Explain how the physical characteristics of an aquatic habitat determine the presence or absence of organisms.

The physical characteristics that determine the exhistance of organisms in aquatic enrironments relate to its pH (which is whether it's acid, neutral or basic), the water clearance (which refers to the particles diluted in it, the presence of toxic substances, etc), the presence of predators (bio chain) and also if there's enough food for the organisms to live, to reproduce and if the water temperature allows them to subsist or if they need to migrate within a year, due to the natural changes.

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.