Interactions between predation risk and temperature affect death, fecundity, and individual growth rates, leading to changes in population growth rate thermal performance curves (TPCs)

Background/Question/Methods

Despite the well-known effects of predation and temperature on organismal traits and the fact that organisms in natural environments may simultaneously experience changes in both, there is little understanding of how temperature and predation risk interact to influence life history traits. We assessed how predation risk altered the response of Daphnia magna life history traits to temperature. We created predation cues by feeding D. magna to a colony of mixed dragonfly larvae. We divided the experiment into a “natal” period (the first 7 days of the experiment) and an “adult” period (days 8 through 21). During the natal period, D. magna were raised at 17^oC (colony temperature) and then moved to one of 7 temperatures (11, 17, 23, 27, 29, 31, 33^oC) for the adult period. Individual D. magna were given one of 4 predator cue treatments 1) no exposure 2) continual exposure 3) exposure only in “natal” period 4) exposure only “adult” period. We analyzed treatment effects on the thermal performance curves of fecundity, age at first reproduction, generation time (T), reproductive rate (R₀), and mortality. We use the r-EAAR model (a recently derived population growth rate model rooted in enzyme kinetics) to analyze how changes in r TPCs correspond to changes in system enzymes.

Results/Conclusions

Reproductive rate (R₀) and population growth were enhanced by the presence of predation cues (hereafter “cue”) during the “natal” period. At the coldest temperature (11^oC), generation time (T) was accelerated in all treatments receiving cue relative to the no exposure control, otherwise T decreased with increasing temperature. Mortality increased with temperature at all treatments but did so more quickly when exposed to cue during the natal period. Population growth rate TPCs showed an elevated r at cooler temperatures for Daphnia exposed to predator cues during the natal period. Furthermore, all Daphnia treatments exposed to cue demonstrated an increased right skew of TPCs compared to the control (no exposure). These changes in r TPC shape are consistent with temperature-dependent responses of Daphnia vital rates to predation risk that are further influenced by timing of exposure. Furthermore, these changes in the shape of the r TPC are consistent with shifts in whole-organism enzymatic properties (e.g., E_ΔCp, E_ΔH, and E_b).