Invasive earthworms of North America
Invasive species of earthworms, specifically from the suborder Lumbricina, have migrated and spread through North America. Their introduction is having drastic effects on the multiple nutrient cycles in temperate or temperate-coniferous 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 rely on thick layers of decaying organic matter for growth and nutrition, they are diminishing in diversity and young plants struggle in these environments. Many species of trees and other plants may be incapable of surviving such drastic changes in available nutrients. This change in the plant diversity directly affects the other organisms of the environment and often leads to increased invasions of other exotic species as well as overall temperate forest decline.
Earthworms and migration
Earthworms are from the suborder Lumbricina, and are primarily engineers of their environment. Earthworms are considered keystone species, since they alter many different variables of their ecosystem drastically since they are detritivores. Of the 182 taxa of earthworms found in the United States of America and Canada, 60 (33%) are invasive species. Specifically, 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. These earthworm species are primarily from Europe and Asia, and they are disturbing many nutrient cycles. 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. In addition, 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. The earthworms are migrating north into forests between 45° and 60° latitude in North America.
Influence on nutrient cycles
Most northern forests rely on a thick layer of decomposing organic matter as their primary nutrient source. These nutrients, carbon, phosphorus, and nitrogen, are necessary for the production of many components of the cells, making it a limiting factor often in growth and maturation. When organisms die, their bodies slowly decompose, forming a layer of nutrients on the forest floor. This provides for the growth of the trees, ferns, and smaller ground plants. When earthworms are introduced into areas where they previously didn't reside, the earthworms break up the organic layer. They often mix the nutrients into the soil where they become leached and unavailable to most species of plants. Without this important source of nutrients, many species of plants cannot survive.
Carbon, nitrogen, and phosphorus are necessary components of different cellular products, including carbohydrates, nucleic acids, and proteins. The primary carbon source for forests is CO2 from the atmosphere. However, all other nutrients are taken up by trees and plants from the soil. These nutrients are replenished from the decomposing organic matter on the surface of the forest floor. When earthworms invade forest soil they break up this layer of organic material. This is the mechanism by which the nutrients become unavailable. Once the organic material is broken up, nitrogen, phosphorus, and carbon can be leached into the soil. This leads to increased mineralization and loss of nutrients. Overall effects include: a decrease in the thickness of the organic layer, increased bulk density, spreading of the organic matter and humus, and increased rate of decomposition. These environmental alterations (drier, brighter, nutrient deficient soil) generally create changes to the ecosystem.
Effects on organisms
When many of the species cannot survive, the diversity of the forest decreases drastically. Without the nutrients available, some species are completely eradicated which provide important biological niches to the ecosystem. In addition, young plants can rarely grow without that available nitrogen source nearby. Since young plants do not have the elongated root systems that older trees have, they often cannot attain enough nutrients to survive. 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 mycorrhizae associations with roots. This adds to the problem of finding available nutrients for plants. 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, since a decrease in overall ground cover and canopy vegetation is witnessed, food for other organisms becomes scarce. 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. 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.
Generally, with the addition of earthworms to a forest, a clear 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, make drastic increases in population density. To summarize, we see a decrease in diversity, seedling populations, organic matter volume, and overall habitat. In addition, we often see increased invasive species, and decreased diversity of non-plant organisms.
Most of the harmful 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 Wisconsonian glaciation had severely impacted the earthworm species native to North America, and the low population of native earthworms allowed the invaders to flourish. Now, recreational practices and construction methods are the primary mode of transportation for the earthworms. Their movement in the soil is quite slow on their own, but with human transportation their migration drastically increases. The earthworms are commonly used as bait for fishing, and many escape or are released. This is a very common way in which they move into new locations. 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. 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.
At this point there is no known way to remove the earthworms from the soil, but simple methods may help slow the migration. 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. It would take years to restore an area of forest, even if the worms could be removed. 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. 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, 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 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.
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