Trophic levels, food chains, and food webs

Organisms within an ecosystem are organised into different trophic levels, which are stages in a food chain or food web that represent the movement of biomass and energy through various organisms.

Key trophic levels include:

• Producers - These organisms make their own food. For example, plants and algae perform photosynthesis, converting light energy into chemical energy.
• Primary consumers - These are herbivores that consume producers for food.
• Secondary consumers - These organisms consume primary consumers for food.
• Tertiary consumers - These organisms consume secondary consumers for food.
• Saprobionts (decomposers) - These organisms decompose complex materials in dead organisms into simpler substances to obtain their food, and release nutrients back into the ecosystem.

A food chain is a sequence of these organisms that are connected by feeding, and a food web is a complex network of interconnected food chains within an ecosystem.

The direction of energy flow from one organism to another is represented by arrows in food chains and food webs.

What is biomass?

Biomass refers to the total mass of living material present at a particular time in a specific place or within certain organisms. It serves as an indicator of energy content.

Biomass is measured in grams per square metre ($g{\text{ m}}^{-2}$) on land, and grams per cubic metre ($g{\text{ m}}^{-3}$) in aquatic environments.

Measuring the energy stored in biomass

The chemical energy store in dry biomass can be estimated using calorimetry.

Using calorimetry to measure biomass:

1. Dry the biomass sample until its mass remains constant to obtain the dry mass.
2. Weigh the dry mass.
3. Burn the dry mass in a calorimeter.
4. Measure the volume and temperature change of the surrounding water.
5. Use these values to calculate an estimate of the heat energy released from the burnt biomass.

Gross and net primary productivity

Gross primary production (GPP) is the total amount of solar energy that plants convert into organic matter, storing it as chemical energy within their biomass.

Net primary production (NPP) is the chemical energy that is available to the next trophic level after accounting for respiratory losses (R). It is the energy that is converted into biomass.

The formula for calculating net primary production is:

$NPP=GPP-R$

Primary productivity is the rate of primary production, measured as biomass in a given area in a given time.

Note: Sometimes the terms production and productivity are used interchangeably, so check the units carefully if they are provided to see what sort of value you are dealing with.

Worked example - Calculating net primary productivity

Calculate the net primary productivity (NPP), given that gross primary productivity (GPP) of a forest is $22\text{,}500{\text{ kJ}}^{-2}$ per year, and its respiratory losses (R) are $9\text{,}500{\text{ kJ}}^{-2}$ per year.

Step 1: Formula

$NPP=GPP-R$

Step 2: Substitution and correct evaluation

$NPP=22\text{,}500-9\text{,}500=13\text{,}000$

Step 3: State units

$13\text{,}000{\text{ kJ}}^{-2}{\text{ year}}^{-1}$

Energy losses between trophic levels in a food chain

Biomass and energy content decrease with each ascending trophic level in a food chain. This results in less energy being available for biomass production higher up in the food chain.

Several factors contribute to energy losses at each trophic level:

1. Not all solar energy is captured by leaves or is of the correct wavelength for photosynthesis.
2. Factors like water availability can limit photosynthesis.
3. Energy is lost during photosynthetic reactions.
4. Not all parts of biomass are edible or digestible by consumers.
5. Energy is lost as heat during movement or respiration.
6. Energy is lost in excretory materials, such as urine and faeces.

How agriculture can manipulate the transfer of biomass through ecosystems

Agriculture involves manipulating the environment to favour plant species that we can eat (crops) and to rear animals for food or their produce.

This creates simple food chains by reducing the number of trophic levels, ensuring as much energy as possible is transferred into biomass that can be eaten by humans.

Calculating ecological efficiency

Ecological efficiency is the efficiency at which biomass or energy is transferred from one trophic level to the next as a percentage.

The formula for calculating ecological efficiency is:

$\text{ecological efficiency}=\frac{\text{energy or biomass available after transfer}}{\text{energy or biomass available before transfer}}\times \text{ 100}\%$

To determine the efficiency of energy transfer between trophic levels, compare the net production or net productivity values.

Worked example - Calculating ecological efficiency

The NPP of plants in a grassland is $36\text{,}000{\text{ kJ}}^{-2}$ per year. Rabbits in the same area have a net production of $4\text{,}800{\text{ kJ}}^{-2}$ per year after consuming the plants.

Calculate the ecological efficiency of energy transfer from plants to rabbits.

Step 1: Formula

$\text{ecological efficiency }=\frac{\text{energy or biomass available after transfer}}{\text{energy or biomass available before transfer}}\times \text{100}\%$

Step 2: Substitution and correct evaluation

$\text{ecological efficiency }=\frac{4\text{,}800}{36\text{,}000}\times \text{100}\%$

$\text{ecological efficiency }=\text{ 13.3}\%\text{ (to 3 s.f.)}$