Energy Losses (DP IB Biology)

Energy Losses between Trophic Levels

• When a consumer ingests another organism not all the chemical energy in the consumer’s food is transferred to the consumer's tissues

  • Only around 10 % of the energy is available to the consumer to store in their tissues

  • This is because around 90 % of the energy is lost to the environment

• Around 90 % of the energy is lost to the environment because

  • Not every part of the food organism is eaten

    • E.g. the roots and woody parts of plants or the bones of animals, meaning that the chemical energy these uneaten tissues contain is lost to the environment

  • Consumers are not able to digest all of the food they ingest

    • E.g. cellulose in plants, or the fur of animals, so some is egested as faeces; the chemical energy in this undigested food is also lost to the environment

  • Energy is lost to the environment in the form of heat when consumers respire

  • Energy is lost to the environment when organisms excrete the waste products of metabolism

    • E.g. urea in urine

  • Not all individual organisms are consumed; some die without being consumed, and their bodies decompose

• The energy that is left after these losses is available to the consumer to fuel their life functions, including being stored in carbon compounds in their tissues during growth

Energy losses diagram

Energy is lost from the food chain as heat during respiration, due to incomplete digestion, and through excretion of the waste products of metabolism. Remaining energy fuels the organism’s life processes or is stored in carbon compounds in the tissues.

The role of decay organisms in energy loss

• Note that while detritivores and saprotrophs are not considered to be part of a food chain, they have a role to play in the loss of energy from food chains:

  • They decompose the parts of organisms that are not eaten, e.g. the bones and teeth of dead prey animals

  • They break down undigested waste material

  • They decompose the bodies of any organisms that die and are not consumed

Heat Loss to the Environment

• The transfer of energy in food chains is not 100 % efficient

• Heat is lost to the environment during cellular respiration and when ATP is used in other cellular processes

  • This applies in producers, consumers, detritivores, and saprotrophs

• This heat energy is lost to the environment at every trophic level, and during decomposition

  • Heat loss occurs by the process of radiation

Energy lost as heat diagram

Heat energy released during respiration and other cellular processes is lost to the environment at every stage of the food chain

Number of Trophic Levels & Energy Loss

• Food chains are limited in length

  • Food chains rarely have more than around four or five trophic levels; this is because with energy losses at each trophic level, there is less and less energy available to the consumer as you go up the food chain

  • When a food chain gets longer than four or five trophic levels it becomes too difficult for a predator to hunt enough prey to gain the energy to survive

• Biomass decreases with each trophic level, e.g. due to fewer individuals being present, or due to individuals being smaller

  • Because only around 10 % of the energy stored in a producer's tissues is available to a primary consumer, primary consumers needs to consume a lot of plant biomass to gain enough energy to survive

  • Again, only around 10 % of the energy stored in a primary consumer's tissues is available to a secondary consumer, meaning that secondary consumers need to consume a lot of prey biomass to gain enough energy to survive

• This leads to a large reduction in biomass at each trophic level and means that when represented in terms of biomass, food chains have a pyramid structure

  • Note that while the biomass decreases at each trophic level of a food chain, the energy stored in that biomass per unit of mass does not change

Pyramid of biomass diagram

The biomass at each trophic level of a food chain can be represented as a pyramid of biomass. The pyramid shape results from the energy losses at each trophic level.