Such diversity produces a rich and robust assemblage of foods, medicines, industrial materials and recreation activities, said University of California-Davis Professor Sharon Strauss.

Strauss and former doctoral student Richard Llankan focused their research on competition among genetically varied plants of one species -- black mustard, Brassica nigra -- and among black mustard and plants of other species.

"This is one of the first studies to show that genetic diversity and species diversity depend on each other," Lankau said. "Diversity within a species is necessary to maintain diversity among species and, at the same time, diversity among species is necessary to maintain diversity within a species.

"And if any one type is removed from the system, the cycle can break down and the community becomes dominated by a single species."

The research, funded by the National Science Foundation, was published in the journal Science.