Answer:
Predation is the relationship.
Explanation:
Predation is the relationship present between the two species of barnacles and the snail because snails feed on barnacles for their living. In the fundamental niche, the snail grow in population due to availability of suitable environmental conditions, but when there is realized niche, predation and competition exists which limits the growth of the population of organism. Here barnacles population is limited due to presence of snail which feeds on it.