Nelson Hairston Sr. (16 October 1917 – 31 July 2008) was an American ecologist. Hairston is well known for his work in ecology and human disease. In the field of ecology he is famous for championing the idea of the trophic cascade, on which he published the provocative “Green World Hypothesis” with colleagues Frederick E. Smith and Lawrence B. Slobodkin. Nelson was also deeply interested in the factors that control human disease and was an adviser to the World Health Organization for many years.