What is formed during primary succession that

doesn't need to be formed during secondary

succession?

During the primary succession soil is formed.

Explanation:

It doesn't need to be formed in the secondary succession because the pioneer plants already broke down the mineral-rich rocks into soil. Which in term allow less hardy species to grow and eventually replace the pioneer species.

I think the answer would be lichens, because lichens are formed during primary succession as a pioneer species. During secondary succession, grasses are usually the pioneer species.