Significant ideas:

• The laws of thermodynamics govern the flow of energy in a system and the ability to do work.

• Systems can exist in alternative stable states or as equilibria between which there are tipping points.

• Destabilizing positive feedback mechanisms will drive systems towards these tipping points, whereas stabilizing negative feedback mechanisms will resist such changes.

• The first law of thermodynamics is the principle of conservation of energy, which states that energy in an isolated system can be transformed but cannot be created or destroyed.

  • The principle of conservation of energy can be modelled by the energy transformations along food chains and energy production systems.

• The second law of thermodynamics states that the entropy of a system increases over time. Entropy is a measure of the amount of disorder in a system. An increase in entropy arising from energy transformations reduces the energy available to do work.

  • The second law of thermodynamics explains the inefficiency and decrease in available energy along a food chain and energy generation systems.

• As an open system, an ecosystem will normally exist in a stable equilibrium, either in a steady-state equilibrium or in one developing over time (for example, succession), and maintained by stabilizing negative feedback loops.

• Negative feedback loops (stabilizing) occur when the output of a process inhibits or reverses the operation of the same process in such a way as to reduce change—it counteracts deviation.

• Positive feedback loops (destabilizing) will tend to amplify changes and drive the system towards a tipping point where a new equilibrium is adopted.

• The resilience of a system, ecological or social, refers to its tendency to avoid such tipping points and maintain stability.

• Diversity and the size of storages within systems can contribute to their resilience and affect their speed of response to change (time lags).

• Humans can affect the resilience of systems through reducing these storages and diversity.

• The delays involved in feedback loops make it difficult to predict tipping points and add to the complexity of modelling systems.