Liam Heneghan

Earthworm invasion into previously earthworm-free temperate and boreal forests.

Earthworms are keystone detritivores that can influence primary producers by changing seedbed conditions, soil characteristics, flow of water, nutrients and carbon, and plant–herbivore interactions. The invasion of European earthworms into previously earthworm-free temperate and boreal forests of North America dominated by Acer, Quercus, Betula, Pinus and Populus has provided ample opportunity to observe how earthworms engineer ecosystems. Impacts vary with soil parent material, land use history, and assemblage of invading earthworm species. Earthworms reduce the thickness of organic layers, increase the bulk density of soils and incorporate litter and humus materials into deeper horizons of the soil profile, thereby affecting the whole soil food web and the above ground plant community. Mixing of organic and mineral materials turns mor into mull humus which significantly changes the distribution and community composition of the soil microflora and seedbed conditions for vascular plants. In some forests earthworm invasion leads to reduced availability and increased leaching of N and P in soil horizons where most fine roots are concentrated. Earthworms can contribute to a forest decline syndrome, and forest herbs in the genera Aralia, Botrychium, Osmorhiza, Trillium, Uvularia, and Viola are reduced in abundance during earthworm invasion. The degree of plant recovery after invasion varies greatly among sites and depends on complex interactions with soil processes and herbivores. These changes are likely to alter competitive relationships among plant species, possibly facilitating invasion of exotic plant species such as Rhamnus cathartica into North American forests, leading to as yet unknown changes in successional trajectory.

The influence of invasive earthworms on indigenous fauna in ecosystems previously uninhabited by earthworms

Recent studies on earthworm invasion of North American soils report dramatic changes in soil structure, nutrient dynamics and plant communities in ecosystems historically free of earthworms. However, the direct and indirect impacts of earthworm invasions on animals have been largely ignored. This paper summarizes the current knowledge on the impact of earthworm invasion on other soil fauna, vertebrates as well as invertebrates.Earthworm invasions can have positive effects on the abundance of other soil invertebrates, but such effects are often small, transient, and restricted to habitats with harsh climates or a long history of earthworm co-occurrence with other soil invertebrates. Middens and burrows can increase soil heterogeneity and create microhabitats with a larger pore size, high microbial biomass, and microclimates that are attractive to micro- and mesofauna. Under harsh climatic conditions, the aggregates formed by earthworms may increase the stability of soil microclimates. Positive effects can also be seen when comminution and mucus secretion increase the palatability of unpalatable organic material for microorganisms which are the main food of most micro- and mesofaunal groups. For larger invertebrates or small vertebrates, invasive earthworms may become important prey, with the potential to increase resource availability. In the longer-term, the activity of invading earthworms can have a strong negative impact on indigenous faunal groups across multiple trophic levels. Evidence from field and laboratory studies indicates that the restructuring of soil layers, particularly the loss of organic horizons, physical disturbance to the soil, alteration of understory vegetation, and direct competition for food resources, lead directly and indirectly to significant declines in the abundance of soil micro- and mesofauna. Though studies of invasive earthworm impacts on the abundance of larger invertebrates or vertebrates are generally lacking, recent evidence suggests that reduced abundance of small soil fauna and alteration of soil microclimates may be contributing to declines in vertebrate fauna such as terrestrial salamanders. Preliminary evidence also suggests the potential for earthworm invasions to interact with other factors such as soil pollution, to negatively affect vertebrate populations.

Policy and management responses to earthworm invasions in North America

The introduction, establishment and spread of non-native earthworm species in North America have been ongoing for centuries. These introductions have occurred across the continent and in some ecosystems have resulted in considerable modifications to ecosystem processes and functions associated with above- and belowground foodwebs. However, many areas of North America have either never been colonized by introduced earthworms, or have soils that are still inhabited exclusively by native earthworm fauna. Although several modes of transport and subsequent proliferation of non-native earthworms have been identified, little effort has been made to interrupt the flow of new species into new areas. Examples of major avenues for introduction of earthworms are the fish-bait, horticulture, and vermicomposting industries. In this paper we examine land management practices that influence the establishment of introduced species in several ecosystem types, and identify situations where land management may be useful in limiting the spread of introduced earthworm species. Finally, we discuss methods to regulate the importation of earthworms and earthworm-containing media so that introduction of new exotic species can be minimized or avoided. Although our focus in this paper is necessarily North American, many of the management and policy options presented here could be applicable to the problem of earthworm invasions in other parts of the world.

European Buckthorn (Rhamnus cathartica) and its Effects on Some Ecosystem Properties in an Urban Woodland

DECEMBER 2004 275 Ecological Restoration, Vol. 22, No. 4, 2004 ISSN 1522-4740 E-ISSN 1543-4079 ©2004 by the Board of Regents of the University of Wisconsin System. The proliferation of invasive species and their effects on local diversity has emerged as a priority issue in ecological conservation (Mooney and Hobbs 2000). Although the majority of invasive species remain minor components of the invaded assemblages, some species become dominant members and may, as a consequence, substantially modify both the composition and the function of ecosystems (Williamson 1996). Researchers have paid increasing attention to the potential of invasive species to alter aspects of ecosystem function (D’Antonio and Vitousek 1992, Lodge 1993, Williamson 1996), but only recently have quantitative data been available about the indirect effects of these species (Vitousek and others 1996, Ehrenfeld and Scott 2001, Heneghan and others 2002). Nonetheless, information about alterations to ecosystem processes is particularly useful when restoring the site following removal of the invasives. European buckthorn is native to Eurasia and North Africa (Godwin 1943). It was introduced to North America in the late 1880s as an ornamental shrub and has become naturalized throughout much of the northeastern United States, the Great Plains, and throughout Canada. In Illinois, European buckthorn is widespread in the northern half of the state, and dominates many oak woodlands in the vicinity of Chicago (McPherson and others 1994). Surveys of European buckthorn populations reveal that it dominates the understory of some red oak (Quercus rubra) forests and can decimate populations of native herbaceous plants (Gourley and Howell 1984, Boudreau and Willson 1992). Despite concern about the effects of European buckthorn on native flora, little is known about how it influences the soil processes in these woodlands. We do know that buckthorn produces considerable amounts of organic matter, mainly in the form of leaves and coarse woody debris. Buckthorn litter, with its high nitrogen (N) content (2.2 percent), decomposes faster than the litter of the dominant trees in an Illinois woodland (Heneghan and others 2002). If rates of decomposition are high, as these observations show, European buckthorn may enrich soils with rapidly mineralized nutrients. Moreover, since modification of site fertility is one of the fundamental drivers of plant community organization (Tilman 1988), a legacy effect of European buckthorn in the soil may have a dramatic effect on restoration sites cleared of buckthorn. In this paper, we report on a study we conducted to characterize the effects on soil functions of rapidly decomposing, European Buckthorn (Rhamnus cathartica) and its Effects on Some Ecosystem Properties in an Urban Woodland

Rapid Decomposition of Buckthorn Litter May Change Soil Nutrient Levels

Ecological Restoration, Vol. 20, No. 2, 2002 ISSN 1522-4740 ©2002 by the Board of Regents of the University of Wisconsin System. Research ecologists and others have paid considerable attention to the potential of non-native species to alter ecosystems. This is especially true for those species that substantially modify both the function and composition of an ecosystem (Vitousek and others 1996, Lodge 1993). Myrica (Myrica faya), for example, has invaded Hawaiian forests and introduced a new ecosystem function—nitrogen fixation—to previously nitrogen-limited primary successional sites. Among other things, this resulted in measurable differences in soil fertility (Vitousek and others 1987). Our recent study in an urban woodland near Chicago revealed to us that buckthorn (Rhamnus cathartica)—an ornamental shrub native to Eurasia and North Africa—has the potential for similar effects on the woodlands it inhabits. Buckthorn is one of the most ubiquitous non-native shrubs found in the woodlands of northeastern United States, the Midwest, and southern Canada. As others have observed (Gourley and Howell 1984, Boudreau and Willson 1992), buckthorn often comes to dominate in these woodland ecosystems, displacing native shrubs and herbs. However, little is known about how it affects the soil processes in these woodlands. In this article we explain how the rapid rate at which buckthorn litter decomposes may enrich the soil in ways that may foster its dominance within a woodland ecosystem. S tu d y S ite an d E x p e rim en ta l D e s ign We conducted our study in a 10-acre woodland that is part of the 130-acre Skokie Preserve in Lake Forest, Illinois. Soils in the preserve are part of an alluvial fan and have the consistency of loamy sand. The dominant trees are Hill’s oak (Quercus ellipsoidalis), bur oak (Q. macrocarpa), white oak (Q. alba), wild cherry (Prunus serotina), and cottonwood (Populus deltoides). The presence of baneberry (Actaea ellipsoidalis) and blue cohosh (Caulophyllum thalictroides) suggested to us that the core of the woodlands remains relatively undisturbed. During the past 12 years, the Lake Forest Open Lands Association has managed the site by removing buckthorn and conducting prescribed burns every two to three years. In the autumn of 1999, we collected recently senesced leaves of cottonwood, buckthorn, wild cherry, and Hill’s oak throughout the woodland. We air-dried the leaves of all species and placed approximately 3 grams (the precise weights were recorded to one-hundredth of a gram) of each individual species into individuallymarked fiberglass litterbags. The litterbags were approximately 10 x 10 cm and had a mesh size of approximately 1.6 x 1.8 mm, which allowed microarthropods and other smaller invertebrates free access to the bags while restricting access to larger Rapid Decomposition of Buckthorn Litter May Change Soil Nutrient Levels

Soil microarthropod contribution to forest ecosystem processes: the importance of observational scale

Investigations of the role of microarthropods (Acari and Collembola) in organic matter decomposition and nutrient cycling have shown that they may contribute to primary productivity in nutrient poor conditions. The potential of microarthropods to affect other ecosystem properties, such as above ground plant diversity or succession, lags somewhat. In this contribution we demonstrate: (1) that the effect on the mobilization of nutrients promoted by microarthropods must be measured at the microhabitat scale appropriate to the scale of the faunal activity, and (2) that small changes in the structure of microarthropod assemblages can have significant effects on the local mobilization of nutrients. In the first of two experiments we measured the nutrients leaching from field mesocosms containing litter and mineral soil, with and without fauna. After eight months, the C:N ratios of the litter differed significantly indicating that the fauna were effective in altering the decomposition rate. However, the patterns of release over time and the concentration of the measured nutrients differed little between the two sets of mesocosms. In a second experiment microarthropod assemblages, which differed only slightly, were introduced into laboratory microcosms and the nutrient fluxes were measured over a ten week period. Significant differences were detected in the concentration of nitrogen, K and Mg leached and in CO2 evolved. We suggest that when the potential influence of microarthropods on ecosystem properties is being assessed, specific knowledge of the relevant details of interactions at the smallest scale must be considered. These details can be incorporated or dismissed when interactions on the next level of the ecological hierarchy are examined. Using such analysis we suggest that the creation of soil nutrient hot-spots by microarthropods may have implications for maintaining plant species of lowered competitive ability in a given system.

Ecological research can augment restoration practice in urban areas degraded by invasive species—examples from Chicago Wilderness

Urban biodiversity conservation needs a firm scientific foundation, one that draws upon active and regionally calibrated research programs. Until recently this foundation has not existed. In this paper we examine the way in which the emerging discipline of restoration ecology in an urban context can learn from the experiences of ongoing restoration projects and in turn how novel insights from research of urban soils can help these projects define and reach their goals. Limitations on collaboration between academic researchers and practitioners continues be problematic. We discuss a model whereby this impediment may be removed. A case study of Rhamnus cathartica, an invader of Midwestern woodlands which modifies some important soil ecosystem properties will be used to illustrate the potential for engagement between research and practice.

Nitrogen dynamics in decomposing chestnut oak (Quercus prinus L.) in mesic temperate and tropical forest

This study examined nitrogen dynamics in decomposing Quercus prinus L. litter, confined in litterbags, in two tropical forests (La Selva Biological Station, Costa Rica and Luquillo Experimental Forest, Puerto Rico) and one temperate forest site (Coweeta Hydrologic Laboratory, NC). Using regressions of %N in the decomposing litter against litter mass remaining. we demonstrated similar concentrations of N at all sites when the amount of litter lost was 50%. By using naphthalene, an arthropod repellent, we examined the effect of microarthropods on the N fluxes in the litterbags. Microarthropods had little effect on the %N remaining. At La Selva, the presence of fauna resulted in a marginally significant increase in litter nitrogen concentrations (p <0.06). At both tropical sites, there was a significant net immobilization of N followed by N mineralization after four months. Although there was a net immobilization of N at Coweeta, this lasted for a longer period and the litterbags had not begun to mineralize N after 10 months. We suggest that the rapid accumulation of N in decomposing litter at the tropical sites during the first few months after leaf fall can result in the retention of mobile nitrogen ions in soils. The subsequent mineralization, in later decomposition stages, can make N available to trees during leaf flush. # 1999 Elsevier Science B.V. All rights reserved.

Below‐ground causes and consequences of woodland shrub invasions: a novel paired‐point framework reveals new insights

Summary 1. Confirming the impacts of invasive plants is essential for prioritizing management efforts, but is challenging, especially if impacts occur below-ground and over long periods as hypothesized for woodland shrub invasions. 2. For this reason, we developed a novel ‘paired-point’ framework capable of detecting the below-ground impacts of slow-growing invasive plants from short-term data sets in an investigation into the below-ground consequences of European buckthorn Rhamnus cathartica L. (hereafter buckthorn) invasions into Chicago-area woodlands. We measured differences between the members of 97 pairs of buckthorn-invaded and buckthorn-free points in 10 below-ground response variables (RVs) that buckthorn is hypothesized to alter (listed below). We then modelled these differences (DRVs) in response to the severity of the invasion found at each invaded point. A relationship (linear or nonlinear) between a DRV and severity, that is slope 6 0, suggests a buckthorn-induced change. An intercept (value of DRV where severity = 0) different from zero suggests a pre-existing difference. 3. Relating differences to an invasion gradient rather than simply noting their presence provided evidence that the higher levels of moisture, pH, total C, total N, NH þ � N and Ca 2+ in invaded soils pre-date and possibly promote invasion (particularly Ca 2+ ) and that neither earthworm biomass nor soil C : N ratios are associated with buckthorn invasion, all of which suggest buckthorn to be less of a ‘driver’ of below-ground change than hypothesized. We did, however, find evidence that buckthorn establishes in areas having greater leaf litter mass and higher rates of decomposition, and then proceeds to accelerate decomposition further and to alter spring soil NO � � N levels. 4. Synthesis and applications. Our findings suggest that decisions regarding regional buckthorn management should be less driven by concerns about buckthorn’s below-ground impacts and that greater consideration of how variation in below-ground factors relates to invader establishment is needed, rather than simply assuming this variation to be invaderinduced. This latter consideration can help to design better targeted monitoring programmes, limiting the further spread of woodland invaders. These insights illustrate the utility of the paired-point framework both for investigating below-ground causes and consequences of slow-growing invasive plants and for guiding the management of these invasions.

A Simple, safe, and effective sampling technique for investigating earthworm communities in woodland soils: Implications for citizen science

ABSTRACT: We evaluated the efficacy of a mixture of ground hot mustard and water as a sampling method for earthworms (Lumbricina) in research projects involving citizen scientists. To do so we conducted a field experiment to determine if sampling earthworms using mustard-powder would reveal similar patterns of earthworm abundance and community composition as relying on the more difficult to prepare, and possibly hazardous, allyl isothiocyanate (AITC) solution. Earthworms were sampled using either mustard or AITC in four pairs of 0.25-m2 plots located in each of four woodland sites that were predicted to exhibit a range of earthworm densities. Soil gravimetric water content (GWC) of each plot was quantified as a covariate. For analyses of changes in abundance and community structure, earthworms were classified as belonging to one of five groups based on where they occur in the soil profile, developmental stage, and level of taxonomic identification. The two sampling techniques revealed similar earthworm abundances and community composition across the four sites (all Ps > 0.16) and across the range in GWC (all Ps > 0.36). We conclude that using the mustard-water mixture to sample earthworms at our study site appears to be just as effective and reliable as using AITC. The mustard-water mixture, which is easier to prepare and is less hazardous than AITC solution, should, therefore, be considered as an appropriate tool to be utilized by researchers who collaborate with citizen scientists to help collect the large datasets needed to reveal how woodland management and restoration programs affect earthworms.