COS 177-2 - Live long if you want to prosper: partitioning reproductive skew into contributions from fixed traits, external factors, and random chance.

Abstract: In many species, the distribution of lifetime reproductive success (LRO) is highly right-skewed: a few individuals produce most of next generation. How much of this skew is driven by variation among individuals in fixed traits, how much by variation in external factors, and how much by random chance? How does this balance change across an individual's life? And what does it take to be truly exceptional, far out in the right tail of the LRO distribution?

In the past, we and others have partitioned the variance of LRO as a proxy for reproductive skew. Here we explain how to partition LRO skewness itself into contributions from fixed trait variation; four forms of “demographic luck” (birth state luck, fecundity luck, survival trajectory luck, and growth trajectory luck); and two kinds of “environmental luck” (birth environment luck and environment trajectory luck). Each of these is further partitioned into contributions at different ages. We also determine what it takes to be in, e.g., the top 1% of LRO, by partitioning the variance in a "success" variable that is one if an individual reaches the cutoff LRO and zero otherwise.

We demonstrate our approach with case studies having contrasting life histories. We find that fixed trait variation generally contributes less than half: like LRO variance, LRO skew is largely driven by luck. But while LRO variance can be driven by different forms of luck depending on life history, LRO skew is always dominated by survival luck in our case studies. In particular, we find that living long is necessary, but not sufficient, for having exceptionally many offspring. Finally, preliminary results suggest that traits become even less important, and survival luck more important, for individuals with exceptionally high LRO.