Invasive earthworms of North America
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Invasive species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...
of
earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. Th ...
s from the suborder Lumbricina have been expanding their range in
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
.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. Earthworms are considered one of the most abundant
macroinvertebrates Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate s ...
in the soil of ecosystems in
temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...
and tropical climates. There are around 3,000 species known worldwide. They are considered
keystone species A keystone species is a species which has a disproportionately large effect on its natural environment relative to its abundance, a concept introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in maintaini ...
in their native habitats of Asia and Europe because, as
detritivore Detritivores (also known as detrivores, detritophages, detritus feeders, or detritus eaters) are heterotrophs that obtain nutrients by consuming detritus (decomposing plant and animal parts as well as feces). There are many kinds of invertebrates, ...
s, they alter many different variables of their ecosystem.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. Their introduction to North America has had marked effects on the
nutrient cycles A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cycli ...
and soil profiles in
temperate forest A temperate forest is a forest found between the tropical and boreal regions, located in the temperate zone. It is the second largest biome on our planet, covering 25% of the world's forest area, only behind the boreal forest, which covers abou ...
s. These earthworms increase the cycling and
leaching Leaching is the loss or extraction of certain materials from a carrier into a liquid (usually, but not always a solvent). and may refer to: *Leaching (agriculture), the loss of water-soluble plant nutrients from the soil; or applying a small amoun ...
of nutrients by breaking up decaying organic matter and spreading it into the
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
. This thins out the soil rapidly because earthworms do not require a mate to reproduce, allowing them to spread fast. Since plants native to these northern forests are evolutionarily adapted to the presence of thick layers of decaying organic matter, the introduction of worms can lead to a loss of biodiversity as young plants face less nutrient-rich conditions. Some species of trees and other plants may be incapable of surviving such changes in available nutrients.Campbell, Neil A., and Jane B. Reece. 2009. Biology. San Francisco: Pearson Benjamin Cummings. This change in the plant diversity in turn affects other organisms and often leads to increased invasions of other exotic species as well as overall forest decline.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. They are considered one of the most invasive animals in the
Midwestern United States The Midwestern United States, also referred to as the Midwest or the American Midwest, is one of four census regions of the United States Census Bureau (also known as "Region 2"). It occupies the northern central part of the United States. I ...
along with feral swine.


Earthworms and range shifts

Earthworms are shifting their ranges northwards into forests between 45° and 69° latitude in North America that have lacked native earthworms since the last ice age. Of the 182 taxa of earthworms found in the United States and Canada, 60 (33%) are introduced species, these earthworm species are primarily from Europe and Asia. Among these, ''
Lumbricus terrestris ''Lumbricus terrestris'' or the ''common earthworm'' is a large, reddish worm species thought to be native to Western Europe, now widely distributed around the world (along with several other lumbricids). In some areas where it is an introduce ...
'', ''L. rubellus'', ''L. friendi'', ''
Amynthas agrestis ''Amynthas agrestis'', the Asian jumping worm,The species goes by many common names, including the rustic jumping worm, the Alabama jumper, and the Asian crazy worm. is a species of worm in the family Megascolecidae. They have a smooth, glossy g ...
'', and ''
Dendrobaena octaedra ''Dendrobaena'' is a genus of annelids belonging to the family Lumbricidae The Lumbricidae are a family of earthworms. About 33 lumbricid species have become naturalized around the world, but the bulk of the species are in the Holarctic reg ...
'' have been studied for their ability to invade previously uninhabited locations and disturb the local ecosystems and 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. 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.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.


Influence on nutrient cycles and soil profiles

When organisms die and plants undergo
senescence Senescence () or biological aging is the gradual deterioration of functional characteristics in living organisms. The word ''senescence'' can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence inv ...
, their detritus fall to the forest floor, where they begin decomposing into their constituent nutrients. In the absence of efficient detritivores such as earthworms, a thick layer of such
organic matter Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
accumulates. Most northern forests in North America lack native earthworms, which were largely wiped out when the ice sheets of the Wisconsin glaciation scoured 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 evolved to rely on it. As it slowly decomposes, it supplies
nutrients A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
, particularly
potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosphe ...
,
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
, and
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
, that are necessary for the production of cellular components such as
carbohydrates In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or may ...
, nucleic acids, and
proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
; these nutrients are often a limiting factor in growth and maturation. This provides for the growth of 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 do this physically by
burrow An Eastern chipmunk at the entrance of its burrow A burrow is a hole or tunnel excavated into the ground by an animal to construct a space suitable for habitation or temporary refuge, or as a byproduct of locomotion. Burrows provide a form of sh ...
ing, consuming and redistributing leaf litter, and leaving behind
castings In metalworking and jewelry making, casting is a process in which a liquid metal is delivered into a Mold (manufacturing), mold (usually by a crucible) that contains a negative impression (i.e., a three-dimensional negative image) of the intende ...
in the soil. 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 Mineralization may refer to: * Mineralization (biology), when an inorganic substance precipitates in an organic matrix ** Biomineralization, a form of mineralization ** Mineralization of bone, an example of mineralization ** Mineralized tissues are ...
, increased
bulk density Bulk density, also called apparent density or volumetric density, is a property of powders, granules, and other "divided" solids, especially used in reference to mineral components (soil, gravel), chemical substances, (pharmaceutical) ingredients ...
, spreading of the organic matter and
humus In classical soil science, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of soil organic matter. It is rich in nutrients and retains moisture in the soil. Humus is the Lati ...
, and increased rate of
decomposition Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is e ...
. These environmental alterations (drier, brighter, less nutrient-rich soil) create changes to the ecosystem. Podzol soils lose their classic banded appearance when earthworms obliterate their eluvial (A2, Ae, or E) horizons.


Effects

Invasive earthworms have caused a decrease in diversity, seedling populations, forest floor organic matter volume, and overall habitat quality. In addition, there is often an increase in invasive species and a decreased diversity of non-plant organisms. Some species that provide important biological niches to the
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
may be eradicated.


Effects on plants

Because earthworms take away valuable nutrients needed by plants, young plants may be unable to grow without the surface nitrogen source provided by the layer of detritus. Since young plants do not have the deep root systems that older trees have, they often cannot obtain 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
mycorrhizal   A mycorrhiza (from Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plan ...
associations with roots. This adds to the problem of finding available nutrients for plants. Many different species of earthworms throughout Northern Minnesota and
Wisconsin Wisconsin () is a state in the upper Midwestern United States. Wisconsin is the 25th-largest state by total area and the 20th-most populous. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake M ...
have been connected with the reduction of native plants. Specifically, trees like poplar,
birch A birch is a thin-leaved deciduous hardwood tree of the genus ''Betula'' (), in the family Betulaceae, which also includes alders, hazels, and hornbeams. It is closely related to the beech-oak family Fagaceae. The genus ''Betula'' contains 30 ...
, and
maple ''Acer'' () is a genus of trees and shrubs commonly known as maples. The genus is placed in the family Sapindaceae.Stevens, P. F. (2001 onwards). Angiosperm Phylogeny Website. Version 9, June 2008 nd more or less continuously updated since http ...
are disfavored by the change in habitat, as are many forest herbs like ''
Aralia ''Aralia'' , or spikenard, is a genus of the family Araliaceae, consisting of 68 accepted species of deciduous or evergreen trees, shrubs, and rhizomatous herbaceous perennials. The genus is native to Asia and the Americas, with most species occu ...
'', ''Viola'', and ''
Botrychium ''Botrychium'' is a genus of ferns, seedless vascular plants in the family Ophioglossaceae. ''Botrychium'' species are known as moonworts. They are small, with fleshy roots, and reproduce by spores shed into the air. One part of the leaf, the ...
''. These plants are strongly negatively effected by the activity of invading earthworms. When a decrease in overall ground cover and canopy vegetation occurs, the few plants remaining are often eaten shortly after
germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fer ...
. 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. It has been observed that the presence of earthworms has a strong positive effect on the biomass of invasive plant species. For example, in newly invaded forests
buckthorn ''Rhamnus'' is a genus of about 110 accepted species of shrubs or small trees, commonly known as buckthorns, in the family Rhamnaceae. Its species range from tall (rarely to ) and are native mainly in east Asia and North America, but found thr ...
and
garlic mustard ''Alliaria petiolata'', or garlic mustard, is a biennial flowering plant in the mustard family (Brassicaceae). It is native to Europe, western and central Asia, north-western Africa, Morocco, Iberia and the British Isles, north to northern Sc ...
, both invasive species, increase notably in population density.


Effects on animals

When invasive earthworms cause a decrease in overall ground cover and canopy vegetation, food for other organisms becomes scarce. As a result, some organisms are forced to leave the areas. 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 may be unable to compete with the introduced species because the native ones are not well adapted to the new conditions of the forest soil. For example, Millipedes in particular are competing for resources with Asian earthworms in the southern
Appalachian Mountains The Appalachian Mountains, often called the Appalachians, (french: Appalaches), are a system of mountains in eastern to northeastern North America. The Appalachians first formed roughly 480 million years ago during the Ordovician Period. They ...
. Moreover, the decrease in plant biomass due to species richness affects earthworms is directly related to
trophic level The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it i ...
s. The lower trophic levels are affected less than the higher trophic levels.


Origins

Most of the invasive earthworms are European or Asian and came over in
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
during the 18th century as Europeans began settling the North American continent. The worms were originally transferred through the
horticultural Horticulture is the branch of agriculture that deals with the art, science, technology, and business of plant cultivation. It includes the cultivation of fruits, vegetables, nuts, seeds, herbs, sprouts, mushrooms, algae, flowers, seaweeds and no ...
trade, probably in the soil bulbs of European plants carried to the Americas. Some earthworms have been brought over to be used as bait for fishing and escaped or got released. For example, at least one species of earthworm was introduced in
Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S., ...
as bait for anglers. The lack of competition from 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 slow, but with human transportation, they can migrate much faster. In addition, many are moved physically in soil through construction practices in dirt loads. Certain characteristics of soil habitat may affect the ability of the earthworms to invade an environment. High
salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...
and sandy soils have greater resistance to earthworm spread. Low pH and the presence of plant matter with a high
carbon-to-nitrogen ratio A carbon-to-nitrogen ratio (C/N ratio or C:N ratio) is a ratio of the mass of carbon to the mass of nitrogen in organic residues. It can, amongst other things, be used in analysing sediments and soil including soil organic matter and soil amendmen ...
(C:N) may promote resistance; conversely, high pH and low C:N ratios appear to confer greater susceptibility.


Asian earthworms

More recently than the invasion of European earthworms, Asian earthworms have been introduced by human activities. Particularly of the genera '' Amynthas'' and '' Metaphire''. These earthworms have a variety of nicknames due to their characteristic thrashing behavior, including "Asian jumping worms", "Alabama jumpers", "crazy worms", and "snake worms". The effects of invasive Asian earthworm species are much less documented than those of European lumbricid earthworms, but there is greater concern over the potential effects of jumping worms on soil structure and chemistry,
nutrient cycling A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cycli ...
, forest regeneration, and animal and plant communities. Evidence shows that Asian earthworms grow more rapidly, reproduce more quickly, and have greater flexibility in their diet than European species. They can also exist at higher densities than European earthworm species. These characteristics may allow jumping worms to outcompete their European earthworm competitors. These traits mean that jumping worms can consume organic matter more rapidly, stripping the forest floor of organic matter and temporarily flooding the system with nutrients. Northeastern forests evolved under the slow decomposition and release of nutrients, and it is still unclear how forests are responding to the rapid breakdown of organic material.


Prevention


Removal

One way invasive earthworms can be removed from the environment is worm grunting. Worm grunting is the act of vibrating a wooden stake that has been driven into soil to bring worms to the surface where they can be collected by hand. Another way to remove invasive earthworms are mustard pours. Mustard pours are 1 gallon of water and 1/3 cup ground yellow mustard seed mixed together. This is better for gardens because it can be poured over soil to drive worms to the surface without harming any plants.


See also

*
Earthworms as invasive species Earthworms are invasive species throughout the world. Of a total of about 6,000 species of earthworm, about 120 species are widely distributed around the globe. These are the peregrine or cosmopolitan earthworms. Some of these are invasive species i ...
* Asian jumping worm (Amynthas agrestis)


References


External links


Minnesota Department of Natural Resources: EarthwormsInvasive Worms in American Soil. All About Wildlife. (Posted: March 3, 2010)
{{Americas topic, Invasive earthworms of
Earthworms An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. Th ...
Annelids