Bottom‐up vs. top‐down effects on terrestrial insect herbivores: a meta‐analysis

Abstract

Primary consumers are under strong selection from resource ('bottom-up') and consumer ('topdown') controls, but the relative importance of these selective forces is unknown. We performed a meta-analysis to compare the strength of top-down and bottom-up forces on consumer fitness, considering multiple predictors that can modulate these effects: diet breadth, feeding guild, habitat/environment, type of bottom-up effects, type of top-down effects and how consumer fitness effects are measured. We focused our analyses on the most diverse group of primary consumers, herbivorous insects, and found that in general top-down forces were stronger than bottom-up forces. Notably, chewing, sucking and gall-making herbivores were more affected by top-down than bottom-up forces, top-down forces were stronger than bottom-up in both natural and controlled (cultivated) environments, and parasitoids and predators had equally strong top-down effects on insect herbivores. Future studies should broaden the scope of focal consumers, particularly in understudied terrestrial systems, guilds, taxonomic groups and top-down controls (e.g. pathogens), and test for more complex indirect community interactions. Our results demonstrate the surprising strength of forces exerted by natural enemies on herbivorous insects, and thus the necessity of using a tri-trophic approach when studying insect-plant interactions.

Intercropping impacts the host location behaviour and population growth of aphids

“…(Costello & Altieri, 1995;Wratten et al, 2007), our study shows that vegetation diversity can also contribute to decreasing the growth of the later wingless aphid populations. Finally, our laboratory results need to be confirmed at a larger scale under actual field crop conditions, where other parameters such as the interaction with natural enemies might play an important part in the final aphid control (Vidal & Murphy, 2018). Although additive vs. substitutive designs are often compared in studies of the effect of intercropping on pest control, little is reported about the influence of plant relative densities on pest dispersal (Lopes et al, 2016).…”

Deconstruction of a plant‐arthropod community reveals influential plant traits with nonlinear effects on arthropod assemblages

“…Similarly, our model of herbivore richness only explained 18.7% of the variation in herbivore richness, whereas our model of herbivore abundance was unsupported. When secondary consumer abundance was included as a predictor variable in our model of herbivore richness, model performance increased by approximately 10% (Figures and S10), which suggests an importance of top down pressures in our system (Vidal & Murphy, ). However, the addition of secondary consumer abundance to our model of herbivore abundance did not lead to a supported model (Figure S10).…”

Linking plant genes to insect communities: Identifying the genetic bases of plant traits and community composition

“…While we have focused on bottom‐up mechanisms underlying insect communities on aspen, we recognize that top‐down factors (e.g., predation and parasitism) also influence insect herbivores (Katano, Doi, Eriksson, & Hillebrand, ; van Veen, Morris, & Godfray, ; Vidal & Murphy, ). A meta‐analysis by Vidal and Murphy () revealed that top‐down forces are often stronger than bottom‐up effects in influencing insect herbivore fitness. Yet, these differences varied across insect groups (e.g., specialist vs. generalist insects and across feeding guilds).…”