What would happen to the grass population if the population of wolves in the ecosystem decreased? why ?

Wolves are a predator, meaning they keep the cattle population down. Take away wolves and you end up with more cattle. More cattle eats more grass, too much grass. The grass is eaten faster than it can grow back. Cattle are being born more quickly than being eaten by wolves. The region would ultimately become uninhabitable and nothing could live there. 

Probably north korea if your family has decided to use only nuclear power.