Transition zones and biogeographic characterization of endemism in three biogeographic provinces of central Mexico

Abstract

Background: Sites of overlap in the species distribution among regions are known as transition zones. Their floristic and biogeographic complexity is rarely discussed.
 Questions: Is it possible to identify transition zones amid the biogeographic provinces of Mexico? Can transition zones also be considered areas of endemism?
 Study species: 315 Asteraceae taxa endemic to Mexico.Study site: Volcanic Belt, Balsas Depression and Sierra Madre del Sur biogeographic provinces.
 Methods: For species of Asteraceae endemic to three biogeographic provinces of central Mexico, we characterized the province to which each species belonged and estimated the distance it could penetrate the neighboring province. We defined transition zones between biogeographic provinces as the average penetration distances of the species. We also evaluated the presence of species with restricted distribution to transition zones.
 Results: Two transition zones were identified in the Balsas Depression, first, at the junction with the Volcanic Belt with a width of 13 km, and second, with the Sierra Madre del Sur, with a width of 12.3 km. There were 45 species whose distribution was restricted to those transition zones.Conclusions: Understanding how richness and endemism are exchanged from one region to another is essential to explain how their floras have been assembling and evolving. This analysis has allowed us to better understand the relationships among regions as complex as those studied here.