Emily S. J. Rauschert

Forest Roads Facilitate the Spread of Invasive Plants

Abstract The distribution and abundance of invasive species can be strongly influenced by habitat suitability and by corridors that facilitate dispersal. We synthesize results from a large-scale invasive plant survey with a patch-scale expansion experiment. The large-scale survey involved transects up to 250 m away from of all roads in a 32,000 ha forest. The patch experiment involved initiating invasions in different habitat types (roadside, wetland, disturbed, and intact forests), and then fitting statistical models to patch spread rates. The large-scale survey highlighted the importance of roads in predicting the presence of invasive plants, also revealing that one invasive plant, Microstegium vimineum, has spread rapidly since its purported introduction in 1994. The patch-scale experiments focused on Microstegium and demonstrated that spread rates are higher in roadsides than in forested and wetland patches, even in the absence of major disturbances. These results highlight the importance of landscape features when designing prevention and management practices aimed at limiting invasive plant abundance and spread. Nomenclature: Japanese stiltgrass, Microstegium vimineum (Trin.) A. Camus.

Invasional Interference Due to Similar Inter- and Intraspecific Competition Between Invaders May Affect Management

As the number of biological invasions increases, the potential for invader-invader interactions also rises. The effect of multiple invaders can be superadditive (invasional meltdown), additive, or subadditive (invasional interference); which of these situations occurs has critical implications for prioritization of management efforts. Carduus nutans and C. acanthoides, two congeneric invasive weeds, have a striking, segregated distribution in central Pennsylvania, U.S.A. Possible hypotheses for this pattern include invasion history and chance, direct competition, or negative interactions mediated by other species, such as shared pollinators. To explore the role of resource competition in generating this pattern, we conducted three related experiments using a response-surface design throughout the life cycles of two cohorts. Although these species have similar niche requirements, we found no differential response to competition between conspecifics vs. congeners. The response to combined density was relatively weak for both species. While direct competitive interactions do not explain the segregated distributional patterns of these two species, we predict that invasions of either species singly, or both species together, would have similar impacts. When prioritizing which areas to target to prevent the spread of one of the species, it is better to focus on areas as yet unaffected by its congener; where the congener is already present, invasional interference makes it unlikely that the net effect will change.

Environmental Factors Influence Early Population Growth of Japanese Stiltgrass (Microstegium vimineum)

Abstract Habitat suitability and disturbance can shape the early stages of biological invasions in important ways. Much that we know about habitat suitability and invasion originates from point-in-time studies, which characterize invasive plant abundance and associated site characteristics. In our study, we tested the influence of habitat suitability by creating small-scale invasions in a range of environments. Seeds of the invasive annual grass Japanese stiltgrass [Microstegium vimineum (Trin.) A. Camus] were planted into six environments in a deciduous forest in central Pennsylvania, and patch growth was monitored for 4 yr. Each of the 30 sites included a subplot subjected to litter disturbance at time of planting. This litter disturbance led to increased seedling recruitment only in the first 2 yr. Although patches were generally larger in wetland and roadside habitats, site influence was highly variable. Environmental variables (soil moisture, ammonium–N, pH, and plant species richness) measured in each plot were better predictors of population success than broad habitat categories. We conclude that risk assessment for species such as M. vimineum should focus not on habitat types but on areas likely to experience the physical changes that release M. vimineum populations. Nomenclature: Japanese stiltgrass, Microstegium vimineum (Trin.) A. Camus (Poaceae).

Plant community associations of two invasive thistles

We assessed the field-scale plant community associations of Carduus nutans and C. acanthoides, two similar, economically important invasive thistles. Several plant species were associated with the presence of Carduus thistles while others, including an important pasture species, were associated with Carduus free areas. Thus, even within fields, areas invaded by Carduus thistles have different vegetation than uninvaded areas, either because some plants can resist invasion or because invasion changes the local plant community. Our results will allow us to target future research about the role of vegetation structure in resisting and responding to invasion.

A unifying gravity framework for dispersal

Most organisms disperse at some life-history stage, but different research traditions to study dispersal have evolved in botany, zoology, and epidemiology. In this paper, we synthesize concepts, principles, patterns, and processes in dispersal across organisms. We suggest a consistent conceptual framework for dispersal, which utilizes generalized gravity models. This framework will facilitate communication among research traditions, guide the development of dispersal models for theoretical and applied ecology, and enable common representation across taxonomic groups, encapsulating processes at the source and destination of movement, as well as during the intervening relocation process, while allowing each of these stages in the dispersal process to be addressed separately and in relevant detail. For different research traditions, certain parts of the dispersal process are less studied than others (e.g., seed release processes in plants and termination of dispersal in terrestrial and aquatic animals). The generalized gravity model can serve as a unifying framework for such processes, because it captures the general conceptual and formal components of any dispersal process, no matter what the relevant biological timescale involved. We illustrate the use of the framework with examples of passive (a plant), active (an animal), and vectored (a fungus) dispersal, and point out promising applications, including studies of dispersal mechanisms, total dispersal kernels, and spatial population dynamics.

Influence of Microsite Disturbance on the Establishment of Two Congeneric Invasive Thistles

The successful establishment of invasive species has been shown to depend on aspects of the invaded community, such as gap characteristics. Biotic resistance may be particularly critical for stopping invaders at early life history stages, but new species can often invade following disturbances, which may create microsites with very different characteristics than are usually present. We examine the response of two invasive thistle species, Carduus nutans L. and C. acanthoides L., to three different microsite characteristics: disturbance type, size, and water availability. The two species initially responded differently to the type of disturbance: C. acanthoides had higher emergence and survival in plots with both above- and belowground disturbance, whereas C. nutans had better early performance in large microsites with above-ground disturbance only. Later in their life cycle, C. nutans performed better in plots that had been disturbed both above- and belowground, whereas C. acanthoides was largely unaffected by disturbance type. Increased emergence and survival, larger size and a higher proportion flowering were observed in larger gaps for both species throughout the life cycle. Watering had a negative impact on C. nutans emergence and fall survival and on C. acanthoides survival to the following summer. Overall, these results suggest that disturbance-generated microsite characteristics (disturbance type and size) may have large impacts on establishment of these two Carduus species, which in turn may persist well beyond the initial stages of growth. Studying invader responses to disturbance can help us to understand under what circumstances they are likely to establish and create persistent problems; avoiding or ameliorating such situations will have significant management benefits.

Primary literature across the undergraduate curriculum: teaching science process skills and content

Emily Rauschert, Joseph Dauer, Jennifer L. Momsen, and Ariana Sutton-Grier presented a workshop titled “101 Ways to Effectively Use Journal Articles as Teaching Tools.” They show us a number (not sure there are exactly 101 but who is counting) of ways to use primary literature to meet a multitude of learning objectives, primarily in the undergraduate classroom, but many of the approaches can be used in other settings. They also present some feedback and comments from the participants, so it is almost as if you were there.

Competition between similar invasive species: modeling invasional interference across a landscape

As the number of biological invasions increases, interactions between different invasive species will become increasingly important. Several studies have examined facilitative invader–invader interactions, potentially leading to invasional meltdown. However, if invader interactions are negative, invasional interference may lead to lower invader abundance and spread. To explore this possibility, we develop models of two competing invaders. A landscape simulation model examines the patterns created by two such species invading into the same region. We then apply the model to a case study of Carduus nutans L. and C. acanthoides L., two economically important invasive weeds that exhibit a spatially segregated distribution in central Pennsylvania, USA. The results of these spatially-explicit models are generally consistent with the results of classic Lotka–Volterra competition models, with widespread coexistence predicted if interspecific effects are weaker than intraspecific effects for both species. However, spatial segregation of the two species (with lower net densities and no further spread) may arise, particularly when interspecific competition is stronger than intraspecific competition. A moving area of overlap may result when one species is a superior competitor. In the Carduus system, our model suggests that invasional interference will lead to lower levels of each species when together, but a similar net level of thistle invasion due to the similarity of intra- and interspecific competition. Thus, invasional interference may have important implications for the distribution and management of invasive species.

Teaching How Scientific Consensus Is Developed Through Simplified Meta-analysis of Peer-Reviewed Literature

Students need to understand the diverse pathways to scientific knowledge and how scientists develop consensus about emerging questions. For students in environmental fields, it is especially important to learn how to reach consensus when not every study shows the same effect. To address these needs, this activity involves conducting a simplified, in-class meta-analysis. The approach is suitable for a variety of topics, and a case study is provided investigating the impact of global change on invasive plant success. After completing this activity, students should be able to (1) interpret and explain graphs in and extract major conclusions from a scientific peer-reviewed article, (2) identify the procedure of a meta-analysis and conduct a simplified meta-analysis, and (3) synthesize results of multiple studies to reach a general conclusion for a question.