Abstract: Irreversibility is a fundamental feature of life, manifested in the asymmetry of population dynamics when played forward versus played backward in time. Despite its early recognition in ecology, irreversibility has largely remained a high-level, unquantifiable concept. Here, we present a new formalism rooted in non-equilibrium statistical physics to quantify irreversibility in general ecological systems and identify the factors that drive it. Our findings show that irreversibility only emerges in ecological systems with non-neutral and non-linear dynamics. We further found that system energy, such as resource availability and external disturbance, is a key determinant of irreversibility in multiple canonical ecological models. We then applied this framework to empirical datasets to explore the roles of body mass, rapid evolution, and temperature in driving irreversibility. Our study provides a rigorous formalism on quantifying irreversibility in ecological systems, with the potential to serve as a `common currency' for exploring the intricate relationship between energy and information.
