Pollution in an ocean ecosystem has caused a significant drop in sea otter population. Within a year, scientists monitoring the ecosystem note that the sea
urchin population on which the otters feed has greatly increased. The prevalence of
sea urchins has caused the kelp and algae populations on which the urchins feed to
decrease. The scientists fear the urchins will also begin to overwhelm the ecosystem's fragile coral population. Which species is most likely the keystone
species in the ecosystem?

Algae
Coral
Sea star
Sea urchin