In this study, Nash et al. measured annual plant community dynamics along transects at several distances away from livestock watering points, to determine how livestock grazing intensity affects annual plant populations in the Chihuahuan Desert. Nash et al. used multivariate analysis and semivariograms to locate boundaries and to determine the number and width of different annual-plant zones (biotic zones) on long-term livestock disturbance gradients in the northern Chihuahuan Desert, New Mexico. The relationships of annual-plant abundance and species richness with distance from water points and with perennial-plant cover were not significant. The number of boundaries and sizes of zones varied with distance from water points, with season, and with duration of grazing. The first biotic zone (most severely impacted by cattle) ranged from 75 to 795 m radius for winter-spring annuals and from 165 to 1065 m radius for the summer annuals. There were fewer and larger zones of summer annuals than of winter-spring annuals. Boundary analysis of livestock disturbance gradients provided a method with replication for assessing the impact of long-term livestock grazing on annual-plant communities. Livestock create nutrient rich patches near water points and these patches support species of annuals that are rare or absent in areas where soils are subjected to low-intensity disturbance.
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