Optimal adult growth of Daphnia in a seasonal environment

1. The cladoeeran Daphnia serves as an example of an iteroparous organism, with overlapping generations, that is eapable of substantial adult growth. The life history of Daphnia was modelIed as the eonsequenee of aseries of decisions about alloeation of energetie resourees to growth and reproduetion. 2. We used numerieal methods to find resouree alloeation patterns that maximized fitness of Daphnia in a temporally variable environment. Temporal variation was modelIed as alternating aetive and dormant seasons; length of the aetive season was uniformly distributed. Fitness was measured by the geometrie mean of resting eggs produeed at the end of the aetive season. We examined effeets of mean and range of the aetive season on the optimallife history; we also examined effeets of inereasing (invertebrate predation), eonstant (non-seleetive) and deereasing (fish) size-speeifie survival rates. For eomparison, we found resouree alloeation patterns that maximized fitness in a eonstant environment, where fitness was measured by the intrinsie rate of inerease r. 3. Life histories optimized for seasonal environments generally showed earlier maturity and greater adult growth than those optimized for eonstant environments. Adult growth oeeurred with non-seleetive predation, and even with fish predation, eonditions under whieh it does not oeeur in the optimal life his tori es for eonstant environments. 4. Greatest size at maturity and adult growth oeeurred in life histories optimized to invertebrate predation in seasonal environments. Smalkst size at maturity and least adult growth oeeurred in life histories optimized to fish predation. 5. In the optimallife histories, size at maturity generally inereased with mean length of the aetive season. Adult growth reaehed a maximum for mean seasons of length equal to about one-half to one life span of Daphnia. 6. Inereasing the variation in season length deereased adult growth in the optimal life history , but had little effeet on size at maturity. 7. We expeet that life histories are adapted to the long-term average of season length and its variation. If the animals ean deteet the type of predator, seleetion eould favour phenotypie variation in resouree alloeation.