What would happen to the owls if there were fewer grasshoppers?

In an ecosystem, there are food chains and a food web, which is made up of interconnected food chains. Owls and grasshoppers can be considered to be part of the same food chain, since owls are natural predators of grasshoppers. 

From the point of view of a food chain in which owls are dependent on grasshoppers, if most of the grasshoppers are removed (that is, the grasshopper population falls drastically) then owls will have scarcity of food and will die out. A new equilibrium will be established with fewer owls supported by fewer grasshoppers. 

However, from the perspective of the food web, owls have more than 1 food source and if grasshoppers are reduced, owls will live on other food sources and hence the effect on the owl population will not be as drastic as in the case of a food chain.

In essence, in an ecosystem, a reduction in the population of grasshoppers will result in a decreased owl population and a newer equilibrium between the various species.

Hope this helps.