Human traffic and fire effects on invasive earthworm abundance in the Boundary Waters Canoe Area Wilderness of northern Minnesota
Exotic earthworm invasion of temperate North American forests may be an important driver of ecosystem change. Earthworms can alter soil structure, nutrient cycles, and the diversity and abundance of plants. However, the extent of earthworm invasion and their impact on northern forests is incompletely understood. To study the role of humans in spreading exotic earthworms and their response to fire, we examined the distribution and abundance of exotic earthworms in the Boundary Waters Canoe Area (BWCA) Wilderness in Minnesota, USA. We hypothesized that worms would be more abundant near BWCA campsites and less abundant in areas burned by the Pagami Creek Fire of 2011. We assessed earthworm abundance in quadrats from four land categories: campsites, non-campsites, burned forest, and unburned forest. Earthworm impacts were assessed by measuring soil nitrogen and pH, plant cover, and litter accumulation.
Humans and fire appear to strongly affect the occurrence of invasive earthworms in the BWCA. Campsites showed significantly higher worm abundance than non-campsite areas, and the burned forest had lower densities. In burned areas, worm abundance decreased as distance as the access entry point increased, indicating human traffic correlates with their spread. Earthworms were associated with lower leaf litter biomass and these effects were especially evident near campsites. Fire increased soil N and pH, but earthworm density did not affect these parameters. The data suggest that although earthworm effects in the BWCA are not yet pronounced, human activities are an ongoing driver of the earthworm invasion and forest fires may temporarily reduce their numbers.