Individual and environmental variability in a population of long-lived marine mammals

Abstract

Environmental fluctuations and individual heterogeneity are two interacting sources of population variation affecting demographic and evolutionary processes. While environmental variation has traditionally been at the core of population ecology, individual heterogeneity has received relatively little attention. Using long-term mark-recapture data, I investigated some specifics of environmental and individual variation in a population of Weddell seals. First, I assessed the impact of an extreme environmental perturbation (occurrence of gigantic icebergs in the vicinity of seal breeding area) on colony attendance, survival, and reproductive rates of females. As expected for long-lived species, survival rates were unaffected by the perturbation. Reproductive rates dropped by ~40% during this iceberg event but rapidly re-increased afterwards. The seal population thus appeared to be resilient and relatively unaffected by this isolated perturbing event, but if such events were to occur too frequently, the effects on reproduction could have important demographic impacts. Second, I showed that individual heterogeneity was an important source of variation in female reproductive rates. Accounting for this heterogeneity allowed detection for the first time of an important cost of reproduction on future breeding probability. This reproductive cost was expressed as a 10% higher probability of reproduction a given year if a female skipped reproduction the previous year. Further, I found that individual differences were relatively consistent over time, although their expression seemed slightly amplified in disturbed conditions. Finally, I addressed the evolutionary consequences of this individual heterogeneity in female reproductive rates. Females reproducing at high frequency produced female offspring with average chances of recruitment, suggesting no trade-off between progeny number and quality. Using simulations, I showed that such females with high reproductive rates were consequently expected to produce more grandchildren through their female progeny and thus potentially have higher fitness, although the contribution of male progeny could not be investigated here. Female reproductive rates also displayed some heritability, although very low, as expected for a fitness-related trait. Taken together, these results suggest that, in this population, traits associated with the observed individual reproductive heterogeneity could be under natural selection.