Résumé
A large area in the Gulf Coast region of Mexico is pastureland, generally dominated by native grass species, yet little is known of the physical, chemical and biological characteristics and limitations of soils beneath them. Furthermore, nothing is known of the effect of converting native to introduced grasses on the soil ecosystem in Mexican pastures. Over the last 30 years 60 samples were taken in 21 sites throughout SE Mexico to evaluate soil macrofauna communities. Of these, 15 samples were taken at five sites in the state of Veracruz, Mexico, taken during the dry (April–May) and/or wet (September–October) seasons of 1998 and 1999, to specifically compare soil macrofauna present in native and introduced pastures. These sites were located in a N–S transect including three biogeographic regions, separated by the transverse Neo-volcanic axis. Taking data from all sites, earthworms, ants and termites dominated in terms of density, while earthworms dominated the soil fauna biomass, commonly surpassing the weight of the grazing cattle per hectare. Of a total of 15 comparisons of soil fauna populations in native and introduced pastures, important differences in the communities were observed on nine occasions, using multivariate analyses. These differences, however, depended on the site sampled, season, and sample year, and tended to be more evident in the rainy season, when populations were at their maximum numbers. Earthworm communities were different between the two pasture types; twice as many species on average were found in native (four species) than in introduced (two species) pastures. Most species were native to Mexico, only a few exotics being found, indicating slow exotic species invasion rates or little replacement of natives by exotics. In conclusion, the present study showed that large communities of soil macrofauna are present in SE Mexican pastures and that, depending on the site’s characteristics and the management practices implemented, the conversion of native to introduced pastures can significantly alter the diversity and abundance of soil-dwelling macro-invertebrates. However, further studies must be undertaken in other pastures, particularly well-managed introduced pastures with or without legume associations, to assess whether these results hold true under a wider range of management situations and sites.