What Is Demographic Lability and When Might We Expect to See It?

AbstractWhen vital rates are convex functions of environmental drivers, temporal variation in those vital rates could increase long-term stochastic fitness (so-called demographic lability). Yet no empirical cases of this phenomenon have yet been documented. We first outline three necessary steps to document lability: estimate how vital rates change with environmental drivers, quantify driver distributions, and compare the fitness effects of variation to a "no-variation" baseline driver value (typically its mean). We then review articles that presented evidence for lability and find that none fully documented it. In addition, we examine for the first time when natural selection would produce adaptive lability de novo, rather than other adaptations to stochastic environments, and we suggest that selection to better exploit the most frequent environmental states may often erode lability. Finally, we consider conditions (including life history "speed," shape of vital rate/environment relationships, and type of environmental driver) that might support lability. We argue that lability is less likely in response to abiotic than biotic drivers but question whether fast and slow life histories differ in their propensity for lability. Our principal aim is to suggest research directions that would put the intriguing idea of demographic lability on a firmer foundation.