Assessing the Impact of Lumbricid Earthworms on Forest Seeds in Northeastern Ohio

Abstract

This study examines the capacity of two nonnative lumbricid earthworms, Lumbricus terrestris and L. rubellus, to ingest and digest the seeds of herbaceous plants and affect their distribution in forest soils of northeastern Ohio. A survey of earthworms in a forested area of Cleveland Metroparks revealed Lumbricus was the most common and widely distributed earthworm genus, although Aporrectodea and Octolasion were also well represented. A total of 120 soil samples taken adjacent to earthworm sampling plots and placed in a growth chamber yielded a total of 35 germinations. The germinations occurred primarily in the upper 8 cm of the soil profile, suggesting limited redistribution of seeds by earthworms. Growth chamber feeding trials using seeds ranging in size from approximately 2 to 5 mm maximum linear dimension revealed that both species of earthworm will consume and digest seeds that measure up to between 3 and 4 mm. Lumbricus terrestris consistently ingested and digested more than twice as many seeds as did the substantially smaller L. rubellus. Comparisons of the germination rates of seeds egested by the earthworms with rates for seeds that were not ingested showed no impact on germination due to egestion, suggesting ingestion and subsequent egestion of seeds by lumbricid earthworms does not enhance germination. By ingesting and digesting seeds, L. rubellus and L. terrestris should reduce the number of viable seeds in the soil and, thereby, contribute to changes in forest vegetation.