Introduced plague lessens the effects of an herbivorous rodent on grassland vegetation

1. Plague, introduced from Eurasia around 1900, today affects many animal species in the western part of North America, including black-tailed prairie dogs Cynomys ludovicianus, widespread colonial herbivores of the Central Plains. Most studies of introduced diseases describe how they affect susceptible individuals or populations. We examined the indirect community and ecosystem-level effects of plague to address the hypothesis that frequent plague epizootics have a mediating effect on the important role of prairie dogs in shaping grassland plant communities. 2. We used 25 years of spatially explicit monitoring data on the Pawnee National Grassland (Colorado, USA) to estimate frequency of plague epizootics in prairie dog colonies. We also examined plant community characteristics of young (3-8 years), old (~20 years) and plague-extirpated (7-12 years when extirpated) colonies intermittently to determine how colony age and status (active or inactive) influence the effects of prairie dogs on plant communities. 3. Approximately 98% of colonies experienced a plague epizootic within 15 years of continuous activity, nearly half remained inactive for at least 5 years following an epizootic, and less than half attained their pre-plague area within 10 years of an epizootic. 4. Prairie dogs lowered plant canopy height, reduced plant biomass and altered plant community composition. These effects were most pronounced for older colonies. Plague-extirpated colonies were not significantly different from their associated off-colony sites for most variables measured. 5. Synthesis and applications. Our work shows that introduced diseases can indirectly affect community- and ecosystem-level processes through their modification of host species dynamics. Frequent epizootics that lessen the effects of prairie dogs on vegetation probably affect colony-associated species and grassland food webs. Over 100 animal species are associated with prairie dog colonies because prairie dogs are an important food source and their colonies provide habitat. Prairie dogs are considered competitors with livestock for forage and are exterminated on private lands at considerable expense to ranchers. Extermination may be unwarranted and cost-ineffective where plague exists because of the high frequency of plague epizootics.