Summer dormancy as a refuge from mortality in the freshwater bryozoan Plumatella emarginata

In expanding populations, individuals that produce dormant offspring when conditions are otherwise suitable for growth and reproduction incur a cost, since the dormant life stage delays reproduction. These individuals are at a disadvantage unless (1) there is not enough time for reproduction to occur before the environment degrades, or (2) the probability of death in the non-dormant state is high. Here we investigate resting stages of the freshwater bryozoan Plumatella emarginata to test the prediction that delayed emergence from dormancy can be related to seasonal fluctuations in mortality. Our results show that emergence in late spring and summer occurs at much lower frequencies than in early spring and is strongly associated with high mortality, at least in part due to predation. We document significant reductions in the growth and survival of plumatellid colonies during the summer in the presence of the crayfish Orconectes limosus. Thus summer dormancy provides a significant refuge from predation. Dormant resting stages during the winter also experience significant mortality. Our results are consistent with the general notions that (1) the proportion of colonies emerging from dormancy reflects tradeoffs in the relative risks of mortality in dormant versus non-dormant states, and (2) temporal shifts in the risk of mortality influence the timing of life-cycle transitions.