Tropical dry forest trees and the relationship between local abundance and geographic range


Correspondence: John N. Williams, Department of Environmental Science and Policy, University of California, One Shields Avenue, Davis, CA 95616, USA.


Aim  To test the macroecological principle that a positive relationship exists between local abundance and geographic range size for tree communities in the tropical dry forest.

Location  Two tropical dry forest (TDF) regions on the Pacific coast of Mexico: one near Chamela, Jalisco; the other near Huatulco, Oaxaca.

Methods  We recorded species presence and relative abundance of trees and lianas from over 40 locales in each of the study regions using transects across an elevational gradient. We then compared the field data with occurrence data from national and online databases to examine how local patterns of abundance relate to putative geographic range areas and latitudinal breadth.

Results  We found no significant correlation between abundance and range size. Overall, many more locally abundant species had small ranges than large ones. We found that most species occupy the majority of the TDF range north of Colombia, and those species present in South America occupy the majority of that continent’s TDF range as well. This pattern was independent of local abundance. We also found no relationship between range size and local niche breadth as measured by elevation, or between local abundance and distance to the range centre.

Main conclusions  The macroecological tenet that posits a positive correlation between local abundance and geographic range size does not appear to hold for TDF trees. The finding that many locally abundant species had narrow ranges also suggests that dry forest endemics may be particularly well adapted to local conditions and make important contributions to community structure. We hypothesize that the absence of abundant species with large ranges is due to opposing environmental constraints that prevent a species from thriving everywhere.