How can the extinction of a single species affect how the ecosystem functions? give an example?

Predators who need a prey for energy wont be able to get it. Extinction affects all of the species predators and prey

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