Early bird dynamics reduce selection imposed by species interactions in boom-and-bust regimes

Interspecific interactions are a major force shaping evolution, yet the magnitude of their effect varies across ecosystems and interactions. What explains this variability remains underexplored. Here, we propose that the temporal dynamics of growth and resources are a key determinant. We study how the degree to which species interactions alter selection forces depends on a system's growth regime, ranging from steady chemostat-like growth to boom-and-bust cycles. Using consumer-resource simulations together with analytical approximations, we show that selection in boom-and-bust cycles is dominated by an early bird effect: because most generations elapse early in a growth cycle, when populations are small and resources are abundant, selection is concentrated in a period where species interactions are relatively weak. As a result, boom-and-bust dynamics diminish the degree to which growth rate reducing interactions alter selection. In contrast, interactions that act through mortality, such as the secretion of lethal toxins, operate regardless of growth phase and are therefore not reduced, and can even be amplified in boom-and-bust dynamics. These findings help contextualize estimates of the evolutionary effects of species interactions across experimental systems, and suggest that the growth regime is a coarse-grained, measurable feature that governs when biotic interactions leave a strong evolutionary mark.