When Pisaster was removed from an enclosed intertidal area of the American Pacific Coast, more than 10 species of ____(i)_______became extinct because of ____(ii)_______.

The correct option is B (i) - invertebrates, (ii) - interspecific competition
Populations of different species live together and interact in several ways. These interactions are called interspecific interactions and can be beneficial, harmful or neutral to the interacting species.

Predation is a type of interaction, in which one organism called the predator eats another organism called the prey.

The predator is represented by ‘+’ sign as it gets benefited and ‘-’ sign is used to indicate prey as it gets harmed or killed.

Predator-prey relationships are of great importance to maintain the ecological balance in nature. One of the roles played by the predators is maintaining the species diversity in a community. This can be understood by considering the following example. The starfish Pisaster is an important predator in the rocky intertidal communities of the American Pacific Coast. This predatory starfish feeds on the mussels (molluscs) and keeps its population under control and creates space for several other invertebrate species to coexist along with the mussels. This is how Pisaster maintains the species diversity of that community.

In a field experiment, when Pisaster was removed, the mussel population increased rapidly due to the absence of its predator. These mussels covered the entire intertidal shore and there was severe interspecific competition for space and hence other invertebrate species could not establish themselves. This severe interspecific competition for space led to the extinction of more than 10 invertebrate species.