Discussion
Our results corroborate previous studies that show climate warming can influence top-down control of herbivores and plants by predators. Our research also contributes to a growing appreciation for the context dependency of warming and demonstrates that the timing of warming can mediate the net effect on a community. Specifically, this study found different results for night warming compared to day and constant warming. We detected evidence of a trophic cascade in night warming treatments that was not apparent in day- or constant-warming treatments. These results shed light on the effects of asymmetrical warming, an underappreciated nuance in environmental change, and the potential consequences of its absence in climate change studies.
In the absence of predators and herbivores (P treatment), we found that all warming treatments had a positive direct effect on above-ground plant biomass. This result is not shocking, as climate warming commonly increases plant biomass (Lin et al. 2010). However, when aphids were added (PH treatment) the positive direct effect of warming on plants was negated. Aphids reduced plant biomass in all warming treatments relative to the biomass of plants grown without aphids. Night-warming plant biomass decreased at a higher magnitude compared to constant- and day-warming treatments. This effect could be attributed to constant- and day-warming having a moderately positive effect on aphids that increased their abundance approximately three times higher than that found in ambient treatments (Figure 2). However, in the night-warming treatments, aphids benefited significantly more and reached the highest abundances of any treatment, approximately 1.5 times as high as the other warming treatments and four times larger than ambient treatments. As such, the positive direct effect of night warming on plant biomass disappeared; aphids suppressed final plant biomass so that ambient- and night-warming treatments did not differ. Such different responses of aphids to day and night warming are not unexpected. For example, night warming has been shown to increase pea aphid abundances (Miller et al. 2017) whereas increased day temperatures can cause heat stress that reduces reproduction rates (Harmon et al. 2009).
The presence of a predator (PHP treatment) reduced aphid abundances in all temperature treatments except day warming (relative to PH treatment) and all warming treatments had higher final aphid abundances than the ambient (Figure 2). When predators were present, the final aphid density did not differ among the three warming treatments. This result is somewhat consistent with data from the predation rate experiment. That is, lady beetles increased predation rates when warmed, with day-warmed beetles consuming the most aphids during the day and night-warmed beetles consuming the most aphids at night (Figure 3). Inexplicably, the total consumption over 24 hours differed among all warming treatments, yet aphids still reached similar abundances in the tritrophic experiment. Such inconsistencies suggest that there are subtle effects of warming within the tritrophic system that are beyond the scope of this study.
Despite the predators in day-warming treatments consuming the most aphids in the foraging experiment, we only detected a trophic cascade with night warming (Figure 1). The mechanisms driving the lack of a trophic cascade in day-warming treatments is unknown but not unprecedented. For example, Flynn et al. (2006) studied the effects of aphids on plant biomass and only documented a reduction in plant biomass in their highest warming treatment, despite similar aphid densities in ambient and warmed treatments. They hypothesized this outcome could be a result of increased aphid development rate or decreased aphid metabolic efficiency. Nonetheless, both Flynn et al. (2006) and our study supports the result that similar aphid population sizes can have different effects on plant biomass.
The importance of trophic cascades has been studied for decades, yet we are still learning how this and many other processes will be impacted by future environmental change. This study provides for a unique investigation of top-down control in different climate warming scenarios. As reported with other species (Barton and Schimtz 2018), we show that night warming and day warming produced completely different indirect effects that cascade through the system. These results are alarming because they demonstrate that inference made from experiments that used warming treatments that are inconsistent with expected patterns of warming can be misleading. Therefore, we urge ecologists to think carefully about the realism of their climate manipulations and the implications of drawing conclusions from unrealistic warming treatments.