Non-native earthworms increase population densities and are important prey of Plethodon cinereus, a common woodland salamander of eastern North America

Background/Question/Methods

Native earthworm species to Québec (Canada) were extirpated during the last glaciation period. Reintroduced by Europeans over recent centuries, non-native earthworms have had substantial impacts on forest ecosystem properties. However, less is known about their impacts on higher faunal species. Here, we report on a study that investigated the effects of non-native earthworms on populations of Plethodon cinereus, a common woodland salamander. We hypothesized that earthworms could adversely affect P. cinereus by consuming the forest floor, thereby decreasing ground cover and soil moisture as well as the abundance of native prey. On the other hand, earthworms could positively affect P. cinereus by providing refugia in their abandoned burrows and by being nutritious prey. Therefore, we conducted a study to determine how non-native earthworms affect the demographics and diets of P. cinereus. In 2019, we installed cover-board arrays at 38 mature sugar maple (Acer saccharum) forest sites. We quantified earthworm population densities using hot mustard solution extractions and surface surveys. During 2020 and 2021, we visited each site on nine occasions and tagged individual salamanders with visible implant elastomers to estimate population densities using capture-mark-recapture techniques. At a subset of four sites, we determined salamander diets using gastric lavage techniques.

Results/Conclusions

As expected, forest floor depth decreased with earthworm abundance. On the other hand, population density and body size of P. cinereus increased with earthworm abundance, contrary to similar studies performed in the U.S. Earthworms composed the majority of the salamander’s diet at sites with high earthworm population densities, volumetrically accounting for more than all other prey groups combined. Our study confirms that the human-facilitated invasion of exotic earthworms into northern temperate forests may have an impact on native vertebrate species such as salamanders. Our results thus provide useful information for guiding nature conservation policies in Quebec.