A hypothetical species of antelope is host to a pathogenic species of bacteria that is spread through the air over short distances of 10 to 20 feet. Scientists have observed that in areas with a low population density, bacteria displaying lower virulence make up a greater presence than virulent strains. While in areas with a high population density, the reverse is true. Below is a graph showing the virulence of the bacterial pathogen compared to the density of the antelope population.
Which of the following BEST explains the relationship shown in the graph?