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

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.