Demographic outputs and their variances for three life history complexes for the Southern Fulmar across contrasted sea ice conditions.

Individuals differ in many ways. Most produce few offspring; a handful produce many. Some die early; others live to old age. It is tempting to attribute these differences in outcomes to differences in individual traits, and thus in the demographic rates experienced. However, there is more to individual variation than meets the eye of the biologist. Even among individuals sharing identical traits, life history outcomes (life expectancy and lifetime reproduction) will vary due to individual stochasticity, i.e., to chance. Quantifying the contributions of heterogeneity and chance is essential to understanding natural variability. Inter-individual differences vary across environmental conditions, hence heterogeneity and stochasticity depend on environmental conditions. We show that favorable conditions increase the contributions of individual stochasticity, and reduce the contributions of heterogeneity, to variance in demographic outcomes in a seabird population. The opposite is true under poor conditions. This result has important consequence for understanding the ecology and evolution of life history strategies. Specifically, three life-history complexes exist in a population of southern fulmar (defined as sets of life-history characteristics that occur together through the lifetime of an individual). They are reminiscent of the gradient of life- history strategy observed among species: 1. Group 1 (14% of offspring at fledging) is a slow-paced life history where individuals tend to delay recruitment, recruit successfully, and extend their reproductive lifespan. 2. Group 2 (67% of offspring at fledging) consists of individuals that are less likely to recruit, have high adult survival, and skip breeding often. 3. Group 3 (19% of offspring at fledging) is a fast-paced life history where individuals recruit early and attempt to breed often but have a short lifespan. Individuals in groups 1 and 3 are considered “high-quality” individuals because they produce, on average, more offspring over their lives than do individuals in group 2. But group 2 is made-up of individuals that experience the highest levels of adult survival. Differences between these groups, i.e. individual heterogeneity, only explains a small fraction of variance in life expectancy (5.9%) and lifetime reproduction (22%) when environmental conditions are ordinary. We expect that the environmental context experienced, especially when environmental conditions get extreme, is key to characterizing individual heterogeneity and its contribution to life history outcomes. Here, we build on previous studies to quantify the impact of extreme environmental conditions on the relative contributions of individual heterogeneity and stochasticity to variance in life history outcomes. We found that the differences in vital rates and demographic outcomes among complexes depend on the sea ice conditions individuals experience. Importantly, differences across life history complexes are amplified when sea ice concentration get extremely low. Sea ice conditions did not only affect patterns of life history traits, but also the variance of life history outcomes and the relative proportion of individual unobserved heterogeneity to the total variance. These new results advance the current debate on the relative importance heterogeneity (i.e. potentially adaptive) and stochasticity (i.e. enhances genetic drift) in shaping potentially neutral vs. adaptive changes in life histories.