<b>Relationship’s magnitude between intraspecific trait variation and ecological responses: A meta-analysis</b>

<b>1.</b> Intraspecific trait variability (ITV) matters in ecology, but we need to evaluate how much changes in ITV can affect ecological dynamics. Specifically, it remains unclear which factors drive the magnitude of relationships between ITV and ecological responses across biological scales.<b>2.</b> We asked whether the magnitude of relationships between ITV and ecological responses differ (<i>i</i>) between intraspecific levels (among- <i>vs</i>. within- populations), (<i>ii</i>) among ecological responses across levels of biological organisation (from the individual to the ecosystem levels), and (<i>iii</i>) among three main trait categories (morphology, physiology and behaviour).<b>3.</b> We used a multi-level meta-analysis, synthesising 1008 effect sizes from 188 studies encompassing relationships between traits measured among individuals or populations of animals across seven taxonomic groups and a broad range of ecological responses across biological levels, from ecological performances (resource acquisition rate, trophic niche, growth rate) and fitness (survival, reproduction), to the whole ecosystem (community structure, ecosystem functioning).<b>4.</b> The average effect size of the magnitude of relationships between ITV and ecological responses was |<i>r</i>| = 0.24 [0.20 – 0.27] (95% confidence intervals). First, effect sizes were twice higher at the population than at the among-individual level, but this difference was not significant. Second, effect sizes were higher for ecological performance (foraging, diet), community and ecosystem responses than for fitness responses (reproduction). Third, effect sizes for physiological and behavioural traits were both on average 28% higher than for morphological traits.<b>5.</b> Our results confirm the central role of ITV in animals for ecological processes and suggest giving a special emphasis on physiological and behavioural traits to improve that issue. Future studies should go further, in adopting a multi-trait approach and considering the covariation among traits. To deepen our understanding of factors mediating the links between ITV and ecological dynamics, we need to emphasise further trait-mediated interactions, keystone and engineering species, as well as non-linearity of ecological relationships.