Unintended consequences of invasive species

An interesting article in this week's PLOS Biology reminds us that when we alter ecosystems, or for that matter any complex system, we cannot do just one thing. Stinson et al show through a set of very convincing experiments that an invasive species, the European Garlic Mustard, has an interesting effect on tree seedling in forests. Apparently this non native mustard reduces the growth of native tress seedlings by disrupting the mutualistic relationship between the roots of trees and the fungi that live with the roots. These sorts of mycorrhizal fungi are particularly important for woody plants but the garlic mustard and many other weedy invasive species apparently do not have these sorts of relationships.





Image from http://www.nps.gov/plants/alien/fact/alpe1.htm

The authors did a complex series of experiments to examine the affects of the mustard on woody seedling growth and colonization of roots by the mycorrhizal fungi. They concluded that the mustard does inhibit the colonization of woody seedling roots by the fungi, and that this inhibition is due to a chemical produced by the mustard that somehow inhibits the colonization of roots by the fungi.

What is unsettling is that the authors note that this effect is more pronounce in undisturbed habitats. They write:

"The strongest effects were observed for woody species most typically found in forested sites. These results indicate that the invasion of garlic mustard is more likely to negatively impact highly mycorrhizal-dependent tree seedlings than less-mycorrhizal-dependent plants. Thus, garlic mustard's successful colonization of understory habitat may be attributed in part to its ability to indirectly suppress woody competitors, and its effect on the native flora may be more detrimental in intact forests than disturbed sites. In addition, the data suggest that invasion by garlic mustard may have profound effects on the composition of mature forest communities (e.g., by repressing the regeneration of dominant canopy trees, and by favoring plants with low mycorrhizal dependency such as weedy herbs)."

The reason this is unsettling is that often in ecology we teach that mature, undisturbed habitats are less prone to invasion by weedy non native species and that these invasive species are problem mainly for disturbed habitats. Clearly this may not be the case!

References:

http://www.invasive.org/eastern/biocontrol/29GarlicMustard.html

Stinson KA, Campbell SA, Powell JR, Wolfe BE, Callaway RM, et al. (2006) Invasive Plant Suppresses the Growth of Native Tree Seedlings by Disrupting Belowground Mutualisms. PLoS Biol 4(5): e140

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