Quantifying seeds egested by field-collected earthworms: a dynamic and overlooked pool in forest soil seed banks

Abstract

Although awareness of the influence of earthworms on soil seed banks in Canadian forests is growing, there have been few direct field measurements. We used a novel pairing of field-collected earthworms from a central Great Lakes forest in Ontario with a laboratory seed egestion assay to obtain a snapshot of the number of seeds passing through earthworms compared with seeds found in the surrounding soil. We identified a pool of seeds egested by earthworms that accounted for 2.4% of all seeds found in the earthworms and the top 0–10 cm of soil. Individual earthworms contained 0–5 seeds. The large-bodied adult anecic non-native Dew Worm or Common Nightcrawler (Lumbricus terrestris) egested a disproportionate number of seeds for its abundance (50% of egested seeds from 17% of earthworms), but smaller earthworms were also an important source of egested seeds (the other 50%). This small-scale proof-of-concept study demonstrates a method of directly measuring earthworm–seed interactions in the field. It can also detect seeds egested by earthworms below ground that would otherwise be missed by other seed accounting methods and it highlights the importance of granivory by non-surface casting earthworms.