PS 36-77
Management of the invasive earthworm, Amynthas agrestis, with prescribed fire
Earthworms function as ecosystem engineers by changing and constructing environments, and by influencing the community structure of other soil organisms. Therefore, invasion of earthworms into ecosystems can cause significant alterations. Amynthas agrestis is known to have invaded the USA from Asia, and has expanded its distribution range to include relatively undisturbed forest ecosystems. First, to clarify the life history of A. agrestis, we reared individuals under 7 different conditions of food provision (only litter, only Oa/Oe, only soil, litter and Oa/Oe, litter and soil, Oa/Oe and soil, all materials), and compared the weights of worms, survival rates, and cocoon production rates. We also collected A. agrestis in the field, and analyzed their stable isotope ratios to investigate their food sources. Second, we examined whether prescribed fire (a common forest management practice) can be used to manage invasive earthworms by direct mortality to earthworms and cocoons, and/or by limiting food resources. We constructed eight experimental beds (1.3x3.0m) with 100 individuals of A. agrestis each, and burned half of these after 8 weeks of A. agrestis habitation. After burning, we counted live earthworms and collected their cocoons. Collected cocoons were incubated for 4 months and then dissected to determine their viability.
Results/Conclusions
The breeding experiment showed that the earthworms reared without soil did not produce cocoons, and died within 140 days. The worms in units containing soil and some form of organic matter (soil and litter; soil and Oa/Oe; all materials) produced many cocoons (60, 79 and 179, respectively). These results show that litter and Oa/Oe are important food resources for A. agrestis. Stable isotope analyses also supported that the litter and Oa/Oe are the main food resource in the field. During the fire experiment, soil temperature at the 5cm depth increased only a little from 29.9 to 34.8ºC. Litter mass was significantly lower in burned relative to unburned beds. Although the numbers of A. agrestis and cocoons recovered from burned and unburned beds were not different, the hatching rates of cocoons were significantly lower in burned beds. Our study revealed that prescribed fire does not increase temperature in the soil enough to directly eliminate the invasive earthworm, but can greatly reduce the hatching rate of cocoons. It can also reduce the survival rate of juveniles in the next year by depriving their food resource. Therefore, prescribed fire may be a viable tool for control of invasive earthworm populations in forest ecosystems.