The adaptive timing of diapause – a search for evolutionarily robust strategies in Calanus finmarchicus

In environments with strong and variable seasonal fluctuations, organisms are selected on the basis of their phenology, such as the timing of diapause, reproduction, or assembling of storage products. A simulation model of the dynamic balance of various phenologies within a population facing density-dependence and varying annual, environmentally determined growth opportunities is presented. The main assumption is that the timing of phenological events is heritable and cued by a single signal (day length). Then, the balance between alternative strategies is regulated by natural selection and reproductive success. The model is developed for the marine copepod Calanus finmarchicus, which must decide when to start preparing for diapause by allocating to storage rather than somatic growth, how much storage (lipids) to bring with it during wintering and when to “wake up” to complete development and to reproduce. The results show that (1) density-dependence may lead to frequency-dependent resting and emergence strategies, (2) environmental stochasticity causes delayed arousal, (3) the selection procedure creates well-adapted individuals which out-compete others with fixed probabilities of maturing or entering diapause, and (4) that the use of day length as a cue to phenological decisions may be beneficial when growth conditions are persistent within years (e.g. warm/cold years).