Invasion of the exotic earthworms!

[edfrank]

Invasion of the exotic earthworms!

http://www.nps.gov/grsm/naturescience/d ... orms10.htm


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You may know this worm already, although when the Asian Jumping Worm (Amynthas agrestis) is sold for bait or composting as the Alabama Jumper or Georgia Jumper, there is no mention of the destruction it can bring to forests.

Composting ads boast that the worm can eat and process more than its body weight in organic matter (vegetable scraps, leaves, lawn trimmings, etc.) each day. That same tenacious appetite means that when people release their bait (or their bait escapes) and the worms make their way to the forest, they consume massive amounts of leaf litter. If you were a leaf-litter-feeder such as a millipede, fly larvae, or springtail, this would be a big problem because your food source would be gone! It would also be bad if you were a creature that ate these invertebrates. In some areas on the western side of the Smokies, the Jumping Worm population is so high there is almost no leaf litter left. Without this food, native animals are disappearing, and the nutrients from decaying plants aren’t there to build new soil!

Continued

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[KoutaR]

In some areas on the western side of the Smokies, the Jumping Worm population is so high there is almost no leaf litter left. Without this food, native animals are disappearing, and the nutrients from decaying plants aren’t there to build new soil!

The earthworms do not carry the nutrients away from forest soil. They speed up the decaying and change the structure of the resulting soil. And of course, change the wood webs. But the nutrients should still be there if the leaching is not accelerated as well. Do they mean a nutrient loss through the leaching?

Kouta

[edfrank]

Kouta,

Lee Frelich has done much work on the effects of earthworms on the soil. Basically the earthworms change the structure of the forest floor. In areas where they are not present there is a thick layer of duff- bits of leaves in various states of decay. Without worms, fallen leaves decompose slowly, creating a spongy layer of organic "duff." This duff layer is the natural growing environment for native woodland wildflowers. It also provides habitat for ground-dwelling animals and helps prevent soil erosion.

Most of the nutrient cycling takes place in the duff layer and organic rich soil and decayed leaf mixture of the upper soil zone. This nutrient cycling is mediated by fungal dominated processes. The earthworms eat this duff. I areas where they are present the soil is not overlain by the duff and becomes more compacted. The decay process are no longer taking place in the duff layer and upper soil horizons but within the mineral layers of the deeper soil. Here they are more likely to become mineralized and leached away instead of being held by the organic layer and available for plants. "According to Peter Groffman, a microbial ecologist at the Cary Institute of Ecosystem Studies in Millbrook, N.Y., northern hardwood forests have relied on thick layers of leaf litter that serve as a rooting medium. The earth­worms, Groffman reports, “come into an area with a thick organic mat, and two to five years later that layer is gone.” As a result, some northern hardwood forests that once had a lush understory now have but a single species of native herb and virtually no tree seedlings. Evidently, earthworms change the forest soils from a fungal to a bacterial-dominated system, which speeds up the conversion of leaf detritus to mineral compounds and thereby potentially robs plants of organic nutrients."


In addition with the absence of the duff layer the soil tends to dry out and erode rather than retain water. The soils tend to dry out quickly and shallow rooted plants become stressed, others starting from seed die before being able to root deep enough to find adequate water. The absence of the duff layer dramatically affects the rate of water infiltration into the soil. Water being nutrients quickly seeps into the deeper subsurface or to the water table rather than being retained in the shallow soil zone and being made available to shallow rooted plants and seedlings.

Many native species root in this duff layer and upper soil zone and are eliminated from the ecosystem. Big trees survive, but many young seedlings perish, along with many ferns and wildflowers. Some species return after the initial invasion, but others disappear. In areas heavily infested by earthworms, soil erosion and leaching of nutrients may reduce the productivity of forests and ultimately degrade fish habitat. The presence of earthworms also facilitates the invasion of non-native plants into the environment through the elimination of many native species unable to reproduce adequately when the duff layer is gone.

You can see the sharp boundary in the forest between areas heavily infested by non-native earthworms and those areas which are not. Perhaps in Europe they are not a problem, but there are not many natural predators that limit the populations of the invasive earthworms in our soils and they are overwhelming the natural soil forming and nutrient cycling processes. There is material on the web talking about some of this. Much of it is in English, and not really very detailed and overly simplified. One site to look at is found here: http://www.nrri.umn.edu/worms/forest/soil.html another blog that is OK is found here: http://www.wildlifegardeners.org/forum/ ... rests.html If Lee responds he can direct you to some better articles in refereed scientific journals.

Ed

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[James Parton]

ENTS,

I remember first seeing these worms many years ago just outside of Atlanta Georgia ( Forest Park ) at my cousin's house. Before going fishing at Southlake or another fishing spot, we would go out back near the fence in his yard to dig worms. Digging in the leaf litter we would find these very active wriggling worms. We called em' Georgia Wrigglers". They would even " jump " out of a bait box! They naturally made excellent bait.

Until ENTS, I always thought earthworms were beneficial. That is what we were taught in school back in the 1970's and early 1980's. And I have heard it all my life that earthworms are always beneficial. It was quite a surprise for me to find out that it was not always true.

Fortunately for the environment, in the past I usually tossed my bait into the water I was fishing at the end of a days fishing trip. I fed the fish! This hopefully helped keep me unknowingly from spreading invasive earthworms into the environment. Now it is something I always do. But providing you refrigerate them, worms can be saved and used on another trip. This is the best option. You get your money's worth outta the bait and it is not released into the forest!

James

[edfrank]

Kouta,

I guess the short answer really is that the nutrients that are slowly being released from the decayng leaves. The earthworms are eating them and rapidly converting them into a more mobile chemical form. At the same time the removal of the leaves and decaying leaves are changing the soil structure. The resulting soil is thinner and does not hold water as well as the original soil. Water from rainfall and snow quickly passes through the altered soil dissolving and carrying the nutrients with it as it infiltrates. This leaches the nutrients from the soil so they are not longer available for palnt growth. The earthworms result in thiner more compact soil, soil that contains much smaller amounts of organic matter, soil that retains less water than the original soil, and soil that is poorer in nutrients because of this leaching.

Ed

[Jeroen Philippona]

Ed, Kouta, James,

It is often the same with introduced exotic species: they are a plague in the new environment. I see the mentioned earth worms are of Asiatic origin. In Europe I have never heard of the same problem with exotic earth worms. The native earth worms are no plague, they are thought of as beneficial and are very common in arable lands and meadowland. In forests they are common on the less acid or more neutral to basic soils. There the decaying of leaves and litter is much quicker than on acid soils. The acid soils have a lot thicker duff layer (so you call it, whe say humus-layer) but very few earth worms. In fact the less acid soils in Europe have more animal and plant life and native earth worms, so the whole system seems to be different. Interesting to see is if the exotic worms are a plague on all sorts of soils or concentrated on specific soils.
The decomposition of leaves also differs for species: especially conifers, beech and oak have more acid leaves wich decompose only slow, whereas leaves of other species like maples, ash, Tilia, Populus, etc. are much quicker decomposed.

Jeroen

[KoutaR]

Thanks, Ed! I think they should not simplify the issue too much in the web site. They could write, for example, "The earthworms eat leaf litter and release the nutrients to the mineral soil where they easily become leached away" or something like that. Now they are saying the earthworms eat the nutrients away. Everyone having basic understanding about ecology knows it is impossible.

James told he was taught earthworms are always beneficial. Probably that was a doctrine directly from Europe (and perhaps from American wheat fields, too) without understanding the thorough difference between European and American soils.

Can somebody explain the natural absence of earthworms in your soils? They were land bridges between Eurasian and American continents up to 5 million years ago, and the earthworms must be of very ancient origin. Were they absent also in the southern US (Florida, Texas, ...)? What about the West?

Kouta

[edfrank]

Kouta,

I am also frustrated by the oversimplification of science as is presented to the public. That is a response to and also responsible for the low level of science literacy in this country. They built a multi million dollar Tom Ridge Environmental Center for visitors near Presque Isle State Park in Erie County, PA. They have so diluted the science that their explanations of vegetation succession in a migrating bar deposit is fundamentally wrong. They have talked about farming by native Americans on the peninsula when if fact they have no archaeological evidence it ever took place, and it is actually unlikely because of the soils there it was ever farmed. And my tax dollars are paying for the fluff with no substance and the misinformation presented to the public. (Did you read my signature that appears with my posts?)

There are many different species of native earthworms here. They are largely absent from the northern areas that were glaciated. The limit of the worm absence does not exactly match the glacial foot print, but approximates it. These area are being invaded by foreign earthworms that have been imported and grown as fishing bait. These are dumped when the days fishing is done. Maps of areas affected correspond to invasive fronts primarily associated with fishing in lakes, pond, and streams. They also have sold worm farms to improve gardens. The impact on cultivated soils is likely much different than it is in woodland areas. In areas with native worms, such as the Great Smoky Mountains, the non-native earthworms are being displaced by the more aggressive invasive species. The invasives are not only from Europe, but there are some Asian species as well. I am not a worm expert, so I can't really provide much more detail than you could find yourself on the internet.

Ed

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[KoutaR]

Ed & others,

In Europe, earthworms are present in cultivated areas and deciduous forests up to the northern Scandinavia. Of course, the northern areas (north from central Germany) were glaciated. I began to think why earthworms were able to re-colonize the European glaciated areas but not the American glaciated areas. What was the difference between the two continents? The human history, particularly cultivation, of course. Could the earthworms have been brought by man to the Northern Europe, too? I began to search the internet, and this doctor thesis, I found, support the theory:

Richter, K. (2009): Genetic structure in European populations of the earthworm Lumbricus terrestris. Kassel University Press.
http://www.upress.uni-kassel.de/online/ ... t.frei.pdf

From the page 9:
"This indicates that today's distribution of L. terrestris was facilitated by human transport of earthworms and cocoons and human impact on habitat soils."

This is one example how profound the human impact have been upon the nature. The northern European nature has evolved with earthworms over millennia, but many northern European habitats would be different if there were not the human-brought earthworms!

Kouta