BIRD COMMUNITIES ASSOCIATED WITH 3 TRADITIONAL AGROECOSYSTEMS IN THE SAN-LUIS-POTOSI PLATEAU, MEXICO

The bird communities associated with three traditional dry farming systems, their borders and their unfarmed surroundings on the San Luis Potosi Plateau, Mexico, were studied throughout the year. The systems studied were (1) a purely rain-fed field in a grassland with shrubs, (2) a field on a bajada irrigated with runoff water in a complex and dense community of cacti and shrubs, and (3) a bajio field irrigated with spring moisture and an ephemeral wash in a savanna. The results are in agreement with the hypothesis that complex agroecosystems are less susceptible to pest outbreaks than simple ones. There was no typical cropland avifauna; communities depended on the particular type of agroecosystem and on the original habitat. The foliage height diversity index did not always parallel bird richness, since there are factors affecting the latter which do not affect the index. Edge effect was not evident in this study. Although two agroecosystems had lower richness than their surroundings, the presence of isolated fields increased regional bird richness.