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

• Published in: Conservation biology Vol. 16; no. 6; pp. 1555 - 1561
• Main Author: Gundale, Michael J.
• Format: Journal Article
• Language: English
• Published: Boston, MA, USA Blackwell Science Inc 01.12.2002; Blackwell Science; Blackwell
• Subjects: Animal, plant and microbial ecology; Applied ecology; Biological and medical sciences; Conservation, protection and management of environment and wildlife; Earthworms; Environmental degradation: ecosystems survey and restoration; Forest soils; Fundamental and applied biological sciences. Psychology; Humus; Microcosms; Mineral soils; Organic soils; Parks, reserves, wildlife conservation. Endangered species: population survey and restocking; Soil biology; Soil ecology; Soil horizons; Soil organic matter; Minnesota; Chippewa County Minnesota; United States; North America; America; Introduced species; Organic soil; Impact study; Forest soil; Pteridophyta; Annelida; Oligochaeta; Rare species; Bioturbation; Earthworm; Plant; Lumbricus rubellus; Animal plant relation; Soil horizons; Invertebrata; Colonization; Filicineae
• ISSN: 0888-8892; 1523-1739
• DOI: 10.1046/j.1523-1739.2002.01229.x

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.