Mass mortality events of predators generate unique community dynamics

Background/Question/Methods

Recent declines and extirpations of predator populations have substantially affected the structure and dynamics of biological communities. The role of predators in communities often focuses on their direct and indirect effects on lower trophic levels (top-down forces), which are routinely evaluated by comparing communities with or without predators. However, predators may be absent from communities due to either natural conditions, such as geographic isolation or environmental mismatches, or population declines and extirpation, such as after a mass mortality event. Although both of these scenarios diminish top-down forces by removing predators, mass mortality events of predators also deposit substantial amounts of nutrient-rich carrion (resource pulses) that may alter the effects of producers on higher trophic levels (bottom-up forces). In turn, these extreme events may evoke temporal shifts in species abundances, frequencies, and interactions that generate different dynamics than predicted by trophic cascade and resource pulse theories. To test this idea, we constructed mesocosms with tri-trophic, freshwater systems (phytoplankton, zooplankton, planktivorous fish) and induced predator removal, resource pulse, mass mortality event of predators, and control treatments. We then used community trajectory and time-series analyses to compare community dynamics amongst these ecological scenarios.

Results/Conclusions

Our results suggest that mass mortality events of planktivorous fish may generate unique community dynamics that differ from predator removals and resource pulses, both separately and additively. The predator removal treatment had the lowest and highest abundances of phytoplankton and zooplankton, respectively. The resource pulse treatment had the highest phytoplankton abundances, but similar zooplankton abundances compared to the control treatment. Comparatively, the mass mortality treatment had slightly lower phytoplankton abundances than the resource pulse treatment, which coincided with an earlier increase in zooplankton abundances compared to the predator removal treatment. These comparisons suggest that mass mortality events of planktivorous fish may generate strong top-down and bottom-up forces that evoke unique temporal shifts in the dynamics of species interactions. In addition, the responses of different phytoplankton and zooplankton functional groups illustrate how some lentic communities may respond to these events, and may potentially help explain previous observations of compositional turnovers within communities after these events. Understanding the effects of mass mortality events on the structure and dynamics of communities is crucial for basic and applied research in our changing world, particularly because these events are predicted to increase in frequency and intensity.