The president of Rhode Island’s flagship public university says it is critical to extend commuter rail service to the southern part of the state. David Dooley, president of the University of Rhode Island, said Wednesday that extension of the existing Massachusetts Bay Transportation Authority rail line to the campus in South Kingstown is essential. Dooley said a station in South Kingstown would probably be more widely used than the one that opened earlier this year in North Kingstown. He said it would have a built-in ridership, given the campus community of 18,000 people. The MBTA began commuter rail service to the new Wickford Junction Train Station in North Kingstown in April. Wickford is now the most southern stop on the Providence-Stoughton line.