The species-specific interactive effects of predation and heat shocks on an insect herbivore

Background/Question/Methods

Species are subjected to numerous types of environmental disturbances, including fluctuating temperatures. How these species respond depends not only on their sensitivities to the environmental disturbances, but also on how they interact with other species. Species interactions can dampen or exacerbate the effects on an environmental disturbance, depending on how the species interact and how the interactions themselves react to the disturbance. The pea aphid, Acyrthosiphon pisum can be sensitive to high temperatures; for heat-sensitive aphid strains, exposure to short-duration shocks of high temperatures reduces pea aphid fecundity and hence population growth rates. We set out to determine how exposure to these heat shocks may interact with another important mortality factor, exposure to predation by one of two species of predatory ladybird beetles. Using large field cages, we investigated the effect of heat shocks and predators on pea aphid populations.

Results/Conclusions

We found that each of these factors individually reduced aphid populations over time as compared to control populations kept without predators at ambient environmental conditions. The interactive effect of heat shocks and predators, however, depended on the species of predator used. For one predatory species, Coccinella septempunctata, exposure to high temperatures and predators had a near additive effect; reducing pea aphid populations even more then either factor did alone. However, when Harmonia axyridis was used as a predatory species, the effect of temperature nullified the predatory effect of the ladybird. Aphid populations exposed to heat shocks were the same with or without the addition of H. axyridis. This interaction between H. axyridis and heat shocks seemed to be indirectly mediated by aphid density. When the aphid density was higher in the ambient temperature cages, the predators were more active and presumably more successful foragers. However, aphids exposed to heat shocks were at a lower density over time which appeared to have had a deleterious effect on the predators and their interest in actively foraging. This study demonstrates the potential importance of understanding how environmental disturbances can be modified in a larger context where species interactions can occur. It also suggests that we will need to consider both direct and indirect effects of disturbances on these interactions.