An Investigation into the Effects of Earthworms as Invasive Species

Invasive Species (Earthworms)
Required Readings:
Chapters 1 and 2 from: Nentwig, W. (Ed) Ecological Studies (Volume 193): Biological Invasions. New York: Springer-Verlag Heidelberg, 2007.
Lawrence, B. et al. 2003. Influence of Nonnative Earthworms on Mycorrhizal Colonization of Sugar Maple. New Phytologist. 158: 145-153.
Optional Readings:
Richardson, D.M. et al. 2002. Naturalization and Invasion of Alien Plants: Concepts and Definitions. Diversity and Distributions. 6: 93-107.
Holdsworth, A. Earthworm Shocking. 19 April 2007. Available: http://www.grist.org/comments/dispatches/2003/09/02/holdsworth-biologist/index.html

Summaries
Nentwig, Chapter 1.
Nentwig discusses the importance of studying invasion for its environmental and anthropogenic impacts. The chapter focuses mostly on the damage that invasive species, such as weeds and insect pests, cause on agriculture – these species cause $1.4 trillion of lost crop productivity per year. He also briefly mentions invasive microbes, such as the bubonic plague. Most invasions are not successful, and protecting against them is very costly. Control mechanisms include biological (releasing predator species), chemical (using pesticides) and manual removal.
Nentwig, Chapter 2.
In chapter 2, Nentwig moves on to discuss the vectors of invasion; he highlights transfer mechanisms through air, sea and ground. He notes that airports often transport mosquitoes and all of the microbes that the mosquitoes carry. Many invasions, even of damaging exotics, occurred intentionally. For example, Europeans originally planted garlic mustard in North America for its medical and edible properties but it has since come to dominate the herbaceous under story of many Northeast forests. Also consider that many exotic species assist humanity. The spread of crops, most recently the potato, and livestock has assisted human development. He concludes that most unintended invasions cause much damage and it is cheaper to institute protective measures than it is to use corrective measures.

Lawrence.
Introduction: Most earthworms in the Northeastern United States are exotics from Europe. Fishers and farmers use these worms because their larger size makes them more effective. Recently ecologists have become concerned that the exotic earthworms may be digesting leaf matter and soil too quickly, clearing under stories and changing soil composition. Mycorrhizal are fungi that form mutalistic relationships with plants by provided small hair follicles that obtain nutrients for plant roots in exchange for energy from the plants. Lawrence investigates the potential of earthworms to disrupt mycorrhizal colonization of Sugar Maple roots.
Methods:
The study measured root colonization length and flux of phosphorus (a limiting nutrient in plant growth)
Results:
Lawrence found that plots without earthworms have higher fungal colonization rates and that different sites had different flux rates of phosphorus for unknown reasons.
Discussion:
Observing that sites without earthworms have increased levels of colonization does not prove that earthworms cause this effect. It is possible that richer soils both attract earthworms and require less mycorrhizal. Furthermore, even if earthworms do decrease colonization it is unclear whether this is because the earthworms are physically (eating) or chemical (changing soil composition) disrupting the mycorrhizal. The article also suggests that earthworm presence causes rapid cycling. There are still many questions to answer as to how this affects tree growth and water usage.

Richardson.
Richardson explains semantic distinctions in the study of invasion. He defines introduction as the point at which a new species enters an area. Colonization is the process of invasives spreading throughout ecosystems and naturalization is the process in which the ecosystem adapts to the exotic species. He also notes the importance of local invasions for succession (development after disturbances).

Holdsworth.
Holdsworth is a graduate student at the University of Minnesota. The grist pages are a kind of blog of his experiments on invasive species. The recommend article details plans to implement electric shocking techniques to remove earthworms in order to observe tree growth in earthworm free zones.

Questions for Discussion:
How should invasion be protected against?
What should we do with current invasives?
What qualities might make an exotic species beneficial?
How does the Lawrence study help us to understand what properties would be useful?
How are earthworms useful and harmful?
How are invasives able to survive?
What factors make some species weak to their new environment and others able to thrive in areas free of natural predators?
What does the earthworm story teach us about invasives in general?

Class Thoughts:
Preventative measures are often hard to implement because of globalization and trade agreements. We can no longer quarantine, or restrict trade from infected countries, areas because of WTO infringements. It is important to understand that such trade agreements exists for the sake of profit not for the sake of environmental protection. There is little that we can to stop invasion, and it may not even be a good idea. For example, spreading crop diversity and biodiversity may not only change ecosystems but also give people more choices as to what to grow.

We tend to simplify invasion both biologically and politically. The mechanisms for invasion are complex and extend beyond initial introduction. For many species, we do not understand how they are able to colonize areas or how to stop them. Furthermore, local invasion and succession are essential for ecosystem development and preservation. In some sense any movement of species is invasion and hence all species are invasive.

Earthworms exemplify these issues because of the controversies surrounding their usefulness. That is, earthworms certainly offer short-term gains for farmers and fishers and it is unclear how they are affecting long-term productivity, stability and ecosystem diversity. Their economic effects are quite uncertain and it is infeasible to control their populations. We should attempt to understand invasion in the context of biodiversity.