pollution in an ocean ecosystem causes a significant drop in the sea star population. within a year, scientists monitoring the ecosystem note that sea urchin and mussel populations on which the sea stars feed have greatly increased. the prevalence of mussels has caused the plankton population on which the mussels feed to decrease, and the prevalence of sea urchins has caused a similar drop in algae. which species is most likely the keystone species in this ecosystem? algae mussel plankton sea stars