Biogeographical transitions in the Sierra Madre Oriental, Mexico, shown by chorological and evolutionary biogeographical affinities of passerine birds (Aves: Passeriformes)

Abstract

Aim: To locate areas of biogeographical transitions within the Sierra MadreOriental (SMO), Mexican transition zone.Location: Eastern Mexico (18° to 27° N ? 90° to 96° W) and the Americas.Methods: We deconstructed passerine fauna into biogeographical affinities tomap their integration in the studied area. We defined and quantified two features of biogeographical affinity based on raw distributional range concordance(chorological affinity), and on ancestral state reconstruction ofpublished phylogenies (evolutionary biogeographical affinity). Then, wedivided the SMO into 500 m elevation intervals and 1.2° latitudinal belts.Point records of resident passerine birds were used to compute species turnoverand to map the dominance and heterogeneity of biogeographical affinitiesalong the gradient.Results: Considering the gradient of biogeographical affinities, we identifiedthe sharpest transition at elevations of c. 1500 m from the southern evergreencloud forests to the canyons of the middle Panuco basin (c. 22° of latitude)and then turning eastwards, following the Panuco river through the semideciduous lowland forest to the river mouth in the Gulf of Mexico.Main conclusions: Our analysis allowed us to map a gradual pattern of variationbased on a quantitative definition of biogeographical affinities. Thisapproach is particularly useful for the analysis of rather small areas where aregional biota cannot be partitioned concomitantly with regionalization procedures. Our findings support previous suggestion of a middle elevation zone of mixture between Neotropical and Nearctic biotas, as shown by their chorotypesand cenocrons, and highlight the Panuco river as a biogeographical boundarydividing northern and southern assemblages based on bird distributions innorth-eastern Mexico.