Perhaps the most significant ecological change that man has initiated in the Black Hills has been the suppression of wildfire. Pictures taken during the Custer exploratory expedition of 1874 show clearly that the hills had a much less dense growth of ponderosa pine. When people began suppressing wildfires a natural thinning process was stopped and the ponderosa began to encroach upon the prairie. In fact it was more dramatic than that. Because of favorable precipitation and temperature in the Black Hills, the regeneration of ponderosa pine is excellent which led to an explosion in the ponderosa pine population. Black Hills spruce is another conifer that benefits from fire suppression.
As so often happens when people change the natural order of things, the ecological balance was upset. In many areas where we once had a greater variety of vegetation, more grasses, shrubs and forbs, we now have a monoculture. In some places the pines grow so dense that the forest floor doesn't receive enough sunlight to support vegetation that would be more beneficial to animal life. Deer, elk, and pronghorn do not get nearly as much sustenance from pine trees as they could from the grasses and shrubs the pine trees have displaced. Other hardwood trees such as the bur oak, American elm, boxelder, birch and aspen are also important food sources that have been displaced by the encroaching pine trees.
Because of increased transpiration from millions more trees it is likely that the streams are not carrying as much water as they did historically. The character of the soil itself is changing.