Resistance of environmental systems is their capacity to withstand or absorb force or disturbance with minimal change. In many cases we can measure it based on, e.g., strength or absorptive capacity. Resilience is the ability of a system to recover after a disturbance or applied force to (or toward) its pre-disturbance condition—in many cases a function of dynamical stability. In my classes I illustrate the difference by comparing a steel bar and a rubber band. The steel bar has high resistance and low resilience—you have to apply a great deal of force to bend it, but once bent it stays bent. A rubber band has low resistance and high resilience—it is easily broken, but after any application of force short of the breaking point, it snaps back to its original state.

These definitions are broadly consistent in use in Earth sciences, systems theory, and engineering. Unfortunately (at least for interdisciplinary communication), the term is used differently in ecology. Ecologists often use resilience in a way that overlaps with resistance as defined above, defining it as the amount of disturbance that an ecosystem could withstand without changing self-organized processes and structures (defined as alternative stable states). This follows C.H. Holling, who in 1973 popularized ecological resilience (though apparently Elton used the term in ecology in 1958), and termed resilience as I defined it above “engineering resilience”. It didn’t help that Holling defined stability as something separate from resilience, when by some definitions dynamical stability is an indicator of resilience.

It gets even more confusing when you consider that a few ecologists use something like my definition of resilience above, and more confusing yet when you account for the varied way the term is used in the literature of, e.g., environmental management and policy, natural hazards, political ecology, sustainability studies, etc.  In fact the term seems to generate a lot of debate in those fields, though more about implications and interpretations of resilience than its definition.

My introduction to resilience concepts was via geosciences and systems theory. Thus it came as a surprise to me when some geomorphology colleagues reported that, when using the term resilience as dynamical stability, they ran afoul of ecologists in the audience, who argued that they were using the term incorrectly. They weren’t, of course—they just weren’t using it the same way that ecologists often do.

There is unlikely to be any agreement across the sciences on a single, unified definition. Thus, the best we can do is define explicitly what we mean when we use the term.