The biggest buyers of U.S. corporate bonds since the financial crisis may be the least likely to stick around when the Federal Reserve raises interest rates.

Mutual funds have purchased about half of the company bonds that have been added to the U.S. market since 2008, equivalent to about $1.5 trillion of assets, according to Morgan Stanley.

That’s helped expand mutual funds’ share of the debt to 16 percent from 13 percent five years ago and contrasts with a decline for buyers such as pension funds and life insurers that tend to hold onto the securities longer.

The shifting base is increasing the risk that five straight years of gains will end for a section of the market that’s been among the biggest beneficiaries of both mutual-fund demand and the Fed’s easy-money policies: shorter-term notes due in one to five years.

“A lot of the mutual-fund ownership has been in the front end, so if we have a significant rise in rates in that part of the market, at some point investors may look to other opportunities,” said Sivan Mahadevan, a credit strategist at Morgan Stanley in New York.