Energy Flow in Ecosystems (DP IB Environmental Systems & Societies (ESS)): Revision Note

Energy flow in ecosystems

• Ecosystems rely on a constant supply of energy and matter to maintain their structure and function

  • Energy is essential for driving biological processes, while matter cycles through the ecosystem, being reused and recycled

• Ecosystems are considered open systems, meaning they exchange both energy and matter with their surroundings

  • Energy enters ecosystems primarily from the sun, entering as sunlight and being converted into chemical energy by producers through photosynthesis

    • This energy is then transferred between trophic levels as organisms consume one another, with some energy lost as heat at each transfer

    • Decomposers break down organic matter, releasing energy and returning nutrients to the environment

  • Matter, such as nutrients and water, flows into and out of ecosystems through various processes like decomposition, nutrient cycling and precipitation

The first law of thermodynamics

• Energy exists in many different forms, including light energy, heat energy, chemical energy, electrical energy and kinetic energy

• The way in which energy behaves within systems can be explained by the laws of thermodynamics

  • There are two laws of thermodynamics

• The first law of thermodynamics is as follows:

Energy can neither be created nor destroyed, it can only be transformed from one form to another

• This is also known as the principle of conservation of energy

  • It means that the energy entering a system equals the energy leaving it

  • It means that as energy flows through ecosystems, it can only change from one form to another

• The transfer of energy in food chains within ecosystems demonstrates the principle of conservation of energy:

  • Energy enters the system (the food chain or food web) in the form of sunlight

  • Producers convert this light energy into biomass (stored chemical energy) via photosynthesis

  • This chemical energy is passed along the food chain, via consumers, as biomass

  • All energy ultimately leaves the food chain, food web or ecosystem as heat energy

The second law of thermodynamics

• The second law of thermodynamics states that:

Energy transfers in ecosystems are inefficient

• This is because energy transfers in any system are never 100% efficient

• The second law of thermodynamics explains the decrease in available energy within ecosystems:

  • In a food chain, energy is transformed from a more concentrated (ordered) form (e.g. light energy from the Sun), into a more dispersed or disordered form (heat energy lost by organisms)

  • Initially, light energy from the Sun is absorbed by producers

    • However, even at this initial stage, energy absorption and transfer by producers is inefficient

    • This is due to reflection, transmission (light passing through leaves) and inefficient energy transfer during photosynthesis

  • The energy that is converted to plant biomass is then inefficiently transferred along the food chain due to respiration and the production of waste heat energy

    • In ecosystems, the biggest losses occur during cellular respiration

    • When energy is transformed, some must be degraded into a less useful form, such as heat

  • As a result of these inefficient energy transfers, food chains are often short (they rarely contain more than five trophic levels)