Evol Ecol Res 3: 413-428 (2001)     Full PDF if your library subscribes.

An evaluation of the geographic area hypothesis using the latitudinal gradient in North American tree diversity

Paul V.A. Fine

Department of Biology, University of Utah, 257 S. 1400 East, Room 201, Salt Lake City, UT 84112, USA

e-mail: fine@biology.utah.edu


One hypothesis to explain the latitudinal gradient in species diversity is the geographic area hypothesis. This hypothesis posits that the size of a biome has considerable influence on its species diversity. Since the tropics are so much larger than any other extra-tropical biome, one would predict the latitudinal gradient to resemble a step function if area and species richness were tightly correlated. When there is a smooth latitudinal gradient in species diversity, it must be because tropical species’ ranges extend into extra-tropical areas, inflating the number of species in the extra-tropical areas nearest to the tropics. Here, using data for North American tree ranges, I test whether tropical species’ ranges do extend into extra-tropical areas. In a second test, I expand my definition of a tropical species to include species from genera with tropical origins (speciation spillovers). This second test searches for the effect of spillover events over evolutionary time. Only a few tropical species also live in the extra-tropics and, therefore, the latitudinal gradient in tree diversity at large scales is a step function. Thus, spillover species do not contribute to the shape of the latitudinal gradient. However, speciation spillovers account for a quarter of subtropical areas’ species richness, and past range expansion was probably important in generating today’s North American tree diversity. The lack of tropical species expanding their ranges into North America may be a result of a trade-off between frost tolerance and growth rate.

Keywords: frost tolerance, geographic area hypothesis, latitudinal gradient, North American tree diversity.

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