Cohort splitting: Mechanisms modeled, relevance revealed

Background/Question/Methods

Understanding the role of adaptive plasticity under random environmental variation is a central challenge of contemporary life history analysis.  An important class of plastic responses is "cohort splitting", in which groups of organisms beginning development at nearly the same time follow distinctly different developmental pathways.  Our review of studies published in ecological journals identified three main mechanisms proposed to explain cohort splitting: initiation asynchrony, in which differences in start-up timing can trigger a split; condition dependence, a bifurcation at some intermediate developmental stage; and bet hedging, spreading risk between the pathways.  We propose a fourth mechanism likely to prove important: (negative) frequency dependence, interactions between mortality rates or fecundities associated with the alternative pathways.  We use optimization models to evaluate production of cohort splits by these four mechanisms and to determine the conditions under which each is most likely to apply.

Results/Conclusions

We show that initiation asynchrony can make cohort splitting optimal but may feature high year-to-year variation in whether a split occurs or in the proportions following different pathways.  Condition dependence requires sufficiently high random variation in developmental rates to generate cohort splits, and the critical thresholds at which the split appears should vary considerably among different climate-based seasonal environmental patterns.  High random variation between pathways in reproductive success is essential for bet hedging to prove effective.  Our results indicate a rather narrow range of conditions in which bet hedging leads to cohort splitting, despite its relative popularity as an explanation in the literature.  Frequency dependence can arise from alternative mating strategies but is notoriously challenging to demonstrate.  If present, frequency dependence can readily lead to cohort splits.  Climate change has the potential to influence all of these mechanisms and to change the spatial distributions over which populations split their cohorts.