The Canadian author and conservationist Graeme Gibson has died at the age of 85. Gibson was the long-term partner of Margaret Atwood, and was with the novelist while she toured to promote her new book, The Testaments.

Atwood said in a statement this afternoon that her family was “devastated by the loss of Graeme, our beloved father, grandfather, and spouse, but we are happy that he achieved the kind of swift exit he wanted and avoided the decline into further dementia that he feared”.

The novelist, who had been with Gibson since the early 1970s, said he “had a lovely last few weeks, and he went out on a high, surrounded by love, friendship and appreciation”.

“We are grateful for his wise, ethical and committed life,” said the author, who is currently promoting her sequel to The Handmaid’s Tale but had cancelled events this week due to an “unforeseen family illness”. Gibson had dementia, but continued to travel with Atwood, who told Time magazine earlier this month that she had completed The Testaments while caring for him.

Gibson’s publisher, Penguin Random House Canada, described him as “such a beloved and distinguished author” and “a friend to several generations of Canadian writers”.

“His influence on the lives of writers in this country has been profound and far-reaching. We are grateful for that superlative legacy, one that will continue to flourish, and also grateful for our own experiences working with Graeme: a true gentleman, whose gracious, elegant, and witty manner touched all who knew him,” said chief executive, Kristin Cochrane.

Gibson’s books included a miscellany of writings about birds, The Bedside Book of Birds, and The Bedside Book of Beasts, in which he wanted to “celebrate the deeply moving truth of creatures that are truly wild”. He wrote novels including Five Legs, the stream-of-consciousness story of two guilt-ridden young men which, when published in 1969, was described by one reviewer as having “more potent writing in it, page for page, than any other young Canadian novel that I can think of”.

An ornithologist and conservationist, he also co-founded the Writers’ Trust of Canada and the Writers’ Union of Canada, served as president of PEN Canada, and won awards including the Harbourfront Festival Prize and the Toronto Arts Award.

An interviewer once said that “every woman writer should be married to Graeme Gibson”. Atwood told the Guardian in 2000 – in a profile which described Gibson as a man who “exudes, bursts, beams with pride” for his partner – that “we got that put on to a T-shirt for him, in fact”.