CANCELLED - Predator regime and resource availability drive rapid evolution of Daphnia life history traits

Background/Question/Methods

Rapid declines and extirpations of predator populations have affected the structure and dynamics of communities across systems. These population declines can directly and indirectly affect consumer populations via multiple pathways, such as reductions in size-selective predation and increases in productivity after the decomposition of dead predators. In turn, the synergistic effects generated by mass mortality events (MMEs) of predators may partially aid the recovery of perturbed systems if trait variation subsequently increases within consumer populations. Such consumer responses may be most applicable to aquatic systems, which often experience strong trophic cascades and have been increasingly affected by fish MMEs. Observational studies of such events have shown rapid consumer compositional turnovers and near system recoveries after a single year. Yet limited data are available about mechanisms that may facilitate rapid community changes after these biological catastrophes. We used an artificial selection experiment on Daphnia employing a 2x2 design with simulated predator regimes (size-selective, uniform) and resource availabilities (low, high) – scenarios representative of a MME, predator removal, resource pulse, and control. We then reared Daphnia F1s and measured life history traits to understand whether these scenarios generate divergent life history changes that may affect how communities respond to these biological catastrophes.

Results/Conclusions

Our results suggest that predator MMEs can lead to rapid changes in consumer life history traits. Specifically, the MME had the largest positive effect on incipient population density and resulted in increased body size and fecundity, as well as reduced development time. By contrast, the resource pulse had negligible effect on incipient population density, body size, and fecundity, although development time was reduced compared to the control. Low resource availability resulted in strong directional life history changes that materialized as low abundances of small-bodied individuals with low fecundity and short development time. The predator removal generated individuals with variable body sizes, increased fecundity, and decreased development time. We hypothesize that the synergistic effects of uniform predation and high resource availability were partially driven by reductions in intraspecific competition that allowed a wider range of individuals to reproduce and contribute toward population-level trait changes. Collectively, these results suggest that rapid consumer evolution plays a pivotal role in the recovery of systems after predator MMEs. Understanding how community structure and dynamics change after biological catastrophes is of growing concern given the lack of global action to mitigate climate change.