Invasive earthworms of North America
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Invasive species of earthworms, specifically from the suborder Lumbricina, have migrated and spread through North America.[1] Their introduction is having marked effects on the nutrient cycles in temperate forests. These earthworms increase the cycling and leaching of nutrients by breaking up decaying organic matter and spreading it into the soil. Since these northern forests are evolutionarily adapted to the presence of thick layers of decaying organic matter for growth and nutrition, they are diminishing in diversity and young plants struggle in these environments. Some species of trees and other plants may be incapable of surviving such changes in available nutrients.[2] This change in the plant diversity directly affects other organisms and often leads to increased invasions of other exotic species as well as overall forest decline[citation needed].
Contents
Earthworms and migration
Earthworms are migrating north into forests between 45° and 60° latitude in North America.[3] The worms in question are primary engineers of their environment. They are considered keystone species because, as detritivores, they alter many different variables of their ecosystem.[3] Of the 182 taxa of earthworms found in the United States and Canada, 60 (33%) are introduced species.[4] Among these, Lumbricus terrestris, L. rubellus, L. friendi, Amynthas agrestis, and Dendrobaena octaedra have been studied for their ability to invade previously uninhabited locations and disturb the local ecosystems.[3][4] These earthworm species are primarily from Europe and Asia, and they are disturbing many nutrient cycles.[2] By redistributing nutrients, mixing soil layers, and creating pores in the soil, they can affect the characteristics of the soil important to the rest of the ecosystem. Earthworms break up decomposing matter on the surface of the soil and carry or mix it into the surrounding soil, often carrying some of the nutrients deeper into the soil, where saplings and other young plants have trouble reaching them.[5]
Influence on nutrient cycles
When organisms die, their remains fall to the forest floor, where they begin decomposing into their constituent nutrients.[6] In the absence of efficient detritovores such as earthworms, a thick layer of such organic matter accumulates. And most northern forests in North America do indeed lack native earthworms, which were largely wiped out when the ice sheets of the Wisconsin glaciation scoured the much of the continent down to the bedrock. A deep detritus layer is thus characteristic of the native ecosystem of the region, and many native plants have come to rely on it.[2] As it slowly decomposes, it supplies nutrients, particularly potassium, phosphorus, and nitrogen, that are necessary for the production of cellular components such as carbohydrates, nucleic acids, and proteins; these nutrients are often a limiting factor in growth and maturation.[2] This provides for the growth of the trees, ferns, and smaller ground plants.
When earthworms are introduced into areas where they previously did not reside, the earthworms break up the organic layer. They often mix the nutrients into the soil, out of the reach of all but the deeper tree roots. Nutrients may then be leached and lost from the ecosystem entirely. Overall effects include a decrease in the thickness of the organic layer, increased mineralization, increased bulk density, spreading of the organic matter and humus, and increased rate of decomposition.[1][3][5] These environmental alterations (drier, brighter, less nutrient-rich soil) create changes to the ecosystem.
Effects on organisms
When many of the species cannot survive, the diversity of the forest decreases. Without the nutrients available, some species that provide important biological niches to the ecosystem may be eradicated. In addition, young plants can rarely grow without that available nitrogen source nearby.[5] Since young plants do not have the elongated root systems that older trees have, they often cannot attain enough nutrients to survive.[1] Thus, few saplings or under-canopy plants grow to full maturity and generally only the larger trees with extensive root systems survive. The addition of earthworms to an environment has been shown to decrease mycorrhizal associations with roots. This adds to the problem of finding available nutrients for plants.[3] Specifically, trees like populus, birch, and maples suffer greatly in the changing habitats, as well as many forest herbs like Aralia, Viola, and Botrychium. These plants are commonly eradicated from the temperate forests after only months of the invasives' presence. Also, when a decrease in overall ground cover and canopy vegetation occurs, food for other organisms becomes scarce.[2] As a result, some organisms are forced to leave the areas, and the few plants remaining are often eaten shortly after germination.
These invasive earthworms destroy the native species habitat and many species suffer besides plants since their available food and shelter becomes greatly limited. As stated above, often animals are forced to leave or starve due to decreased overall vegetation. With decreased ground level vegetation, many terrestrial organisms like insects, small mammals, and other vertebrates must compete for fewer resources, leading to decreased diversity and population.[7] In addition, the native species of worms have to compete with the invasives, which they often can't do since their environment changes too drastically. Many of these worms die since they are not well adapted to the new conditions of the forest soil.[6]
Generally, with the addition of earthworms to a forest, a decrease in diversity is observed, and often other exotic species follow that can survive the nutrient diminished environment better than the natives. For example, in newly invaded forests buckthorn and garlic mustard, both invasive species, increase notably in population density.[3][7] To summarize, there is a decrease in diversity, seedling populations, organic matter volume, and overall habitat. In addition, there is often a increase in invasive species and decreased diversity of non-plant organisms.
Origins
Most of the invasive earthworms are European or Asian and came over in soil during the 18th century as Europeans began settling the North American continent. The worms were originally transferred through the horticultural trade, probably in the soil bulbs of European plants being carried over to the Americas. The lack of competition from native earthworms allowed the invaders to flourish.[8] Now recreational practices and construction methods are the primary mode of transportation for the earthworms.[3] Their movement in the soil is slow on their own, but with human transportation they can migrate much faster. The earthworms are commonly used as bait for fishing, and many escape or are released. In addition, many are moved physically in soil through construction practices. Either they can be moved in dirt loads from one location to another, or be trapped in dirt attached to wheels of larger trucks.[1] Some propose a major mode of transportation is through logging trucks, which move from location to location with large amounts of dirt attached to their wheels.
Solutions
Lua error in package.lua at line 80: module 'strict' not found. At this point there is no known way to remove the earthworms from the soil, but simple methods may help slow the migration.[3] One way to help reduce their migration would be to reduce the number of worms released during fishing practices. Releasing them increases the number of available mates for the worms, assisting their proliferation and migration efforts. The movement of dirt from one location to the other could be regulated so that dirt from areas where earthworms are common is not moved into forests without the invasive species. This could be used for many different species of invasives found in dirt, but is difficult to regulate.[1] It would take years to restore an area of forest, even if the worms could be removed.[1][3] Allowing seedlings, native species, and previous nutrient conditions to stabilize would take a great deal of time and effort.
Future experiments are examining evidence that certain characteristics of the soil habitat could have a great effect on the ability of the earthworms to invade an environment. High salinity and sandy soils have been shown to have increased resistance to these invasive earthworms.[3] Additionally, low pH and high carbon to nitrogen ratio plant material may assist in resistance of forest ecosystems to invasive earthworms. Alternatively, high pH, and low C:N ratios appear to be more susceptible. These environmental factors may be points of interest when designing procedures or protocols to protect forests and destroy the invasive earthworms before they destroy the northern forests. Some have advocated the introduction of the New Zealand Flatworm (Arthurdendyus triangulatus) which is invasive to Europe and is in the process of wiping out the earthworms native there,[9] but such 'cures' often prove worse than the disease (e.g., Australia's cane toad). One approach that helps is the removal of introduced shrubs such as common buckthorn (Rhamnus cathartica) and honeysuckle (Lonicera x bella), which produce high quality leaf litter. Removal experiments in two northern hardwood forest stands reduced exotic earthworm populations by roughly 50% for the following 3 years.[10]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Nico Eisenhauer, Stephan Partsch, Dennis Parkinson and Stefan Scheu. 2007. Invasion of a deciduous forest by earthworms: changes in soil chemistry, microflora, microarthropds, and vegetation. Soil Biology and Biochemistry 39: 1099-110.doi:10.1016/j.soilbio.2006.12.019
- ↑ 2.0 2.1 2.2 2.3 2.4 Campbell, Neil A., and Jane B. Reece. 2009. Biology. San Francisco: Pearson Benjamin Cummings.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 Lee E. Frelich, Cindy M. Hale, Stefan Scheu, Andrew R. Holdsworth, Liam Heneghan, Patrick J. Bohlen and Peter B. Reich. 2006. Earthworm invasion into previously earthworm-free temperate and boreal forests. Biological Invasions 8: 1235–245. doi:10.1007/s10530-006-9019-3
- ↑ 4.0 4.1 Blakemore, R. J. 2006. American Earthworms from North of the Rio Grande—a Species Checklist. Rep. Yokohama, Japan: YNU.
- ↑ 5.0 5.1 5.2 Bohlen, Patrick J., Derek M. Pelletier, Peter M. Groffman, Timothy J. Fahey, and Melany C. Fisk. 2004. Influence of earthworm invasion on redistribution and retention of soil carbon and nitrogen in northern temperate forests. Ecosystems 7: 13–27. doi:10.1007/s10021-003-0127-y
- ↑ 6.0 6.1 Hendrix, P. F., G. H. Baker, and M. A. Callaham Jr. 2006. Invasion of exotic earthworms into ecosystems inhabited by native earthworms. Biological Invasions 8: 1287-300.
- ↑ 7.0 7.1 Katalin Szlavecza, Sarah A. Placellaa, Richard V. Pouyatb, Peter M. Groffmanc, Csaba Csuzdid and Ian Yesilonis. 2006. Invasive earthworm species and nitrogen cycling in remnant forest patches. Applied Soil Ecology 32: 54-62. doi:10.1016/j.apsoil.2005.01.006
- ↑ Mac A. Callaham Jr., Grizelle Gonzalez, Cynthia M. Hale, Liam Heneghan, Sharon L. Lachnicht, Xiaoming Zou. 2006. Policy and management responses to earthworm invasions in North America. Springer Science+Business Media B.V: 2-5.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Michael D. Madritch & Richard L. Lindroth, "Removal of invasive shrubs reduces exotic earthworm population", Biol Invasions (2009) 11:663–671
