Exotic Plants: Winged by natural enemies?

New study disproves previous theories on the spread of invasive plant species

Giant Bear Claw © USDA
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Introduced exotic plants spread very quickly with us because they lack the natural enemies - at least so far the common doctrine. But now, a study just published in Science seems to prove the opposite: it shows that exotic plants thrive even better in the presence of their natural predators.

Plants that were not originally native to a region, but introduced or deliberately introduced, are becoming more and more of a problem: these so-called invasive species are spreading wildly, displacing native species and may unbalance entire ecosystems. In Germany, for example, the giant hogweed or the late bird cherry have already made headlines. For the US, the annual damage from invasive species is estimated at $ 120 billion.

The reason for the unbridled spread of invasive species was previously the lack of natural enemies. Researchers at the Georgia Institute of Technology around biologist John Parker have now verified this. They analyzed 63 studies on a total of more than 100 exotic and 400 native plant species. The focus was not on insects but on vertebrates as herbivores.

"Common enemies" promote frequency and diversity

The result of the study: Contrary to previous assumptions, the presence of their - usually also introduced or even deliberately introduced natural enemies - seems to boost the spread of the exotics: not only the proportion of invasive plant species, their diversity was higher everywhere where her natural enemies were also present.

"Exotic herbivores can promote the growth of exotic plants by selectively eating native plants. It frees up resources for the exotic plants that can better withstand these herbivores, "explains Parker. "These results are particularly interesting because on most continents, the original native herbivores were almost wiped out by early settlers to make room for their imported herbivorous livestock." In America, for example, the early immigrants brought their cows and sheep and rotted Over time, the bison look like supposed competitors of their animals. display

Preferential food for domestic herbivores

So far, the assumption has been that the invasive species spread so successfully in the new area, because the native herbivores do not know them and therefore spare. However, the results of the new study also contradict this: Some studies, including one by Parker himself, have shown that native herbivores may sometimes even prefer the new "immigrants" to their normal diet and thereby inhibit their spread,

In one case, the local herbivores reduced the relative abundance of exotic plants by 28 percent. According to the researchers, this could be because the invasive plant species were not yet able to adapt to the threat posed by their natural enemies. As a result, they lack appropriate defense measures and they are easy prey for the native herbivores.

Not only insects, also vertebrates suitable for the control

Most recent studies on the "Enemy Enemy Hypothesis" on invasive plant species have focused on insects. But Parker notes that while insects effectively reduce the growth and biomass of the plants they infest, they rarely destroy it. In contrast, the far larger, herbivorous vertebrates literally eat the plants with stumps and stems and so often have a similar or even greater impact on plant communities.

These results are therefore important for the control of invasive plants that have already become plague in many regions and for the preservation of domestic ecosystems. "The reintroduction and promotion of native herbivores in their original habitats and the concomitant reduction of exotic herbivores could be an effective tool to restore invasive plants" "Parker explains.

(Georgia Institute of Technology, 10.03.2006 - NPO)