Is Mike Darnell’s exit beginning of the end for ‘American Idol’?

Fox and “American Idol” are losing their secret weapon now that reality TV chief Mike Darnell is ambling out the door in his snakeskin cowboy boots.

TV professionals, filing out of town for the Memorial Day weekend, let out a collective gasp Friday with the news that the diminutive Darnell will leave Fox this summer, capping an 18-year career as a reality-TV pioneer. Inevitably, his exit poses troubling questions about the future of “Idol,” the former No. 1-rated phenomenon whose plummeting ratings have shaken the network to its core.

One of the most colorful executives in the business - a British newspaper once described him as a “cowboy-hatted, ringletted munchkin” -- Darnell has been a tireless cheerleader for Fox’s unscripted schedule and for “Idol” in particular. Although he did not engineer the transfer of Britain’s “Pop Idol” to the U.S. back in 2002, Darnell helped shepherd the show and worked closely with the producers to solve problems and exploit opportunities. It was fun while it lasted, and so much more -- “Idol” totally dominated TV for years running.

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Darnell was also a brave experimenter in the genre. Many of his concoctions made critics wince but viewers watch. That includes the sensationalistic “When Animals Attack,” “Who Wants to Marry a Millionaire” and “Celebrity Boxing” -- efforts that, in the current overstuffed media environment, would probably merit shrugs rather than the outrage they precipitated years ago.

A few seasons back, in fact, Darnell had amassed so much power on the Fox lot that talent representatives buzzed he would soon wind up running the entire network.

By all appearances, the decision to leave now seems to have been his alone. He has said he wants to pursue his own projects, and the Wrap reported he had signed with talent agency WME (a spokesman could not confirm this). News Corp. chief Rupert Murdoch, whose empire includes Fox Broadcasting, on Friday called Darnell “a smart and fearless executive.”

But it was also probably a good time to check out. Beset by competition and a never-ending parade of new judges and gimmicks (including the possibility of former contestants serving as judges next season), “Idol” is probably headed nowhere but down. With his sure grasp of cultural cycles, Darnell may have realized it was best to step away while some gleam is still left on “Idol’s” pedestal.

“Idol” will survive Darnell’s departure -- for the time being. But it was Darnell who rode it to the top of the mountain. Whoever takes his place will be soldiering on in the other direction.

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Twitter: @scottcollinsLAT