Night warming alters mean warming effects on predator-prey interactions by modifying predator demographics and interaction strengths

Temperatures are increasing globally, but this increase is not symmetrical: instead, nighttime minimum air temperatures increase faster than daytime maximum temperatures. However, we still know little about when and how this differential increase in maximum vs. minimum air temperatures affects ecological interactions between species. Understanding the connection between different types (night vs day) of warming and interspecific interactions is essential to predict the ecological consequences of climate change for natural ecosystems.

Here we used experiments in a common predator (lady beetle) - prey (aphid) system to examine how night warming, thermal regime (constant vs. fluctuating) and prey density influence demographic rates and per capita feeding rate and thus predation pressure. We mainly focused on the demographic variables and prey consumption as well as energy-related variables of the predators to understand how changes in nighttime temperature, temperature regime and prey density affect predator performances and to differentiate these effects between constant and fluctuating thermal conditions.

We found that warming effects on most of the individual demographic rates and per-capita consumption rates of the predator differed between constant and fluctuating regimes. These differences scaled up to alter population stage-structure and population growth rate of the predator and ultimately the predation pressure in our system. Furthermore, warming had stronger and negative consequences on predator population growth and thus predation pressure in constant regime, but little or even positive effects under fluctuating conditions. Importantly, these results are consistent across prey densities.

Our results suggest that compared to night warming, predictions based on mean warming can under- or overestimate species demographic rates and per-capita predation effects, causing inaccurate or even misleading expectations on population structure, long-term population growth and interspecific interactions. These results emphasize the importance of accounting for both ecologically relevant diurnal temperature cycle and lifetime-long demographics and per-capita consumption when assessing how warming affects interspecific interactions. Our findings thus provide better understanding of how night warming will affect predator-prey interactions and energy flux within trophic cascades and also have implications for predicting the consequences of top-down control in natural and agricultural ecosystems under climate change.