Question

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

Answer 1

If there were no secondary consumers, there would be an increased population of primary consumers. Since primary consumers are herbivores, they would be eating plant life (producers) at a much higher rate. This could lead to many implications.

Plant life may be threatened. Which would cause increased difficulty for primary consumers to survive after they overpopulate.

If plants are over consumed, microorganisms that rely on plants as hosts (either parasitic, or many times symbiotic) will suffer.

If those plants are flowering, local bee and pollinator populations can also be affected.

Overpopulation of primary consumers can lead to diseased of primary consumers. since there would be more of them. They could also encroach on new territory an harm wildlife in another area.

Loss of plant life can lead to erosion and loss of land nutrients, along with any farming capability. Can also be desert forming.

If you removed all the primary consumers from an ecosystem, there would be no way for energy to get from the producer (plant) level to higher level consumers which eat only primary consumers. So initially you would have see population increases in producer (and probably producer diversity) and decreases in secondary, tertiary and higher order consumer populations.