Human-facilitated invasion of exotic earthworms into northern boreal forests

Abstract

Recreational and industrial development is rapidly expanding in the boreal forest of western Canada. Concerns exist that such activity is facilitating the introduction and spread of non-native species such as exotic earthworms. The mechanisms by which earthworms are introduced and spread in forested systems are not well understood. Given the poor intrinsic dispersal abilities of earthworms, we propose 4 major introduction mechanisms: 1) abandonment of worms brought as fishing bait; 2) dispersal via vehicles; 3) dispersal via vertebrate predators; or 4) dispersal via waterways. To test these hypotheses, we sampled earthworms in forest stands near boat launches, roads, seismic lines, forest interiors, and remote shorelines of lakes in the boreal forest of northern Alberta, Canada. Boat launches and roads had a significantly higher probability of earthworm occurrence (54% of plots occupied) compared with the other locations (13% of plots occupied). Species commonly used as fishing bait occurred more often near boat launches (18% of plots occupied) than near roads alone (1% of plots occupied). These results suggest that vehicle transport and bait abandonment may both be mechanisms of earthworm introduction. The consequences of these introductions remain unclear but suggest that reduced road construction and regulations prohibiting the discarding of bait need to be considered to slow earthworm invasions.