"The complete change in grassland ecosystems brought with it more damaging fire regimes, shorter grazing seasons, and altered water and carbon cycles" said Eric Seabloom, an OSU assistant professor of zoology and co-author of the study. "Annual grasses like wild oats, barley, and ripgut brome helped to damage what had been one of the richest and most diverse plant communities in the world."
What the OSU researchers found, however, was that the invading grasses might never have become dominant were it not for a plant pathogen called barley yellow dwarf virus, which reduces plant growth, lowers seed production and increases mortality. In the absence of this pathogen, the native plants would have been able to rebuff the invaders and persist, rather than being overtaken, the study concluded.
"Not only were the native grasses under constant challenge from this disease, but the characteristics of the new grasses helped to spread the disease much more efficiently," Borer said. "It just beat them down, and they were not able to compete successfully."
This virus had been in the native California grasses for a long time, the scientists said, but when the annual grasses were added to the mix, they changed the infection rates in the native perennial grasses. The higher infection rate in the natives probably allowed the annuals to invade and take over this vast grassland.
The research was conducted with a $1.8 million grant from the National Science Foundation, under the Ecology of Infectious Disease grant program, and in collaboration with scientists at Princeton University.