Time and Chance: Using Age Partitioning to Understand How Luck Drives Variation in Reproductive Success.

Area of Science:

• Evolutionary Biology
• Population Ecology
• Demography

Background:

• Individual traits often explain less variation in lifetime reproductive output (LRO) than luck.
• Understanding the interplay between luck and traits across the life cycle is key to population dynamics.

Purpose of the Study:

• To partition age-specific contributions of luck and trait variation to LRO variance.
• To quantify distinct components of luck and their demographic impact.
• To identify life stages where luck is most influential.

Main Methods:

• Developed a framework to partition LRO variance into luck and trait components by age.
• Quantified three specific components of luck.
• Applied the framework to empirical case studies.

Main Results:

• Luck consistently overwhelms trait variation effects on LRO across all ages.
• Randomness in individual state dynamics ('state trajectory luck') is the primary driver.
• Early-life luck is most critical, often determining initial reproductive opportunity.

Conclusions:

• Luck, particularly early-life state trajectory luck, is the dominant factor in LRO variation.
• The importance of luck often peaks early in life or shows a bimodal distribution.
• Peak luck periods may represent critical targets for population interventions.