Exergy is a measure of both a system and its environment, more specifically a measure of how a system departs from its environment. Here exergy is a measure of the amount of work that can be done before the system comes to equilibrium with its surrounding. As such exergy is a measurement of the potential for “usefulness”. Exergy is of central interest within non-­equilibrium thermodynamics and it is often used in the analysis of an ecosystem’s resilience; when exergy is zero the system is in equilibrium with its environment, which would equate to the complete collapse of an ecology.