Neotropical-Nearctic limits in Middle America as determined by distributions of bats

The Neotropical-Nearctic transition in Middle America was analyzed using distributional patterns of bats. The 169 non-insular bat species of Middle America were classified into four categories: shared with North and South America, shared only with North America, shared only with South America, and endemic. A theoretical biogeographic index was developed and compared with null frequency distributions based on the multivariate hypergeometric model. A total of 1,054 half-by-half-degree quadrats were used to determine the biogeographic zone (Nearctic, Neotropical, or transitional) of sites in Mexico and Central America. The null distribution allowed statistical assessment of each quadrat, permitting the quantitative determination of the limit between the two biogeographic regions. Results showed a pattern that coincided with previous studies in the position of the southern limits for the Nearctic region but differed in locations of Neotropical areas, and in the extent of the transitional zone, which was much broader in the present study.