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Buffelgrass (Pennisetum ciliare) has been planted in large areas of desert and thornscrub of northwestern Mexico in order to increase forage production. However, no previous study has documented the influence of land conversion on the native species that persist on buffelgrass pastures. In this paper, we describe the influence of buffelgrass conversion on the regeneration and reproduction of the columnar cactus, Pachycereus pecten-aboriginum, in the state of Sonora. We selected three different pastures of different age and adjacent undisturbed thornscrub where we estimated abundance and population structure of this cactus. During the flowering season, we estimated several reproductive parameters. We also set up an experiment to explore whether seedlings are able to establish in the buffelgrass pasture. Our results indicate that land conversion has no significant effect on plant abundance, but a major effect on size distribution. For most reproductive parameters, no statistically significant differences were detected between thornscrub and pastures. Seedlings were able to establish only on thornscrub, as all seedlings that emerged in pasture died within I year. Land conversion has apparently a minor effect on plant reproduction, but a significant effect on the regeneration of this columnar cactus. Our data suggest that adult populations that persist on pastures are unable to self-replace and native populations will be driven into local extinction. (c) 2007 Elsevier Ltd. All rights reserved.