He investigates the direct and indirect interactions between plants and with other organisms. More specifically, these interactions include resource competition, allelopathy, facilitation/mutualisms and interactions with invasive species, as well as soil microbe, herbivore and competitor-mediated interactions.
↑Callaway, Ragan M.; Ridenour, Wendy M. (Oct 2004). "Novel weapons: invasive success and the evolution of increased competitive ability". Frontiers in Ecology and the Environment. 2 (8): 436–443. doi:10.1890/1540-9295(2004)002[0436:NWISAT]2.0.CO;2.
↑Callaway, Ragan M.; Walker, Lawrence R. (October 1997). "Competition and facilitation: a synthetic approach to interactions in plant communities". Ecology. 78 (7): 1958–1965. doi:10.1890/0012-9658(1997)078[1958:CAFASA]2.0.CO;2.