Earthworm invasion: Aliens causing more harm than good?
Earthworms are generally considered to be good news. As highly effective composters, they are often referred to as the "best friend" of gardeners and farmers.
Earthworms break down and recycle decaying plant matter, and loosen, mix and fertilise soil by bringing nutrients closer to the surface. But their efficiency is not always so welcome.
There is a global earthworm invasion taking place. Alien species of the slow-moving, innocuous-looking creatures have conquered almost every continent.
They are out-competing native wildlife, adapting to deforested and cultivated soils more effectively than native species, and changing soil structure.
The race is now on to understand how they are doing it and, more importantly, what damage they may be causing.
"Non-native earthworm invasion is a truly global phenomenon in which invasive earthworm species are invading every continent except Antarctica," explains Dr Bruce Snyder of the University of Georgia, USA. A recent study on the subject by Dr Snyder and colleagues is published in the journal Soil Biology & Biochemistry.
Most forests in northern North America were once devoid of earthworms. When Europeans settled North America, they inadvertently introduced European earthworms in ballast soil and in the earth used to transport plants.
The worms have been spread further by harvested timber and individuals using earthworms for composting and as fishing bait.
"European species are invading forests in northern Wisconsin, Minnesota, New York and parts of Canada," Dr Snyder told BBC Nature.
A number of scientific studies have been published in recent years in an attempt to document how far these invasive worms have travelled and with what consequences.
Last year, research published in the journal Human Ecology found that invasive earthworms can alter the carbon and nitrogen cycles in woodland, as well as undermining native plant species.
Another study published last year discovered that invasive earthworms can help their new environments. It examined the role of worms in the carbon cycle of tropical forests and found the creatures had a positive function of locking carbon into the soil.Formidable foes
To date, most studies have focused on invasive European earthworms from the Lumbricidae family.
But Asian earthworms are also invading North America, particularly species from the genus Amynthas.
A recent journal published online in Biological Invasions, by Dr Holly Greiner of Oakland University, Michigan, USA, and colleagues, reports findings of their study on the impact of both European and Asian earthworms within temperate deciduous Michigan forests.
The study found that the Asian species A. hilgendorfi grows more quickly in its new environment and increases concentrations of mineral forms of nitrogen and phosphorous in the soil, though it makes forest litter decompose less quickly.
Other observations by the team suggest that A. hilgendorfi may displace other earthworm species to become the only surface-dwelling worm present.
That finding is backed by the study undertaken by Bruce Snyder and colleagues.
They studied an invasion of the related Asian earthworm, Amynthas agrestis, that is taking place in the forests of the Great Smoky Mountains on the Tennessee-North Carolina border, USA.
The team's microcosm environments mimicked conditions on the forest floor, containing leaf litter and food of different sizes from the forest and either earthworms or a native millipede species (Sigmoria ainsliei), or both together.
Results of the nine-month study showed that the two species consumed and competed for the same partially decomposed leaf material.
If this food source wasn't available, the millipedes died nearly three months earlier than usual.
"Millipedes died more rapidly when earthworms were present," said Dr Snyder.
Earthworms demonstrated a further advantage: when surface resources were limited they could burrow, whereas millipedes were restricted to the surface.
"This study, combined with other studies, suggests the earthworm invasion can directly or indirectly change the way carbon is stored in the soil systems, damage native plant and animal biodiversity, and cause erosion problems," explained Dr Snyder.
The team also found that the earthworms did not have it all their own way. In the microcosm environment, the presence of millipedes appeared to reduce the earthworms' ability to reproduce.
Where earthworms and millipedes were both present, no earthworm eggs were found, whereas earthworm eggs were reported in the same environment when no millipedes were present.
This "biotic resistance" suggests that native millipedes may be able to slow down or even halt the progress of invasive earthworms.But is it all bad?
The global earthworm invasion is not limited to forests; different species invade different ecosystems with diverse effects.
For example, invasions of Pontoscolex corethrurus worms are conditioning and aggregating the soil in deforested areas of the Amazon, protecting the exposed soils from eroding away.
This worm species can reproduce particularly quickly, with each adult producing up to 80 cocoons, or eggs, a year compared to the handful produced by most species.
According to Professor Patrick Lavelle from the French and European Academies of Science, "This worm is adapted to the new conditions created by deforestation and cultivation, which was not the case for native species".
He and his colleagues have just published a study of the impact of this earthworm in deforested soils in Amazonia, in the journal Pedobiologica.
Researchers found that these worms may increase mineral levels and stimulate plant growth.
On the surface this might appear to be a good thing, but there is a cost.
"[The] drawback is that native species will likely not come back," Prof Lavelle told BBC Nature. "We all feel sad at losing species," he added.
The loss is especially poignant as invasive earthworms may be driving out native earthworm species that scientists have not yet had a chance to even describe.