Question 5 unsaved in a tide pool, 15 species of invertebrates were reduced to 8 after one species was removed. the species removed was likely a(n)

The species was a keystone species.

Keystone species are the species that ''hold the ecosystem together''.
They have an important role in the trophic networks (food chains) and often they can even afflict changes in the abiotic part of the ecosystem (change the composition of soil, purify the water, lower the effect of the wind etc)
Therefore, when a keystone species is removed it affects greatly the whole ecosystem.