In a study published today in the journal Nature, Ragan Callaway of the University of Montana and his colleagues first grew C. maculosa in sterilized soil samples collected from the weeds native turf in Central Europe and its colonized territory in North America. Weeds cultivated in the sterile European sample grew up to nine times faster than those in natural soil, suggesting that the bacteria normally present in the plants native soil keep growth in check. Conversely, weeds grown in the North American bacteria-free soil tended to show reduced growth, indicating that microbes there are more conducive to colonization. In further experiments, researchers directly tested the biogeographic effects of the soil microbes on the weed: when planted in European soil in which C. maculosa had previously grown, the weed developed more slowly than when grown in soil pre-cultured with other native European species. U.S. soil produced the opposite effect.
Soil microbes that exert negative feedback on a plant might promote ecological diversity by reining in the plant's imperialistic tendencies, the researchers conclude. Foreign soil lacking such a brake pedal enables the weed to spread out of control. This may be because pathogenic soil bacteria tend to be species-specific, Callaway explains, noting that "when something is eating something else, theres a lot of selective pressure to win the arms race." But in the invaded ecosystem, he adds, "these kinds of evolutionary pressures may not be so strong for mutualistic relationships." --Alla Katsnelson