My field site is a typical urban forest, loaded with invasive species. The primary woody invaders are buckthorns (Rhamnus catartica and frangula). I’ve wondered why the mono-patches of buckthorn usually had no litter layer beneath them. It turns out this is likely due to a synergistic relationship between buckthorn and exotic earthworms.
First, a little background on the worms. Michigan has no native earthworms. Neither does most of the upper Midwest, much of New England, or Canada. The Wisconsin glaciation exterminated whatever species may have occurred in these areas. Click on the map; the blue area is the extent of this last glaciation. If you live in these areas, all the worms you find when you turn a spade of earth likely have European or Eurasian origins. Some arrived in soil and plant material brought by colonial settlers. More recently, there has been a much larger influx from worms imported and raised as fish bait and for vermiculture. North America’s non-native wormifauna is comprised of nearly four dozen species, including the common species we call nightcrawlers (Lumbricus terrestris) and red wigglers (L. rubellus). Non-native earthworms are common in urban, suburban, and rural areas. Wilderness areas are invaded by the dumping of unused fish bait, and the transport of soil in logging operations, for example.
Northern forests evolved to function with a thick layer of organic matter which acts as a mulch. Soil properties, including delicate fungal relationships and low soil pH, are maintained when this litter is broken down slowly by native soil decomposers. Earthworms are very efficient decomposers. They quickly break down leaf litter, and reduce or eliminate the organic soil horizon and change soil nutrient composition and availability. These actions have a cascading effect in the ecosystem, altering the soil microbial community and the composition of plant life. In one instance, a rare fern was extirpated because the loss and alteration of the litter layer disrupted the fungi relationship it needed to survive. Declines in some salamanders may be linked to exotic earthworms invasions, which reduce litter-dependent prey available to young salamanders.
I ran across a paper in the Chicago Wilderness Journal on the synergy of introduced earthworms and introduced buckthorn. It notes that buckthorn leaves are high in nitrogen. This makes them attractive to earthworms and other decomposers, so they break down rapidly. Mixed-species leaf litter, if it also contains buckthorn, also decomposes faster than it would if buckthorn were not present. Thus, buckthorn accelerates the reduction of forest floor litter, especially where non-native earthworms are present, exacerbating the problems associated with the lack of litter and changes in associated processes. Where buckthorn and worms are present together, ecosystems receive a double-whammy, since buckthorn-dominated patches have elevated nitrogen, higher pH, and higher water content, all factors that can make restoration of habitats difficult even after the removal of the buckthorn.
I’m intrigued by the impacts of non-native species on ecosystems. It seems we are just now beginning to scratch the surface of non-native/non-native and native/non-native interactions.
- Invasion of north temperate forest soils by exotic earthworms — Institute of Ecosystem Studies — great diagram on impacts on earthworms on soils.
- Heneghan, L. 2003. And when they got together: impacts of Eurasian earthworm and invasive shrubs on Chicago woodland ecosystems. Chicago Wildnerness Journal 1:27-31.
- Gundale, M.J. 2002. Influence of exotic earthworms on the soil organic horizon and the rare fern Botrychium mormo. Conservation Biology 16:1555-1561.
- Gundale, M.J., W.M. Jolly, and T.H. Deluca. 2005. Susceptibility of a northern hardwood forest to exotic earthworm invasion. Conservation Biology 19:1075-1083.
- Invasive earthworms — a threat to North American forests — Brooklyn Botanical Garden.
- Minnesota Worm Watch
- Earthwormed Over — Audubon Magazine
- The Earth Moved: On the Remarkable Achievements of Earthworms by Amy Stewart.