Bet hedging against pre- and post-dispersal mortality risks in heterocarpic Grindelia ciliata (Asteraceae)
Bet-hedging strategies maximize long term mean geometric fitness at the cost of reduced mean arithmetic fitness. Heterocarpy is a bet-hedging strategy that offsets post-dispersal offspring mortality risks by producing two or more fruit types that vary in dormancy and dispersal ability. It is not known if heterocarpy can also offset pre-dispersal mortality risks. We investigated heterocarpy in Grindelia ciliata (Asteraceae) to test if it also offsets pre-dispersal offspring mortality risks posed by the seed predator Schinia mortua(Noctuidae). We conducted two manipulative experiments from which vital rates including pre-dispersal achene mortality, post-dispersal seed bank mortality, and seed emergence were modeled in stochastic population growth simulations to calculate optimal achene type allocation strategies in simulated environments.
Dormant achenes persisted for over one year in the seed-bank and were less vulnerable to herbivory by S. mortua. In contrast, non-dormant achenes experienced low seed bank mortality due to rapid germination but high pre-dispersal herbivory. Population growth models show that observed G. ciliata achene proportions were optimal for a narrow range of environments in an herbivore independent model, yet were optimal in a wider range of environments in an herbivore dependent model. Depending on the degree of pre-dispersal mortality by S. mortua, production of multiple achene types was shown to be adaptive in environments that varied widely in the frequency of bad years. Our theoretical evaluations are the first to show that the bet-hedging strategy, heterocarpy, could evolve in the absence of post germination mortality. This study demonstrates that heterocarpy also offsets risks of pre-dispersal achene mortality, which increases the adaptive value of this strategy beyond what has been described in previous bet-hedging models of heterocarpy.