Mulch That Shrub!

It may look great as a hedge around your garden, but the Chinese privet shrub has become an ecological eyesore across more than 1 million hectares of forest in the United States. Now a study from Georgia finds that aggressively mulching the invasive bushes can help bring back forest butterflies.

Some researchers “consider invasive plants to be the single greatest invasive threat to federally listed insect species and a major threat to Lepidoptera (butterflies and moths) in eastern North America,” James L. Hanula and Scott Horn of the U.S. Department of Agriculture’s Forest Service write in Forest Ecology and Management. One notable offender is the Chinese privet, Ligustrum sinense, which was introduced in the 1850s and is now common in southeastern forests from Texas to Georgia. “The thick growth of Chinese privet evident along roadside, stream and field to forest edges may be especially harmful to butterflies,” the authors note, since it can form a leafy barrier to  fluttering insects.

To see if an all-out bush war might help the butterflies, the researchers selected four heavily-infested sites in northeast Georgia in 2005. And they didn’t hedge their bets: Hired contractors used mulching machines and handtools to oust the invaders from the 2-hectare plots. Two years later, the researchers surveyed vegetation and insects on the cleared plots, and compared the results to both nearby uncleared plots and three plots in a nearby forest that had not been invaded by privet.

Both mulched and felled plots “had more butterflies two years after privet removal than untreated control plots,” they report, although the mulched plots had “nearly twice as many butterflies” as the plots cleared by hand felling. And although the researchers didn’t do a “direct” comparison, the cleared plots also had butterfly abundance and species diversity numbers that were similar to the plots that had never been invaded by privet. The quick rebound may have been due, in part, to nearby roads and clearings that provided edge habitat for immigrant butterflies, and a combination of greater availability of host plants or nectar sources on a forest floor that was getting more sunlight. They also found that one butterfly species — the Carolina satyr, Hermeuptychia sosybius — was “the best indicator of forests where privet had never invaded.”

The “evidence suggests that butterfly communities in other temperate forests could benefit from removal of extensive shrub layers dominated by a single species,” the authors conclude. “Privet is not the only shrub to affect plant communities,” they add, noting that shrub honeysuckles (Lonicera species) even “native shrubs that increase in abundance due to changes in land use or forest management practices” can also put a chokehold on butterflies. Time to rev up the mulcher?David Malakoff | May 26, 2011

Source: Hanula, J.L., Horn, S. Removing an exotic shrub from riparian forests increases butterfly abundance and diversity. Forest
Ecol. Manage. (2011), doi:10.1016/j.foreco.2011.04.040

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