How do plant populations be affected if the populations of decomposers such as earthworms decreased dramatically

A decrease in the number of earth worms will also decrease the growth of plants.

Explanation:

Earths worms are the most essential dwelling invertebrates. They affect the biotic as well as abiotic properties of the earth. They are the decomposers in soil and provide free nitrogen to plants. So, if they are dramatically decreased in the soil they will affect the plant growth directly.

The plant population may reduce with decrease in earthworms population

Explanation:

Earthworms are very essential for plants especially the garden plants. They are one of the major source of nutrients for the plants. They decompose the dead organic matter in soil and excrete waste which is rich in nitrogen, phosphorus, magnesium etc.

If the population of earth worm decreases, the rate of conversion of dead organic matter into essential nutrients will reduce and hence the plants will not get sufficient nutrition from soil and they may die.