Energy and ecosystems

In a nutshell

Producers are the starting point in an ecosystem as they use energy from the sun to produce the biomass that will then be passed down the food chain. However, not all of the energy will be transferred to the next trophic level. This summary explains the reasons for this and how productivity can be calculated.

Ecosystems

Definition

An ecosystem is all living organisms and the abiotic factors in an area.

Producers use light energy and carbon dioxide to produce organic molecules, such as glucose, in a process known as photosynthesis. This glucose can then be used to produce biological molecules, such as starch and cellulose. These biological molecules make up the plant's biomass.

Energy is passed through an ecosystem when organisms eat one another. Primary consumers are herbivores that eat the producers. Then, secondary consumers eat the primary consumers. The tertiary consumers will eat the secondary consumers and so on.

Biomass

Definition

Biomass is the total mass of living material in a specific area at any specific time. When biomass is being measured in an area like a grassland, the unit of measurement is grams per square metre ( $gm{^-}{^2}$ ). When biomass is being measured in an area like a pond, the unit of measurement is grams per cubic metre ( $gm{^-}{^3}$ ).

How to measure biomass

Biomass can be measured in two ways. These are explained below.

Dry mass

A sample of an organism is taken and heated in an oven. The mass is measured every day and the dry mass is reached when the mass becomes constant. This is because all of the water has evaporated.

Calorimetry

The chemical energy stored in the biomass can be estimated using a bomb calorimeter. A sample of dry material is burnt and the energy that is released will heat a known volume of water. It is known how much thermal energy is required to raise the temperature of

1\ g

of water by

1\ \degree C

. Therefore, the energy released from the biomass can be calculated. The units for this method would be

kJ\ kg{^-}{^1}

.

Efficiency of photosynthesis

Photosynthesis is not a $100\%$ efficient process as not all of the light energy that hits the leaf is used. This is shown below.

Biology; Ecosystems; KS5 Year 12; Energy and ecosystems — 1. Light energy hits the leaf, 2. $5\%$ of the light energy is reflected, 3. $50\%$ of the light energy is not absorbed but causes evaporation from the leaf, 4. $5\%$ of the light energy passes straight through the leaf, 5. $40\%$ of the light energy is absorbed by the chlorophyll.

Primary production

Gross primary production

Gross primary production (GPP) is the amount of chemical energy stored in plant biomass in a given area (or volume) and in a given time.

Net primary primary production

Plants will use between $20\%$ and $50\%$ of their total energy on respiration. Therefore, net primary production (NPP) is the amount of energy left for growth and reproduction. This energy is also what is available for organisms in the next trophic level.

Calculating primary production

$net\ primary\ production = gross\ primary\ production - respiration$

$NPP=GPP-R$

Note: Primary production is often measured in $kJ\ ha{^-}{^1}\ yr{^-}{^1}$ (which means kilojoules per hectare per year) or $kJ\ m{^-}{^2}\ yr{^-}{^1}$

Example

What is the net primary productivity of trees in an ecosystem when their gross primary productivity is $90000\ kJ\ m{^-}{^2}\ yr{^-}{^1}$ and $50000\ kJ\ m{^-}{^2}\ yr{^-}{^1}$ is lost as heat through respiration?

$NPP=GPP-R\\NPP=90000-50000\ =\underline{40000\ kJ\ m{^-}{^2}\ yr{^-}{^1}}$

$\underline{The\ net\ primary\ productivity\ is\ 40000\ kJ\ m{^-}{^2}\ yr{^-}{^1}}$

Secondary production

Secondary production is the net production of consumers. Consumers gain energy through eating plant matter or other matter that has consumed plant matter. However, around $90\%$ of the consumed energy is not transferred to the next trophic level. There are many reasons for this and some are explained below.

Reason	Explanation
Not all of the plant gets eaten	Some parts of the plant like the roots and twigs may not be eaten by the animals.
Some of the energy is egested	Animals may be unable to digest molecules like cellulose as they don't have the correct enzymes to do so. Therefore, they will be egested in faeces.
Respiration	A lot of the energy intake of a primary consumer will be lost during respiration.
Chemical reactions	Primary consumers may need to regulate their body temperature. This process requires energy and this will mean there is less energy to pass to the next trophic level.

Calculating secondary production

$N=I-(F+R)$

$N$	Net production.
$I$	Chemical energy from ingested food.
$F$	Chemical energy lost in faeces and urine.
$R$	Energy lost through respiration.

Example

What is the net productivity of squirrels in a woodland ecosystem if they ingest $15000\ kJ\ m{^-}{^2}\ yr{^-}{^1}$ of energy but lose $8000\ kJ\ m{^-}{^2}\ yr{^-}{^1}$  through faeces and urine and lose a further $5500\ kJ\ m{^-}{^2}\ yr{^-}{^1}$ through respiration?

$N=I-(F+R)\\N=15000-(8000+5500)\ =\underline{1500\ kJ\ m{^-}{^2}\ yr{^-}{^1}}$ 

$\underline{The\ net\ productivity\ is\ 1500\ kJ\ m{^-}{^2}\ yr{^-}{^1}}$

Calculating the efficiency of energy transfers

The efficiency of energy transfers between trophic levels can be calculated.

Example

The NPP of grass was $25000\ kJ\ m{^-}{^3}\ yr{^-}{^1}$ . The net productivity of the grasshoppers that eat the grass is $1500\ kJ\ m{^-}{^2}\ yr{^-}{^1}$ . What is the efficiency of this energy transfer?

$\dfrac{1500}{25000}\times100=6\%$

$\underline{The\ efficiency\ of\ the\ energy\ transfer\ is\ 6\%}$

Farming practices impacting production

In order to maximise the amount of energy their crops are able to transfer to humans, farmers can reduce the number of pests and the losses from respiration.

Reducing pests

Pests will reduce the biomass of the crops so farmers can target these to increase the energy available for the plant to grow.

Control method	Explanation
Insecticides	Insecticides are chemicals that kill the pests, which means they won't be able to feed on the crops. As a result, there will be more energy available for the plant to use to grow. This will result in a higher NPP.
Herbicides	Herbicides are chemicals that kill weeds, which will reduce competition for light. As a result, the crops can photosynthesise more as there is more light available. This will increase the amount of glucose produced and also the biological molecules that plants use glucose to produce. As a result, the NPP will increase.
Parasites	Parasites can infect pests or they can lay their eggs on the pests. This will reduce the number of pests. Example Some wasps can lay their eggs inside caterpillars. When the eggs hatch, the caterpillar is killed.
Pathogenic bacteria and viruses	Bacteria and viruses can infect the pests and reduce their population size. Example Bacillus thuringiensis produces a toxin called Bt. This toxin kills caterpillars.

Reducing loss from respiration

$20\%$ to $50\%$ of the total energy of an organism can be lost through respiration. Farmers can reduce the amount of energy lost through respiration and therefore increase the amount of energy transferred to humans through consumption. They achieve this through limiting activities that may increase the rate of respiration.

Examples

Animals respire more when they move around, so farmers may limit the movement of their animals.
Maintaining body temperature requires energy and this energy is transferred through respiration. Farmers can therefore keep their animals inside in warmer conditions to reduce this respiration loss.

However, the examples described above have ethical issues surrounding them as some may argue that keeping the animals in an environment that is unlike their natural environment is inhumane. The animals may also be distressed.