Portland's Got Worms! How earthworms are transforming the ecosystem.
There is a body of published research from the Midwest and the East coast that clearly points to introduced earthworms as drivers of ecological change. Their introduction has led to transformative shifts in the plant community, whereby some understory species and some tree and shrub species disappear, and others benefit. The plant community after earthworm introduction is comprised of mostly r-selected species--those that produce many offspring and grow very quickly. Many of these are introduced species, including graminoids (grasses and sedges). Earthworms break down the duff layer at higher rates than would otherwise occur, causing its disappearance. This has cascading effects that affect soil moisture, seed bed conditions, and impact ground-nesting birds, soil invertebrates, and salamanders. The duff layer, which we know from those old-growth hikes, is spongy, soft, and filled with organic matter, mycorrhizae and works as a sponge. Earthworms, and the duff-layers’ disappearance, reduces soil moisture, shifts the soil from fungal-dominated to bacteria-dominated, and has shown to increase drought stress in sugar maple forests. Cindy Hale, a researcher from the University of Minnesota, (who also wrote this book that you should buy) explains worm-invaded areas as being the source for eutrophication by unlocking nitrogen and phosphorus in the organic layer and leading to its increased runoff in waterways (The appearance of aquatic weeds could be a symptom of earthworm effects!). In a large meta-analysis, the authors found earthworms to favor non-native plants and graminoids. Besides completely up-ending the soil structure, earthworms also eat seeds, bury seeds, create channels, and are constantly stirring the soil, similar to a plow. All of this points to introduced earthworms as being a huge driver in changing the ground rules of the ecosystem. Also, there is some research that shows deer coupled with earthworms create a further driving force that creates conditions not conducive to most native plant species, including trillium. Do we have the same introduced species in Oregon? Yes. Are they in similar abundances? Yes. Are they as well studied in the west? No.
There is a 2002 published study that was hoping to re-discover the native Oregon Giant Earthworm, which hasn’t been officially collected in over ten years but was found after this study. It can grow over 3 feet long and can burrow up to 20 feet (and it smells like lilies!). The authors did not find this species, but they did find that in Willamette Valley “remnant forests” 97.2% of the biomass of worms was that of introduced species. There were some native worm genera found, but they were very few.
In Toby’s study, they used a mustard extraction method to find the abundances of earthworms in various natural areas throughout Portland. They found earthworms to be most abundant in hardwood forests, with some areas having densities of over 2 million per acre. There were a few plots where there were no earthworms, and they were in an undisturbed conifer-dominated area of Forest Park. Introduced earthworms were nearby, however (Low pH, as well as rocky and sandy soils, can be a limiting condition for introduced earthworms.). Most plot areas surveyed had crossed a threshold where the introduced worms have created non-historic soil conditions that contain no duff layer. The data showed introduced earthworms to be abundant and common in most natural areas in Portland. This points to reimagining how we contextualize invasive plant species. Earthworms are one of many drivers of rapid ecological change that favors r-selected introduced plant species. We have explained invasive non-native plants as outcompeting native plants, but this is not the story where introduced earthworms live. In earthworm areas, the field and published research point to earthworms as driving a large shift in ecosystem processes, with one of the outcomes being the favoring of introduced, r-selected plant species.
An audience member mentioned Dr. Bernd Blossey, who is a professor at Cornell and author of numerous earthworm studies. He was working on a biocontrol for garlic mustard but ultimately decided to abandon the project. His research showed that earthworms, along with non-native slugs and deer overpopulation, were favoring non-native plant species, like garlic mustard. He decided to forgo his efforts to pursue the biocontrol for garlic mustard because he felt that the out of place plants were the symptom of larger changes in the ecosystem and the biocontrol would not address the deeper issues.
Clearly, more research on earthworms in our region is needed. In the meantime, we can begin to take notice of earthworm signs in the field and consider the possible effects of earthworms on our restorations on a site-by-site basis.