How can pioneer species decrease 

Pioneer species can decrease if other species move it out. If the pioneer species in a forest area is a type of bush, that bush might decrease in number as more species come in.

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