B. Michael Walton

Edge effects and intraguild predation in native and introduced centipedes: evidence from the field and from laboratory microcosms

Human alteration of habitat has increased the proportion of forest edge in areas of previously continuous forest. This edge habitat facilitates invasion of exotic species into remaining fragments. The ability of native species to resist invasion varies and may depend on intrinsic variables such as dispersal and reproductive rates as well as external factors such as rate of habitat change and the density of populations of introduced species in edge habitat. We examined the distributional and competitive relationships of two members of the class Chilopoda, Scolopocryptops sexspinosus, a centipede native to the eastern US, and Lithobius forficatus, an exotic centipede introduced from Europe. We found that L. forficatus was most abundant in edge habitat and S. sexspinosus was most abundant in the interior habitat at our field sites. Although L. forficatus was present in habitat interiors at 11 of 12 sites, there was no correlation between fragment size and numbers of L. forficatus in interior habitat. The native centipede was rarely found occupying fragment edges. We used laboratory microcosms to examine potential competitive interactions and to indirectly assess prey preferences of the two species. In microcosms both species consumed similar prey, but the native centipede, S. sexspinosus, acted as an intraguild predator on the introduced centipede. Native centipedes were competitively superior in both intraspecific and interspecific pairings. Our results suggest that intraguild predation may aid native centipedes in resisting invasion of introduced centipedes from edge habitat.

Determinants of mussel diversity in Lake Erie tributaries

Abstract We examined recent faunal changes in freshwater mussels (Unionidae) in northern Ohio. We assessed species diversity in 2 agricultural watersheds, the Vermilion and Huron Rivers, and compared these faunas among 7 neighboring rivers within the Lake Erie watershed that varied in levels of human impact, geological structure, and fish diversity. We found 19 mussel species in each of the Huron and Vermilion Rivers, and 89% of species overlapped between the 2 rivers. Museum records suggested that as many as 4 species might now be missing from each river, and across the 9 tributaries in the region, species loss has been common. The total number of species once present in each river was positively correlated with watershed area, but the size of watersheds no longer provides a significant estimate of mussel or fish diversity. Current mussel faunas also were weakly correlated with the fish species present, suggesting that mussel species ranges have not been limited by a lack of available fish hosts.

Top-Down Regulation of Litter Invertebrates by a Terrestrial Salamander

Abstract: Terrestrial plethodontid salamanders are abundant predators within the forest floor litter of eastern North America, and are hypothesized to regulate soil and litter invertebrate density and species composition. I tested this hypothesis during a 6-yr study of the effects of the Eastern Red-backed Salamander (Plethodon cinereus) on the invertebrate community of a forest site in northeast Ohio. Salamander surface density, invertebrate abundance, and community composition were monitored within 30 open, circular plots. Variation in plot occupancy by P. cinereus was achieved by supplying plots with differing amounts of artificial cover (0, 1, or 4 ceramic tiles) that served as refuges for the salamanders. Salamander plot occupancy, invertebrate density, leaf litter mass, and leaf litter moisture were quantified each spring and fall from 2003 through 2008. Statistically significant effects of salamander plot-occupancy on invertebrate densities were found for several taxa of mesofauna, including several Collembola taxa, oribatid mites, pseudoscorpions, and psocoptera. The strength and direction of salamander effects varied among taxa and included negative, positive, and no effects on invertebrate densities. The magnitude and sign of salamander effects on invertebrate densities were predicted by seasonal and interannual variation in leaf litter mass and, to a lesser extent, litter moisture content. Salamander effects decreased with increasing litter mass and were more often negative when litter mass was high, whereas positive effects on invertebrate densities were more likely when litter mass was low. For several taxa, the positive effect of P. cinereus also increased with litter moisture. I propose two mechanistic hypotheses for these dynamics that integrate behavioral ecology of salamander prey selection and territorial defense with variation in litter mass and litter moisture.

Interactions among Forest-floor Guild Members in Structurally Simple Microhabitats

Abstract Intraguild predation in structurally complex habitats is thought to weaken trophic cascades and increase food web stability. However, many predators commonly found in leaf litter become restricted to simple microhabitat beneath rocks and logs during periods between rains. It is within this structurally simple microhabitat that some predators defend rich prey resources and are likely to interact strongly as the surrounding forest becomes too dry to forage broadly in space. We conducted a 4-y press experiment where we removed focal predators from unfenced field plots. To evaluate the effects of predators on one another we removed either salamanders or centipedes from beneath artificially placed cover objects and compared abundances of these and other intraguild predators to those in non-removal controls. We predicted that salamanders and centipedes would have strong negative effects on each other and on carabid beetles and spiders. We removed a total of 1288 salamanders and 1056 centipedes over 98 sampling dates. In salamander removal plots spider abundance increased by 34%, and carabid beetles decreased by 15% relative to the control. In centipede removal plots salamanders increased by 18% and carabid beetles increased by 29%, but spider abundance decreased by 15%. Interaction strengths were strongest in the drier summer months when territorial predators were confined in spatially fixed microhabitats. It is during these periods that predators may strongly regulate the abundances of guild members. In territorial species that defend areas beneath natural cover, the effect of intraguild predators may be an important mechanism that regulates distribution and abundance of forest floor predators.

Status of the exotic ant Nylanderia flavipes (Hymenoptera: Formicidae) in northeastern Ohio

In 2005, the non-native Nylanderia flavipes was first recorded from Ohio. Here, we present the results of a baseline study designed to assess the status of this exotic species in northeastern Ohio and to explore its potential impacts on local ant communities and the extent to which it has been incorporated into the diet of a native predator, the red-backed salamander. At the sites where N. flavipes occurred, we found a sixfold increase in total ant abundance, with 87% of all ants collected being N. flavipes. The high numerical dominance of N. flavipes did not lead to observable changes in the species richness and abundance of the native ant community. At baits, N. flavipes did not engage in interspecific combat and did not exhibit aggression towards native ants. Thus, aggression and interference competition are not likely factors explaining the high local abundance of N. flavipes. Red-backed salamanders have incorporated N. flavipes into their diet, but further research is needed to understand the interactions of N. flavipes within the forest-floor food web. Although we did not detect changes in the local ant communities in the presence of N. flavipes, we argue that this species’ high local abundance and ability to forage at cooler temperatures may give it a competitive advantage and thereby affect native ants through exploitative competition.

Eastern Red-backed Salamanders Regulate Top-Down Effects in a Temperate Forest-Floor Community

Abstract Understanding the role of species interactions as regulatory mechanisms for ecosystem processes presents a challenge to ecologists working in systems with high species diversity and habitat complexity. Recent studies suggest that interactions among intraguild predators, such as terrestrial salamanders and large arthropods, might be important for the regulation of detritivores, fungivores, and perhaps detritus within terrestrial webs. A key prediction is that interactions among predators weaken trophic cascades. Our research examined this prediction by removing predators for 4 yr from unfenced field plots to investigate the effects on litter arthropods, the microbial community, and rates of leaf litter decomposition. We manipulated predator abundance in three treatments (salamander removal, centipede removal, and multiple predator removal) compared to a control in which no predators were removed. Despite difficulties in suppressing centipede numbers, we observed increases in salamanders, millipedes, isopods, slugs, numbers of ant colonies, and gamasid mites in the centipede removal plots. Additionally, several phospholipid fatty acid markers for bacteria were suppressed in plots where salamanders were most abundant. Finally, we detected treatment effects on the rate of litter disappearance from leaf bags in our field plots: those with the most salamanders had the lowest levels of litter decomposition. Overall, we found some evidence for top-down effects of predators in a temperate forest-floor web. Our study is one of few that have employed an unfenced field design and the only study examining the effects of salamanders on forest soil microbes. The results contribute to a growing body of evidence indicating that territorial predators, such as terrestrial salamanders, can be strong regulators of species composition at lower trophic levels in a system that is commonly thought to be regulated primarily through bottom-up effects of organic matter supply.

Evaluating the mussel fauna of the Chagrin River, a state-listed "scenic" tributary of Lake Erie.

Abstract Levels of environmental protection vary among watersheds, and assessing how well conservation efforts protect threatened faunal groups is a critical need for management. Almost the entire 114 km of the Chagrin River is designated as scenic by the state of Ohio, which implies good water quality and community efforts to maintain and improve water resource integrity. We examined mussel diversity and abundance across 30 sites. The mussels present remained largely restricted to the upper reaches. One species, Lampsilis radiata luteola, dominated the assemblage of the upper Chagrin, which now includes only six additional species, none of which are very abundant: Lasmigona costata, Lasmigona compressa, Pyganodon grandis, Strophitus undulatus, Utterbackia imbecillis, and Anodontoides ferussacianus. One additional species, Lampsilis cardium, was the only species found living in the lower Chagrin. Applying the Shannon index of diversity indicated that the Chagrin River has a more depauperate fauna than neighboring watersheds, and therefore, the current passive conservation efforts may be insufficient to protect these small isolated populations of remaining species.