Opuntia species have been poorly studied ecologically and taxonomically in the eastern United States. This study deals with the ecology of Opuntia species in the mid-south United States and covers not only the high degree of morphological variation exhibited by taxa, but also the taxonomy and distributions of the group for Mississippi. The taxa in the mid-south have distinct habitat preferences and can be separated based on habitat characterization. Information from this work provides valuable data useful in predicting possible routes that an invasive species, Cactoblastis cactorum (the cactus moth), might use in its potential westward migration. Phenotypic plasticity exhibited by Opuntia pusilla subjected to experimental conditions exemplifies the care that should be taken when making species delineations. Spine production in certain species is more a function of abiotic environmental pressures than genetic heritage. Two taxa that previously were put into synonymy with other species are recognized from this work.