Assistant professor biology Florida State University, Tallahassee, 1968—1973, associate professor, 1973—1978, professor, 1978—1997, Robert O. Lawton Distinguished professor, 1986. Nancy Gore Hunger professor environmental studies department ecology and evolutionary biology University Tennessee, since 1997. Visiting professor University Michigan, 1974, University Minnesota, 1980, Hebrew University, Jerusalem, 1984.
Board directors National Science Board, 2000—2006. Member species survival commission International Union Conservation Nature and Natural Resources.
Editor: Journal Biogeography, since 1974, Biodiversity and Conservation. Co-editor: Ecological Communities: Conceptual Issues and the Evidence, 1984. Co-editor: (with D. Schmitz and T. Brown) Strangers in Paradise: Impact and Management of Nonindigenous Species in Florida, 1997.
Member editorial board: Journal Biogeography, Northeast Gulf. Science, Environmental and Ecological Statistics, Raffles Bulletin of Zoology, Ecologia, Oecologia, BioSci., Biological Invasions.
Contributor articles to science journals.
1968 - 1973