PS 32-36
Testing for thresholds in a semiarid grassland: the influence of prairie dogs and plague on vegetation dynamics
State and transition models for semiarid grasslands in the Great Plains of North America suggest that the presence of herbivorous black-tailed prairie dogs (Cynomys ludovicianus) on a site (1) creates a vegetation state characterized by increased dominance of annual forbs and unpalatable bunchgrasses, and increased bare soil exposure, and (2) requires long-term (>40 yr) prairie dog removal to transition back to a vegetation state dominated by palatable perennial grasses. We examined (1) how the history of prairie dog occupancy on a site (1 to 10 years) influences the magnitude of prairie dog effects on vegetation composition, and (2) how occupancy history affects vegetation dynamics following removal of prairie dogs. We used a natural experiment where prairie dogs were removed from multiple sites during an outbreak of epizootic plague. Within each site, we examined vegetation dynamics at locations that had been occupied by prairie dogs for varying lengths of time prior to plague.
Results/Conclusions
On sites occupied by prairie dogs for 1–4 years prior to removal, plant cover and composition recovered to conditions similar to unoccupied (control) sites within a single growing season. Significantly larger reductions in perennial C4 grasses were observed on sites occupied for the prior 7 – 10 years compared to sites with shorter occupancy histories (<6 years). C4 perennial grasses did recover over 5 years following prairie dog removal from sites occupied the prior 7–10 years; in addition, C3 perennial graminoids and forbs remained more abundant (compared to sites with no history of prairie dogs) throughout the 5 year period. Our findings show that prairie dogs did not induce irreversible shifts in vegetation state in this semiarid grassland. Rather, this semiarid ecosystem was highly resilient to the effects of prairie dog grazing. State and transition models for semiarid grasslands should explicitly incorporate the time frame over which colonial, burrowing herbivores, such as prairie dogs, occupy a site.