THE MBTA is paring back service on 31 bus routes and the Mattapan trolley because it doesn’t have enough workers to provide the scheduled service.

Starting December 19, T officials said some service will be added on a handful of routes, but overall service levels during the winter will be declining by about 3 percent to reflect a large dropoff in the number of drivers available. The transit authority on Monday put out a call on Twitter for more drivers, saying 300 positions need to be filled.

The driver shortage has been building for some time. Over the last several months, the T has been canceling about one of every 20 scheduled bus trips because it doesn’t have enough drivers, the T said.

MBTA General Manager Steve Poftak said drivers are scarce across the board in the transportation sector and at many of the nation’s transit agencies. He said attrition is outpacing the T’s ability to add new workers.

“These service changes are not a cost-control measure,” he said in a statement. “The MBTA is budgeted for a full level of service, and ready to add back services when we have hired and trained new bus and train operators. We have teams at the MBTA working to streamline the hiring process and attract new employees, and I encourage all prospective candidates to visit mbta.com/apply to learn more about the many benefits that come with a career at the T.”

The Mattapan trolley, which runs between Ashmont Station in Dorchester and Mattapan Square, is also feeling the pinch. While service is being added on Saturday and Sunday evenings, reducing the time between trains from 23 minutes to 13 minutes, service is being pared back during morning and evening weekday periods, increasing the time between trains from 5 to 7 minutes. No other subway changes are planned.

For a complete list of the upcoming service changes, click here.