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Intraspecific trait variation shows the predominant influence of functional divergence on species co-occurrence

Walker Mitchell, Grace1; Gilbert, Benjamin1

Published Dec 06, 2025 on Dryad. https://doi.org/10.5061/dryad.cz8w9gjj8

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Abstract

Intraspecific trait variation (ITV) has been recognized as an important source of variation in communities. We separated among-population ITV (differences in mean trait values among populations) from within-population ITV (trait variation within populations) to test three theoretical predictions: i) communities should show greater among-species trait diversity when among-population ITV is accounted for due to trait means shifting away from neighbouring species, ii) the increased differences among species should be greatest in species poor communities, and iii) within-population trait variation should also be greatest in species-poor communities.

We tested these predictions in an experimental old-field metacommunity, measuring four functional traits known to be important for plant function in 1842 individuals across 33 species and 84 patches that varied in size (area), species diversity, and composition.

We found that when among-population ITV was accounted for, species diverged in their average trait values (nearest neighbour distances and functional dispersion), consistent with theory that trait differences among species promote coexistence. This divergence was slightly greater in species-poor communities. Within-population ITV showed a different pattern than predicted, with the highest within-population ITV occurring in large patches and in communities where species were more functionally differentiated.

Overall, among-population ITV revealed the influence of ecological interactions on trait divergence among species. Failing to incorporate ITV underestimates among-species trait differences and may miss opportunities to detect trait-based assembly and coexistence mechanisms. Nonetheless, estimates that ignore ITV are strongly correlated with those that include ITV, suggesting that the relative differences in functional diversity among communities can be estimated without ITV. Within-population ITV showed less consistent results, suggesting that other factors may have a stronger influence within populations.

Synthesis. Theory predicts that variation within species may alter species coexistence and the functional diversity of communities. We found support for the hypothesis that species show higher divergence in their average traits when ITV is accounted for. However, we found little support for the hypothesis that ITV within populations is greater in patches with few species. These results suggest that variation among populations moderates species interactions, facilitating species co-occurrence and shaping community assembly.