Effects of an invasive earthworm on plethodontid salamanders in the Great Smoky Mountains National Park

Craft, Jeremy John

Amynthas agrestis is an epigeic (litter-dwelling) invasive earthworm found in Blount County, TN and within the Great Smoky Mountains National Park. Epigeic Amynthas spp. convert leaf litter into granular surfaces with little or no leaves, contributing to declining abundances of litter-dwelling arthropods. The reduction of the litter layer by A. agrestis, and carbon and nitrogen loss associated with earthworm activity, should decrease woodland salamander (Plethodon spp.) foraging success, body size, and body condition. The possibility of top-down control on A. agrestis exists if Plethodon recognize A. agrestis as prey. I collected ten intact 1500 cm3 cores of soil and leaf litter from each side of an A. agrestis invasion front in May, June, and July 2008 (60 total) and placed them into plastic pots with individual Plethodon metcalfi or P. teyahalee. After five days salamanders were removed. Their feces were collected and analyzed for percent total organic carbon (TOC) and percent total nitrogen (TN). I conducted 67 feeding trials for P. metcalfi and 49 trials for P. teyahalee in which a salamander was given the choice between one A. agrestis and an isopod. Salamanders were observed for prey consumption for one hour and again after 12 hours. Ten 0.25 m2 leaf litter samples (20 total) were sampled from random locations on each side of the invasion front both in June and in July 2008, and analyzed for differences in dry mass. Mean C:N ratios in salamander feces were significantly higher for the earthworm-invaded treatment than for the non-invaded treatment for both salamander species (P. metcalfi difference 0.27, P = 0.0004; P. teyahalee difference 1.18, P = 8.4 x 10-6). Mean TOC (P. metcalfi difference 7.2%, P = 6.3 x 10-9; P. teyahalee difference 3.4%, P = 0.0046) and mean TN (P. metcalfi difference 1.2%, P = 3.3 x 10-10, P. teyahalee difference 0.8%, P = 0.0001) were both significantly less in earthworm-invaded treatments for both salamander species. Plethodon metcalfi (26 isopods to 2 earthworms, P < 0.0001) and P. teyahalee (16 isopods to 4 earthworms, P < 0.0004) consumed significantly more isopods than earthworms. Mean leaf litter mass was significantly less (40%) in invaded plots in July (P = 0.0012) but not in June. Nutrient analyses suggest that salamanders foraging in A. agrestis invaded areas obtain significantly less carbon and nitrogen in their diets, as well as less nitrogen relative to carbon. Reduced leaf litter mass in invaded areas diminishes salamander habitat as well as salamander prey habitat. Together these data suggest that invasive earthworms have a bottom-up influence on terrestrial salamanders. Feeding trials results indicate that salamanders prefer isopods over A. agrestis when both were offered simultaneously. This suggests that salamanders might exhibit little top-down control on A. agrestis, but additional tests using other common prey items are necessary. Further studies of foraging success and salamander densities in earthworm-invaded habitats will clarify the extent of the earthworm impact on salamanders.

Effects of an invasive earthworm on plethodontid salamanders in the Great Smoky Mountains National Park

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