The second terminal at Minneapolis-St. Paul International Airport, which was mostly empty for many years after its 2001 opening, is now so busy that airport executives are shuffling one of its more recent, fast-growing entrants, Spirit Airlines, to the big terminal.

"Terminal 2 has been experiencing double-digit growth for the last several years," Metropolitan Airports Commission spokesman Patrick Hogan said Thursday. "It's reaching the point we need to either add on to Terminal 2 or move an airline."

The immediate decision, he said, is that Spirit will move to concourse E of Terminal 1 in January.

That will free up the gate that Spirit has been using, along with a second gate that it uses part-time, at Terminal 2 for other airlines. Sun Country is the biggest user of Terminal 2, but it is also occupied by Southwest, Condor, AirTran and Icelandair.

To make the move work, the airport is taking advantage of the shifting desires of other airlines at Terminal 1. Following its merger with US Airways, American Airlines asked airport officials to bring all of its gates together. The two airlines both flew out of Concourse E at the airport, but their gates were separated.

Hogan said the American and former US Airways gates will be consolidated and Great Lakes Air will move from Concourse E to either Concourse A or B. That will free up two gates on Concourse E for Spirit, which now has 11 daily departures from MSP but tends to increase its flights to warm-weather destinations during the winter months.

Spirit, known for its low-fare, no-frills operating style, started flying to and from MSP in 2012. It served about 217,000 MSP passengers that year and more than 600,000 last year.

Meanwhile, the biggest operator at Terminal 2, Sun Country, also has seen a huge jump in service, flying 1.5 million people in and out of the terminal last year, up from 1.2 million in 2012 and 1.1 million in 2011.

Known also as the Hubert H. Humphrey Terminal, the 10-gate facility opened in 2001 to fewer-than-expected airlines and passengers. The airports commission originally expected 17,000 arrivals and departures in its first year, but had only 7,100.

For many years, its fortune was tied to that of Sun Country, which went through bankruptcy in both 2001 and 2008 and had three owners over the course of a decade. In 2008, during Sun Country's second restructuring and the onset of the economic downturn, the number of passengers getting on at the terminal plunged 38 percent to just 783,000.

Last year, just over 2 million got on a plane at Terminal 2. "Long-term, we may still need to add some gates to ­Terminal 2," Hogan said.