The major effect of genetic drift is

Random changes in specific trait of a given small population

Explanation:

Genetic drift is a phenomenon which is more specific in population of small size. Usually under the genetic drift effect , a population with a specific trait tries to reproduce with population that has no such traits and hence, in long run the resulting population eventually loses this specific trait.

Thus, genetic drift causes loss of alleles that are rare and also reduces the size of gene pool. The new population is genetically distinct than the original population.

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.

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms