Cyclic pathogen epidemics favour the evolution of delayed germination

Abstract

Delayed germination is classically explained as a bet-hedging strategy against environmental variability: by withholding a fraction of seeds from germination, plants spread establishment risk across years that vary in temperature, precipitation, and other conditions governing seedling survival. Whether biotic interactions can generate analogous selective pressure has received comparatively little theoretical attention. Here we examine the evolution of germination timing in a host-pathogen model in which a saturating (Holling Type II) transmission function couples seed bank dynamics to epidemic cycles. The model produces three qualitatively distinct ecological regimes: a pathogen-free equilibrium, a stable endemic equilibrium, and an oscillatory endemic regime arising through a bifurcation, a transition from stable to oscillatory endemic dynamics. Using adaptive dynamics, namely invasion analysis, we show that the evolutionarily stable germination rate depends critically on which regime the resident population occupies. When pathogen dynamics settle to an equilibrium, selection favours ever-faster germination with no finite optimum, regardless of pathogen presence. When dynamics are limit cycles, periodic epidemic peaks create recurrent windows of high establishment mortality that function as biotic analogues of abiotic interactions (environmental), and selection drives the germination rate toward the bifurcation boundary. The evolutionary attractor thus coincides with an ecological bifurcation point. Trait substitution sequence simulations confirm convergence to this attractor, and multi-trait eco-evolutionary simulations provide evidence that the attractor is evolutionarily stable, consistent with a continuously stable strategy (CSS).These results extend classical bet-hedging theory to biotic drivers and suggest that pathogens capable of sustaining population cycles may be an underappreciated selective force on germination timing and, more broadly, on the pace of life-history evolution.