Background/Question/Methods Predators can strongly influence prey populations not only through consumptive effects (direct killing) but also through non-consumptive ‘fear’ effects. Most studies, however, only focused on the role of direct killing in driving evolutionary changes in prey populations. By integrating experimental evolution in outdoor mesocosms and resurrection ecology, we tested and compared the roles of fear and direct killing in driving evolution in a population of the water flea Daphnia magna. This natural population experienced strong changes in fish predation pressure within ⁓16 years where a period without fish (no-fish period) was followed by a strong increase (high-fish period) and then a strong reduction (reduced-fish period) in fish predation pressure. All resurrected genotypes were pooled, inoculated in outdoor mesocosms, and exposed to free-ranging as well as caged fish predators. Results/Conclusions We found that fear induced rapid, repeatable changes in the clonal composition and associated genotypic trait changes in life history, morphology and behavior that were similar in magnitude and direction to those imposed by direct killing. Moreover, these genetic changes were in the same direction as the adaptive trait shifts observed in situ through resurrection ecology. Both direct killing and fear caused a shift toward a dominance of clones from the high-fish period of this natural population which can better deal with fish predation. Our results show that fear may induce rapid, adaptive evolution in natural prey populations with a similar magnitude as direct predation, and thus may represent an overlooked driver of the eco-evolutionary dynamics.
