Action: increasing the amount of oxygen in the water

variable is independent.
variable is dependent.

x would be the most and the least would be y because there are more fruit   and seeds than flock z has insects

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.