Demystifying individual heterogeneity

• Published in: Ecology letters 2021-10, Vol.24 (10), p.2282-2297
• Main Authors: Forsythe, Amy B., Day, Troy, Nelson, William A., Coulson, Tim
• Format: Article
• Language: English
• Subjects: dynamic condition; fixed condition; Genetics; Heterogeneity; individual heterogeneity; individual stochasticity; life‐history theory; Mathematical analysis; Mathematical models; Population dynamics; Population studies; Quantitative genetics; Statistical analysis; Statistical models; Terminology; vital rates
• ISSN: 1461-023X; 1461-0248
• DOI: 10.1111/ele.13843

Among‐individual variation in vital rates, such as mortality and birth rates, exists in nearly all populations. Recent studies suggest that this individual heterogeneity produces substantial life‐history and fitness differences among individuals, which in turn scale up to influence population dynamics. However, our ability to understand the consequences of individual heterogeneity is limited by inconsistencies across conceptual frameworks in the field. Studies of individual heterogeneity remain filled with contradicting and ambiguous terminology that introduces risks of misunderstandings, conflicting models and unreliable conclusions. Here, we synthesise the existing literature into a single and comparatively straightforward framework with explicit terminology and definitions. This work introduces a distinction between potential vital rates and realised vital rates to develop a coherent framework that maps directly onto mathematical models of individual heterogeneity. We suggest the terms “fixed condition” and “dynamic condition” be used to distinguish potential vital rates that are permanent from those that can change throughout an individual's life. To illustrate, we connect the framework to quantitative genetics models and to common classes of statistical models used to infer individual heterogeneity. We also develop a population projection matrix model that provides an example of how our definitions are translated into precise quantitative terms. Extensive research shows that individual heterogeneity exists across a wide variety of species and environments, yet our understanding is still limited by inconsistencies across conceptual frameworks in the field. Here, we synthesise the existing literature into a single and comparatively straightforward framework with explicit terminology and definitions. This integrative framework is also intended to bridge theoretical models and empirical studies.