Meta‐analysis of induced anti‐herbivore defence traits in plants from 647 manipulative experiments with natural and simulated herbivory

Abstract

Induced anti‐herbivore defences in plants can shape ecosystem structure and functions. Since these are known to be highly variable, quantifying their sources of variation remains important to understand their eco‐evolutionary roles. We conducted meta‐analysis of 647 experiments (paired control and treatment) from 192 studies to address sources of variation in induced anti‐herbivore defensive traits. This covered different agents of herbivory (insect, mammal and clipping), studies in greenhouse and in field settings, and covered 24 types of defence traits in 163 species from 50 families across 24 angiosperm orders. We used meta‐regression models—multi‐level random‐effects (MLREs) and robust variance estimators (RVEs)—to quantify variation due to study setting, herbivore type, chemical identity, angiosperm order, volatility, fertilization, and publication year as moderator variables. There were strong indications for publication bias in favour of large effect sizes, and studies with adequate sample size and precision were rare. Strength of induction was distributed uniformly over the phylogeny of 24 angiosperm orders. Stronger effects were reported from greenhouses than in field conditions, and induction was uninfluenced by herbivore type. Volatiles showed stronger response than non‐volatiles in greenhouses for insects. Biosynthetic precursors (e.g. jasmonates) and many defensive chemicals were induced in the greenhouse, but when measured in field conditions, they were unresponsive to herbivory and appear to be constitutive. Similar to constitutive response, induced responses were also unaffected by fertilization. Direction and magnitude of induction differ greatly from previous assessments as these have also changed over time, representing natural epistemological growth. A third of the variability was explained by moderators (marginal R2: chemical identity, angiosperm order, study setting, herbivore type and publication year); another third (conditional R2) was attributed to the identity of individual studies and observations. Synthesis. Manipulative experiments reveal many intrinsic differences among defensive chemicals and traits. They suggest plants in greenhouses may respond very differently to herbivores from those in field conditions. Although biosynthetic pathways of chemical responses, their modes of action and their effects on herbivores are well‐understood, studies with greater statistical power under ecologically relevant settings are needed to discern induced and constitutive defences in how plants respond to their natural enemies

Elevated atmospheric CO2 suppresses silicon accumulation and exacerbates endophyte reductions in plant phosphorus

“…Glasshouse experiments previously reported that H. armigera can induce Si defences in L. arundinaceum (Cibils-Stewart et al, 2022), although to a greater extent (+ 94%) than in the current study (+24%). This is consistent with a recent meta-analysis of 647 experiments which demonstrated that plant defences, in general, are induced to a greater extent when measured in glasshouse settings compared to field situations (Ojha et al, 2022).…”

Leaf and root chemical and physical defence traits mediate monoculture yield decline in a grassland experiment

“…These species-specific shifts along the collaboration gradient highlight that long-term monoculture growth exerts a strong selective pressure against DIY species. Despite these trait temporal changes may be due to phenotypic plasticity (Ojha et al 2022), the genetic and epigenetic trait divergence previously found in the same monocultures of this study (van Moorsel et al 2018(van Moorsel et al , 2019 suggests that the shift along the collaboration gradient may be partially due to plant microevolution in response to belowground processes, such as a potential accumulation of root antagonists (Didiano et al 2014). Moreover, the missing link between leaf defence gradients and yield decline, suggests that belowground antagonists or other belowground processes are more important drivers of monoculture yield decline than aboveground processes (Bennett et al 2012, Benitez et al 2021.…”

“…However, plant defence traits show high phenotypic plasticity in response to current selective pressure through antagonists, even within short time frames (i.e. one growing season) (Poorter et al 2019, Ojha et al 2022. Given more time, strong selection by antagonists can result in altered plant defence trait expression through microevolution (Didiano et al 2014).…”

Improving quantitative synthesis to achieve generality in ecology

“…It is important to present the range and variability of effect sizes alongside main effect interpretation, using e,g., orchard plots 43 (e.g. as in 44,45 ), and density plots (as in 46 and 47 ).…”