Offspring size-number tradeoffs and food quality feedbacks impact population dynamics in a Daphnia-algae system

Population fluctuations can be affected by both extrinsic (e.g. weather patterns, food availability) and intrinsic (e.g. life-history) factors. A key life-history tradeoff is the production of offspring size versus number, ranging from many small offspring to few large offspring. Models show that this life-history tradeoff in offspring size and number, through maturation time, can have significant impacts on population dynamics. However, few manipulative experiments have been conducted that can isolate life-history effects from impacts of extrinsic factors in consumer-resource systems. We experimentally tested the effect of an offspring size-number tradeoff on population stability and food availability in a consumer-resource system. Using Daphnia pulex, we created a shift from many, small offspring being produced to fewer, larger offspring. Two sets of experiments were performed to examine the interaction of an extrinsic factor (light levels) and intrinsic population structure on dynamics, and we controlled for the ingestion pressure on algal prey at the time of the manipulation. We predicted that the tradeoff would impact internal consumer population characteristics, including biasing the stage structure towards adults, increasing adult size, and increasing average population-level reproduction. This adult-dominated stage structure was predicted to then lead to instability and a low quantity-high quality food state. Under all light levels, the manipulated populations became dominated by large adults. Contrary to predictions, the amplitudes of fluctuations in Daphnia biomass were lower in populations shifted to few-large offspring, representing higher stability in these populations. Furthermore, in high light conditions, a stable low Daphnia - high algae biomass (low food quality) state was observed in few-large offspring treatments but not in control (many-small offspring) treatments. Our results show a strong link between light levels as an extrinsic factor and the life-history tradeoff of consumer offspring size versus number that impacts consumer-resource population dynamics through feedbacks with resource quality.