When does stochastic dispersal select for bet-hedging strategies and which life history traits matter most?
We sought to understand how turbulent coastal eddies influence selection on the reproductive strategies of benthic marine organisms with a pelagic larval period. We developed a general mathematical model that draws from theories on bet hedging, offspring size-number trade-offs, and stochastic dispersal in coastal currents. In our model, the dispersal of larvae is driven primarily by turbulent eddies that collect larvae into groups that move as coherent “packets”. The larvae in a packet succeed or fail as a group, depending on whether their packet ends up near suitable habitat or not during larval competency, which generates a form of within-generation variance in parental fitness. The expected mean fitness of parents is influenced by the outcome of a classic offspring size-number trade-off (Smith and Fretwell) and depends on the rate of planktonic mortality and development times of offspring. The fitness variance is influenced by the number of independent offspring releases and the number of offspring within each release, which is in turn influenced by offspring size. Instead of finding the offspring size that maximizes expected parental fitness, we ask when the stochasticity in packet success favors changes in offspring size, or other life history traits, to increase mean fitness versus decrease variance in fitness.
Bet hedging strategies in response to stochastic larval dispersal become important whenever: 1) effective fecundity is high (e.g., planktotrophs) or 2) if there are fewer larval packets (e.g, because the population is spread over a smaller spatial domain or ocean eddies are larger). Stochasticity in packet success not only selects for spreading reproductive events over time, but also an offspring size that is different from the classic Smith and Fretwell optima that maximizes mean fitness. The difference arises because of the importance of lowering the variance in settlement success caused by nearshore eddies rather than minimizing planktonic mortality to increase the total number of settlers. Sensitivity analysis revealed that changing offspring size was often the most efficient way to reduce fitness variance, compared to other traits like repeated reproduction, reproductive investment, or size at settlement. These results present new hypotheses for the evolution of benthic marine reproductive strategies and new insights into conditions favoring within-generation bet-hedging more generally.