Final answer:
A stable population of 300,000 prairie dogs has been maintained in the Black Hills over 40 years, indicating the species' role as a keystone in the ecosystem, with its burrowing activities having significant ecological impact.
Step-by-step explanation:
The number of 300,000 prairie dogs in the Black Hills of South Dakota would best be described as a stable population. This is indicated by the relatively consistent population size over the last 40 years, with only minor fluctuations of a few thousand individuals each year. Prairie dogs are considered a keystone species in their ecosystem due to their extensive burrowing activities which provide shelter and protection for various small animals and influence the presence of large predators. These underground burrows also aid in soil aeration and can affect the growth of vegetation above ground.