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To the untrained eye, the spotted knapweed looks innocent enough. To ecologists, however, it is the Genghis Khan of the plant world, one of the most aggressive invaders known. Researchers have long attributed the spread of the spotted knapweed, or Centaurea maculosa, and other invasive plants to the absence of native predators in their new environment. But a new report brings to light the importance of the interaction between these newcomers and microbes in the soil.
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 weed¿s 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 plant¿s 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, there¿s 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