Bottom-up and top-down effects on insect herbivore fitness under global change

Insect herbivores can be affected by selective forces from natural enemies (top-down) and host plants (bottom-up). The quality of plants as food for insect herbivores as well as the potential pressure by natural enemies can vary directly and indirectly with environmental factors. Many of these environmental factors are already and will continue to rapidly change due to global environmental changes. These global changes can have variable effects on insect herbivore fitness via top-down and bottom-up forces, which makes the response of insect herbivores hard to predict. For instance, plants can respond positively to some environmental changes (e.g., nutrient runoff), resulting in positive effects for herbivores, whereas other factors can reduce plant quality (e.g., eCO₂), resulting in an indirect negative effect on herbivores. Natural enemies are likely going to be negatively affected by many environmental changes (e.g., change in precipitation, temperature), which may result in enemy release and a positive effect on herbivores. We conducted a meta-analysis of 262 papers to synthesize the current knowledge on the indirect effects of global change on insect herbivores via bottom-up and top-down forces. We further tested the effects of different types of global change stressors on the responses of insect herbivores to top-down and bottom-up forces, and how these responses vary with insect guild, type of habitat, fitness measure taken, and type of natural enemies (parasitoids vs. predators). Knowing the many possible effects that global change can have on top-down and bottom-up controls of insect herbivores can help with conservation efforts and pest controls.