Germination variation facilitates the evolution of seed dormancy when coupled with seedling competition

Abstract

Fluctuating environmental conditions have consequences for the evolution of life histories because they cause fitness variance. This variance can favor risk-spreading strategies, often known as bet-hedging strategies, in which growth or reproduction is spread over time or space, with some costs, but greater certainty of success. An important example is seed dormancy in annual plants, in which some fraction of seed remains dormant at any given germination opportunity with the potential of germinating later when environmental conditions may differ. Previous theory shows that environmental variation is critical for the evolution of dormancy. However, these studies have focused on temporal variation in reproduction, ignoring the strong observed effects of environmental variation on the germination fraction, a major contributor to fitness variance. We ask what effects germination fluctuations have on selection for dormancy by adding germination fluctuations to existing density-independent (d.i.) and density-dependent (d.d.) models of annual plant dynamics, extending previous analyses by including temporally fluctuating germination. Specifically, we ask how germination variance affects selection on the temporal average germination fraction, here used to define dormancy. When present alone, or when independently varying with other fitness components, germination fluctuations do not affect selection for dormancy in the d.i. model, despite generating fitness variance because this variance contribution is not reduced by higher dormancy. Germination fluctuations have strong effects in the d.d. model, favoring dormancy when present either alone or coupled with variation affecting plant growth. This is because germination variation causes seedling density to vary, which causes variable reproduction through variable intraspecific competition. Dormancy is advantaged under variable reproduction because it creates a more convex relationship between population growth and reproduction leading to benefits from nonlinear averaging. Predictive germination, a positive statistical association between germination and growth, weakens selection for dormancy under strong competition and strengthens it when competition is weak. Our results suggest that variable germination is a potential explanation for high levels of dormancy observed in nature, with implications for life-history theory for fluctuating environments.