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.