  Collectives form non-equilibrium social structures characterised by a
volatile dynamics. Individuals join or leave. Social relations change quickly.
Therefore, differently from engineered or ecological systems, a resilient
reference state cannot be defined. We propose a novel resilience measure
combining two dimensions: robustness and adaptivity. We demonstrate how they
can be quantified using data from a software developer collective. Our analysis
reveals a resilience life cycle, i.e., stages of increasing resilience are
followed by stages of decreasing resilience. We explain the reasons for these
observed dynamics and provide a formal model to reproduce them. The resilience
life cycle allows distinguishing between short-term resilience, given by a
sequence of resilient states, and long-term resilience, which requires
collectives to survive through different cycles.
