Astute reader Betsy Dowling, a Seattle native now studying urban horticulture at Mesa Community College in Arizona, noticed the similarity between the Seward Park 2011-2017 before and after photos which introduce this blog, and this pair, from the 5th edition of Soil Science and Management, Edward Plaster, 2008 - copied here (alas) without permission:

On a quick trip to Seward Park last night I performed a ridiculously ad hoc assay, digging and examining a cubic foot of soil from three spots: ground zero (GZ: 1 worm), active die-off zone (ADZ: 4 worms) and a healthy dense sword fern community off the Erratic Trail (ET: 4 worms). This establishes nothing other than that earthworms (no species ID) are present.

Here is a hodgepodge of academic papers. 2002 research by Michael Gundale apparently introduced the topic, describing on the effects of invasive earthworms on a rare, mostly underground fern:

Influence of Exotic Earthworms on the Soil Organic Horizon and the Rare Fern Botrychium mormo

The research is summarized in the Conservation Magazine. The original paper is listed here, with this abstract:

Forests north of the last glacial extent have no native earthworms. Exotic earthworms are now colonizing forests that are naturally free of earthworms. It is currently unknown how these exotic earthworms might affect rare plants. To determine whether there is an association between the presence of an exotic earthworm species and extirpation of the rare fern Botrychium mormo, I surveyed 28 populations documented and counted previously. I estimated current population sizes of B. mormo, soil horizon thickness, earthworm species present, and carbon content, nitrogen content, and pH of the A soil horizon. Two earthworm species were abundant, Lumbricus rubellus and Dendrobaena octaedra. Dendrobaena octaedra had no significant association with any soil variable or with B. mormo extirpation. Lumbricus rubellus was significantly associated with B. mormo extirpation and a mull humus type. Where L. rubellus was present, O1 and O2 horizons were significantly thinner. I conducted a laboratory microcosm experiment to determine whether L. rubellus could create the conditions it was associated with in the field. Microcosms with L. rubellus resulted in a significant reduction in the thickness of the O1 and O2 horizon and a significant increase in the thickness of the A horizon. This experiment suggests that L. rubellus created the conditions with which it was associated in the field. The intrinsic rate of increase (r) of B. mormo was best explained by the O2 depth, which implies that this soil layer supplies a critical resource. My results support the idea that exotic earthworms alter the forest floor, leading to negative changes in native vegetation.