Question

What problems would you expect to observe in an ecosystem without primary consumers?

Answer 1

Some problems include overgrowth of producers and extinction of secondary consumers.

Answer 2

Stephstar is right.

Do you remember the levels of the food chain?

Primary Producers-->Primary Consumers-->Secondary Consumers-->Tertiary Consumers-->Quaternary Consumers

(Primary Producers are at the bottom. Quaternary Consumers are at the top.)

Take a look at the order of the food chain, above. You see how Primary Consumers are ABOVE Primary Producers?
That means Primary Consumers normally EAT Primary Producers.

But if you don't have Primary Consumers, who will eat the Primary Producers??

No one! And if nobody eats the Primary Producers...the Primary Producers will thrive. There would be a WHOOOLE LOT of Primary Producers.