Materials and methods Introduction Invasive species often have striking primary negative effects on native species, in such a manner that secondary effects can be overlooked. Some studies have shown that although invasive species impact new ecosystems negatively, they can also have positive secondary effects, for example by increasing selective pressures on natives and driving evolutionary diversification (Vellend, 2007) or controlling the presence of other invasive species through competition (Zavaleta, et al. 2001). Terrestrial mollusks can have negative and positive effects on ecosystems. Slugs have been found to have negative effects on seedling establishment (Scheidel, 2005; Hanley 1995), vegetation biomass and diversity (Buschmann 2004) and positive effects on litter consumption and nutrient cycling (Jennings and Barkham 1979). Specifically, Arion spp. consumed approximately 8.4% of leaf litter input in a Virginia forest (Jennings and Barkham 1979) and their mucus and feces significantly increased nutrient leeching from beech leaf litter and microbial biomass (Theenhaus, 1996a). Other studies suggest higher abundances of slugs increase soil invertebrate populations (Ferguson, 2004), which can positively affect understory plants through increased nutrient availability. As an example, plants increased growth rates in soil with bacteria and nematodes compared to bacteria alone because of increased nitrogen availability (Ingham et al. 1985). Although these studies stress the importance of either negative (herbivory) or positive effects (increased litter decomposition and nutrient leeching), few studies have simultaneously looked at both. This study attempted to examine the ecological impacts of an invasive slug on native understory plants to determine the relative magnitudes of positive and negative effects. Because understory plants benefit from nutrients released into the soil from slug feces regardless of whether they are a result of herbivory or the consumption of leaf litter, it was hypothesized that slugs have a positive effect on plants. ResultsConclusions Since the invasive slugs had a significant negative effect on leaf area in plots with high slug densities (Fig. 3) while positive effects on litter invertebrates (Fig. 5) or nutrients did not differ between plots, we conclude that A. ater has a negative net ecological effect on M. nervosa. In addition, M. nervosa in plots with low slug density had more new growth, and therefore higher final number of leaves, compared to those with higher slug density (Fig. 4a). Plant growth analyses The greatest difference in plant measurements occurred between None and Exclusion plots, where significant differences were observed in change in leaf area (Fig. 3, p<0.005), number of branches (Fig. 4a, p<0.005), and number of leaves (Fig. 4b, p<0.05). Although plots without fences were not significantly different from control plots, this indicates that the presence of fences has some effect on slug foraging behavior. The final number of branches was significantly higher for Inclusion and Exclusion plots when compared to None plots (Fig. 4a). This same significance was found at the beginning of the experiment (data not shown) and can only be explained by random chance because the seedlings were planted at random. As M. nervosa grows, it produces new branches which subsequently have more and more leaves. Therefore a plant with more branches is more likely to produce a branch with more leaves. While this may be the case, inclusion plots showed significantly more loss in leaf area than exclusion plots—in other words, the herbivory effect is so strong that any additional variables produced by number of branches have no significance. Soil and litter analyses Interestingly, there was a higher relative abundance of litter invertebrates in inclusion plots (Fig. 5a) which supports previous findings that slugs positively affect the presence of other invertebrates (Ferguson, 2004). The difference cannot be attributed to the existence of the fence, as the abundances in the Control (with fence) and None (without fence) plots were very similar. Laboratory experiments The non-significant differences in dry litter mass indicate that slugs do not consume large amounts of litter, a fact which is supported by the lab consumption rates (Fig. 4A). More slugs ate leaf litter than those which ate fresh leaves, suggesting that although the mean consumption of fresh leaves was significantly higher, litter is more universally palatable among individuals. These conclusions cannot be absolute, because the food types were presented separately to slugs and preference cannot be determined. The insignificance for positive effects such as change in nitrate may be explained because it takes more time before they are evident. Herbivory affects the plant through loss of leaf area and therefore photosynthetic potential immediately. However, the digestion of plant matter and subsequent “recruitment” of bacteria and nitrate availability take a longer time to become evident. Hillary Z.G. Lauren* & W. Lindsay Whitlow Department of Biology, Seattle University, 2009 Figure 1. Illustration of the ecological effects of slugs, including negative such as herbivory (a) and positive such as litter decomposition (b), nutrient leeching (c), and effect on litter invertebrates (d). The good, the bad, and the slimy: Analyzing the net ecological effects of invasive slugs on an urban forest None: no fence Inclusion: slugs enclosed Control: free movement Exclusion: slugs excluded Ten sites were chosen in Seward Park, Seattle. Each site had four treatment 0.25m2 plots for controlling slug presence (Figure 2) using fences with or without copper tape. Each plot had three Mahonia nervosa (Cascade Oregon grape) seedlings. At the end of the trial period (August 2008 – April 2009), measurements were taken of dry leaf litter mass soil pH change in soil nitrate % soil moisture, and litter invertebrate abundance and diversity. number and length of leaves and branches change in leaf surface area Experiments in the lab included calculating the consumption and assimilation rate of fresh M. nervosa leaves and Acer macrophyllum (Big leaf maple) litter. Figure 2. Four plot treatments at each field site. Inclusion and exclusion plots had copper tape to discourage slugs from crossing the barrier. Acknowledgments We would like to thank the M.J. Murdock Charitable Trust for funding, the Seattle Parks and Recreation Department for permission to study Seward Park, and our colleagues at Seattle University, especially John Vincent, for technical and field assistance. Contact *Please contact Hillary Lauren at More information on this and related projects are available at www2.seattleu.edu/scieng/biology/ a b c d Figure 3. Effect of slug density on change in M. nervosa leaf area per leaf. There was a significant decline in M. nervosa leaf area in treatment areas of high slug density. (p<0.05) ***p<0.005 **p<0.01 *p<0.05 Plant growth analyses Plants in plots where there was a lower slug density showed less herbivory damage than those where slugs had higher densities (Fig. 3). Data analyses showed that change in leaf area per plant had a significant interaction between site and treatment (p<0.05) but no significant differences for either main effect. The change in leaf area per leaf showed that slug exclusion plots were significantly different from the none (p<0.005), inclusion (p<0.01), and control (p<0.05) plots (Fig. 3). Although there was also a significant difference between sites (p<0.01), there was no interaction effect. Number of branches per plant was inconsistent between high and low slug density (Fig. 4a), but number of leaves per plant was highest in plots of low slug density (Fig 4b). There was a significant difference between treatments concerning number of leaves (p<0.05) and branches (p<0.01), but neither showed significance between sites or an interaction effect. Leaf length showed no significant differences between sites or treatments, and branch length showed a significant difference between sites (p<0.01) but not treatments. Soil and litter analyses Percent moisture content did not differ significantly between treatments or sites. All other measurements showed significant differences between sites, including pH (p<0.001), dry leaf litter mass (p<0.001), change in nitrate (p<0.05), and abundance (p<0.01) and diversity (p<0.001) of litter invertebrates per gram litter. None of these showed differences between treatments. Interestingly, the abundance of invertebrates in the inclusion plot was relatively high compared to the other plots (Fig. 5a), though not significantly. Laboratory experiments The slugs showed a significantly higher consumption rate of fresh leaves than litter (Fig. 6a, p<0.01), and a slightly higher assimilation rate for leaf litter, though not significantly (Fig. 6b). Figure 4. Effect of slug density on number of leaves and branches per M. nervosa plant. (a) There was an increasing amount of leaves from none, inclusion, control to exclusion plots, significant between None and Exclusion plots (p=0.106). (b) Exclusion plots had a significantly higher number of branches than None and Control plots but not Inclusion plots (p<0.05). **p<0.005 *p<0.05 Figure 5. Effect of slug density on litter invertebrate abundance and diversity. Inclusion plots showed higher mean litter invertebrate (a) abundance (p=0.421) and (b) diversity per gram litter than other treatment groups (p=0.731). Figure 6. Slug consumption and assimilation rates on leaf litter and fresh leaves. There was a significantly higher (a) consumption rate (p<0.01) of fresh leaves than gram leaf litter per live slug weight per day but no significant differences concerning (b) assimilation (p=0.223). a a b b b a Literature cited Buschmann, H, et al The effect of slug grazing on vegetation development and plant species diversity in an experimental grassland. Functional Ecology 19: Hanley, ME, Fenner, M, & Edwards, PJ An experimental field study of the effects of mollusk grazing on seedling recruitment and survival. Journal of Ecology 83: Ingham, RE, et al Interactions of bacteria, fungi, and their nematode grazers: Effects on nutrient cycling and plant growth. Ecological Monographs 55: Jennings, TJ, & Barkham, JP Litter decomposition by slugs in mixed deciduous woodland. Holarctic Ecology 2: Scheidel, U & Bruelheide, H Effects of slug herbivory on the seedling establishment of two montane Asteraceae species. Flora 200: Theenhaus, A & Scheu, S The influence of slug (Arion rufus) mucus and cast material addition on microbial biomass, respiration, and nutrient cycling in beech leaf litter. Biology and Fertility of Soils 23: Vellend, M, et al Effects of exotic species on evolutionary diversification. Trends in Ecology and Evolution 22: Zavaleta, ES, Hobbs, RJ, & Mooney, HA Viewing invasive species removal in a whole-ecosystem context. Trends in Ecology & Evolution 16: