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After leaving Roan Grammar, he was employed at the British Museum (Natural History), where he worked in the Bird Room. After a couple of years here he left for National Service. After this, Owen worked from 1951 to 1958 as a Field Assistant at the Edward Grey Institute for Field Ornithology, Oxford University, under its Director, the evolutionary biologist Dr David Lack. Amongst other things, he was responsible for maintaining the records of tits at the Wytham Woods site close to Oxford.
David Lack recommended Owen for a Zoology degree course at Oxford University. By the time Owen graduated, in 1958, he had already written 43 research papers. During this time he met his first wife Jennifer Owen (née Bak) who was also an undergraduate zoologist. The two collaborated on a number of research projects and married following their graduation in 1958. Both moved to the United States to become teaching fellows and PhD students at the University of Michigan.
During four years in the United States, Owen completed his doctoral project on owls and also researched insect ecology, collecting the first New World data on industrial melanism in the peppered moth as well as working on the ecological genetics of spittle bugs.
Owen's appointment as Lecturer in Zoology at the University College of Makerere (now Makerere University), Uganda in 1962 led to a four-year extended visit in Africa, where he studied the ecological genetics of butterflies and snails. Upon leaving Uganda in 1966, he took up the Chair of Zoology at Fourah Bay College, later to become the University of Sierra Leone at the age of 35. His research on butterfly ecology and genetics continued, culminating in his 1971 book, Tropical Butterflies Between 1967–68 he was also Director of a UNESCO Biology Teaching Project for Africa, based in Ghana.
Owen's next move was in 1971 to the University of Lund as Professor of Tropical Ecology, continuing his research on tropical ecology, and also temperate and Arctic ecology, including the genetics of Cepaea snails in Iceland. Owen's final appointment, in 1973, was as Principal Lecturer in Biology at Oxford Polytechnic later to become Oxford Brookes University. By coincidence he worked closely with David Lack's son Andrew Lack, who was also employed in the same department. Denis Owen remained at Brookes until his retirement in 1996.
Owen was a visiting professor at a number of international universities including the United Nations University (1977–79), the University of Bergen (1990–91) and the University of Florida (1991–92).
As well as carrying out his own research, Owen supervised or co-supervised over 20 PhD students, many of whom went on to establish research careers of their own, including Professor Tim Shreeve (Oxford Brookes University), Dr Rob Hammond (University of Leicester), Professor Jeff Ollerton (University of Northampton) and Professor Dave Goulson (University of Sussex).
During his career he published 240 scientific papers, 40 popular articles and 10 books. His first paper was published in 1949 when he was just 18 years of age.
Owen's most well-known book, What is Ecology?, was published in 1975. The second edition was revised by Jennifer Owen and subsequently translated into five languages. During this period Owen also gave radio broadcasts on Spanish natural history for the BBC World Service (with John Burton) as well as writing a series called "What's in a Name?", published as a book by the BBC.
- "Study Sites". The Edward Grey Institute of Field Ornithology. Retrieved 2011-11-21.
- David A.S. Smith (1996-10-24). "Obituary: Denis Owen". The Independent. Retrieved 2011-11-21.
- Carol Kaesuk Yoon (1996-11-12). "Parallel Plots In Classic Of Evolution". New York Times. Retrieved 2011-11-21.
- "Denis Owen". Oxford Brookes University. Retrieved 2011-11-21.