Spatial variations in the abundance of Sporophila seedeaters in the southern neotropics:: Contrasting the effects of agricultural development and geographical position

Human alterations of the habitat may interfere with the natural processes that determine spatial patterns of species abundance. We examine the geographical position hypothesis and the agricultural transformation hypothesis to explain spatial patterns in the abundance of seedeater species (Sporophila spp.) in the southern Neotropics. The geographical position hypothesis predicts decreasing abundance with increasing distance from the center to the edge of a species' geographical range, and the agricultural transformation hypothesis predicts changing abundance as a response of variations in agricultural intensity. Bird abundance and the proportion of agricultural land were estimated for 16 transects covering 500 km along a gradient of both increasing agricultural intensity and increasing distance from the center of the species' geographical ranges. We found no evidence of distance effects on seedeater abundance. Responses to agricultural intensity varied among species. Neither the geographical position nor the agricultural transformation seemed to explain the pattern of abundance of Double-collared seedeaters (S. caerulescens). Agricultural intensity accounted for 63% and 99% of the spatial variation in the abundance of Dark-throated (S. ruficollis) and Tawny-bellied (S. hypoxantha) seedeaters, supporting the agricultural transformation hypothesis. The Dark-throated seedeater seem to be more tolerant to agricultural transformation than the Tawny-bellied seedeater, as they were not recorded in areas with more than 60% and 20% of agricultural lands, respectively. Our results indicate that the Dark-throated seedeater and the Tawny-bellied seedeater will most likely face (or may be already facing) a reduction in the southern part of their geographical range due to habitat loss to agriculture.