No studies have attempted to quantify the loss of biodiversity that results from the widespread practice of converting native plant communities to exotic buffelgrass (Pennisetum ciliare) pastures in northern Mexico. We examined the effects of land conversion on the diversity of perennial plant communities in the state of Sonora where the extent of buffelgrass pastures has grown exponentially over the past four decades. Land conversion is occurring along a rainfall-driven gradient of primary productivity across which native vegetation communities transition from desertscrub to thornscrub to tropical deciduous forest. We sampled perennial plant diversity in buffelgrass pastures and native plant communities to compare the form of the productivity-diversity relationship in these two distinct habitats that lie along the same environmental gradient.
Diversity increased linearly with productivity in both buffelgrass pastures and native plant communities. We found that land conversion to buffelgrass pastures reduced species richness by approximately 50% at both local and regional scales.
In contrast, the effects of land conversion on dominance were significantly modified by productivity. In addition, native vegetation communities exhibited higher beta-diversity (among-site diversity) than buffelgrass pastures. We conclude that land conversion to buffelgrass pastures results in a large loss of perennial plant diversity and significant changes to spatial patterns of diversity that are likely to affect ecosystem functioning. If rates of land conversion remain unchecked, the native desertscrub and thornscrub ecosystems of central Sonora may be replaced by an exotic grassland that exhibits substantially reduced levels of biodiversity with significantly altered ecosystem functions.