An Accidental Spill of a Pesticide Locally Wiped Out One

An accidental spill of a pesticide locally wiped out one species of an endangered butterfly in coastal California. Several bird species had fed on this butterfly, but they switched prey items fairly quickly to more abundant moths, and thus the birds showed little change in population size. The endangered butterflies were:

A) keystone species, and thus they were driven to extinction very easily.
B) part of a predator-prey interaction.
C) ecological dominants, but they were not keystone species.
D) pioneer species.