Patterns and Processes in the Development of the High Andean Flora

Abstract

Previous work has suggested that the floras of the separate regions of the high Andes are the products of disparate geographical, ecological, and geological factors. Among the important factors that differ among the regions are distances from source areas of propagules, climates, and differential effects of Pleistocene glaciations. To ascertain the relative importances of these factors in shaping the modern floras, and to test hypotheses about the causes of observed generic diversity patterns, models of abundances of species per genus were tested against data from three samples representing the major vegetation zones of the high Andes (páramo, puna, and austral alpine). While originally proposed for intraspecific phenomena, abundance models (broken stick, log normal, geometric series, and log series) appear to be applicable to supraspecific taxa and were used to assess alternative hypotheses. Potential sources of genera now present in the sample areas were compared, and coefficients of similarity were computed between these samples and between each of them and two North American high-elevation floras. Propagule types in the three Andean samples were also scored to see if methods of fruit dispersal differed. Results showed that all areas fit the geometric series model if abundance plots of ranked genera were compared, but these results were heavily influenced by the large number of genera represented by only one species. Both the puna and austral alpine samples fit the log series models, indicating that the habitats of these areas are relatively uniform and that arrival of propagules was random over time. The lack of a fit of the Colombian data to the log series or log normal models is due primarily to a large number of species-rich genera relative to other areas. These findings support the hypothesis that there has been a pronounced level of autochthonous speciation in the Colombian Andes relative to other areas. Coefficients of similarity at the generic level show that a given flora is most similar to that of the nearest sample area within the same continent. However, the floristic similarity between Tierra del Fuego and the high mountains of Colorado is as high as that between the Tierra del Fuego sample and the neighboring Argentina sample, indicating that similarities in climate can be as important as distance in determining the composition of some Andean floras.