Trophic levels of food chains

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Slide 13

There are three different types of trophic pyramids. In the pyramid of numbers, each level of the pyramid represents the number of individual organisms at the corresponding trophic level of the food chain. In the pyramid of biomass, each layer of the pyramid represents the total biomass of the organisms at the corresponding trophic level of the food chain. Finally, the pyramid of energy represents the amount of energy produced and transferred by each trophic level of the food chain.

Slide 14

The pyramid of number shows the actual numbers of individual organisms present at each trophic level in the food chain. It emphasises the large difference in numbers of individual organisms at each level of the food chain. In the pyramid on the right, the producers are represented by 100,000 leaves. These are munched on by 10,000 caterpillars, which, in turn are fed on by 100 blue tits. There is one hawk present as an apex predator to hunt the 100 blue tits. In this ecosystem, the number of producers far outstrips the number of organisms at each higher trophic level. There are 10,000 primary consumers, and far fewer (200) secondary consumers. Finally, there is only 1 apex predator.

Slide 15

In the pyramid of biomass, each layer represents the biomass of the members of the food chain present at the corresponding trophic level. It clearly shows the decrease of the biomass at each trophic level from the base to the apex of the trophic pyramid. For example, the total mass of the producers ingested by the herbivores will always be more than the total biomass of the herbivores in the ecosystem. On the right, the biomass of phytoplankton, which is the producer, is 1 million kilograms. The mass of the zooplankton, which is the primary consumer, is significantly smaller at 100,000 kilograms. There is a still smaller biomass of the secondary consumer, or the herrings at 10,000 kilograms. Finally, the biomass of the tertiary consumer, the sea lion, is only 1,000 kg.
 
Slide 16

The pyramid of energy shows the energy flow throughout the food chain. It shows how much energy flows from one trophic level to the rest, and also shows the roles of the various organisms in the transfer of energy. It clearly demonstrates how much energy is required to sustain the next trophic level.

Slide 17

Each trophic level only passes 10% of the energy it receives to the next trophic level. The shape of an energy pyramid reflects this. The energy stored in each trophic level, which is the food available to the next trophic, is far less than the amount it takes in. So, the successive layers of the energy pyramid get smaller. Scientists have calculated that, on average, 90% of the energy that each step of the food chain receives is lost to the environment. The energy that enters a population of organisms as food from the preceding trophic level is lost to the living world as heat.

Slide 18

Energy flows through a food chain in one direction only. The only form of energy that can enter the food chain is solar radiation. Green plants are able to trap the energy from the Sun (an abiotic factor) and convert it into food through a process called photosynthesis. The organisms at the next trophic level ingest this energy by eating the plants, but only about 10% of it is stored in them, ready to be accessed by the organisms at the next trophic level. This process is repeated at each trophic level: the organisms only store 10% of the energy that they gain by eating the organisms at the preceding trophic level.
 
Slide 19

The position of an organism in a food chain determines how much energy is available to it at each meal. Energy flows through a food chain from bottom to top, but progressively less energy is available to each trophic layer as it is used up. Some energy is lost to the environment as heat, some is used for life processes such as movement and growth, and more energy is lost to the environment through waste products such as faeces. This last energy source is available to decomposers in another food chain. The energy stored in a trophic level is available to the next trophic level in the form of chemical energy when they eat the organisms at the preceding level. Only about 10% of the chemical energy available at one trophic level is available for the next trophic level. We call this figure of 10% the trophic efficiency of the food chain.
 
Slide 20

The same organism can exist at different levels of a food chain, depending on what it eats. This is particularly true for omnivores like us. If we eat a salad for lunch, then we are eating producers, so we are acting as a primary consumer and are only one trophic level away from the producers. If we eat a burger for lunch, we’re eating a primary consumer in the form of a cow (if it’s a beef burger). Then we’re acting as a secondary consumer, so we’re two trophic levels away from the producers. There’s more food energy available to us, the closer we are in the food chain to the producers, so, in fact, there’s more food energy available to us by eating salad than by eating burgers. Of course, this is tempered by the fact that there are fewer cows than vegetables available.
 
Slide 21

Scientists use trophic levels to categorise organisms into classes that are characterised by the way they feed and transfer energy. Each trophic level provides biomass available for consumption by organisms at the next trophic level. Energy is transferred from one trophic level to the next as food (or chemical) energy. However, this transfer of energy is not efficient. Scientists estimate that only about 10% of the energy received by a trophic level is transferred to the next level. The rest is lost to the environment as heat energy, used for the biological processes of the organisms at the previous trophic level or excreted as waste. Energy enters the ecosystem from abiotic elements such as the Sun and travels in only one direction along the food chain. The trophic levels of each organism determine how efficient it is in gaining energy from the abiotic factors of the ecosystem. Those which are closer to the producers gain more energy from the Sun. Those which are further away gain less energy from the Sun. In the food chain on the right, the shark is at a higher trophic level as an apex predator than the zooplankton, which is a primary consumer.
The zooplankton are more efficient at gaining energy from the Sun by eating the producers than the shark.