Jonathan T. Bauer

Invasive species: “back-seat drivers” of ecosystem change?

Invasive species are often assumed to be the cause (drivers) of declines in native species and alterations of native ecosystems. However, an alternative model suggests that many invasive plants are better described as passengers of altered disturbance regimes or other changes in ecosystem properties. Some species do seem to be easily categorized as passengers or drivers, but others may be better described as “back-seat drivers”. Back-seat drivers require or benefit from disruptions of ecosystem processes or properties that lead to declines of native species but also contribute to changes in ecosystem properties and further declines of native species. Among these possibilities, drivers are a direct cause of the decline of native species through the introduction of novel traits or functions to an ecosystem, whereas back-seat drivers interact with ecosystem change to cause native species declines. Passengers are better considered as a symptom of an underlying problem, rather than the cause of native species declines. Driver, back-seat driver and passenger models suggest different associations between invasive species, ecosystem change and native species declines, and these models provide a framework for predicting and understanding the response of native species to invasive species management.

Experimental evidence for indirect facilitation among invasive plants

Summary Facilitation among species may promote non-native plant invasions through alteration of environmental conditions, enemies or mutualists. However, the role of non-trophic indirect facilitation in invasions has rarely been examined. We used a long-term field experiment to test for indirect facilitation by invasions of Microstegium vimineum (stiltgrass) on a secondary invasion of Alliaria petiolata (garlic mustard) by introducing Alliaria seed into replicated plots previously invaded experimentally by Microstegium. Alliaria more readily colonized control plots without Microstegium but produced almost seven times more biomass and nearly four times as many siliques per plant in Microstegium-invaded plots. Improved performance of Alliaria in Microstegium-invaded plots compared to control plots overwhelmed differences in total number of plants such that, on average, invaded plots contained 327% greater total Alliaria biomass and 234% more total siliques compared to control plots. The facilitation of Alliaria in Microstegium-invaded plots was associated with an 85% reduction in the biomass of resident species at the peak of the growing season and significantly greater light availability in Microstegium-invaded than control plots early in the growing season. Synthesis. Our results demonstrate that an initial plant invasion associated with suppression of resident species and increased resource availability can facilitate a secondary plant invasion. Such positive interactions among species with similar habitat requirements, but offset phenologies, may exacerbate invasions and their impacts on native ecosystems.

Effect of Removal of Garlic Mustard (Alliaria petiolata, Brassicaeae) on Arbuscular Mycorrhizal Fungi Inoculum Potential in Forest Soils

Garlic mustard (Alliaria petiolata), a biennial species, is considered to be among the most troublesome of the invasive plants in the Eastern Deciduous forest of North America. It has been shown to prevent or reduce mycorrhizal colonization of native herbaceous ground layer plants and trees in these forests. It is estimated that 70-90% or more of herbaceous native ground layer plant species form associations with arbuscular mycorrhizal fungi (AMF). Loss of the mycorrhizal association can reduce growth, reproductive success, and competitiveness of plant species. Using a corn root bioassay, we examined the effect of garlic mustard removal on the soil AMF mycorrhizal inoculum potential (MIP), in control plots and plots that had second-year garlic mustard removed annually for the past five years (2005-2009). Removal treatment plots had significantly (P = 0.0240, df = 28) greater MIP than control plots (25.72±2.26% and 18.29±2.04%, respectively). MIP was negatively correlated with cover of garlic mustard (r 2 = 0.1325, P < 0.05, df = 30), which accounted for 13.2% of the variation in MIP. Cover of native vegetation in removal treatment plots (104.50±2.6%) was greater than that of the control plots (95.14±3.66%), (P = 0.0236, df = 115). These results show that removal of garlic mustard results in an increase in soil MIP and cover of native species; however, there is not a complete loss of MIP associated with garlic mustard invasion. Following removal of garlic mustard, our data suggest that mycorrhizal plants recover more slowly than non-mycorrhizal species, apparently due to a delay in the establishment of a well-functioning mycorrhizal association. Our study is the first to demonstrate that the MIP of native soils and cover of native species increase following reduction in the cover of garlic mustard.

Context dependency of the allelopathic effects of Lonicera maackii on seed germination

Allelopathic effects of invasive plants on native flora may be mitigated by the abiotic and biotic environment into which the allelochemicals are released. Lonicera maackii (Amur honeysuckle), an invasive plant of the eastern deciduous forest, suppresses seed germination in laboratory assays. We investigated how L. maackii leachate interacts with abiotic conditions and with the soil microbial community. First, we tested the effects of leaf extract from L. maackii on germination of the native woodland herb, Blephilia hirsuta, under different light and soil conditions. We found that germination of Blephilia hirsuta was reduced by L. maackii extract, but abiotic conditions did not interact with this effect. We also tested the effects of leaf extract on germination of five native woodland species and L. maackii placed in sterile or live soil. There was an overall suppressive effect of L. maackii extract on itself and the other five native species tested. However, L. maackii extract interacted with live soil in ways that differed with the species being tested and, in some cases, changed over time. Our results indicate that allelopathic potential of L. maackii shows context dependency with respect to soil microorganisms and native species identity but not to light conditions or soil type. Our results imply that restoration of invaded areas may require active reintroduction of species sensitive to allelopathy in live soil. Further, laboratory assays of allelopathy should consider the interaction of allelochemicals with biotic and abiotic conditions to more accurately predict the impacts of allelopathy on plant communities.

Suppression of the Woodland Herb Senna hebecarpa by the Invasive Grass Microstegium vimineum

Abstract 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 21% 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.

Plant-soil feedbacks between invasive shrubs and native forest understory species lead to shifts in the abundance of mycorrhizal fungi

AimsNon-native shrubs are important invaders of the Eastern Deciduous Forest, dramatically altering forest structure and functioning. Study of invasion mechanisms in this system has emphasized aboveground processes, and plant-soil feedbacks are relatively unexplored as a mechanism of shrub dominance. We tested whether plant-soil feedback in this habitat is affected by competition and whether arbuscular mycorrhizal fungi (AMF) are involved in plant-soil feedback.MethodsWe used a standard two-phase plant-soil feedback experiment run concurrently for each of three invasive shrub species, measuring feedback effects on AMF colonization, aboveground biomass, and the responses of native plant species in greenhouse mesocosms.ResultsLonicera maackii and Ligustrum vulgare reduced AMF colonization of native roots, both with legacy effects (prior growth in soil) and direct effects (current growth in soil). Elaeagnus umbellata grown with natives left a legacy of increased AMF colonization of native communities.ConclusionsOur results suggest that woody invasive species can alter the AMF associations of native plants even after the invasive is no longer present. Such consequences merit study with other native species and where environmental factors, such as light availability, might be expected to compound the effects of changes in AMF.

Comparison of the effect of early and late removal of second-year garlic mustard (Alliaria petiolata) on first-year plants and deciduous forest spring and summer dominant herbaceous groundlayer species in central Illinois, USA.

Garlic mustard, a biennial Eurasian species, has extensively invaded eastern North American deciduous forests. We studied effects of 3 years (2005–2007) of annual removal of second-year garlic mustard plants on first-year plants and native spring herbaceous species in upland and lowland woods. Treatments compared removal of second-year plants in mid-March (early treatment) or mid-May (late treatment) to a control. We recorded first- and second-year plants and native herbaceous species percent cover on April 19 and 20. First-year plant cover was higher on control than treatment plots; however, in the upland woods only control and late treatment plots differed significantly. First-year plant cover was less in removal than control plots, indicating reduced seed input; however, we found no difference in cover of second-year plants between late treatment and control plots. Results suggest second-year plants strongly compete with younger conspecifics, and their removal decreases first-year plant mortality. Removal of second-year garlic mustard did not significantly affect total cover of native herbaceous species. Second-year plants complete vegetative growth before late May and might impact early developing native species more than later growing species. We tested effect of removal of garlic mustard on native species in 2 phenological categories: spring- and summer-dominant species. We found no treatment effects on summer-dominant species. However, early treatment plots had significantly more cover of spring-dominant plants than late treatment and control in the upland woods. Indicator Species Analysis indicated a majority of spring (75%) and summer (50%) dominant species maximized performance in the early treatment.

Effects of between-site variation in soil microbial communities and plant-soil feedbacks on the productivity and composition of plant communities

Summary A critical challenge in the science and practice of restoration ecology is to understand the drivers of variation in restoration outcomes. Soil microbial communities may have a role in explaining this variation due to both site-to-site variation in the composition of soil microbial communities and due to variation that can arise due to plant-soil feedbacks. We tested the relative importance of between-site variation in soil microbial community composition and plant-soil feedbacks in shaping plant community composition and ecosystem function. We used a standard two-phase plant-soil feedback design. Soil inoculum was collected from four tallgrass prairie sites. Then, soils were conditioned separately with nine plant species, and conditioned soils were used to inoculate prairie community mesocosms. In a separate experiment using soil from an additional site we tested conditioned soil samples for the abundance of arbuscular mycorrhizal fungi (AMF) and rhizobia. Site of soil origin and plant-soil feedbacks both had effects on the composition and productivity of our plant communities, and the magnitudes of these effects were similar. We also found changes in the abundance of AMF and rhizobia due to plant-soil feedbacks and that AMF abundance were associated with differences in plant community composition. These results indicate that the composition of soil communities due to site-to-site variation and plant-soil feedbacks are both important determinants of plant community composition and productivity. Our results also suggest that AMF and rhizobia are key microbial functional groups underlying plant-soil feedback effects. Synthesis and applications. Site-to-site variation in soil communities can explain some variation in restoration of plant communities. Since plant-soil feedback effects of restored plant species do not overcome this variation, knowledge of soil microbial communities present at a site prior to initiation of restoration efforts may improve predictability of restoration outcomes, and reintroduction of some components of the soil community may be necessary to achieve restoration goals. Additionally, by understanding variation due to plant-soil feedbacks, restoration practitioners can choose plant species for reintroduction that will create favourable soil conditions, including promoting microbial mutualists. Plant-soil feedbacks should also make it possible to increase heterogeneity in soil microbial communities, leading to increases in beta diversity in plant communities.