Impacts of invasive earthworms and deer on native ferns in forests of northeastern North America

Research Summary by Abby Bezrutczyk

Bowe, A., Dobson, A., & Blossey, B. (2020). Impacts of invasive earthworms and deer on native ferns in forests of northeastern North America. Biological Invasions, 22(4), 1431–1445.


In the understories of northeastern forests, overabundant deer and invasive earthworms are ecosystem stressors. But how do these two stressors – deer and earthworms – interact, and what does that mean for understory plants like woodland ferns? Bowe and colleagues (2020) studied the impacts of deer and earthworms on four fern species (Northern Maidenhair fern (Adiantum pedatum) , Hay-scented fern (Dennstaedtia punctilobula), Christmas fern (Polystichum acrostichoides), and Marginal Wood fern (Dryopteris marginalis)), by planting ferns in areas with and without earthworms, and with and without deer. Also, the authors developed a lab-based experiment – where they planted ferns in pots with and without worms and leaf litter to mimic field conditions, and followed growth and survival of ferns and earthworms over the growing season. Because deer tend to avoid eating ferns, they expected that the presence of deer would favor the ferns – but instead found that the presence or absence of deer made no difference. However, they found that in the field, worms were a driving force: favoring Christmas Fern and Maidenhair Fern. And ferns might be a driver themselves; worms added to pots with Christmas Fern experienced lower survival.

Take-home points 

  • Deer overabundance and earthworm invasions are two stressors on ecosystems.
  • Invasive earthworms favored the growth and establishment of Christmas fern and Maidenhair fern over other ferns.
  • Native ferns can have negative impacts on the survival of invasive worms.

Management implications 

  • Earthworms invasions change the understory community by disturbing roots, disrupting mycorrhizal associations, and altering seedling survival.
  • Because stressors interact with each other, the cause behind an unhealthy ecosystem may not be as simple as it seems. This makes it important to study complex interactions in the field, because a lab-based experiment can produce different results.
  • Reducing high deer populations may be important to prevent browse on desirable species, but preventing distribution of earthworms through heavy equipment and forestry practices may be equally important to reduce fern dominance and sustain desirable and diverse forest understories.