Invasive Earthworms: Affects on Native Soils


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In North America, earthworms exist as native species in the warmer Eastern and Western coasts of the United States, having avoided the glaciation of the last Ice Age series. Native earthworms found refuge in these areas as the northern plains and upper latitudes of North America were scoured and covered in ice (Fender and McKey-Fender 1990, Fender 1995, Gates 1982, James 1995). After the last glacial epoch migration of the earthworm has been limited by its own mobility. Not until the migration of humans to middle America did the earthworm move into the American Midwest and northern plains (Gates 1967, Lee 1985). Unfortunately these earthworms were exotic European species brought in with the soil in ship ballasts and potted with imported roots by immigrant farmers. The unintentional carriers of invasive earthworms were later followed by modern international importers selling soil and plants (Hendrix and Bohlen 2002). Of the eight European species introduced, only three species have flourished in American soils.

The three types of earthworms moving and being moved through the northern plains and forests are: Epigeic, Endogeic and Anecic (A fourth combined type called Epi-Endogeic is sometimes used).

EPIGEIC earthworms live within the first inch inch of the forest soils. They are litter dwellers and measure less than three inches in length. Their diet consists of surface litter, fungi, microorganisms and soil enriched with organic matter. Red Worms Eisenia foetida are part of this first group. They neither burrow nor migrate in the soil. Redworm castings are reported have a N-P-K ratio of 3.2 - 1.1 - 1.5.

ENDOGEIC species live in the rhrizosphere burrowing twenty inches deep into the soil and returning to the surface to eat litter and exude castings. 1-5 inches in length, Endogeic worms dwell in the soil and consume soil organic matter, fungi and organisms. They collect fallen leaves near their burrow and eat the inner cells.

ANECIC earthworms are the largest of the three groups and form vertical burrows up to 6 feet into the soil horizons. Five to eight inches long, they return to the surface to eat fresh surface litter at night. The Night Crawler L. terrestris is a well-known Anecic species widely used and scattered on land by fisherman, causing major soil changes in northern Minnesota forests. (L. Freilich, 2008).

 

The diet of Anecic earthworms focuses on the high C:N ratio leaf litter. Coniferous tree litter and acid soils are unsuited to the earthworm’s palate. Maple, Basswood, and Oak trees are prime food sources after the initial herbaceous litter has been consumed. The previous leaf fall is eaten in order of preference. In the Spring the earthworms consume Basswood leaves and by June Maple leaves have begun to dissapear. By early Fall only scant piles of Oak leaves can be seen (L. Freilich, 2008). Access to leaf litter in the Fall is controlled by weather. A long warm autumn will allow for extended leaf litter comsumption.


Dispersal
Earthworms entered Middle America around 1900, and Central Minnesota in the 1930s through human importation. For crop and pasture soils the earthworms helped break down organic matter and aerate the soil. For plowed fields this is adequate, as the aerated soil is not allowed to build bulk density. Recently earthworms are being used for bioremediation of degraded soils (Satchell 1983, Lee 1995, Edwards and Bohlen 1996). Ranchers will inoculate their pastures with plugs of earthworm rich soils. In grass prairie and farmland earthworms work in tandem with the local ecology (personal observation).

Ecological Effects
In the forest, the local ecology is modified as earthworms become a driver of local plant species. One effect caused by earthworms is an increase in bulk density by cementing soil particles together while burrowing, removing organic matter in the forest floor and displacing the biota and organisms that would otherwise dig, burrow, and aerate the soil (McLean and Parkinson 1998a, b; Scheu and Parkinson 1994a; Alban and Berry 1994; Migge 2001). Bioturbation by burrowing and the deposition of castings changes the forest floor composition (Scheu 1987; Blanchart 1990).

European forests maintain low bulk density via native fauna not existing in America (Bal 1982; Schaefer 1991). Studies in Minnesota have shown soil bulk density increases with the number of earthworms.

With the invasion of deep dweller Lumbricus–Aporrectodea in Northern Minnesota, Hale (2005) found increased bulk density and reduced ammonium, nitrate and phosprous availability. A comparable earthworm community in New York State was found by Bohlen (2004) to have lower C:N ratios and reduced C in the upper soils. The change in soil is affected by the metobilism of differing earthworm species.


Bulk Density Plotted, Minnesota Research

 

Total Earthworm Biomass (ash free dry g/m2)


Bulk Density Plotted, Minnesota Research
Scatter plots of A horizon bulk density in relation to mean total earthearthworm biomass in each study site. n = 27 in each site except Ottertail where n = 24. (Hale and others, 2005)




Listed below are ten reasons to avoid soil compaction, From readings it would seem to reflect the effects of increased bulk density caused by earthworms. The worms are, over time, slowly compacting the soil to a greater depth. A penetrometer would help understand this effect.

1. Causes nutrient deficiencies
2. Reduces crop productivity
3. Restricts root development
4. Reduces soil aeration
5. Decreases soil available water
6. Reduces infiltration rate
7. Increases bulk density
8. Increases sediment and nutrient losses
9. Increases surface runoff
10. Damages soil structure

Source: Iowa State University Extension publication PM 1901g

In the United Kingdom, a controlled study of endogeic enchytraeid earthworms (Oligochaeta) in extracted soil cores found microbial increased respiration by 35% in the surface (0–4 cm) horizon and “six times more ammonium and four times more carbon being leached from the surface” (L. Cole, R. D. Bardgett, P. Ineson, 2001). The study found that “enchytraeids indirectly drive the processes of decomposition and nutrient mineralization in organic upland soils” finding no loss of biomass, doubled organic C and no release effect of N or P from the contained soil horizons.

Unlike Anecic L. terrestris, which eats fresh leaf litter and leaves time for plants to adapt to a slowly diminishing ground cover, a population of Epi-Endogenic L. rubellus can consume 10 cm. of ditrius forest floor in one season, a time too short for herbaceous plants to adapt. Root mass is depleted as dead plant material is consumed and the resulting effect changes the deep Mor organic O horizon and thin A and deep E horizon to a Mull structure with only the Oi horizon, and a deep A horizon (25-30 cm). Rich in organic material and similar to plowed fields in structure (Lee 1985; Edwards and Bohlen 1995).

Epigeic Dendrobaena octaedra mixes the organic material or the Oa and Oi soil horizons, leaving the Oe and lower mineral horizons relatively untouched. They have little effect on soil density or C distribution (McLean and Parkinson 1997). Each type of earthworm ultimately changes the soil dynamics over time and quickly if combined.

Nutrients and Minerals
Earthworm castings have five times more nitrogen, seven times more phosphorus, eleven times more potassium, three times more exchangeable magnesium, and 1.5 times the calcium than most soils. However, this does not mean the nutrients or minerals are available in the root bed. N is collected in the castings as if having been distilled from leaves in the area and concentrated in one package. Removing the N from the forest floor’s cache and exposing it to weathering elements. Castings less than ten days old are more likely to leach away in surface water run-off than increase soil capacity, as does the Phosphorus. Surface run-off during weather events causes increased nutrient loads in wetlands, lakes,  and streams.

Epigeic earthworms assist the decomposition and mineralization of surface litter. Deep burrowing anecic species integrate organic material deep into the soil profile increasing aeration and water movement (Lee 1985, Edwards and Bohlen 1996). The combined effect of multiple species in an area is dramatic on the surface but even more so underground where the borrows of uncountable earthworms aerate and infiltrate nutrients. The initial infusion of nutrients from earthworm metabolization decreases just as the expanding network of burrows begin to leach out the soil nutrients beyond the root zone (Hale 2008).

Conclusion
Contradictory results in research may be due to some short term or containerized experiments. However, the effect of invasive earthworms on soils in forest or grassland is well documented. The additional under-story depletion that occurs from deer populations eating remaining herbaceous plants, seeds unable to germinate for lack of fibrist layers, and seedlings with no duff cover, accelerates the loss of established habitat. The change in soil structure and elluviation of nutrients to the deeper soil limits nutrient availability to plants and increases leaching. The raised bulk density combined with slick channels from earthworm burrows lowers moisture capacity and the soil drains more quickly. With the expanding invasion of European earthworms nitrogen availability declines (Hale et al. 2005a). Phosphorous leaches away in deep horizons or runs off in surface water (Suarez et al 2004), and cascading ecosystem impacts continue undeterred (Freilich, 2006).

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