Abstract Intraspecific trait variation can strengthen or weaken species interactions in ecological communities. A mechanistic understanding of how intraspecific variation affects species interactions will help identify general rules about when intraspecific variation strengthens versus weakens species interactions. We used a series of greenhouse experiments to investigate the effect of larval body size variation of an herbivorous insect on the total damage and distribution of damage on its host plant, and to determine the mechanisms underlying the effects of size variation. Using groups of larvae with the same mean body size but different levels of variation in body size, we tested three mechanisms: (1) nonlinear averaging, (2) disproportionate effects of large individuals and (3) feeding facilitation. We found that groups of herbivores with high size variation damaged plants more and dispersed their damage more among leaves compared to groups of herbivores with low size variation. A nonlinear relationship between individual herbivore body size and plant damage suggests that the effects of size variation on total damage are likely due to nonlinear averaging. Though none of the mechanisms we tested fully explained the effect of size variation on the spatial distribution of damage, our results suggest that this effect may be due to size‐dependent changes in larval behavioural response to density. Our results suggest the effects of intraspecific trait variation can be mediated by different mechanisms for different response variables, and the mechanism can be context dependent. Read the free Plain Language Summary for this article on the Journal blog.
Montoya et al. (2026) studied this question.