“Pluck or luck: does trait variation or chance drive variation in lifetime reproductive success?”
Robin E. Snyder and Stephen P. Ellner (Apr 2018)
Lifetime reproductive success is determined largely by luck, not fitness-enhancing traits
What makes highly successful individuals (those living a long time or producing many offspring over their lives) successful? How much of it is due to being special (having traits associated with high fitness) and how much of it is due to being lucky? Similarly, if we observe a highly successful individual, what can we conclude about their likely traits and with what confidence? Snyder and Ellner show how to define and answer these questions mathematically and apply their analyses to a series of case studies and models of varying complexity. For reasonable parameter choices, they find that most of success is about luck, not having high-fitness traits. Along the same line, having high-fitness traits is often necessary but not sufficient for being highly successful—an individual also needs to be lucky. These findings leave evolutionary dynamics intact—beneficial traits will spread by natural selection in populations large enough for luck to average out. However, when population-level ecological outcomes (such as birth rate or prey capture rate) are dominated by luck, within-population trait variation may be difficult to detect and may not contribute much to explaining ecological patterns.
While there has been extensive interest in how intraspecific trait variation affects ecological processes, outcomes are highly variable even when individuals are identical: some are lucky while others are not. Trait variation is therefore only important if it adds substantially to the variability produced by luck. We ask when trait variation has a substantial effect on variability in lifetime reproductive success (LRS), using two approaches: 1) we partition the variation in LRS into contributions from luck and trait variation; 2) we ask what can be inferred about an individual's traits, and with what certainty, given their observed LRS. In theoretical stage- and size-structured models, and two empirical case studies, we find that luck usually dominates the variance of LRS. Even when individuals differ substantially in ways that affect expected LRS, unless the effects of luck are substantially reduced (e.g. low variability in reproductive lifespan or in annual fecundity), most variance in lifetime outcomes is due to luck, implying that departures from “null” models omitting trait variation will be hard to detect. Luck also obscures the relationship between realized LRS and individual traits. While trait variation may influence the fate of populations, luck often governs the lives of individuals.