Biogeography of tree species richness and frequency of species peak abundance: Through the lens of latitudinal diversity gradient and Rapoport’s rule

Youngsang Kwon¹ and Chris Larsen² 

¹ University of Memphis, Department of Earth Sciences

² The State University of New York at Buffalo, Department of Geography

A latitudinal gradient in species richness is recognized for most taxonomic groups, with more species in higher latitudes and progressively fewer species at higher latitudes. In contrast, Rapaport’s Rule is that the geographic range of species is smallest at low latitudes and is progressively larger at high latitudes.  We hypothesize that the combination of these two gradients should result in a constant proportion of species that are present in a latitudinal band having their peak abundance in that band. We tested our hypothesis using tree species data in the 31 easternmost US states from the Forest Inventory and Analysis program. The Importance Value (IV) of all species was calculated in 20 x 20 km grids. The mean importance value for each species was calculated in each one-degree latitudinal band from 25 to 49 degrees north latitude. The latitude at which a species exhibited its peak abundance was determined using HOF models.  A total of 187 tree species were observed in the FIA database for eastern USA. Species richness and the number of species with their peak latitude in latitudinal band both exhibited a bell-shaped distribution; the former peaked at 35 degrees and the latter at 36 degrees north latitude. Both variables were correlated with environmental forcing factors such as evapotranspiration and frost frequency days. For each latitudinal band, a ratio was calculated of the number of species with their peak abundance in it and the species richness in it. Regression analyses indicate no latitudinal trend in that ratio.