Cyclical environments drive variation in life history strategies: a general theory of cyclical phenology

Cycles, such as seasons or tides, characterize many systems in nature. Overwhelming evidence shows that climate change-driven alterations to environmental cycles--such as longer seasons-- are associated with phenological shifts around the world, suggesting a deep link between environmental cycles and life cycles. However, general mechanisms of life history evolution in cyclical environments are still not well understood. Here I build a demographic framework and ask how life history strategies optimize fitness when the environment perturbs a structured population cyclically, and how strategies should change as cyclicality changes. I show that cycle periodicity alters optimality predictions of classic life history theory because repeated cycles have rippling selective consequences over time and generations. Notably, fitness landscapes that relate environmental cyclicality and life history optimality vary dramatically depending on which trade-offs govern a given species. The model tuned with known life history trade-offs in a marine intertidal copepod T. californicus successfully predicted the shape of life history variation across natural populations spanning a gradient of tidal periodicities. This framework shows how environmental cycles can drive life history variation--without complex assumptions of individual responses to cues such as temperature--thus expanding the range of life history diversity explained by theory and providing a basis for adaptive phenology.