The role of developmental plasticity in eco‐phenotypic population dynamics

Abstract Individual variation in development can strongly impact population density via its impact on demography. When this variation in development is caused by developmental plasticity, the resulting effects on population density are characterised as ‘eco‐phenotypic dynamics’ If plasticity in turn is sensitive to variation in population density, ‘eco‐phenotypic feedbacks’ can arise. When individual development is density‐dependent due to resource limitation, we expect eco‐phenotypic feedbacks between development rate and population density to be widespread. Yet, exactly how plasticity in development rates, termed ‘heterokairy’ interacts with population density to influence eco‐phenotypic dynamics is unclear. Here, we provide conceptual and empirical starting points to assess the role of heterokairy in eco‐phenotypic population dynamics. We present a conceptual framework to identify how heterokairy impacts, and is impacted by, population density. This framework is validated using existing empirical evidence, focusing on polyphenisms that comprise alternate phenotypes that differ in development rate: ‘heterokairic polyphenisms’. We find that heterokairic polyphenisms typically comprise fast developing (‘minor’) and slow developing (‘major’) phenotypes that are prevalent among ectotherms. How heterokairy shapes eco‐phenotypic feedbacks depends on whether minors (or majors) are mostly expressed at low (or high) density, or vice versa. Minor‐at‐low‐density systems, such as those found in salamanders, aphids and nematodes, reflect classic life history predictions that growing, low density, populations favour short development times, whilst high‐density populations favour slower development. Alternatively, minor‐at‐high‐density systems allow resource‐stressed juveniles, most common at high‐densities, to still mature but as minors by reducing developmental and metabolic costs. We propose such opposing responses to density can prompt different eco‐phenotypic dynamics and feedbacks. Our work highlights the crucial role that development plays in phenotype construction, and the consequences it can have for (meta)population dynamics and phenotype evolution. We call for a shift away from a focus on developmental outcomes in adulthood to the developmental processes that construct phenotypes. Ultimately, this can lead to a more mechanistic, and thus predictive, approach to understanding the adaptive role of alternative paces of development within ecological and evolutionary contexts. Read the free Plain Language Summary for this article on the Journal blog.