Gambling in the desert: Bet hedging in Sonoran Desert winter annual plants
In the face of environmental uncertainty, organisms must have strategies to maximize fitness as conditions change. These strategies fall along a spectrum from adaptive tracking to bet hedging. In bet hedging, organisms sacrifice mean fitness in order to reduce variance in fitness, thus maximizing fitness in a variable environment. Delayed germination is a classic example of biological bet hedging, in which a fraction of a plant’s seeds remain dormant as a hedge against the risk of reproductive failure. Plants can delay germination over two different time scales: among years (or growing seasons) and within years. While most studies have focused on among-year germination delays, the timing of germination within seasons can also be critical in highly variable environments. Here we use 30 years of demographic data to test whether among-year germination timing acts as a bet hedging strategy, investigate relationships between among- and within-year germination delays, and determine the fitness consequences of germination timing for 12 species of Sonoran Desert winter annual plants.
Our results indicate that among-year delayed germination meets the criteria as a bet hedging strategy for all 12 winter annual species, and that competition significantly affects the adaptive value of among-year germination delay. Patterns of within-year germination timing varied among species, which was significantly related to among-year germination delays. Species with high germination fractions tend to germinate early in the season and vice versa, and differences in observed within-year germination timing among species is adaptive. Our results highlight that understanding the evolution of germination timing in variable environments requires integrating the effects of both among- and within-season germination timing and that multiple strategies may evolve in response to environmental variability.