Interrrupting an Invasional Meltdown

Research shows removing privet allows native earthworm communities to recover

by Zoё Hoyle, SRS Science Delivery Group
Southeastern forest infested with Chinese privet. Photo by David Moorhead, courtesy of bugwood.org.

Southeastern forest infested with Chinese privet. Photo by David Moorhead, courtesy of bugwood.org.

Earthworms have been described as “ecosystem engineers” because they can transform soil environments in ways – physical, chemical, and biological – that in turn lead to aboveground ecological changes. Most of the 8,000 species of the world’s earthworms stay in areas where they evolved, some occupying very narrow niches, but about 120 “cosmopolitan” or “peregrine” species have spread throughout the world, some invading and displacing native species.

A recent article by U.S. Forest Service and University of Georgia (UGA) researchers reports the effects of removing invasive Chinese privet on soil properties and earthworm communities in floodplain forests of the U.S. Southeast. Scientists found that removing privet disrupts the “invasional meltdown” that can occur when changes to soil due to privet infestation make it easier for exotic earthworms to invade — and allows native earthworm communities to recover.   

The Asian jumping worm (Amynthas agrestis) was one of the invasive earthworm species found in the experimental plots. Photo by National Park Service.

The Asian jumping worm (Amynthas agrestis) was one of the invasive earthworm species found in the experimental plots. Photo by National Park Service.

The researchers — Mac Callaham and Jim Hanula from the Forest Service Southern Research Station (SRS) with UGA graduate student and lead author Joshua Lobe and UGA professor emeritus Paul Hendrix – used the experimental forest plots established by Hanula for his long-term study on the effects of privet removal on aboveground native plant and pollinator communities. The team sampled every three months for a year, comparing soil characteristics and earthworm communities in plots with privet, those where privet had been felled, and those where privet had not yet invaded. 

“Other than a couple of studies on root biomass and soil properties, there have been surprisingly few studies on the effects of privet on belowground biotic communities,” said Callaham, research ecologist for the SRS Center for Forest Disturbance Science. “This is the first study to look at the effects of removing an invasive plant on soil properties and earthworm communities in these southeastern riparian forests.” 

Researchers found 14 different species of earthworms in the privet experiment plots, five native to North America, the others originating from Europe and Asia. Overall, they found the lowest abundance of native earthworms in the plots with privet, greater abundances in reference plots and in plots where privet was felled and cleared. Analysis showed that the soil pH was significantly higher (i.e., less acidic) in the privet plots than that in reference plots and those where privet was felled.

“More research is needed, of course, but we speculate the presence of privet caused pH to rise, favoring some exotic earthworm species and potentially leading to an invasional meltdown,” said Callaham. “We found that where privet was removed, pH was as low as reference plots, and exotic earthworms seemed to lose their competitive advantage and so native earthworm communities began to recover.”

“The study supports the idea that by removing a key invasive species like privet, land managers could decrease interactions with other exotics and short-circuit the invasional meltdown process,” added Callaham. “It also shows that native earthworm species have the potential to recover after the removal of an invasive plant despite the continued presence of exotic earthworm species.”

 Read the full text of the article, published online in April 2014.

For more information, email Mac Callaham at mcallaham@fs.fed.us.

Read more about recent SRS research on privet removal effects

Read more about about Callaham’s research on invasive earthworms.

Access the latest publications by SRS scientists.

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Posted in Bottomland Hardwoods, Forest Operations, Invasive Plants, Restoration, Threats