Impacts of Invasive Earthworms on Early Life Stages of the Threatened American Hart's-Tongue Fern

Abstract. Conservation of rare ferns requires identification and management of drivers of species declines. Here we focus on potential threats of invasive earthworms introduced from Europe and Asia on the threatened American hart's-tongue fern (Asplenium scolopendrium var. americanum). Earthworms are ecosystem engineers that alter physical and chemical soil properties and can trigger cascading effects on plant and soil communities. Using a paired field sampling and mesocosm study approach, we sought to assess the potential impact of earthworms to A. scolopendrium. We sampled earthworms at eight A. scolopendrium populations in New York State and documented widespread occurrence of a diversity of invasive earthworms in seven fern populations. Fern populations exist mid-slope and earthworm biomass was higher upslope than within or downslope of fern populations. In a two-year mesocosm experiment we evaluated impacts of two epi-endogeic earthworms (Lumbricus rubellus and Metaphire hilgendorfi) on A. scolopendrium gametophytes and young sporophytes by adding earthworm treatments to potted plants and following growth and survival of fern transplants. While L. rubellus reduced gametophyte survival and number of leaves produced by transplanted ferns, we found no effect of M. hilgendorfi on survival, number of leaves or leaf size. Our findings indicate negative, but limited, impacts of one invasive earthworm species on fern early life stages. Our results suggest that impacts on existing fern populations could be low, as few earthworms were detected within fern populations. Continued assessment of earthworm invasions and other associated stressors is necessary to inform future management and conservation efforts to facilitate A. scolopendrium recovery.