A steady-state equilibrium is a type of stable equilibrium in which storages remain approximately constant because inputs and outputs are balanced. A system can still be in stable equilibrium while changing gradually over time, provided stabilizing processes maintain its overall stability.
This is core IB ESS Topic 1.2 Systems content for both SL and HL.
| Concept | Meaning | Example |
|---|---|---|
| Stable equilibrium | The system tends to maintain stability or return toward equilibrium after a disturbance. Its condition may remain constant or develop gradually over time. | During ecological succession, species composition and soil depth change, but negative feedback can maintain an organized, resilient ecosystem. |
| Steady-state equilibrium | Inputs and outputs continue, but their average rates are approximately equal, so storages remain broadly constant over time. | Lake Nasser can be modelled over a selected period as a water storage receiving river inflow while losing water through discharge and evaporation. |
In a steady-state system:
Therefore, there is little net change in the storage. However, matter and energy still flow through the system. For example, water may enter and leave a lake continuously even when its average volume remains similar.
Stable equilibrium is the broader idea. Negative feedback loops counteract disturbances and reduce change, helping the system avoid a tipping point. A forest damaged by a moderate storm may regenerate toward its previous structure; if disturbance overwhelms its resilience, however, it may shift to a different equilibrium.
A common misconception is that equilibrium means “nothing changes.” In ESS, ecosystems are open systems, so flows and transformations continue. Steady-state equilibrium is dynamic, not motionless.
Exam technique: If asked to distinguish the terms, state that steady-state equilibrium requires approximately balanced inputs and outputs and constant storages. Then explain that stable equilibrium concerns the system's overall tendency to maintain or regain stability, including systems developing over time. Use systems terminology and a relevant example.
