Time delays are integral features in the life history of resources and consumers. Theory has shown that these time delays have a major impact on the dynamics of predator – prey systems. For example, the over-production of large numbers of juveniles may suppress their stage-specific growth rates as well as adult fecundity, and lead to particular classes of population cycles in Daphnia-algal systems. Stage-specific growth rates determine the time delay associated with juvenile development and recent theory highlights the importance of juvenile stage duration in producing different types of resource-consumer cycles. Using a Daphnia pulex - algal system, we examined the impact of altering the time-delay of juvenile maturation on population dynamics of D. pulex and their algal prey. We directly manipulated the duration of the juvenile stage in microcosms using “donor-receiver” experiments, controlling for “food per head” in the D. pulex population. At fixed intervals, we removed small newly recruited juveniles from the “receiver” population and replaced them with adolescent D. pulex from “donor” populations. Calculations of the appropriate number of replacement individuals were conducted using size-based ingestion-rates for D. pulex to not alter the “resource-consumer” interaction.
Our results show that we successfully influenced the population structure; during the course of a population cycle, the proportion of the biomass of control populations comprised of adult individuals ranged from 0.02 to 0.35, whereas adults tended to comprise between 0.20 and 0.65 of the total biomass in manipulated populations. Maturation time in manipulated populations was estimated to be approximately two-thirds shorter than in control populations during the trough of the cycles. We characterized the cycle dynamics in predator-prey systems where juvenile stage duration has been manipulated with control systems. The novel experimental approach used here allowed us to decouple the juvenile maturation rate from the population-level ingestion rates of consumers, and examine the effects on population dynamics in interacting predator-prey systems.