Deterministic assembly rules should produce sets of locally coexisting species that are non-random subsets of the regional species pool. Recent analyses of trait distributions within communities have demonstrated patterns consistent with assembly rules, especially in terrestrial plants. We tested for assembly rules in an adaptively radiating clade of temperate marine fish: Sebastes rockfishes of the northeast Pacific. For each species we quantified local-scale resource use (the α-niche) using stable isotopes, and habitat along a depth gradient (a β-niche axis). We then identified morphological principal component traits that relate to each of these niche components. We identified gill raker morphology as an α-trait that might mediate competition for resources, and relative eye size as a β-trait that reflects position along the depth gradient. We predicted that gill raker morphologies would be overdispersed within rockfish assemblages due to limiting similarity, and that environmental filtering would reduce the range of relative eye sizes within assemblages. We used a simple null model to compare the distributions of traits in 30 published rockfish surveys with the null expectation under random assembly.
Results/Conclusions
Few assemblages showed significantly overdispersed or clustered traits when analyzed individually. However, when we combined data from the 30 assemblages in a meta-analysis, we detected a significant trend supporting our prediction of even spacing of the gill raker trait. We also detected a significant a trend toward evenness in body size, an ecologically important trait for which we did not make an a priori prediction. These patterns are consistent with limiting similarity promoting niche differentiation among coexisting species. We also detected a lower range of relative eye sizes than expected by chance, as predicted if environmental filtering allows only species with certain traits to occur in a given depth habitat. The range of gill raker morphologies also tended to be lower than expected, suggesting that environmental filtering and limiting similarity act simultaneously on this trait. Our results support niche-based assembly in rockfish, and show that conclusions derived largely from terrestrial plant communities may apply to a much wider range of organisms and ecosystems. Incorporating functional morphology to identify a priori traits associated with the α- and β-niches can provide for powerful tests of assembly rules.
