What are the different natural events that could alter the diversity index? this question has got me stuck on biology and i'm getting i might have more questions after this but probably not.

the truman doctrine was an american foreign policy whose stated purpose was to counter soviet geopolitical expansion during the cold war. it was announced to congress by president harry s. truman on march 12, 1947, and further developed on july 12, 1948, when he pledged to contain threats to greece and turkey.