In subduction zones, dense pieces of oceanic crust are pulled downward toward the mantle. what is the result of this?

Volcanic eruption

During plate tectonics, the continental and oceanic plates collides. The oceanic plates being lighter gets submerged under continental plate. The subducted oceanic plate reaches the deep layers of the earth crust. The magma  present in the earth mantle melts the bottom layer of the subducted plate. The pores are formed on the subducted plate, the hot magma diffuses inside the plate. The plate becomes bouyant as the density of plate decreases. If the magma solidifies in the subducted plate, it will not have any impact on the surrounding water, if it remain in the molten stage can cause underwater volcanic eruption.

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.