2009 NORTHEASTERN NATURALIST 16(2):269–284 Influence of Soil Buffering Capacity on Earthworm Growth, Survival, and Community Composition in the Western Adirondacks and Central New York Michael J. Bernard1,2, Matthew A. Neatrour1,3, and Timothy S. McCay1,* Abstract - We examined how buffering capacity affected natural earthworm communities by comparing well-buffered soils in Madison County in central New York and poorly buffered soils in the western Adirondacks. We also investigated how liming and interspecific competition influenced growth and survival of 2 exotic taxa (Eisenia foetida and Amynthas agrestis) in Adirondack and central New York soils using laboratory microcosms. Earthworms were more abundant and diverse in central New York soils than in western Adirondack soils. Interspecific competition had no effect on growth or survival of either species in microcosms. Survival of A. agrestis was low in Adirondack soils without lime, but liming increased survival to that of central New York soils. Growth rates of E. foetida were lowest in Adirondack soils without lime, but highest in Adirondack soils with lime. Our results suggest that high soil acidity may be preventing exotic earthworms from successfully invading the western Adirondacks. Introduction Earthworm species native to North America have been slow to recolonize hardwood forests and mixed hardwood forests once covered by the Wisconsinan Glaciation over 10,000 years ago (Hendrix and Bohlen 2002, Reynolds 1995). Consequently, exotic earthworm species introduced from Europe and Asia either dominate local earthworm communities or are currently colonizing many of these forests (Hendrix and Bohlen 2002, Shakir and Dindal 1997, Stegman 1960). The possibility of exotic earthworms invading hardwood forests lacking native or exotic species in the northeastern United States is of particular concern because earthworms can drastically alter soil structure, chemistry, soil microflora communities, and nutrient uptake in plants (e.g., Bohlen et al. 2004, Frelich et al. 2006, Hale et al. 2005b, Súarez et al. 2006a). However, some forests are still devoid of both native and exotic species despite widespread opportunities for introductions through bait fishing, timber harvesting, or road-building over the last century (Gundale et al. 2005). It is not clear why exotics have not colonized these areas, but certain environmental parameters may be acting as abiotic filters to exclude exotics. There have been no studies of exotic earthworm invasions in the Adirondack Park (Adirondacks) of New York, even though exotic earthworm 1Biology Department, Colgate University, 13 Oak Drive, Hamilton, NY 13346. 2Current address - School of Forest Resources, 117 Forest Resources Building, University Park, PA 16802. 3Current address - Biology Department, St. Lawrence University, Canton, NY 13617. *Corresponding author - tmccay@colgate.edu. 270 Northeastern Naturalist Vol. 16, No. 2 introductions have likely occurred in the Park since its inception in 1892. The Adirondack Park is comprised of both private and public land holdings and is the largest publicly protected area in the contiguous United States. It has been logged periodically, has an extensive road network, and is a hub for tourism, particularly outdoor enthusiasts. Certain properties of Adirondack soils, however, may be preventing successful colonization of exotic species. These soils are poorly buffered and more sensitive to acidic inputs compared to soils in other areas of New York, such as central New York, mainly due to a low concentration of calcite in soils and in the underlying parent material (Hanna 1981, Kuhl et al. 1975). Calcite buffers soils from atmospheric deposition of sulfuric and nitric acids, which have lowered the median pH of Adirondack soil to 4.3 in the B-horizon and 3.5 in the O-horizon over the last half century (Driscoll et al. 2001, Sullivan et al. 2005). Atmospheric acidic inputs facilitate the loss of physiologically important cations, such as calcium, from cation exchange sites, and increase solubility of aluminum in soils (Blake et al. 1999, Driscoll et al. 1996). Reduced soil calcium is associated with low foliar concentrations of calcium and, consequently, low litter calcium content (Driscoll et al. 2001, Juice et al. 2006, Minocha et al. 1997). Earthworm density and diversity is generally low in soils with a pH under 4.5 (Ammer and Makeschin 1994, Curry 1998, Rusek and Marshall 2000), such as those characteristic of the Adirondacks. Acid stress can cause the loss of electrolytes from earthworms (Rusek and Marshall 2000). Earthworm densities have also been shown to be positively correlated with extractable calcium in soil and leaf litter (Nielson 1951, Reich et al. 2005), and monomeric aluminum can be toxic to earthworms (van Gestel and Hoogerwerf 2001). However, earthworm populations have been observed to increase when crushed limestone (CaCO3) is applied to soils (Ammer and Makeschin 1994, Baker 1998, Rusek and Marshall 2000, Springett and Syers 1984). In well-buffered soils (ph 4.5–5.0) of southern and central New York, exotic European lumbricids, such as Lumbricus rubellus, L. terrestris, Octolasion tyrtaeum, and Dendrobaena octaedra, are advancing along detectable invasion fronts (Súarez et al. 2006b). Similar species have been found in invasion fronts in hardwood forests of the western Great Lakes region (Hale et al. 2005b, Tiunov et al. 2006). Exotic lumbricids in this region have eliminated the O-horizon, reduced percentage organic matter and fine root density in the A-horizon, and lowered overall soil N and P availability. (Hale et al. 2005b, Tiunov et al. 2006). European lumbricids differ in their ability to invade new areas based on differences in reproductive and feeding strategies and tolerances for environmental stressors, such as cold or soil acidity (Holdsworth el al. 2007). For example, D. octaedra, an epigeic species that reproduces parthenogenetically, has high cold and acid tolerance and often occurs in isolation at the leading edge of invasions (Hale et al. 2005a, Holdsworth et al. 2007); whereas another epigeic lumbricid, Eisenia foetida (Savigny) (Red Wriggler), is found throughout North America, but is confined to nutrient-rich composts and manure heaps, especially in northern temperate forests (Tiunov et al. 2006). 2009 M.J. Bernard, M.A. Neatrour, and T.S. McCay 271 Exotic Asian species in the genus Amynthas once were considered less invasive than many European lumbricids because they were found exclusively in composting areas and were absent from natural settings (Reynolds 1978, Tiunov et al. 2006). However, Amynthas species have recently been found at high densities in isolated forest locations in the southern Appalachian Mountains (Callaham et al. 2003) and in southern New York (Burtelow et al. 1998). They have caused drastic changes to the forest floor, eliminating the O-horizon, and increasing denitrification and carbon flux in New York (Burtelow et al. 1998). Amynthas species are expanding their range northward in New York (Groffman and Bohlen 1999) and were found in compost heaps within 35 km from the Adirondack Park in 1956 (Gates 1958). It is not known, however, whether they will be able to invade Adirondack soils or how they will interact with established species. Most earthworm competition studies have focused on exotic replacement of natives, and demonstrated that habitat alteration (usually driven by humans) typically is needed for an exotic species to colonize and dominate an area already containing an established earthworm community (Kalisz and Dotson 1989, Kalisz and Wood 1995, Stebbings 1962). Laboratory studies have shown that competitive interactions among earthworms can sometimes be negative between species occupying similar ecological niches (Dalby et al. 1998, Garvín et al. 2002, Lowe and Butt 1999); however, the outcome of competitive interactions can shift with changes in habitat quality (Winsome et al. 2006), Therefore, it is not known whether Amynthas could successfully invade potentially stressful acidic soils in the presence of a competitor, native or exotic. With a field survey, we investigated how soil-buffering capacity affected natural earthworm community composition by comparing well-buffered soils in Madison County in central New York with poorly buffered soils in the Western Adirondacks. We also investigated the susceptibility of these soils to invasion by exotic earthworm species by measuring growth and survival of Amynthas agrestis (Goto and Hatai) and E. foetida in soils treated with lime to alter soil-buffering capacity. Furthermore, we examined whether interspecific competition affected survival and growth of either species. We chose E. foetida as a competitor of A. agrestis because E. foetida is epigeic and abundant in composts, where Amynthas species also are known to be common. Additionally, E. foetida is readily available commercially and often is used in laboratory studies, where it has been shown to be a strong competitor (Abbott 1980). We expected that earthworm densities would be lower in Adirondack soils compared to Madison County soils in central New York and expected that earthworm species native to North America would be rare at both sites. We also predicted that earthworm survival and growth would be greatest in Madison County soils and in limed soils. Site Descriptions We surveyed 5 sites in the western Adirondacks (Fig. 1), Herkimer County, NY in July 2006. Four sites (43°44'N, 74°58'W) were located near Old Forge, 272 Northeastern Naturalist Vol. 16, No. 2 NY but were separated by at least 200 m. A fifth site (43°48'N, 74°51'W) was located farther north near Eagle Bay, NY. The Adirondack sites near Old Forge were located near several likely points of invasion for exotic earthworms, which included unpaved roads used for recreational activities (e.g., snowmobiling and off-road vehicle use) and logging (less than 100 m from each site), surface waters used for fishing and swimming (Little Safford Lake or North Branch Moose River, less than 1 km away), and the town limits of Old Forge (2.5 to 4 km away). In addition, these sites were recently logged in the 1970s (Steve Bick, Northeast Forests, LLC, Thendara, NY, pers. comm.), and skid trails used to remove the logs were still visible. Although not recently disturbed by logging, the site near Eagle Bay was located less than 100 m from a paved road and less than 250 m from Big Moose Lake. All Adirondack study sites were part of an ongoing study to determine the effects of soil calcium depletion on forest-floor food webs. Each site was divided into 2 plots (1590 m2, 22.5 m radius): a control plot and an experimental plot treated with lime (CaCO3). Lime was spread by hand in each experimental plot at a total dosage of 10 Mg/ha over 2 equal applications in September 2005 and April 2006. By July 2006, soil pH in the upper 10 cm of soil (Oa- and A-horizons combined) was 5.6 and 3.7 in limed Figure 1. Location of Adirondack and Madison County study sites in New York State. 2009 M.J. Bernard, M.A. Neatrour, and T.S. McCay 273 and unlimed plots, respectively. Dominant overstory species included Fagus grandifolia Ehrh. (American Beech), Acer rubrum L. (Red Maple), Picea glauca (Moench) Voss (White Spruce), and Betula alleghaniensis Britton (Yellow Birch). Soils were spodosols (typic haplothods) (Richard April, Colgate University, Hamilton, NY, pers. comm.). We also sampled 3 sites in central New York in Madison County (Fig. 1) in July 2006. Sites were in intact forest stands that were surrounded by agricultural fields or residential areas. One site was located at the Bewkes Nature Preserve (42°48'N, 75º37'W), another site was near the Colgate University Quarry (42°49'N, 75°32'W), and a third site was located at Turkey Hill Natural Area (42°49'N, 75º25'W). The Madison County sites also were divided into two 1590-m2 plots, but neither plot was limed. These sites were near several points of earthworm invasion. The Bewkes Nature Preserve site was 30–50 m from an unpaved road and 100 m from Seymour Pond, which is heavily used during summer for fishing and swimming. The Colgate University Quarry site was near (<200 m from) the Colgate campus, and the Turkey Hill Nature Preserve site was < 300 m from a paved road. Acer saccharum Marsh. (Sugar Maple), and to a lesser extent, Fraxinus americana L. (White Ash) and American Beech were dominant at the Madison County sites. Soils were inceptisols: typic dystrochrepts at the Colgate University Gate Quarry and Turkey Hill Natural Area and typic fragiochrepts at the Bewkes Nature Preserve (Hanna 1981). Methods Field survey We extracted earthworms from two 0.37-m2 quadrats (0.61 x 0.61 m) placed at random in each Madison County and Adirondack plot using a hot-mustard solution in July 2006 (Lawrence and Bowers 2002). We poured the mustard solution over each quadrat and collected earthworms for 20 minutes. Earthworms were combined by plot, stored in 70% ethanol, and later counted and identified using keys of Eaton (1942), Olson (1940), and Reynolds (1977). Juveniles were identified as either species in the genus Lumbricus or other (Súarez et al. 2006a). Microcosm study design We used E. foetida and A. agrestis in microcosms (the experimental unit) arranged in a fully randomized design with 3 factors: soil, lime, and competition. Soil types were Adirondack or Madison County soils, lime consisted of a lime treatment and an unlimed control, and competition consisted of single-species and paired-species treatments with 1 and 2 individuals per microcosm, respectively. Each treatment combination (soil type x lime x competition) was replicated 5 times for a total of 60 microcosms. We collected soils (Oa- and A-horizons combined) and leaf litter (Oi-horizon) from one Adirondack site (43º48'N, 74º51'W) in Herkimer County and from the Colgate University Quarry (42º49'N, 75º32'W) in Madison County. Soils were sieved (5-mm aperture) to remove large rocks and woody debris. 274 Northeastern Naturalist Vol. 16, No. 2 We constructed earthworm microcosms (8 cm high x 10 cm diameter) using PVC pipe and placed cheesecloth at the base to allow drainage (Zimmer et al. 2005). Microcosms were filled with soil to 3 cm below the top of the microcosm. The soil volume (550 cm3) was similar to the 600-cm3 vessels Butt (1998) used for L. terrestris, which is close in size to Amynthas agrestis. We applied a single dose of 7.9 g (10 Mg/ha) of lime to half of the microcosms for each soil type and evenly mixed it into the soil. We added ≈2.5 g of fresh litter over the surface of the soil in each microcosm and covered the microcosms with petri dishes to retain moisture. Amynthas agrestis was collected locally in Madison County, NY, from soils that were not used in the field survey. Eisenia foetida was purchased from Carolina Biological Supply Company® (Burlington, NC). Eisenia foetida is readily available in the United States and is commonly used for vermicomposting. Microcosms of each soil type and lime treatment were randomly assigned to single-species or paired-species treatments (i.e., competition treatments). In single-species treatments, we placed either one E. foetida individual or one A. agrestis individual into a microcosm. In paired-species treatments, we added one E. foetida and one A. agrestis to a microcosm. We used adult clitellate earthworms only. All microcosms were placed in a growth chamber (Thermmax Scientific Products®) at 19 ºC and 70% relative humidity receiving 12 hours of light followed by 12 hours of dark. Microcosms were watered weekly using a spray bottle to maintain soil moisture between 20–25%. Earthworms were weighed wet prior to being placed in the microcosms and weekly thereafter for 10 weeks from October to December 2006. Mortalities were replaced during the first 6 weeks. We replaced soils and litter after 5 weeks in each microcosm to ensure adequate food supply, and reestablished the same treatments. We determined soil pH, exchangeable soil calcium and magnesium, and litter calcium and magnesium content after 5 weeks to test the effectiveness of the liming and soil-type treatments. Soil pH was measured in experimental microcosms with earthworms and in four additional microcosms without earthworms following protocols in Hendershot et al. (1993). Litter and soils in unlimed microcosms with earthworms, and in the extra microcosms without earthworms, were dried to a constant weight at 60 °C. Litter was wet digested with sulfuric acid and hydrogen peroxide (Parkinson and Allen 1975), and total calcium content was determined using an inductively coupled plasma atomic emission spectrometer (ICP-AES, PerkinElmer Life and Analytical Sciences, Inc., Waltham, MA). Exchangeable calcium of soils was analyzed using an atomic absorption spectrometer (PerkinElmer AAnalyst 200, PerkinElmer Life and Analytical Sciences, Inc, Waltham, MA) following extraction with 1N NH4Cl after 5 and 10 weeks. We also determined organic content of extra Adirondack and Madison County soils not used in the microcosms following Karam (1993). Statistical methods We compared earthworm abundance (adults and juveniles) between Madison County and Adirondack sites using 1-way ANOVA. Changes in 2009 M.J. Bernard, M.A. Neatrour, and T.S. McCay 275 mass (measured as weekly proportional change in mass) of earthworms in microcosms were compared separately for each species using a repeated measures 3-way ANOVA with soil type, lime, and competition as main effects. Because weekly mortality was high in some cases, we measured survival rates per microcosm, which was the number of weeks without a mortality in a microcosm divided by the number of weeks during which mortalities were replaced (i.e., 6 weeks). Survival rates over the first 6 weeks were compared using a 3-way ANOVA after arcsin transformation (Sokal and Rohlf 1995) with soil type, lime, and competition as main effects. We used a 2-way ANOVA to test for significant changes in soil pH from the soil type and liming treatments. We tested for differences in exchangeable and litter calcium and magnesium between unlimed Adirondack and Madison County soil in the first 5 weeks of the study using a 1-way ANOVA. All data were tested for normality using the Kolmogorov-Smirnov Test. Levene’s Test was used to test for homogeneity of variances. We performed all statistical analyses with SPSS (version 14, SPSS Inc., Chicago, IL). Results Field survey Earthworm abundance was greater in Madison County soils than in Adirondack soils (F1,14 = 6.08, P < 0.05; Table 1). Aporrectodea tuberculata and Eisenia rosea, both exotic species, were common in Madison County soils. Another exotic species, Dendrobaena octaedra, was the only species found in the Adirondacks. Two native earthworm species, Bimastos tenuis and Bimastos parvus, were collected in Madison County, but comprised only 6% of all Madison County adults. Non-lumbricus juveniles were more common than lumbricus juveniles in both Madison County and Adirondack soils. Table 1. Number of adult and juvenile (lumbricus and non-lumbricus) earthworms (per m2) collected in Madison County and Adirondack (limed and unlimed plots combined) sites. Percentage of the total earthworms is listed in parentheses. Total area sampled was 4.4 m2 for Madison County sites and 7.4 m2 for Adirondack sites. Species Madison County Adirondacks Aporrectodea tuberculata1 (Eisen) 5.5 (33) 0.0 (0) Bimastos parvus2 (Eisen) 0.2 (1) 0.0 (0) Bimastos tenuis2 (Eisen) 0.5 (3) 0.0 (0) Dendrobaena octaedra1 (Savigny) 0.0 (0) 0.3 (25) Eisenia rosea1 (Savigny) 2.5 (15) 0.0 (0) Lumbricus rubellus1 Hoffmeister 0.7 (4) 0.0 (0) Lumbricus terrestris1 L. 0.7 (4) 0.0 (0) Lumbricus castaneus1 (Savigny) 0.5 (3) 0.0 (0) Octolasion cyaneum1 (Savigny) 0.2 (1) 0.0 (0) Octolasion tyrtaeum1 (Savigny) 0.2 (1) 0.0 (0) Lumbricus juveniles 0.5 (3) 0.0 (0) Other juveniles 5.0 (31) 0.8 (75) Total 16.5 1.1 1Exotic. 2Native to North America. 276 Northeastern Naturalist Vol. 16, No. 2 Figure 2. Mean survival rates (± s.e.) of earthworms subjected to different liming regiments (limed and unlimed), soil types (Adirondack and Madison County), and competition treatments (single or paired) for Amynthas agrestis (A) and Eisenia foetida (B) for the first 6 weeks of the study. AU = unlimed Adirondack soil, AL = limed Adirondack soil, MU = unlimed Madison County soil, and ML = limed Madison County soil. Table 2. Mean soil chemistry (± 1 standard deviation) of Madison County and Adirondack soils used in the microcosm experiment. Organic matter content of the Oa- and A-horizons was determined from leftover soil not used in the microcosms. All other soil attributes were measured from soil in microcosms. Adirondacks Madison County Soil attribute Unlimed Limed Unlimed Limed Soil pH 3.8 (0.2) 7.1 (0.4) 4.1 (0.1) 7.2 (0.3) Organic matter content (%) 52.6 - 15.3 - Litter nutrients (mg/g litter) Ca 7.0 (1.1) - 13.3 (3.0) - Mg 1.0 (0.2) - 1.5 (0.5) - Base cations (cmolc/kg soil) Ca 3.1 (1.2) - 11.3 (1.5) - Mg 0.9 (0.2) - 1.2 (0.1) - Microcosm study Madison county soils had higher soil pH (F1,60 = 9.7, P < 0.01), litter calcium (F1,18 = 39.5, P < 0.001), litter magnesium (F1,18 = 7.7, P < 0.05), exchangeable calcium (F1,30 = 297, P < 0.001), and exchangeable magnesium (F1,30 = 22.6, P < 0.001) than Adirondack soils (Table 2). Organic matter content was >3x greater in Adirondack soils relative to Madison County soils (Table 2). Lime additions increased soil pH in microcosms (F1,60 = 2696, P < 0.00). Soil type (F1, 32 = 24.6, P < 0.001) and lime (F1, 32 = 18.6, P < 0.001) effects were significant for survival rates of A. agrestis, but the interaction (lime x soil-type interaction, F1, 32 = 18.6, P < 0.001) indicated that only lime additions to Adirondack soils affected survival rates (Fig. 2). 2009 M.J. Bernard, M.A. Neatrour, and T.S. McCay 277 Competition with E. foetida (F1, 32 = 0.02, P = 0.9) had no effect on survival rates. Soil type (F1, 32 = 4.6, P < 0.05) had a significant effect on survival rates of E. foetida, but survival rates remained high in both Madison County and Adirondack soils (Fig. 2). However, neither lime additions (F1, 32 = 1.17, P = 0.29) nor compeititon with A. agrestis (F1, 32 = 1.17, P = 0.287) affected survival rates. Mass of Amynthas agrestis individuals decreased in all treatments (Fig. 3). Mass loss did not differ between Madison County and Adirondack soils (F1, 10 = 0.14, P = 0.72) or between competition treatments (F1, 10 = 3.0, P = 0.12). However, lime additions tended to reduce the weekly loss of mass (F1, 10 = 4.8, P = 0.054). There were significant soil (F1, 23 = 5.2, P less than 0.05; Fig. 3) and lime (F1, 23 = 53.2, P < 0.001) effects on growth rates of E. foetida; however, the effect of lime additions on E. foetida growth rates were greater in Adirondack soils compared to Madison County soils (lime x soil type interaction, F1, 23 = 33.7, P < 0.001) even though growth rates were lower in unlimed Adirondack soils than in unlimed Madison County soils. Growth rates of E. foetida were not influenced by competition with A. agrestis (F1, 23 = 1.2, P = 0.29). Discussion Field survey Both Adirondack sites and Madison County sites were dominated by exotic earthworm species. The only native species found were Bimastos tenuis and B. parvus at Madison County sites. Our data are consistent with the work of others who have found few native earthworms in central New York (Shakir and Dindal 1997, Stegman 1960), which supports the hypothesis that natives have been slow to recolonize areas north of Wisconsinan Figure 3. Mean weekly growth rates (± s.e.) of earthworms subjected to different liming regiments (limed and unlimed), soil types (Adirondack and Madison County), and competition treatments (single or paired) for Amynthas agrestis (A) and Eisenia foetida (B). AU = unlimed Adirondack soil, AL = limed Adirondack soil, MU = unlimed Madison County soil, and ML = limed Madison County soil. 278 Northeastern Naturalist Vol. 16, No. 2 glacial margins (Gates 1970). The two species used in the microcosm study, A. agrestris and E. foetida, were not found at any of the sites. Amynthas agrestis individuals used in our microcosm study were collected from soils in a residential area near Tuscarora Lake in Madison County rather than in an intact forest stand. However, this species has been found in undisturbed forest stands in the southern Appalachians (Callaham et al. 2003). Also, Burtelow et al. (1998) did not collect any A. agrestis individuals from their sampling sites in forest stands of southeastern New York where A. agrestis were known to occur. Madison County soils tended to have more exotic earthworms than Adirondack soils, which contained only D. octaedra. It is possible that most exotic earthworm species have not yet reached our Adirondacks sites, but we think this is unlikely. These sites are located <100 m from unpaved roads that have been used extensively for logging and recreational activities (e.g., snowmobiling and off-road vehicle use) since the mid-1960s and <1 km from lakes or streams used for fishing. Assuming a colonization rate of 7.5 m/yr (Hale 2004), exotics already should have reached our Adirondack sites due to their proximity to unpaved roads. Additionally, these sites were recently logged in the 1970s (Steve Bick, Northeast Forests, LLC, Thendara, NY, pers. comm.). Areas with a history of logging have been shown to contain more exotic earthworms than undisturbed forests (Gundale et al. 2005, Kalisz and Dotson 1989). Finally, exotics currently are invading hardwood forests of Minnesota and Michigan, which are farther north than the Adirondacks (Gundale et al. 2005; Hale et al. 2005a, b). The abundance of exotic earthworms in soils of northern Minnesota and Michigan raise questions as to why exotics have not been successful in the western Adirondacks. High soil acidity and associated base cation depletion and aluminum mobilization may be preventing exotic earthworms, including A. agrestis, from successfully invading the Adirondacks. Adirondack soils are acidic, which is caused by a natural accumulation of organic acids in the O- and A-horizons (Kuhl et al. 1975) and the addition of inorganic acids (i.e., sulfuric and nitric acids) through acidic deposition (Driscoll et al. 2001). Earthworm densities often are depressed in acidic soils (Ammer and Makeschin 1994, Reich et al. 2005); very few earthworm species can survive in soil with a pH of below 3.5 (Curry 1998). Strong inorganic acids also can deplete base cations from cation exchange sites and mobilize aluminum, which is toxic to earthworms (van Gestel and Hoogerwerf 2001). Exchangeable base cations and soil pH were both lower in Adirondack soils compared to Madison County soils, and mean soil pH of our Adirondack soils was 3.8, which approached the threshold pH for earthworm survival established by Curry (1998). Furthermore, the only species of earthworm found at Adirondack sites was D. octaedra, a well-known acidophile (Rusek and Marshall 2000). Although earthworm invasions typically begin with colonization by D. octaedra (Hale et al. 2005a), most invasive lumbricids found in northeastern North America are acid intolerant (Tiunov et al. 2006) and consequently may not be able to invade the Adirondacks. 2009 M.J. Bernard, M.A. Neatrour, and T.S. McCay 279 In addition to high soil acidity, litter quality may be influencing invasions by exotics in the Adirondacks. Earthworm density and activity are negatively correlated with litter C:N ratio and lignin concentration (Hendriksen 1990, Schönholzer et al. 1998, Shipitalo et al. 1988) and litter calcium concentration (Hobbie et al. 2006, Reich et al. 2005). We found that calcium concentrations were lower for Adirondack litter compared to Madison County litter. Depletion of calcium from acidic deposition and lack of calcite minerals in Adirondack soils (Kuhl et al. 1975) may have contributed to lower calcium concentrations in litter there. However, differences in species composition of the litter between the Adirondacks and Madison County sites also may have affected litter quality. For example, American Beech was a more abundant species at our Adirondack sites compared to our Madison County sites. Litter of American Beech has higher lignin concentrations than litter of most other hardwood species in the northeastern United States (Lovett et al. 2004, Melillo et al. 1982). In addition, litter calcium concentrations of American Beech were lower relative to Red Maple and Yellow Birch at our Adirondack sites (M.A. Neatrour, unpubl. data). Although litter species composition may be affecting earthworm density, we feel it is unlikely that the species composition of litter influenced the probability of invasion into Adirondack sites. Hale and Host (2005) found that beech-maple forests in the western Great Lakes region supported similar earthworm assemblages of exotic earthworms as Sugar Maple-dominated forests even though earthworm biomass was lower in beech-maple forests. Microcosm study Survival of both A. agrestis and E. foetida and growth of E. foetida was greater in Madison County soils than in Adirondack soils, which may have reflected higher pH and extractable calcium in Madison County soils compared to Adirondack soils. Our results are consistent with another study that has shown reduced growth or survival of some European lumbricids in soils with low soil pH or calcium (Ammer and Makeschin 1994). Other studies have reported low tolerance to acidic conditions for E. foetida (Gunadi and Edwards 2003); however, little is known about acid tolerance of A. agrestis (though another Amynthas species, A. hawayanus Rosa, was found in patches where soil pH was 5.5 in southern New York [Burtelow et al. 1998]). Poor survival rates of A. agrestis in Adirondack soils may indicate a geographical limitation to invasion even though this species is prevalent in southern New York State and has been found within 35 km of the park (Gates 1958, Groffman and Bohlen 1999). Since acidic deposition has lowered soil pH in Adirondack forests (Driscoll et al. 2001), A. agrestis may not be able to colonize this region extensively unless soil pH increases. Lime additions have been shown to positively affect earthworm survival and growth, particularly in acidic soils (Ammer and Makeschin 1994, Rusek and Marshall 2000, Springett and Syers 1984). Our results largely support these findings. Both survival and growth rates of A. agrestis were greater in limed soils than in unlimed soils, and growth rates of E. foetida were higher in limed Adirondack soils relative to unlimed soils. 280 Northeastern Naturalist Vol. 16, No. 2 The effect of liming was different between Adirondack and Madison soils. Growth rates of E. foetida were higher in limed Adirondack soils than in limed Madison County soils even though E. foetida individuals lost mass in unlimed Adirondack soils. Liming may have allowed E. foetida to take advantage of the high organic matter content in these soils, which often determines carrying capacity for earthworms (Curry 1998, Edwards and Lofty 1982). These data suggest that a lack of food resources most likely does not limit earthworm densities in the Adirondacks; however, amelioration of acidic conditions is necessary to make these food resources available. Interspecific competition did not affect growth and survival of either E. foetida or A. agrestis in any of the soil or liming treatments even though both species are epigeic earthworms that potentially consume the same litter resources (Burtelow et al. 1998, Hendrix and Bohlen 2002). Others have reported neutral effects of competition in laboratory microcosms for species requiring similar food resources (Dalby et al. 1998, Garvin et al. 2002). Poor performance or abundant food resources may have prevented measurable competition from occurring at a density of 2 worms per microcosm, which is likely low for compost earthworms often found at high densities. Both litter and soils were replaced once during the 10-week experiment. These experimental weaknesses limit insights into whether A. agrestis would be able to invade the Adirondacks in the presence of competition with species already inhabiting the region. Additionally, inferences drawn from interactions with E. foetida would have been questionable considering that D. octaedra was the only earthworm we found in the Adirondacks. Conclusion Widespread bait fishing, timber harvesting, and road building in the Adirondacks have provided many opportunities for introductions of exotic earthworm species, which are successfully invading other areas in the upper midwestern and northeastern regions of the United States, including New York State. However, we found that exotic earthworms were rare in the Adirondacks and common in Madison County. Furthermore, both A. agrestis and E. foetida performed better in Madison County soils than in Adirondack soils in the laboratory microcosm study. The high organic matter content of soils in the Adirondacks suggests that these soils have abundant food resources for earthworms. Favorable responses of both A. agrestis and E. foetida to liming indicate that high soil acidity may be preventing earthworm colonization in the Adirondacks. Soil properties altered by decreases in soil pH, such as calcium leaching or mobilization of monomeric aluminum, also may be significant factors affecting colonization that should be explored in future studies. Recent declines in acidic deposition resulting from federal controls placed on sulfur dioxide and nitrogen oxide emissions (Driscoll et al. 2001) may result in higher soil pH and allow successful invasions of exotic earthworms in the Adirondacks, which could dramatically alter soil structure, chemistry, and biology. 2009 M.J. Bernard, M.A. Neatrour, and T.S. McCay 281 Acknowledgments We thank Sam James of Kansas University and Peter Ducey of the State University of New York at Cortland for help with species determinations. We are grateful to Jose Medina, Irina Bromberg, and Jake Krong for assisting with data collection, Jeff Fish for help with soil and litter collection, and Dejan Samardžić and Rob Frankel for help with chemical analyses. 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