A growing number of wealthy art collectors are choosing to build their own museums rather than donate works to existing institutions, leaving many museum directors nervous about the future of their collections, the Wall Street Journal reports.

Though still relatively rare, private museums opened by wealthy collectors such as Wal-Mart heiress Alice Walton and Gap founder Don Fisher are increasing in both number and size, resulting in a dip in donations to more established institutions. According to the Association of Art Museum Directors, the number of art objects donated to top museums fell 5.7 percent last year from the previous year, even as the value of those donations increased 130 percent, to almost $607 million.

The reemergence of the private museum isn't the only model to roil the art scene in recent years, however. Museum directors around the country increasingly find themselves outbid by wealthy collectors for major masterpieces at auction, while a little-noted change in the tax law affecting "fractional" donations of art — a strategy that allowed collectors to "donate" artworks to tax-exempt institutions while keeping those works on their walls, often for decades — has made giving away art less attractive. "Nothing has been more destructive for museums in the last five years," said San Francisco Museum of Modern Art director Neal Benezra. "We have to work very hard to justify to collectors why they should donate."

Some museum directors are working to combat these trends by launching aggressive acquisition campaigns, appointing collectors to their boards, and expanding their space to display more works and appease donors who don't want their gifts sitting in storage. (At many major museums, only about 5 percent of the collection is on view at any time.) According to the American Association of Museums, some 40 percent of U.S. museums are undergoing or have just completed an expansion.

One of them, the Museum of Contemporary Art, Los Angeles, is currently featuring an exhibition called "Collecting Collections," which displays approximately two hundred and fifty objects from more than a dozen collections given to the museum over the years. According to museum director Jeremy Strick, the desire to attract future gifts partly motivated him to mount the exhibition. "Displaying great collections from our patrons always serves as a magnet for future donations," he said. "People don't give in a vacuum."