Revisiting Darwin’s hypothesis: does greater intraspecific variability increase species’ ecological breadth?

Background/Question/Methods

Darwin first proposed that species with larger ecological breadth have greater phenotypic variation. We tested this hypothesis in Rocky Mountain subalpine meadows by comparing intraspecific variation in specific leaf area (SLA) for 21 species to each species’ local elevational range. We also examined the importance of external (abiotic) filters on local trait composition by determining if intraspecific changes in SLA within species along an environmental gradient have similar directional shifts and slope magnitudes to the interspecific rate of change of the community-weighted mean SLA value.

Results/Conclusions

In support of Darwin’s hypothesis, we found a significant positive relationship between species’ coefficient of variation for SLA with species’ local elevational range, indicating that wide-ranging species are indeed characterized by greater intraspecific variation. Intraspecific changes in SLA had the same sign, but lesser magnitude than the community mean SLA during peak flowering. Thus, it appears that species’ phenotypes shift across environmental gradients to match the changing community phenotypes determined by the local abiotic environment. Nonetheless, across species, the rate of intraspecific trait change, which may reflect plastic and/or adaptive changes across populations, is limited and ultimately prevents species from adjusting to environmental gradients as quickly as interspecific changes that result from community assembly.