Robert G. Goelet, a civic leader, naturalist and philanthropist whose marriage merged two families that date to 17th-century New Amsterdam and made the couple stewards of Gardiners Island, a storied sanctuary off the tip of Long Island, died on Tuesday at his home in Manhattan. He was 96 .

His death was confirmed by his son, Robert Gardiner Goelet.

The scion of a real estate dynasty, Mr. Goelet (pronounced guh-LET) was 52 when he married Alexandra Gardiner Creel in 1976. Under a trust from her aunt, she held Gardiners Island jointly with her idiosyncratic uncle Robert David Lion Gardiner, and when Mr. Gardiner died in 2004, the Goelets took full possession of it — all 3,300 acres, four times the size of Central Park, complete with 27 miles of coastline, lush white pine and oak forests, colonial buildings, a 200-year-old windmill, a family cemetery and considerably more ospreys than people.

The couple went on to maintain the island as a bird sanctuary while restoring its colonial buildings and natural habitat. Mr. Goelet also established a large penguin reserve in Patagonia and collected some 20,000 bees and wasps, which he donated to the Museum of Natural History. A genus of bee found in Peru, Goeletapis , was named after him.

Independently wealthy, Mr. Goelet devoted much of his time to civic causes. By late 1975, when he was named president of the American Museum of Natural History, he had served in the same role a t the New-York Historical Society and the New York Zoological Society (now the Wildlife Conservation Society). He was later the museum’s chairman, until 1989, when he retired from the post.