and reproduction and to other trophic levels.
• I can calculate the net production of a consumer using N = I − (F + R).
• I can state that productivity is the rate of production, measured as biomass in a given area in a given time, and derive appropriate units.
• I can calculate the efficiency of energy transfer within an ecosystem, and percentage yields.
• I can explain how farming practices increase the efficiency of energy transfer by simplifying food webs to reduce energy losses to non-human food chains, and by reducing respiratory losses within a human food chain.

Related notes

• Photosynthesis
• Respiration
• Nutrient cycles
• Populations in ecosystems