Friday, September 26, 2008

Why Some Invasive Plants Thrive


Invasive plant species sometimes flourish better in their new homes than in their place of origin, according to researchers from the Helmholtz Centre for Environmental Research (UFZ). Their research on Buddleia or the Butterfly bush (Buddleja davidii) and Oregon grape (Mahonia aquifolium) showed that genetic changes and the lack of insects that would normally munch on their leaves gives invasive plants like the Buddleia a striking advantage.

Buddleia was introduced to Europe about 100 years ago from China and has been cultivated since then. Although the blossoms of the Buddleia are aesthetically pleasing and provide a food source for butterflies, their beauty masks a dark side. The butterfly bush can easily go to seed and form dense populations - potentially displacing native species and becoming a safety risk along railway embankments due to its rampant growth.

In order to improve knowledge about the mechanisms responsible for the spread of invasive species, UFZ researchers compared ten populations of Buddleia in Germany with ten populations in its original homeland, the southwest Chinese province Yunnan. Although the climatic conditions are more favorable in China, the bushes were larger in Germany and produced more and heavier seeds.

"From the plants in the Chinese homeland, 15 percent of the leaves had been eaten by insects. By comparison, in Germany only 0.5 percent" said Susan Ebeling from the UFZ. "The intruder is not yet on the menu for our insects. Because there are no relatives of Buddleia in Central Europe, the insects need longer to adapt."

The two Asian insect species that were used in an attempt to control the bushes in New Zealand are not yet present in Europe.

The situation is different with another species that researchers investigated more closely. Originating in the western United States, the Oregon grape is an evergreen bush with yellow blooms; it is even the official flower of the state of Oregon. In Europe the Oregon grape has a similar relative: the European Barberry (Berberis vulgaris). Native insects had millions of years time to adapt to the European Barberry and could now comparatively easily "switch" to the Oregon grape.

The Oregon grape on the other hand could not develop any defense mechanisms against its herbivores.

Nevertheless, it prospers so well in Central Europe that the Swiss Commission for the preservation of wild plants requests owners of gardens to do without the cultivation of Oregon grape as a preventative measure: "Should one already have this species in one’s garden, one must absolutely prevent any further propagation, on the one hand by removing the infructescence, and on the other hand by constantly removing any young shoots."

In spite of herbivores, the Oregon grape is able to flourish, cover forest floors completely and consequently become a problem. "Its success obviously lies in cultivation. Through selection and hybridization it came to a genetic change, enabling the Oregon grape to grow larger in Europe than in its American homeland", said Harald Auge of the UFZ. The biologist and his colleagues from Halle had collected seeds from Oregon grape plants in the USA, Canada, Germany and the Czech Republic and had grown the plants under controlled conditions in a greenhouse.

-- LiveScience Staff

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