Biodiversity, at the level of the species, is basically set by the balance between speciation and extinction. If the rate of speciation is higher than the rate of extinction, the number of species present will grow, and conversely, if extinction rate is higher, the number of species present will fall.
If I were a doctor treating a patient with staphylococcus aureus i would prescribe penicillin because it controls bacterial growth. It also helps defend the body against dangerous diseases.
