David H. Koch, a philanthropist who has given millions of dollars to the American Museum of Natural History in New York but whose businesses in energy and other industries have drawn criticism from climate scientists and environmental groups, has left the museum’s board after serving on it for 23 years.

A museum spokeswoman, Anne Canty, said Mr. Koch’s last day on the board of trustees was Dec. 9 and that his departure was not related to the criticism, but simply because his term was ending.

Mr. Koch, who served on the board since 1992, has donated some $23 million to the museum and his name adorns its dinosaur wing.

More than nine months ago, dozens of members of the scientific community signed a letter that called for museums of science and natural history to “cut all ties” with fossil fuel companies and philanthropists like Mr. Koch, who also sits on the advisory board of the Smithsonian National Museum of Natural History. A separate petition by environmental activists urged the museums to remove him from their respective boards.

The letter expressed deep concern about “the links between museums of science and natural history with those who profit from fossil fuels or fund lobby groups that misrepresent climate science.” The letter was the project of the Natural History Museum, a mobile museum “that highlights the socio-political forces that shape nature,” according to its co-founder and director, Beka Economopoulos.

But Cristyne Nicholas, a spokeswoman in New York for Mr. Koch, said that the letter and petition had nothing to do with Mr. Koch’s departure. “He was not swayed by that at all and it absolutely did not factor into his decision,” Ms. Nicholas said.

She noted that Mr. Koch is on about 20 different boards around the country and that he is cutting that number back while he focuses more time on cancer research. Last year, Mr. Koch, 75, who was once diagnosed with prostate cancer, donated $100 million to New York-Presbyterian Hospital and $150 million to Memorial Sloan Kettering Cancer Center to help researchers develop a cure for prostate cancer.

Ms. Nicholas said that Mr. Koch had missed board meetings at the natural history museum and other institutions because of scheduling conflicts. “He remains supportive of the museum,” she said. “It is just that he does not have time to attend the board meetings.”

Ms. Canty said that Mr. Koch had served as an at-large trustee. These types of board members are up for re-election every year, she said.