NEW YORK - Katie Couric's successor as "CBS Evening News" anchor faces an extraordinarily difficult job in lifting the network out of the ratings cellar.

Couric will be leaving her post after nearly five years, with her contract expiring on June 4, according to a network executive who spoke to The Associated Press on condition of anonymity because Couric has not officially announced her plans. The 54-year-old anchor is expected to launch a syndicated talk show in 2012.

She took over a broadcast that had spent many years in third place behind NBC and ABC in the ratings, and leaves it in third place. The competitive danger for CBS is that the evening news becomes what the morning news has been for half a century: the third choice of viewers no matter what network chiefs try.

New CBS News Chairman Jeff Fager will choose Couric's successor, and will look both inside and outside the company. Many people at CBS News believe that Scott Pelley of "60 Minutes" is the lead contender.

For the first three months of 2011, Couric's telecast was seen each night by an average of 6.4 million viewers, the Nielsen Co. said. That's roughly two-thirds of the typical audience (9.8 million) for Brian Williams and NBC's "Nightly News." ABC's "World News" with Diane Sawyer averaged 8.7 million viewers.

It is believed to be the smallest quarterly audience ever for the CBS evening newscast (comparable records have only been kept since the early 1990s), Nielsen said.

That's a huge gap to overcome for a format in which the audience is steadily shrinking and tends to be older than for most shows. Ratings changes among evening newscasts usually move at glacial speed as a result, and NBC's Williams is at his most dominant now.

News consultant Andrew Tyndall, who tracks the content of the evening newscasts, said he sees "no real way at all" for CBS to escape the basement anytime soon.

CBS is harmed by poor ratings for local newscasts at many affiliates and CBS-owned stations, offering poor lead-ins to network newscasts, Tyndall said. Many viewers don't switch networks from local to national news.

"You can't separate the performance of the evening news from the performance of the local news," Tyndall said.

CBS' ratings problems date to the 1990s. A handful of CBS affiliates changed ownership hands in cities such as Detroit and Atlanta, which made it more difficult for viewers to find the network and led to ratings erosion. Former anchor Dan Rather was often a polarizing figure, as well.

It's not necessarily CBS as a whole: The network's Sunday morning newscast remains the network standard-setter, as is "60 Minutes" on Sunday evenings, he said.

The evening ratings race was usually close in the late 1990s, and CBS battled it out regularly with ABC for second place at a time Fager was the executive producer. In Rather's later years, CBS settled regularly into third place.

CBS' surges from third place were the rare exception: CBS slipped past ABC into second place a few times while Bob Schieffer was the anchor in 2006. The "CBS Evening News" finished first during Couric's first week in September 2006, with an average of 10.2 million viewers, but within a month the broadcast was back in third again.

A new anchor, particularly a longtime CBS hand such as Pelley, could entice viewers who did not like Couric to sample his newscast, said Marcy McGinnis, a former CBS News executive who now teachers journalism at the State University of New York at Stony Brook.

"Any new anchor generates new eyeballs and the way you keep the eyeballs is if you produce a very good show," McGinnis said.