Woods Hole Oceanographic Institution Woods Hole, Massachusetts, United States
Background/Question/Methods Individuals of any species will vary in the fates they experience across their lifetime. Some will live long lives; others short ones. Some will produce many offspring; others will produce few, and many will have none. When individuals are identical in the rates of growth, survival, and fertility they experience over their lifetime, variation in life history outcomes such as longevity and lifetime reproductive output will be due to luck (individual stochasticity). When individuals are heterogeneous in the rates they experience at any stage of the life cycle, such individual heterogeneity will contribute to variation among individuals alongside luck. Questions on the importance of luck in the life cycle, and its role in causing variation and uncertainty among individuals in life history outcomes are of interest in the context of the ecological dynamics and evolutionary fate of populations. Using a simple theoretical stage-structured life history model and the Markov chain with rewards framework for quantifying stochasticity, I will show how much variation and uncertainty (as measured by skewness and kurtosis) among individuals is possible for different configurations of life history rates. I will furthermore quantify the stages and/or rates where it is most crucial to be lucky.
Results/Conclusions The two-stage Juvenile-Adult model is a basic theoretical model for life histories. The model already provides several life history scenarios: development of juveniles into adults can be fast or slow (i.e., precocious / delayed maturation), and adults may have few or many opportunities to reproduce (semelparity / iteroparity). Results show that variance in both lifetime reproduction and longevity depends on adult survival, and is higher when adults are iteroparous. Measures of uncertainty in individual fates, however, depend strongly on whether or not maturation is delayed in juveniles. Luck in survival contributes most of the variance in these scenarios; only in life cycles with precocious juveniles and semelparous adults does luck in fertility play a substantial role. Treating the different life history configurations as heterogeneous strategies within a population shows a large contribution of individual stochasticity, and a contribution of approximately 13% by heterogeneity. Although we need more analyses of stochasticity and heterogeneity within life cycles for a wide array of species in the wild, representing different types of life histories, the use of theoretical models can elucidate general patterns and associations, such as the decoupling of drivers for mean and variance in outcomes from drivers of life cycle uncertainty.