Professor Cornell University Ithaca, NY, New York, United States
Background/Question/Methods How much is lifetime success (lifetime reproductive output, lifespan,...) driven by fixed traits, how much by external factors, and how much by random chance? And how does this balance change across an individual's life? We and others have found that trait variation contributes at most a third of the variance in success, but this leads to more questions. Is luck dominant at every age, or are traits sometimes more important? Can we relate the relative importance of luck in survival, growth, and environment to life history? Are there generally vulnerable periods when luck in environmental conditions and in demographic transitions are both important, or do the critical periods for environmental and demographic luck differ? We present a partitioning of the variance of individual lifetime success into contributions from fixed trait variation; four forms of “demographic luck” (luck in birth state, fecundity, survival, and growth) and two kinds of “environmental luck” (birth environment and environment trajectory). Each of these is further partitioned into contributions at different ages. We demonstrate our approach with four case studies with contrasting life histories.
Results/Conclusions Species that have slow, relatively deterministic growth (e.g. trees) tend to be driven by luck in survival, while those with more labile growth (e.g. shrubs) can be driven by luck in growth for much of their lives. Environment trajectory luck can be important for species that depend on disturbance. However, traits account for less variance in success than demographic or environmental luck at all ages. Luck of all forms tends to be important near birth because you can’t be successful if you die young. Luck of one form or another often peaks a little before or a little after individuals typically first become successful, but the details are different for different forms of luck.