The latitudinal gradient in functional diversity: Higher species richness does not arise from larger functional trait space

Background/Question/Methods

The latitudinal gradient in species richness is one of the most pervasive patterns in biogeography, yet its origins remain elusive. Leading explanatory hypotheses make predictions about how species occupy niche space along a latitudinal gradient. Here we examine patterns of functional diversity at alpha (within community), beta (between communities) and gamma (regional pool) scales in New World tree communities.  To measure functional diversity, we construct hypervolumes from four traits that represent major axes of plant diversity and life history strategies: specific leaf area, wood density, seed size and maximum height. 

Results/Conclusions

We find no latitudinal gradient in alpha functional diversity, but gamma functional diversity in the temperate zone is greater than that of the tropics. At the beta scale, we find a steeper decay of similarity in functional diversity with climatic and geographic distance within the temperate zone relative to the tropics. These patterns suggest that individual communities are similarly impacted by environmental filtering in temperate and tropical regions, but that the wider range of climates in the temperate zone allow for a greater diversity of strategies at the regional level. We conclude that the latitudinal diversity gradient does not arise from a larger functional trait space in the tropics, contrary to the predictions of all the most prominent hypotheses for its drivers.