Several species of fish and marine life eat plankton. how is that diversity important to the ecosystem as a whole?

If there weren't so many species eating the plankton, two main things could possibly happen.

1) The amount of plankton could significantly rise, to possibly dangerous levels.
2) Other species would be expected to be abundant, so that the animals can eat something else.

See, if plankton is the building block of the food web, imagine how different the animals who eat plankton would be if the building block was removed or changed?

Hope this helps! <3