One of the most important conservation issues in ecology is the imperiled state of grassland ecosystems worldwide due to land conversion, desertification, and the loss of native populations and species. The Janos region of northwestern Mexico maintains one of the largest remaining black-tailed prairie dog (Cynomys ludovicianus) colony complexes in North America and supports a high diversity of threatened and endangered species. Yet, cattle grazing, agriculture, and drought have greatly impacted the region. We evaluated the impact of human activities on the Janos grasslands, comparing changes in the vertebrate community over the last two decades. Our results reveal profound, rapid changes in the Janos grassland community, demonstrating large declines in vertebrate abundance across all taxonomic groups. We also found that the 55,000 ha prairie dog colony complex has declined by 73% since 1988. The prairie dog complex has become increasingly fragmented, and their densities have shown a precipitous decline over the years, from an average density of 25 per ha in 1988 to 2 per ha in 2004. We demonstrated that prairie dogs strongly suppressed woody plant encroachment as well as created open grassland habitat by clearing woody vegetation, and found rapid invasion of shrubland once the prairie dogs disappeared from the grasslands. Comparison of grasslands and shrublands showed markedly different species compositions, with species richness being greatest when both habitats were considered together. Our data demonstrate the rapid decline of a grassland ecosystem, and documents the dramatic loss in biodiversity over a very short time period concomitant with anthropogenic grassland degradation and the decline of a keystone species.

Megaherbivores and small burrowing mammals commonly coexist and play important functional roles in grassland ecosystems worldwide. The interactive effects of these two functional groups of herbivores in shaping the structure and function of grassland ecosystems are poorly understood. In North America's central grasslands, domestic cattle (Bos taurus) have supplanted bison (Bison bison), and now coexist with prairie dogs (Cynomys spp.), a keystone burrowing rodent. Understanding the ecological relationships between cattle and prairie dogs and their independent and interactive effects is essential to understanding the ecology and important conservation issues affecting North American grassland ecosystems. To address these needs, we established a long-term manipulative experiment that separates the independent and interactive effects of prairie dogs and cattle using a 2 x 2 factorial design. Our study is located in the Janos-Casas Grandes region of northwestern Chihuahua, Mexico, which supports one of the largest remaining complexes of black-tailed prairie dogs (C. ludovicianus). Two years of posttreatment data show nearly twofold increases in prairie dog abundance on plots grazed by cattle compared to plots without cattle. This positive effect of cattle on prairie dogs resulted in synergistic impacts when they occurred together. Vegetation height was significantly lower on the plots where both species co-occurred compared to where either or both species was absent. The treatments also significantly affected abundance and composition of other grassland animal species, including grasshoppers and banner-tailed kangaroo rats (Dipodomys spectabilis). Our results demonstrate that two different functional groups of herbivorous mammals, burrowing mammals and domestic cattle, have distinctive and synergistic impacts in shaping the structure and function of grassland ecosystems.

The Angel de la Guarda deer mouse Peromyscus guardia on Estanque Island, in the Angel de la Guarda archipelago of the Gulf of California, was probably driven to extinction by a single introduced domestic cat. P. guardia was trapped on the island in October 1995, at which time the species was still relatively abundant. In 1998 a domestic cat was spotted on the island; no deer mice were found at that time nor during subsequent field work in 1999 and 2001. In 1998, c. 100 cat scats were collected, 2% of which contained P. guardia bone remains and 90% P. guardia hair. The cat, which was eradicated in 1999, was the only introduced predator on the island. Our results confirm the extreme vulnerability of island rodent populations to the introduction of alien mammalian predators. To our knowledge, apart from the extermination of Stephens Island wren Xenicus lyalli in New Zealand in 1894 by the lighthouse keeper's cat, this is the first report of the destruction of the total population of an insular species by a single cat. With two of the three subspecies of P. guardia now extinct the only potentially extant population is on the larger Angel de la Guarda Island, where the species was last seen in 1991. A comprehensive survey of the island is required, with subsequent action for the species recovery and conservation if it is found to be extant.