Perhaps the most sweeping example of the novelization of ecosystems in eastern North America is the invasion of exotic earthworms into previously earthworm-free soils. There has been burgeoning interest in studies of earthworms as granivores with preferences for different seeds. It remains unclear, however, how earthworms interact with ecological restoration practices involving the addition of seed to the soil surface. We examined how the common exotic earthworm Lumbricus terrestris interacts with widely used commercial grass seed mixes.
We used a cafeteria feeding experiment to examine the seed preferences of L. terrestris based on seed size (large, small) and the presence/absence of a coating designed to increase water uptake (uncoated, coated). For each coating type, earthworms were presented with 20 large and 20 small seeds. After 18 hours, we assessed the number of remaining and ingested seeds. We conducted a subsequent mesocosm experiment with large nursery pots to examine the effects of L. terrestris on the growth success of the uncoated and coated seed mixes. We used a two-by-two factorial design with two seed mix types (uncoated, coated) and two levels of earthworm amendment (present, absent). After 16 days, aboveground grass biomass was harvested, dried, and compared between trials.
Earthworm ingestion of grass seed was influenced by seed size and coating, with preferences for smaller and coated seeds. Total ingestion rates were highest for small coated seed (49%), moderate for uncoated small (32%) and coated large seed (32%), and lowest for uncoated large seed (8%). In the mesocosm experiment, earthworm presence was observed to significantly reduce grass biomass (p < 0.005), with a greater reduction for the coated seed (35.7% reduction for uncoated, 78.5% reduction for coated). The earthworm impact on grass growth was visibly evident as a reduction in overall cover and an increase in patchiness, with tufts of growth predominantly aggregated around earthworm burrows and casts.
The key finding was that our experiments identified preferences of a prominent exotic earthworm amongst commonly used grass seed. These preferences were mirrored in varying rates of granivory that impacted the overall performance of the different mixes. Selective earthworm granivory of surface-applied seed has implications for both natural plant communities and restoration. Considering the ubiquity of exotic earthworms as ecosystem engineers in eastern North America and their continuing dispersal, our research is an important advancement in understanding the implications of these organisms in their new ecosystems in the Anthropocene.