Life history plasticity in a damselfly: Effects of combined time and biotic constraints

Optimal values for life history traits are expected to depend upon. environ mental conditions during development and the period within which developmen t is constrained (e.g., biotic factors and time constraints, respectively). Theory predicts that life history responses to both biotic factors and tim e constraints may be both direct and behaviorally mediated. Few experimenta l studies of life histories have considered the joint effects of biotic fac tors and time constraints, and fewer still have been able to disentangle di rect from behaviorally mediated effects. We studied such interactions by ma nipulating the perceived time to the onset of winter, predation risk. and f ood resources level in larvae of the damselfly Lestes sponsa. In the first experiment (predation x time constraint), the presence of a predator caused an overall reduction in foraging activity, development rate, and mass at e mergence. However. larvae that had less time available before the end of th e season, increased foraging activity and development rate, while mass at e mergence decreased. These results suggest that the observed changes in life history characters were behaviorally mediated in the presence of predators . In contrast, life history responses of time-constrained larvae occurred i ndependently of the behavioral changes and, therefore, were direct. In the second experiment (food level x time constraint) larvae under high food lev els had a higher foraging activity, increased development rate, and higher growth rates, compared to low food-level treatments, Time-cons trained larv ae accelerated development and had a smaller mass at emergence at high food levels than larvae that were not time constrained. In contrast, and opposi te to predictions, time-constrained larvae at low food levels had the slowe st development rate and the largest mass at emergence. We suggest that larv ae in the latter group were aiming to delay emergence to the next season (c ohort splitting). Our results suggest that both behaviorally mediated and d irect responses to biotic factors and time constraints are a feature of the life history of this damselfly.