Invasive Species, Habitat Restoration, and Reintroduction Biology of the Spotted Salamander
In contrast to the positive effect that round gobies have had on Lake Erie watersnakes, invasive species more typically have negative effects on native species. This is certainly true for European buckthorn, an invasive woody shrub that now occurs across a large portion of eastern North America and has displaced native plants, changed soil characteristics and patterns of nutrient cycling, and may be triggering declines of a variety of invertebrate and vertebrate species. European buckthorn is especially successful at invading flatwoods forest habitats. Such forests are underlain by a clay pan that resists water percolation, and are characterized by hydric soils and ephemeral ponds that are important amphibian breeding sites. The rarity of flatwoods forests has resulted in their recognition as globally imperiled and has made them the target of management and restoration efforts. One such effort is that of the Lake County Forest Preserve District in northeastern Illinois to restore the MacArthur Woods flatwoods forest through European buckthorn removal and drainage tile disablement. Following these restoration efforts, the District approached Dr. Carl von Ende and me with an offer to fund a PhD dissertation project aimed at assessing the feasibility of reintroducing three species of amphibians that had been extirpated from the site, spotted salamanders, woodfrogs, and spring peepers. A national search resulted in our hiring Allison Sacerdote to undertake this project. Allison conducted a series of in situ 
enclosure experiments that demonstrated similar hatching success and survival to metamorphosis between restored ponds and source ponds in a neighboring county, suggesting that reintroduction should be possible. However, these experiments also revealed large among-pond differences in hatching success that appeared to correlate with levels of dissolved oxygen (DO). This led us to conduct a series of laboratory experiments aimed at determining DO thresholds in spotted salamanders and blue-spotted salamanders (a species that has persisted at MacArthur Woods). We anticipated that these species might differ in DO threshold because of differences in egg deposition patterns. Spotted salamanders lay a single tightly packed egg clutch through which oxygen diffusion is slow whereas blue-spotted salamanders lay eggs singly or in loose clusters through which oxygen diffusion should be more rapid. Our results confirmed this expectation.
Spotted salamanders consistently experienced hatching failure at less than 50% DO saturation whereas blue-spotted salamanders hatched successfully at all DO concentrations (Sacerdote and King, in press). Further restoration efforts aimed at maintaining sufficient DO within restored ponds (e.g., leaf-litter removal, selective canopy thinning) have been initiated as a consequence. We have also investigated whether European buckthorn might affect amphibians in other ways. One explanation for the invasion success of European buckthorn is that it produces emodin, a chemical which acts to prevent herbivory and inhibit growth of other plants. Emodin has also been implicated in disruption of mammal metabolism and development and so we wished to test its effect on amphibian development. To do so, we used the Frog Embryo Teratogenesis Assay - Xenopus (FETAX), a standard procedure for assessing developmental effects of environmental contaminants on amphibians. Our results were quite startling. Emodin resulted in 100% embryo mortality at concentrations greater than 50 ppm and increased developmental malformations even at the lowest concentrations tested (0.1 ppm). Our understanding of emodin concentrations, movement, and persistence in the environment remains incomplete but we have recorded soil concentrations exceeding 2 ppm in areas heavily infested with European buckthorn; thus, the potential for emodin to impact amphibian development in nature seems real.