I'm considering vermicomposting as a next step to hot composting, and plan to release the red worms directly into my garden bed (using a worm bucket), but wanted to check before ordering a batch of worms -- are these worms considered invasive in any way? It would be a bit of a faux pas to harm the ecosystem while trying to help it.
For reference, I'm in southeast Kansas, USA.
Red worms (Eisenia Fetida) are non-native to the USA, but whether they are harmful is difficult to say.
Earthworms in general can reduce the thickness of the layer of organic materials on the ground and thus change the amount of available nutrients. It's certainly possible that this drives away plants that require specific conditions for reproduction or survival. For example this article of the Minnesota Department of Natural Resources describes some of the negative effects caused by non-native worms in Minnesota's hardwood forests
Invading earthworms eat the leaves that create the duff layer and are capable of eliminating it completely. 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 article also mentions that
they [earthworms] are not known to survive Minnesota winters. However, if they or other species are able to survive winter and escape from compost piles they could further harm native forests. If you have a compost pile in a forested area, do not introduce additional non-native earthworms. If you are concerned about spreading non-native worms with your compost, you can kill worms and their eggs by freezing the compost for at least 1 week.
Another interesting article on the matter is this blog on redwormcomposting.com. It lists some of the research findings which seems to be inconclusive on red worms. It then goes on to say that it's unlikely that red worms would thrive in the wild, at least not in Northern US states.
So in short, red worms can potentially be invasive but so far there is no evidence that they are. If you live near a forest you may want to play it safe and not start an open vermicompost heap. You would also need to make sure you kill all worms and eggs before spreading the vermicompost in your garden.
The article Earthworms as invasive species in Wikipedia starts with a clear statement: "Earthworms are invasive species throughout the world." So there seems to be no doubt about general invasion of worms - They are invasive and they are already distributed over the world. However, questions should be others, like "are they already in my area?" and "If so, did they change things, like driving out other species or threaten our basis for life?" and in this case "What about the "Red Wiggler?"
About North America the article doesn't stress about the red worm (wiggler), which at least implies that red worms didn't come up as a major problem yet.
Redwormcomposting: "For one thing, these worms don’t tend to occur in “natural” habitats at all – but rather, tend to be found in concentrations of very rich organic matter such as manure and compost heaps. It’s not so much that they won’t survive in decaying leaf litter (they likely would) – it’s more that the chances of them actually thriving in this environment seems quite unlikely. Some experts cite the inability of Red Worms to withstand cold winter temperatures as one of the main reasons for there being less of a concern with them – I’ve actually found them to be quite cold-tolerant, so I’m somewhat less inclined to build an argument around that, but I guess it would be location dependent (they seem to do ok with minimal protection here in Southern Ontario – but in more northerly regions, and/or in more of a “natural” environment, perhaps they don’t overwinter nearly as easily)."
Further sources: Allaboutwildlife and NYtimes
It seems that most people consider red worms as beneficial and probably not dangerous for nature, but all consequences are not yet known.
In many ways, history repeats itself, when the introduction of most none natives (with-in an ecosystem) the impact often is not good. I'm not studied on the red worm per se, but I promote the feeding of soil. I found this link http://nesoil.com/properties/horizons/sld003.htm that talks about "O Horizon" being the layers of soil similar to those found in nature. It is wise to know if one's methods promote or deter soil biodiversity. One needs to be aware of the need for soil biodiversity however in the context of balance wherein production, product and waste all meet within a complete circle and do not interfere with the natural ecosystem. If the red worm in question was introduced into a balanced ecosystem there would presumably be no concern. However, the concern would be the misuse of a model and to assume it okay. It might be beneficial to know if there are any natural predators of red worms which might be within the ecosystem where red worms are being used. Then those natural predators would balance the red worms. Besides reptiles and frogs, there are centipedes and millipedes that attack worms so there should be a balance within to encourage the overall practice worthy.
