| In addition to creating or modifying habitat, ecosystem engineers interact with members of their communities as predators, prey, or competitors. Determining the relative strengths of these biotic verses engineering pathways by organisms may clarify how ecosystem engineering complements more well-studied species interactions, such as competition and predation, in shaping communities. I examined interactions between earthworms and the red-backed salamander, Plethodon cinereus, near Mountain Lake Biological Station in the southern Appalachian Mountains. Earthworms are well-known as ecosystem engineers and I predicted that they would interact with P. cinereus via several pathways: 1) ecosystem engineering, by providing burrows that could be used as a refuge, 2) direct, trophic effects as a prey item, and 3) indirectly, by competing with micro-invertebrates, another important prey item for P. cinereus. In addition, both native and invasive earthworms co-occur with P. cinereus, allowing me to explore the responses of a species to closely related native and invasive ecosystem engineers. I found that P. cinereus readily uses earthworm burrows made by the native earthworm Eisenoides carolinensis and by invasive earthworms including Lumbricus terrestris and Octolasion tyrtaeum. When food resources are limited, such as in an enclosure setting or perhaps in drier years, the direct trophic effect of earthworms as prey for salamanders is > 3X as important than the indirect or ecosystem engineering pathways. In the field, where microinvertebrates are more abundant, the indirect effects of only the invasive earthworm O. tyrtaeum negatively influenced counts of P. cinereus. However, when underground habitat is limiting, earthworm burrows can provided a refuge from predators and increase survival of adult salamanders over the winter. With this project I incorporated ecosystem engineering, direct trophic, and indirect competitive effects throughout and showed that the importance of these three pathways is mediated by season, salamander age class, and earthworm species. My dissertation work highlights the importance of ecosystem engineers, not only through habitat provisioning, but also via direct (trophic) and indirect pathways. In addition, this work may help in understanding of the role of ecologically similar native and non-native ecosystem engineers in ecological communities. |
