Evidence for human‐mediated dispersal of exotic earthworms: support for exploring strategies to limit further spread

Abstract

As potent ecosystem engineers, non-native earthworms are altering the fundamental structure and function of previously earthworm-free cold-temperate hardwood forests in North America. Discarded earthworms used for fishing bait has been presumed to be an important vector for the continued spread of non-native earthworms because epicentres of invasion often include boat landings, lakeshores and roads. However, controversy has remained about the overall importance of human-mediated spread vs. natural expansion of established earthworm populations. In this issue of Molecular Ecology, Cameron et al. explore the continued role of humans in dispersing non-native earthworms.

Broad and local-scale patterns of exotic earthworm functional groups in forests of National Wildlife Refuges of the Upper Midwest, USA

“…Forests with no earthworms encountered were found at Seney and Tamarac, and many stands sampled were not invaded by the full suite of earthworm groups. This lends support to the use of strategies to prevent further introduction and spread of earthworms, such as restricting soil movement between sites by equipment or vehicles and limiting the use of earthworms as fishing bait (Cameron et al 2007;Hale 2008). These strategies also have merit at invaded sites, as multiple introductions have been shown to increase genetic variability and may be linked to increased impacts (Hale 2008).…”

Human Dimensions of Earthworm Invasion in the Adirondack State Park

“…European settlers arriving in North America dumped ballast, a mixture of soil and gravel used to balance ships, thereby inadvertently depositing exotic earthworms hidden within the soil. Today, recreational fishing (through improper disposal of fish bait), gardening (planting, mulching, and composting of materials transported to a new site), vermicomposting (intentional use of exotic worms in compost systems), and unintentional translocation of egg cases in the tires and underbodies of vehicles are all mechanisms by which humans may disperse exotic earthworms (Callaham et al 2006;Hale 2008;NRRI 2008). In addition, land disturbance helps to prepare the landscape for invasion.…”

“…Although some earthworm invaders have effective modes of natural dispersal, recent molecular evidence supports a primary role for humans in moving them into and within new areas (Hale 2008). Understanding earthworm invasion, therefore, requires research on the human activities that facilitate it.…”

“…Impacts include a possible reduction in plant species richness (Holdsworth et al 2007), including the endangered goblin fern, Botrychium mormo (Gundale 2002). In addition, a degraded forest litter layer potentially reduces arthropod, salamander, and small mammal populations, and may disturb communities of ground-nesting birds (Hale 2008). In central New York State, Bohlen and colleagues (2004a, b) find that invasive earthworms both reduce carbon storage in the soil by 28% and alter the nitrogen cycle through the formation of nitrogen-rich earthworm casts, which stimulate nitrification and microbial activity.…”

“…For example, soil moisture and pH, leaf-litter quality and quantity, and level of land disturbance are key factors affecting the quality of the environment for earthworms (Hale et al 2005;Ammer et al 2006;Tiunov et al 2006;Bernard et al 2009). Even with favorable conditions, however, earthworm invasiveness is constrained by an average dispersal rate from an initial introduction site of about 10 m per year (Tiunov et al 2006;Hale 2008;Hendrix et al 2008).…”