Jesús Martín Castillo-Cerón

Beta Diversity in a Highly Heterogeneous Area: Disentangling Species and Taxonomic Dissimilarity for Terrestrial Vertebrates

Quantifying differences in species composition among communities provides important information related to the distribution, conservation and management of biodiversity, especially when two components are recognized: dissimilarity due to turnover, and dissimilarity due to richness differences. The ecoregions in central Mexico, within the Mexican Transition Zone, have outstanding environmental heterogeneity and harbor huge biological richness, besides differences in the origin of the biota. Therefore, biodiversity studies in this area require the use of complementary measures to achieve appropriate information that may help in the design of conservation strategies. In this work we analyze the dissimilarity of terrestrial vertebrates, and the components of turnover and richness differences, among six ecoregions in the state of Hidalgo, central Mexico. We follow two approaches: one based on species level dissimilarity, and the second on taxonomic dissimilarity. We used databases from the project “Biodiversity in the state of Hidalgo”. Our results indicate that species dissimilarity is higher than taxonomic dissimilarity, and that turnover contributes more than richness differences, both for species and taxonomic total dissimilarity. Moreover, total dissimilarity, turnover dissimilarity and the dissimilarity due to richness differences were positively related in the four vertebrate groups. Reptiles had the highest values of dissimilarity, followed by mammals, amphibians and birds. For reptiles, birds, and mammals, species turnover was the most important component, while richness differences had a higher contribution for amphibians. The highest values of dissimilarity occurred between environmentally contrasting ecoregions (i.e., tropical and temperate forests), which suggests that environmental heterogeneity and differences in the origin of biotas are key factors driving beta diversity of terrestrial vertebrates among ecoregions in this complex area.

FEEDING ECOLOGY OF ASTROHIPPUS STOCKII FROM THE LATE HEMPHILLIAN OF CENTRAL MEXICO: STABLE CARBON ISOTOPES ANALYSIS AND DENTAL WEAR PATTERNS

Abstract. The small-sized Astrohippus stockii was a one-toed horse restricted to the latest Hemphillian (Hh4) NALMA (5.8–4.6 Ma). Although this species is relatively common in fossil localities of the United States and Mexico, much remains to be known regarding its feeding ecology. We applied a multidisciplinary approach to reconstruct the dietary behavior of A. stockii based on a sample from Arroyo La Carreta locality, latest Hemphillian of Guanajuato, central Mexico. Carbon isotope and meso- and microwear analyses were conducted. The &dgr;13C isotope data indicate a diet consisting of both C3/C4 plants with a preference for C4 plants (average &dgr;13C of -1.9‰ ± 1,3‰ [SD]), whereas the micro-(high number of scratches and a low number of pits) and mesowear (round to blunt cusps and low relief) signatures implied processing of abrasive sources (biogenic and abiogenic). The information about the feeding ecology of A. stockii from Guanajuato, Chihuahua, Florida and Texas showed that this North American horse had broad feeding habits, ranging from almost exclusive grazing to browsing. The dietary flexibility in this horse species is probably related to local environmental conditions, suggesting an adaptable dietary regime across its geographic distribution. This is the first study that uses three of the most widely applied analyses to reconstruct the feeding ecology of a Mexican fossil ungulate species.