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

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.