Final answer:
Vertical zonation on rocky shores is mostly due to the differences in organisms' tolerance to wave action, which affects their ability to withstand the physical impact and changes between submersion and exposure.
Step-by-step explanation:
Vertical zonation on rocky shores is primarily the result of differences in tolerance to wave action. This involves the intertidal zone, where organisms are exposed to the air during low tide and submerged under water during high tide. Creatures living in this zone are well adapted to withstand the pounding of the waves, which may include having tough exoskeletons like that of the shore crab, Carcinus maenas, to protect against desiccation and mechanical damage. Few algae and plants can establish themselves in the rocks, sand, or mud due to the constant movement caused by the waves.