Understanding how the structure and dynamics of food webs will change under global climate change is a pressing but challenging issue in ecology. Our current understanding suggests that food chains will shorten with warming, specifically by losing top trophic levels. While there is some support for this pattern in experimental microcosms, the data remains nascent results are often conflicting. Current explanations for this impact on food web structure generally invoke changes to phenotypic traits that are critical to predator-prey interactions, such as body size. However, temperature does not affect all species equally and so there is potential for asymmetric impacts of temperature across trophic levels. How these asymmetries might play out is largely unknown, but important effects on trophic interaction strength, and thus food web structure, are likely. Here, we use a simple three-species food web module to assess how warming alters food web structure and dynamics. Our food web module comprises a resource, an intermediate predator, and an omnivorous top predator that feeds upon both other species. We explicitly include known temperature dependencies in important demographic rates and in parameters controlling feeding interactions such as death and attack rates. We also include potential asymmetric responses to changes in temperature among the different demographic rates and across trophic levels.
Results/Conclusions
We find that the omnivorous top predator is almost always more susceptible to warming than the intermediate predator, even when temperature-dependencies are comparable or stronger in the omnivore. This leads to continuous declines in the mean trophic level of the food web, as the top predator is forced to change which species it consumes. Eventually, a discrete change in trophic level is observed when the top predator is totally extirpated from the system. Importantly, we find that this effect can be totally reversed whenever asymmetrical responses to temperature among top predators and intermediate predators across multiple demographic rates and traits controlling feeding interactions are introduced. These results therefore suggest that asymmetric responses to temperature across trophic levels may mediate and potentially drive entire food web responses to warming. Moreover, it suggests an important next step towards predicting changes in food web structure with temperature: the need to assess whether top or intermediate predators are more or less sensitive to changes in temperature.