2008-2015, The University of Tennessee, Knoxville,
Director, National Institute for Mathematical and Biological Synthesis

2017-present: The University of Tennessee, Knoxville,
Director, National Institute for Mathematical and Biological Synthesis

2018-present The University of Tennessee, Knoxville
Chancellor’s Professor

2015-2017: The University of Tennessee, Knoxville,
Director Emeritus, National Institute for Mathematical and Biological Synthesis

2010-present: The University of Tennessee, Knoxville
Alvin and Sally Beaman Distinguished Professor

2008-2011: The University of Tennessee, Knoxville
James R. Cox Distinguished Professor

2002-2004: The University of Tennessee, Knoxville,
Associate Head, Department of Ecology and Evolutionary Biology

1998-present: The University of Tennessee, Knoxville,
Director, The Institute for Environmental Modeling

1997-present: The University of Tennessee, Knoxville,
Professor, Departments of Ecology and Evolutionary Biology and Mathematics

1992-1997: The University of Tennessee, Knoxville,
Professor, Department of Mathematics and Graduate Program in Ecology

1985-1992: The University of Tennessee, Knoxville,
Associate Professor, Department of Mathematics and Graduate Program in Ecology

1987(Summer): Distinguished Visitor, Mathematics Department, University of California, Davis

1986 (Summer): Visiting Biomathematician, Botany Department, University of California, Davis

1979-1985: The University of Tennessee, Knoxville, Assistant Professor, Department of Mathematics and Graduate Program in Ecology

1982 (Fall): Grassland Research Institute, Berkshire, Great Britain, Visiting Researcher, Biomathematics Division.

1982 (Fall): International Centre for Theoretical Physics, Trieste, Italy, Invited Lecturer for Course on Mathematical Ecology

1982-1983(Summers):
Atmospheric Turbulence and Diffusion Lab, Oak Ridge, Tennessee. Faculty Research Participant, Forest Meteorology

1974-1979: Cornell University, Research and Teaching Assistant, Ecology, Biometrics, Mathematics, Theoretical and Applied Mechanics

Philmont Scout Ranch, Raton, New Mexico. Ranger (1970)

Philmont Scout Ranch, Raton, New Mexico. Dispatcher (1974)

Philmont Scout Ranch, Raton, New Mexico. Back-country Camp Director (1975)

1970-1973: National Radio Astronomy Observatory, Charlottesville, VA. Research Assistant (cooperative education – six months each year)