Bet-hedging against larval herbivory and seed bank mortality in the evolution of heterocarpy

Bet-hedging strategies maximize long-term geometric fitness at the cost of reduced arithmetic fitness by offsetting different mortality risks. Heterocarpic systems accomplish bet-hedging through the production of two or more fruit types that vary in dormancy and dispersal ability. It is unknown whether heterocarpy also offsets predispersal mortality risks. To address this question, we investigated whether heterocarpy in Grindelia ciliata (Asteraceae) also offsets mortality risks posed by a seed predator Schinia mortua (Noctuidae) to increase plant fitness.

We conducted two manipulative experiments to quantify critical life history components of this plant-insect interaction. We measured predispersal achene mortality from herbivory, postdispersal achene mortality in the seed bank, and seedling emergence. These measurements were then used in deterministic models to evaluate evolutionary consequences of predispersal seed mortality in G. ciliata.

Dormant achene types were less vulnerable to herbivory but more susceptible to mortality in the seed bank due to delayed seed emergence. Nondormant achene types experienced high predispersal mortality but low seed bank mortality due to rapid germination. Our herbivore-dependent model improved fit between observed and expected proportions of dormant and nondormant G. ciliata achenes and showed that heterocarpy could evolve in the absence of postgermination mortality.

Our study provides empirical support of how predispersal herbivory can be equally important to postdispersal seed mortality risks in the evolution and maintenance of a heterocarpic reproductive system and expands understanding of how bet-hedging theory can be used to understand this unique reproductive strategy.