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Although studies often focus on the direct effects of invasive species on native taxa, invasive species may also alter interactions among native species. For example Solenopsis invicta, the red imported fire ant, may directly alter native seed survival by consuming seeds, but also indirectly alter seed survival, by altering the abundance and/or behavior of native granivores. We tested the effects of invasive S. invicta on rodent and arthropod granivory by quantifying seed removal from seed depots that differed in granivore access (arthropods and rodents or arthropods only) and distance from an S. invicta mound (0.1 m or 4.0 m). We hypothesized the effect of S. invicta on native granivores would be stronger at depots located near (0.1 m) a mound than at depots located 4.0 m from a mound. Use of two different seed species (Rubus cuneifolius and Prunus serotina) allowed us to evaluate the consequences of S. invicta for small-seeded plant species consumed by both arthropods and rodents (R. cuneifolius) as well as for large-seeded species that can only be consumed by rodents (P. serotina). We found overall removal of P. serotina was low, regardless of seed depot location or exclosure type. Near 5. invicta mounds, the removal of R. cuneifolius was also low, with no difference between depots that allowed or excluded rodents. In contrast, removal of R. cuneifolius by arthropods 4.0 m from a mound was nearly twice that of removal next to a mound but only when rodents were excluded. Our results indicate S. invicta may create hotspots of granivory by native arthropods in the areas between S. invicta mounds, but these effects may not extend to large-seeded plants that are consumed by rodents. By influencing seed survival as a function of plant species and proximity to a mound, nonnative S. invicta generates heterogeneity in native seed survival, which may affect plant community composition., INTRODUCTION Invasive species can have negative direct effects on ecological communities by reducing abundance and diversity of native species and disrupting ecosystem processes (e.g., Elton, 1958; Mooney, 2005; Vila et [...]

Throughout Midwestern forests, invasion by the exotic plant garlic mustard [Alliaria petiolata M. Bieb. (Cavara & Grande)] has become increasingly problematic. A multicriteria risk model was developed to predict invasion in the Upper Peninsula of Michigan where garlic mustard is present but not yet widely distributed. The model uses geographic information system (GIS) data to predict the risk of invasion at three phases: introduction, establishment and spread. Known occurrence data for garlic mustard were used to validate the model predictions, with 89% of points correctly identified at moderate to high risk for invasion. The risk model predicted 13% of the Upper Peninsula to be at high risk and 33% at moderate risk for establishment of garlic mustard. Field sampling of randomly generated points across the Upper Peninsula provided only two additional observations of garlic mustard presence. The low encounter rate during field sampling may indicate that garlic mustard has not yet reached its full invasion potential in the Upper Peninsula. This presents an opportunity to use the model predictions and associated risk maps for monitoring and management in a relatively uninvaded region., INTRODUCTION Garlic mustard [Alliaria petiolata M. Bieb. (Cavara & Grande)] is one of the most pervasive exotic plants in Midwestern forests (Blossey et al., 2002). It is an obligate biennial [...]

Biological invasions are associated with declining biodiversity in many ecosystems, but it is often unclear if invasions are directly responsible for such changes, or if invasions are a symptom of environmental degradation. In addition, the mechanism underlying the effects of many invasive species is unknown. To determine if invaders are driving changes in invaded communities, and to identify causal mechanisms, studies that manipulate the presence of invaders are needed. We experimentally introduced the invasive grass Microstegium vimineum into replicated field plots that had been planted with the native woodland herb Senna hebecarpa. After 3 y, we quantified Senna establishment, growth and reproduction. We then conducted a greenhouse experiment to determine if changes in Senna success were due to alteration of soil microbial communities or nutrient depletion in invaded plots. Microstegium-invaded plots had 74% fewer Senna plants and Senna growing in invaded plots were 91% shorter and weighed 64% less than in control plots. The proportion of Senna plants that reproduced was 67% lower and plants produced 78% fewer seeds on average in invaded than in control plots. In contrast to the field results, there were no differences in the growth of Senna when grown in Microstegium-invaded or control soil in the greenhouse, and the invasion treatment did not alter the effects of soil sterilization or fertilization. Further, we found no evidence that Microstegium success is determined by feedbacks with the soil community. Our results demonstrate that Microstegium has negative effects on a native species, but we found no evidence that the suppressive effects of Microstegium invasions are mediated by plant-soil interactions in invaded areas., INTRODUCTION Biological invasions are one of the leading causes of declining biodiversity (Wilcove et al., 1998), but in many cases it is unclear if invasive species are directly causing extirpations [...]

Red imported fire ants (Solenopsis invicta) have negative impacts on a broad array of invertebrate species. We investigated the impacts of fire ants on the upland arthropod community on 20~40 ha study sites in southern Mississippi. Study sites were sampled from 1997-2000 before, during, and after fire ant bait treatments to reduce fire ant populations. Fire ant abundance was assessed with bait transects on all sites, and fire ant population indices were estimated on a subset of study sites. Species richness and diversity of other ant species was also assessed from bait transects. Insect biomass and diversity was determined from light trap samples. Following treatments, fire ant abundance and population indices were significantly reduced, and ant species diversity and richness were greater on treated sites. Arthropod biomass, species diversity and species richness estimated from light trap samples were negatively correlated with fire ant abundance, but there were no observable treatment effects. Solenopsis invicta has the potential to negatively impact native arthropod communities resulting in a potential loss of both species and function., INTRODUCTION The invasion of non-native species into new habitats has been aided by anthropogenic transformation of land cover and land use, as well as the increasing extent and volume of [...]

Introductions of non-indigenous species in ballast water are one of the greatest threats to freshwater and marine ecosystems worldwide. New approaches to reducing the release of organisms from ballast water are under consideration nationally and internationally. Unfortunately, the development of scientifically defensible policy for controlling introductions from ballast water has been retarded by the lack of relevant ecological theory and a shortage of information about the identity and numbers of organisms in ballast. Here, we present a novel quantitative approach to estimating the risk of species establishment by combining a model for population spread with known allometric correlations between body size and population growth rate for broad taxonomic categories. Our approach is applicable to sexually reproducing, planktonic taxonomic groups including ctenophores, cnidaria, arthropods, annelids, mollusks and (as an approximation) echinoderms and fishes. As expected, the allowable volume of ballast discharge depends strongly on the acceptable level of risk (which is a societal decision), the taxonomic group of concern and the characteristics of the receiving environment. For example, for a risk tolerance equivalent to establishment of one in one hundred introduced species, independent releases of untreated ballast water should not exceed around 50,000 metric tons. Because of differences in horizontal mixing in different environments, releases in harbors are more risky than releases in open water. These results provide quantitative guidelines that could immediately lower the risk of species invasion while other more permanent technological solutions are developed.

Introduced American bullfrogs (Rana catesbeiana) come in contact with native amphibians on four continents and are well established in lowlands of western North America. To date, research on the effects of introduced bullfrogs on native frogs has focused on competition and predation, and is based largely on larval interactions. We present observations of interspecific amplexus between bullfrogs and two native ranid frogs (R. aurora and R. pretiosa) from six sites across the Pacific Northwest that imply that this interaction is more widespread than currently recognized. Our observations indicate that R. catesbeiana juveniles and subadults in this region are of appropriate size to elicit marked amplectic responses from males of both native species. Our literature review suggests that greater opportunity may exist for pairings between R. catesbeiana and native R. aurora or R. pretiosa than among syntopic native ranids in western North America. We hypothesize that interspecific amplexus with introduced R. catesbeiana could result in reproductive interference with negative demographic consequences in native ranid populations that have been reduced or altered by other stressors.

Chinese tallow (Sapium sebiferum) is an invasive tree in the southeastern United States that has been a management concern in Florida. We used observational and experimental approaches to explore factors influencing Chinese tallow invasion around the perimeter of Lake Jackson, Leon County, Florida. Twenty-one transect sites were randomly chosen, 12 with, and 9 without Chinese tallow. We used abundances of co-occurring species and abiotic environmental data to study factors associated with Chinese tallow abundance among and within transects. Canonical correspondence analysis indicated that soil moisture availability and other soil characteristics correlated with Chinese tallow invasion, but that Chinese tallow was not associated with other species distributions. A seeding experiment demonstrated that higher elevations with lower soil moisture have higher germination and seedling survival. This experiment predicts specific elevations around the lake that are most vulnerable to Chinese tallow invasion. Presence or absence of Chinese tallow among transects was not a significant predictor of germination success, suggesting that not all sites suitable for invasion have been invaded. In addition, controlled burning decreased germination probability and may therefore be useful for invasion control.