| In order to understand the ecological characteristics of three dry farming systems in the semiarid San Luis Potosi Plateau, Mexico, a one year study was conducted. The systems studied were a purely rainfed field, a field on an alluvial fan irrigated with runoff water, and a field in a bottomland irrigated with water diverted from an ephemeral stream. Three treatments, farmed, edge and unfarmed, were established in a Randomized Block design, with three replications, for each system.;The major conclusions of this research were the following. During the summer, climate is responsible for a concentration of the communities' production and reproduction. Climate is also of paramount importance to agriculture. Rainy periods, on the other hand, decreased the activity of animals. None of the farming systems had detrimental effect on soil fertility. Only slight modifications of soil temperature and air temperature and humidity resulted from farming.;A general overview of all the results did not provide evidence that all farming systems decrease biotic richness and diversity. The effects depended on the type of system, its isolation, and the natural vegetation adjacent to it. Herbs were enhanced by farming only when the natural system was relatively free of them. Farming did not have important effects on invertebrates. Birds were negatively affected by farming, whenever the unfarmed areas included an arboreal stratum. Also, insectivorous birds responded differently than non-insectivores. Rodents were affected negatively by farming in two of the systems, and this could be linked to habitat simplicity. In one case, dense herb cover was associated with very high rodent populations.;In general the data adjusted to the hypothesis that structurally more heterogeneous agroecosystems hold more diverse biotas. The concepts of "farmland biota" and "edge effect" were not supported by this study. Edges were occasionally superior, and only when they included more complex plant communities than either side. A mosaic of heterogeneous farmlands and natural vegetation attracts certain rodents and birds, increasing environmental diversity. |
