Virginia Museum of NATURAL HISTORY PUBLICATIONS JEFFERSONIANA Contributions from the Virginia Museum of Natural History Number 26 6 October 2011 Potential impacts of the invasive herb garlic mustard (Alliaria petiolata) on local ant (Hymenoptera: Formicidae) communities in northern temperate forests. Kaloyan Ivanov and Joe Keiper ISSN 1061-1878 (print) ISSN 2163-8020 (online) Virginia Museum of Natural History Scientific Publications Series The Virginia Museum of Natural History produces five scientific publication series, with each issue published as suitable material becomes available and each numbered consecutively within its series. Topics consist of original research conducted by museum staff or affiliated investigators based on the museum’s collections or on subjects relevant to the museum’s areas of interest. All are distributed to other museums and libraries through our exchange program and are available for purchase by individual consumers. Memoirs are typically larger productions: individual monographs on a single subject such as a regional survey or comprehensive treatment of an entire group. The standardized format is an 8.5 x 11 inch page with two columns. Jeffersoniana is an outlet for relatively short studies treating a single subject, allowing for expeditious publication. The standardized format is a single column on a 6 x 9 inch page. Guidebooks are publications, often semi-popular, designed to assist readers on a particular subject in a particular region. They may be produced to accompany members of an excursion or may serve as a field guide for a specific geographic area. Special Publications consist of unique contributions, usually book length, either single-subject or the proceedings of a symposium or multi-disciplinary project in which the papers refiect a common theme. Appearance and format are customized to accommodate specific needs; page size and layout varies accordingly. The Insects of Virginia is a series of bulletins emphasizing identification, distribution, and biology of individual taxa (usually a family) of insects as represented in the Virginia fauna. Originally produced at VPI & SU in a 6 X 9 inch page size, the series was adopted by VMNH in 1993 and issued in a redesigned 8.5 X 11 inch, double column format. Copyright 2011 by the Virginia Museum of Natural History Printed in the United States of America ISSN 1061-1878 (print) ISSN 2163-8020 (online) Jeffersoniana, Number 26 pp. 1-14 Virginia Museum of Natural History Potential impacts of the invasive herb garlic mustard (Alliaria petiolata) on local ant (Hymenoptera: Formicidae) communities in northern temperate forests. Kaloyan Ivanov^’* and Joe Keiper^ ABSTRACT Garlic mustard, an invasive shade-tolerant species introduced to North America from Eurasia in the late 1860s, now is widely distributed throughout the US and Canada. The presence of garlic mustard results in displacement of understory species and subsequent deeline in native plant diversity. By displaeing native plants, garlie mustard can affect resource availability and habitat quality, and thereby affect animals across different trophie levels. However, these impaets have been doeumented infrequently. Our study foeused on the small-seale effects of garlic mustard invasion on an abundant and important group of forest-floor arthropods. We evaluated the effeets of garlie mustard on forest ant assemblages in invaded and non- invaded areas of two mesophytie forest fragments of northeastern Ohio. Plots invaded by garlie mustard showed redueed leaf litter depth, and an inereased abundanee of nonnative Amynthas earthworms. Sample- based rarefaetion and similarity analyses revealed that the presenee of garlic mustard, and the assoeiated deerease in leaf litter depth, had no deteetable effeet on the observed and expeeted ant speeies riehness and eommunity eomposition. Rank-abundanee distributions also were largely unchanged in garlic mustard invaded plots. Our results suggest that regional sylvan ant eommunities are unaffeeted by the generally presumed negative effects of garlic mustard invasions, or these effects may be more subtle or confounded by other dominating faetors. Keywords: Ohio, speeies riehness, abundanee, eommunity eomposition, leaf-litter, exotie earthworms INTRODUCTION Invasion of alien plant speeies is eonsidered among the most signifieant threats to biodiversity (Blossey 1999). Non-native speeies ean displaee native plants, change trophie strueture and alter eeosystem proeesses (Maek et al. 2000; Ehrenfeld 2003). Problems assoeiated with invasive plant speeies have inereased greatly in reeent times due to inereased species exehange between geographie areas, largely as a result of human-aided dispersal (Vitousek et al. 1997; Chapin et al. 2000). In addition, human-mediated habitat disturbanees ean aeeelerate the ineidenee of sueeessful biologieal invasions by enhaneing the establishment of exotie speeies (Eiebhold et al. 1995; King and Tsehinkel 2006). Despite the inereased awareness of the negative impaets of invasive plants, there is little quantitative evidence for the ecological impacts they pose (Blossey 1999). The widespread invasive garlie mustard {Alliaria petiolata [Bieb.] Cavara and Grande) is no exception, and its eeologieal impaets though eommonly suspeeted have been poorly doeumented (Blossey et al. 2001; Davalos * Corresponding author K. Ivanov 1. Cleveland Museum of Natural History, 1 Wade Oval Drive, University Circle, Cleveland, OH 44106. e-mail: antzmail@gmail.com. telephone: 216-231-4600 ext. 3314. fax: 216-231-5919 2. Virginia Museum of Natural History, 21 Starling Avenue, Martinsville, VA24112 2 Jeffersoniana and Blossey 2004). Garlic mustard (Brassicaceae) is a shade- tolerant, short-lived, obligate biennial herb native to western Eurasia (We Ik et al. 2002). It was introdueed to North Ameriea from Europe in 1868 when it was hrst reeorded in Eong Island, NY (Nuzzo 1993), and has quiekly spread throughout the US and Canada. It is most abundant in the northeastern and midwestern US (We Ik et al. 2002; Blossey et al. 2002). Garlie mustard produees hrst year rosettes that over-winter, it howers early in the following year, and plants die after seed produetion. A single plant ean produee hundreds of seeds, with maximum yields of 7900 seeds (Nuzzo 1993). Garlie mustard spreads exelusively by seeds, whieh require a dormaney period of 8 to 22 months to germinate and ean remain viable for up to hve years (Nuzzo 1999; Blossey et al. 2002). Alliaria petiolata typieally oeeupies disturbed habitats and often is most abundant in urban forest fragments and forest edges, although it has the unusual eapaeity to invade and proliferate within intaet forest eommunities (Nuzzo 1999; Davalos and Blossey 2004; Rodgers et al. 2008). Garlie mustard is one of the most problematie invaders in North Ameriean forest eommunities where it beeomes a permanent member upon introduetion (Rodgers et al. 2008). Among the most often reported negative impaets are the displaeement of native understory plants and deelines in the diversity of native plant communities (MeCarthy 1997; Meekins and MeCarthy 1999; Stinson et al. 2007), although quantitative evidenee often is ineonelusive (Blossey et al. 2001). Garlie mustard deereases the abundanee of myeorrhizal fungi in the soil and on plant roots (Stinson et al. 2006), whieh may reduee the eompetitive ability of many native plant speeies (Roberts and Anderson 2001). The disruption of myeorrhizal assoeiations within understory plant eommunities has been related to the produetion of seeondary eompounds by A. petiolata used to deter herbivory and to suppress the growth of plant and fungal speeies (Eahey et al. 2001; Cipollini and Gruner 2007; Rodgers et al. 2008). Given garlie mustard’s impaets on loeal forest flora, arthropod diversity may deeline, as it eommonly is thought to be positively eorrelated with floristie and struetural plant diversity (Siemann et al. 1998). Arthropod herbivore riehness is generally greatest in areas of highest plant riehness, primarily as a result of greater availability of resourees (Siemann 1998; Knops et al. 1999). This in turn results in a greater diversity of arthropod predators and parasitoids (Hunter and Priee 1992; Siemann 1998). Inereased plant diversity also may direetly inerease diversity of higher trophie levels through an inerease in floral resourees (Jervis et al. 1993). Plant invasions and the assoeiated deerease in plant diversity and homogenization of loeal flora ean alter invertebrate assemblages. Changes in invertebrate abundanee, riehness and community composition have been reported for a variety of arthropod groups in areas invaded by non-native plants (Standish 2004; Gratton and Denno 2005; Wilkie et al. 2007; Bultman and DeWitt 2008). The presenee of garlie mustard has been shown to interfere with the development of the native butterflies Pieris napi oleracea Harris and P. virginiensis Edwards (Eepidoptera: Pieridae). Females of these speeies oviposit on garlie mustard plants, instead on their native toothwort hosts, but garlie mustard inereases the time required for larval development, larvae take longer to pupate, and exhibit redueed pupal mass and pupation rates (Porter 1994; Huang et al. 1995; Keeler and Chew 2008). Davalos and Blossey (2004) reported no effeet of garlie mustard presenee on ground earabid beetle eaptures and speeies riehness, and on overall invertebrate abundanee in New York deeiduous forests. However, the authors were able to doeument a signifleant deerease in leaf litter depth at garlie mustard invaded plots whieh was assoeiated with the presenee of non-native earthworms. A positive eorrelation between garlie mustard eover and non-native earthworm biomass, and a negative eorrelation with leaf litter depth also have been reported by Nuzzo et al. (2009) in New York and Pennsylvania. Garlie mustard invasions therefore also may be symptomatie of invasions by non-native earthworms. The displaeement of native understory plant Ivanov, Keiper: Impacts of invasive gareic mustard on eocae ant communities 3 species associated with the presence of garlic mustard and the creation of near-monospecific stands in the areas where this species occurs are likely to alter resource availability, habitat quality and microclimate and thus alfect organisms from different trophic levels. In addition, areas invaded by garlic mustard may suffer reduction in the leaf litter layer as a direct result from the activity of non-native earthworms. This is likely to negatively affect forest dwelling invertebrates that rely on leaf litter as nesting and foraging resource. Our research focused on the small-scale effects of garlic mustard invasions on ground dwelling forest ant communities. Ants are important and nearly ubiquitous members of terrestrial ecosystems. They play major ecological roles as predators, scavengers, mutualists, seed gatherers and dispersers (Holldobler and Wilson 1990). Ants participate in a variety of mutualistic relationships with plants, fungi and animals, and also are an important prey item for both vertebrates and invertebrates (Holldobler and Wilson 1990; Folgarait 1998). Their ease of sampling, stationary colonies, and responsiveness to environmental disturbances have made them an attractive monitoring tool for tracking changes in ecosystem conditions (Andersen 1990, 1993, 1997; Alonso 2000; Kaspari and Majer 2000). The changes initiated by garlic mustard invasions could affect local ant communities both directly and indirectly and result in changes in local species richness, community composition and structure. Direct impacts can result from changes in habitat quality and resource availability associated with the presence of garlic mustard. In addition, the changes in local plant diversity may give rise to indirect impacts through disruption of mutualistic interactions between ants and honey dew-producing ‘Homoptera’ and/or between ants and ant-dispersed (myrmecochorous) plants. Although not obligately dependent on their mutualistic partners, the food resources in the form of honeydew or elaiosomes are an important part of the diet of many ant species in temperate deciduous forests. Changes in the species richness and composition of the local ant communities may lead to modifications of species interactions (competition, predation, parasitism. pollination, and seed dispersal), and alteration of ecosystem processes (Holldobler and Wilson 1990; Murcia 1995; Folgarait 1998; Laurence et al. 2002 ). To our knowledge, no study has explored the impacts of garlic mustard invasions on ant communities. Here we report the results of a study designed to evaluate the potential impacts of garlic mustard invasions on local temperate- forest ant communities. We investigated whether ant communities dilfer in abundance, species richness, and composition between sites invaded or not invaded by garlic mustard. We expected lower species richness and abundance of ants in areas invaded by garlic mustard as a result of alterations in habitat quality, resource availability, and/or disruption of important mutualistic interactions. MATERIALS AND METHODS Study sites We established two study sites in temperate mixed deciduous forest fragments part of the Cleveland Metroparks in the Cleveland area of northeastern Ohio. One site was located at Bedford Reservation (BED; 890 ha; 41° 22’ N, 81° 33’ W), and a second one at Brecksville Reservation (BRE; 1406 ha; 41° 18’ N, 81° 36’ W). At each site we established 10 plots, of which five were invaded by garlic mustard and five were reference plots. Invaded plots were selected on the basis of visual estimates of the presence of the target species in 30% or more of the cover in the plots. Garlic mustard was present at all of the invaded plots and in none of the non- invaded plots. A few garlic mustard rosettes were present near the edges of some reference plots, but none occurred within the plots. Each plot covered an area of 25m x 25m, and plots within sites were separated by a minimum distance of approximately 150m. The elevation of all plots ranged between 200 and 285m a.s.l. Both research sites were located within mixed deciduous forest types, dominated by mixtures of red and white oak (Querciis rubra E. and Q. alba E. ), sugar and red maple {Acer saccharum Marshall and 4 Jeffersoniana A. rubrum L.), American beech (Fagus grandifolia Ehrh.), tulip tree (Liriodendron tidipifera L.), and hickories {Cary a spp.), supplemented to varying degrees by black cherry (Prunus serotina Ehrh.), cucumber tree {Magnolia accuminata (E.)), and basswood {Tilia americana E.). The understory is characterized by seedlings and saplings of the canopy trees; the shrubs Lindera benzoin E., Hammamelis virginiana E., and Viburnum spp.; the vines Toxicodendron radicans (E.), Parthenocissus quinquefolia (E.); and various herbs and graminoids. Plant nomenclature follows Gleason and Cronquist (1991). Sample collecting and processing Ants were collected via Winkler litter extraction in the period June- July during the peak of annual ant activity in the region. Eitter collection was conducted between 10.00 and 16.00h, and at least a day after a heavy rain as ants are extracted less effectively from wet litter (Eisher 1998). Eive Im^ quadrats were placed in a dice pattern within each plot, with the center of the dice corresponding to the center of the research plot. The closest quadrats (diagonal from center to rest of quadrats) were separated by 7m, and all quadrats were more than 6m away from the edges of the plot to increase the likelihood that foragers originated from within the study plot. We collected the leaf litter and the top layer of loose soil within each quadrat and sifted the collected material through a sifter with a mesh opening of approximately 10mm to exclude larger leaves, twigs and stones. In the laboratory, we loaded the sifted litter into Winkler extractors and left them to operate at room temperature for 72h. Three days are sufficient to extract nearly all ant species and the majority of individuals from the collected samples (Ivanov et al. 2010). At the end of the extraction time, we rinsed the content of each collecting container into a labeled vial containing 95% ethanol. We sorted, counted and identified all individuals to species using the taxonomic keys in Smith (1957), Convert (2005), and Francoeur (2007). We consulted the ongoing work of A. Francoeur, who is revising the North American Myrmica, to account for the presence of a yet undescribed species in our samples. Vouchers will be deposited at the Cleveland Museum of Natural History, Department of Invertebrate Zoology, and the remaining materials are in the first author’s collection. Ant nomenclature follows Bolton et al. (2007). We measured garlic mustard density by counting the number of stems present in two 0.25m^ quadrats placed within each invaded plot. These quadrats were positioned 5m away, on each side, from the central ant-sampling quadrat. In addition, we recorded the depth of the litter layer, to the nearest mm, at the four corners and the center of each sampling quadrat and averaged the five measurements for a single value for each sampling quadrat. In cases of large discrepancies between the individual measurements (>25%), caused by uneven litter accumulation or unevenness in the surface profile of the underlying soil, we discarded the lowest and highest values and took two additional measurements. At the onset of our sampling at BED, we observed high numbers of earthworms present in the leaf litter layer of the garlic mustard invaded plots and we decided to include an estimate of the density of the earthworms present at the research plots in our study. As we did not record the earthworm abundance in the initial plots sampled we limited our earthworm counts to the plots located at BRE. We counted all earthworms present in the leaf-litter layer and on the soil surface within each of the five one square meter samples, and identified representative specimens using the keys in Hale (2009). We did not use earthworm-specific extraction techniques, such as application of mustard solution or formalin, as temperate Amynthas typically are epigeic (residing in the leaf litter layer), or epi-endogeic (surface soil; Hendrix and Bohlen 2002). Our approach therefore allows for a valid estimation of only the surface density of Amynthas earthworms at the research plots. Data analysis We compared density between BED and BRE invaded plots using a two-sample t-test; leaf- litter depth using a two-way Analysis of Variance (ANOVA), with garlic mustard presence and site Ivanov, Keiper: Impacts of invasive gareic mustard on eocae ant communities 5 as main factors; and Amynthas density between BRE invaded and reference plots using the non- parametric Mann- Whitney U test. Normality and homoscedasticity of the data were evaluated prior to the analyses with a Kolmogorov- Smirnov (K-S) test and Levene’s test, respectively. We used a two-way ANOVA to compare ant abundance and species density, with garlic mustard presence and site identity as main factors. We used sample-based rarefaction to calculate and compare the observed and expected species richness across the pooled invaded and non-invaded plots, and across sites (rarefaction curves were scaled to number of occurrences; Gotelli and Colwell 2001). Using incidence and not abundance data is necessary as the sociality of ants often leads to clumping of individuals within samples that can skew species-richness comparisons and species- abundance relationships (King and Porter 2005). We created all sample-based rarefaction curves using the analytical method of Colwell et al. (2004), implemented in Estimates 8.2 (Colwell 2009). To estimate the asymptotic (total) species richness we calculated the Chao2 (Chao 1987) estimator, using 100 randomizations of sample accumulation order. This estimator relies only on presence/absence data, and has been found to be one of the least biased and the most precise estimation methods (Colwell and Codington 1994; Walter and Moore 2005; Hortal et al. 2006). For the analyses, data were pooled for all replicate samples within the invaded and non-invaded plots and compared using the 95% confidence intervals provided in the Estimates output. In addition, we used the calculated measures of expected species richness to assess the degree of sampling completeness in our study. We used hierarchical clustering (group-average linking algorithm with Bray-Curtis similarity measure) to identify natural groupings among the study plots according to similarities in their ant species composition. We used a nonparametric two-way Analysis of Similarity (ANOSIM, Clarke 1993), with garlic mustard and site as main factors, to test for differences in species composition between the invaded and non-invaded plots. Both analyses were performed with the software package PAST (version 1.97, Hammer et al. 2001). We constructed rank-abundance plots and compared them with the nonparametric Kolmogorov- Smirnov test to assess any differences in the structure of the ant communities between the invaded and non-invaded plots (Gotelli and Ellison 2004; Ellison et al. 2007). We used SPSS 16.0 (SPSS Inc, Chicago, Illinois) for all statistical analyses, unless otherwise noted in the text. RESULTS The average number of garlic mustard stems per quadrat was similar in the invaded plots at the two study sites (98.7 ± 14.6SE at BRE, and 146.3 ± 29.3SE at BED; t = 1.45; df = 18; P = 0.16). We collected a total of 9922 ant workers representing 25 species from 12 genera (Table 1). Of these, 4470 ants were collected at BRE and 5452 at BED, with 4639 ants collected at the reference plots, and 5283 at the invaded plots. Ant abundance per sample was higher for the non-invaded plots at BED, but exhibited the opposite trend at BRE, being higher at the invaded plots (Fig. 1). A two- factor analysis of variance showed no significant Fig. 1 Average ant abundance at garlic mustard invaded and non-invaded plots. Error bars are ± ISE. 6 Jeffersoniana Table 1. Ant species, with corresponding abundances, collected in garlic mustard invaded and non- invaded plots at Bedford and Brecksville. BEDFORD BRECKSVILLE TOTAL Species non- invaded invaded non- invaded invaded non- invaded invaded Amblyoponinae Amblyopone pallipes (Hal deman, 1 844) 44 12 15 13 59 25 Ponerinae Ponera pennsylvanica Buckley, 1 866 132 290 66 21 198 311 Myrmicinae Myrmica punctiventris Roger, 1 863 389 132 509 270 898 402 Myrmica semiparasitica Francoeur, 2007 3 0 0 11 3 11 Myrmica sp. 0 19 0 0 0 19 Stenamma brevicorne (Mayr, 1886) 154 232 6 32 160 264 Stenamma impar Foroi, 1901 124 152 10 76 134 228 Stenamma schmitti W.M. Wheeler, 1903 142 13 1 9 143 22 Aphaenogaster picea (W.M. Wheeler, 1908) 387 457 301 859 688 1316 Aphaenogaster tennesseensis (Mayr, 1 862) 0 1 0 0 0 1 Temnothorax curvispinosus (Mayr, 1 866) 177 53 141 42 318 95 Temnothorax longispinosus (Roger, 1 863) 1 6 3 2 4 8 Temnothorax schaumii (Roger, 1 863) 0 1 0 1 0 2 Myrmecina americana Emery, 1 895 663 489 150 575 813 1064 Dolichoderinae Tapinoma sessile (Say, 1836) 12 1 4 0 16 1 Formicinae Prenolepis imparis (Say, 1 836) 18 47 0 4 18 51 Lasius alienus (ToQXSiQX, 1850) 590 605 347 795 937 1400 Lasius nearcticus W.M. Wheeler, 1906 0 0 0 1 0 1 Lasius umbratus (Nylander, 1 846) 0 19 33 0 33 19 Formica neogagates Emery, 1893 0 0 98 0 98 0 Formica subsericea Say, 1 836 33 0 45 10 78 10 Camponotus chromaiodes Bolton, 1 995 3 7 12 1 15 8 Camponotus pennsylvanicus (De Geer, 1773) 14 16 1 4 15 20 Camponotus nearcticus Emery, 1 893 6 1 1 1 7 2 Camponotus subbarbatus Emery, 1 893 4 3 0 0 4 3 Ivanov, Keiper: Impacts of invasive gareic mustard on eocae ant communities 7 effect of garlic mustard presence = 0.74, P = 0.39), and site identity = 1.72, P = 0.19) on ant abundance, with no interaction between the main factors = 3.13, P = 0.08; Fig. 1). Similar results were obtained when abundances were compared at the plot, rather than the sample level. At BRE, we collected a total of 22 species of which 1 8 were recorded from the non-invaded and 19 species from the invaded plots, with 15 species being common to both plots. At BED, we recorded a similar number of species (23), of which 19 were present at the non-invaded plots, and 21 at the invaded plots. Seventeen species were common to both plots at BED (Table 1). The most abundant species in our samples were Lasius alienus (Foerster, \S50), Aphaenogaster picea (W.M. Wheeler, 1908), Myrmecina americana Emery, 1895, and Myrmica punctiventris Roger, 1863 comprising 68% and 85% of the total captures at BED and BRE, respectively (Table 1). No non-native ant species occurred in the collected samples. Both observed and expected species richness were higher at the invaded plots, but the differences were not significant. Our rarefaction analyses showed no significant difference in observed species richness between the invaded and non-invaded plots (Fig. 2). In addition, there was no difference in observed species richness between the invaded and non-invaded plots when compared across sites. The expected number of species was estimated at 28.7 and 21.7 species for the invaded Number of occurrences Fig. 2 Rarefaction curves for pooled invaded and non-invaded plots. For clarity the 95% confidence intervals (lighter dotted lines) are shown only for the invaded plots. Fig. 3 Species density at garlic mustard invaded and non- invaded plots. Error bars are ± ISE. and non-invaded plots, respectively. The difference was not significant as evidenced by the overlap of the 95% confidence intervals. Based on these estimates, we were able to sample an estimated 84% and 97% of the local ant fauna in invaded and non-invaded plots, respectively. Small scale species richness (i.e., species density per sample) was higher at BED than at BRE (two-way ANOVA: F(i = 47.88, P < 0.001), but not different across the invaded and reference plots within a site (F^^ = 0.13, P = 0.72), with no interaction between the main factors (F^^ = 2.78, P = 0.10; Fig. 3). The captures of some species differed between the non-invaded and the invaded plots (Table 1), however these patterns were often inconsistent across sites. For example, Myrmica semiparasitica Francoeur, 2007 was found only at the reference plots at BED, but was found only at the invaded plots at BRE. A reversed pattern was observed for Lasius umbratus (Nylander, 1846). Four species occurred only at the invaded plots, but three of those were restricted to only one of the two study sites. All four of these species showed very low frequency of occurrence, being present in only one or two samples (Table 1). Formica neogagates Emery, 1 893 was the only species that occurred at just non-invaded plots, however it was only found 8 Jeffersoniana at BRE and represented only 1% of the total of all ants eolleeted. Our hierarehieal eluster analysis did not reveal elear separation of plots, based on either presenee of garlie mustard or site (Fig. 4). A two-way ANOSIM revealed no signiheant differenee in ant speeies eomposition between invaded and non- invaded plots (R = 0.006, P = 0.47) and between sites (R = 0.056, P = 0.26). Our analyses thus show no evidenee for differenees in loeal ant eommunity eomposition between sites invaded and not invaded by garlie mustard. In addition, we found no ehanges in the strueture of the loeal ant eommunities between the invaded and non-invaded plots (Kolmogorov- Smirnov two-sample test: Z = 0.707, P = 0.70; Fig. 5). Leaf-litter depth was signiheantly lower in the invaded plots at both sites (two-way ANOVA: = 40.76, P < 0.001), with no signiheant effeet of site (F(i = 1.32, P = 0.25), and no interaetion between the main faetors (F^j = 2.81, P = 0.10; Fig. 6). At BRE, we found a signiheantly higher number of Amynthas earthworms present at the invaded plots (64.4 ± 20.6SD; Mann Whitney U test: Z = -5.63, P < 0.001), as eompared to the non-invaded plots (0.6 ± 1.7SD). DISCUSSION At the loeal seale at whieh we eondueted our study, and eontrary to our expeetations, we were unable to deteet impaets of garlie mustard presenee on native forest ant assemblages. The presenee of garlie mustard was not assoeiated with deteetable ehanges in the ant eommunities at the study sites, and our results showed no signiheant differenees in terms of abundanee, speeies riehness or eomposition 1400 1200 1000 CD 0 TO 800 c 1 600 400 200 0 • invaded Anon-invaded ® ^ A A 10 20 Fig. 5 Rank abundance plots for garlic mustard invaded and non-invaded plots Fig. 4 Dendrogram for hierarchical clustering of invaded and non-invaded plots according to similarities in ant species composition. Invaded plots are presented in black bold type, and reference plots are shown in grey Fig. 6 Average leaf-litter depth at garlic mustard invaded and non-invaded plots. Error bars are ± ISE Ivanov, Keiper: Impacts of invasive gareic mustard on eocae ant communities 9 between the invaded and non-invaded plots. Although ants have been shown to be good indieators of environmental perturbations and habitat disturbanee (Alonso 2000; Kaspari and Majer 2000), the eeologieal effeets of garlie mustard may be too subtle for them to respond, or these effeets may be masked by other more potent faetors. Site eharaeteristies are important in affeeting the outeome of the presenee and therefore the impaets of invasive weeds (Samways et al. 1996; Yeates and Williams 2001). The loeal ant assemblages, at our study sites, likely have been affeeted by faetors related to urbanization and habitat modiheation prior to the establishment of garlie mustard. Urban forests typieally eonsist of isolated patehes with redueed area and high proportion of edges. Anthropogenie habitat alterations sueh as elearing of native vegetation, fragmentation and destruetion of natural habitats and the assoeiated ereation of habitat edges operate on large spatial seales and are among the key environmental features of urban landseapes that are known to affeet loeal diversity (Bolger et al. 2000; Gibb and Honehuli 2002). These environmental disturbanees are among the major forees shaping the physieal and biologieal ehanges oeeurring in urban landseapes (Halme and Niemela 1993; Mureia 1995; Ewers et al. 2007). Generalist and opportunistie ant speeies are known to respond more sueeessfully to environmental disturbanees than speeialist speeies (Didham et al. 1996; Gibb and Hoehuli 2002), and ehanges in the proportion of opportunistie ant speeies have been eorrelated with levels of disturbanee (Andersen 1990). Urban forest fragments typieally are eharaeterized by a large proportion of generalist speeies and disturbed- habitat speeialists that have adapted to these, often degraded, environments (King and Tsehinkel 2006; Clarke et al. 2008). Thus the absenee of signiheant responses from the loeal ant eommunities to garlie mustard presenee may be a result of the eommon oeeurrenee, at our study sites, of speeies eapable of tolerating high levels of disturbanee. The impaets assoeiated with garlie mustard presenee may not be strong enough to elieit further ehanges in the abundanee, riehness, and eomposition of these eommunities. Another reason for a laek of deteetable differenees between the invaded and non- invaded plots may be the relatively reeent deteetion of garlie mustard in the Cleveland area, and in northeast Ohio in general. Although garlie mustard has been present in parts of the US for over a eentury, it was hrst deteeted in ‘natural’ settings in the Cleveland area in the early 1980s (C. Thomas, pers. eommunieation). This speeies was likely present earlier in gardens and other human habitations, however it was not a prominent part of the loeal flora until the late 1980s. Older settlements in Berea, Chagrin Falls, Bedford and Breeksville likely were epieenters for the plant’s establishment from whieh it naturalized urban green spaees (C. Thomas pers. eommunieation). A few ant speeies responded to the presenee of garlie mustard with ehanges in their frequeney of oeeurrenee and/or abundanee but these speeies were rather uneommon and their responses often were ineonsistent aeross sites. These differenees may be a result of patehy distributions and low probability of deteetion rather than a funetion of garlie mustard presenee. Formica neogagates was the only speeies in our study that showed preferenees to areas not invaded by garlie mustard. This woodland speeies is rare in Ohio, and in the Cleveland region where it is eurrently known only from Breeksville Reservation (Ivanov unpublished). It is possible that this speeies is negatively affeeted by the presenee of garlie mustard, but the faet that it only oeeurred in three samples from a single plot prevents us from making any definitive eonelusions about its response to garlie mustard presenee. The laek of signiheant differenee in terms of eommunity eomposition and strueture suggest that the presenee of garlie mustard did not lead to alterations of loeal speeies makeup and eommunity strueture. We were able to doeument a deerease in the leaf- litter layer at the garlie mustard invaded plots whieh was eorrelated with inereased abundanee of non- native Amynthas earthworms. Our results thus are eonsistent with the findings of Davalos and Blossey (2004) and Nuzzo et al. (2009). Garlie mustard presenee thus may be indieative of underground invasions by non-native Amynthas earthworms. These earthworms were more abundant at the garlie 10 Jeffersoniana mustard plots, but were also present at a few referenee plots. A deerease in the leaf-litter layer has been assoeiated with redueed invertebrate abundanee and ehanges in eommunity strueture (Koivula et al. 1999), likely as a direet result of deereased mierohabitat and resouree heterogeneity. Although we found signifieant reduetion in the leaf-litter layer at the garlie mustard invaded plots, the deerease was small (3.2mm at BED and 5.5mm at BRE) and may not be sufheient to translate into ehanges in the loeal forest ant eommunities. Exotie Amynthas have been deteeted in the region only reeently (M. Walton, pers. eommunieation) and it may be that more time is neeessary before the impaets assoeiated with their presenee are translated into ehanges in the loeal leaf-litter arthropod assemblages. Despite our findings and the similar results reported by Davalos and Blossey (2004), it is likely that garlie mustard has negative eeologieal impaets, but the biennial nature of the plant and the pattern of spread may produee subtle effeets that are diffieult to deteet. Moreover, these impaets may be masked by stronger faetors sueh as land use history and habitat disturbanee assoeiated with urbanization. Sueh faetors operate at larger spatial seales and may obseure the loealized impaet of an invasive plant. Future studies foeusing on the effeets of garlie mustard on loeal invertebrate eommunities may benefit by targeting ‘undisturbed’ natural areas in whieh garlie mustard is present. Sueh an approaeh may help to diseriminate between the impaets assoeiated with anthropogenie habitat disturbanee and those related to the presenee of this invasive speeies. In addition, the eeologieal impaets of garlie mustard in temperate deeiduous forest may be undeteetable when using ants (our study), or ground-dwelling earabid beetles (Davalos and Blossey 2004) as indieators. The effeets of invasive plants on loeal animal eommunities have been shown previously to be strongly dependent on the taxonomie group investigated (de Groot et al. 2007). Eong-term investigations, experimental manipulations of garlie mustard densities and use of other animal indieators are neeessary to more fully reveal the impaets assoeiated with^. petiolata invasions (Blossey 1999). ACKNOWLEDGMENTS We thank the Cleveland Metroparks for aeeess to the study sites and permission to eolleet samples. We also thank J. Milligan for help with various eomponents of the field work. Comments and suggestions from R. 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