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Master of Science / Department of Biology / David C. Hartnett / Invasive plants pose a global threat by changing natural communities and ecosystems in ways that may be irreversible. Marrubium vulgare L. (horehound), native to Eurasia, is an invasive exotic plant found throughout the United States. Little is known about M. vulgare in the U.S. and this study aimed to understand the basic biology, habitat, and population dynamics of M. vulgare in its invasive habitat as well as the role of disturbance in the invasions. Wind Cave National Park (WCNP), an area comprising ponderosa pine and mixed grass prairie vegetation types, was used as the site for this study. In a GPS and GIS mapping study, M. vulgare was found to be significantly associated with prairie dog towns, an Aristida purpurea - Dyssodia papposa (threeawn and fetid marigold) vegetation community (a vegetation type found exclusively on prairie dog towns), and certain types of loamy soils. M. vulgare was rarely found off of prairie dog towns and then only in other disturbed areas such as bison trails or tree falls. Population studies showed M. vulgare populations were mostly stable or slightly decreasing in density (though not significantly) from 2010 to 2011; however, a few of the populations did increase dramatically in density from 2010 to 2011. M. vulgare populations existed in high densities (78.3-322.9 ramets/m2) and in low densities (0.1-2.2 genets/m2) throughout WCNP. Disturbances including prairie dog burrows, bison trails, prairie dog and bison presence, mowing, and percent cover of bare ground were measured in relation to population dynamics. While M. vulgare populations were disturbance dependent, there was no clear relationship between disturbances and population dynamics. Examination of M. vulgare life history traits found that while M. vulgare had relatively low biomass allocation to reproduction as compared to native perennial prairie species, M. vulgare produced an extremely high number of small seeds (estimated 1487/plant). Seedling establishment rates were also high. While not quantified, M. vulgare appears to be effectively dispersed through bison epizoochory. These results help to inform management of M. vulgare populations: minimizing disturbance, decreasing propagule supply, and minimizing dispersal may help to reduce M. vulgare invasions.

Understanding the health of rare and at-risk plant species is crucial for the conservation of biodiversity. Rare and at-risk species are often specialists with very specific requirements for growth and reproductive success. Some parameters associated with growth and reproductive success of rare aquatic species may include biotic and abiotic factors such as herbivory, competition, water depth, pH, and other factors associated with habitat quality. In this thesis, the population health of the rare and at-risk aquatic species Plantago cordata in Southern Illinois streams is assessed in relation to stream quality of sites containing Plantago cordata and sites without Plantago cordata.The objectives of this study were to (1) Determine if there is a relationship between population size structure and biotic habitat factors among populations and sites, and (2) Determine if there is a relationship between population size structure and abiotic habitat factors among populations and sites. Hypotheses were constructed using the hierarchy-of hypotheses method and most narrow alternative hypotheses are as follows: (1) There will be significant relationships between biotic and abiotic factors and Plantago cordata growth and reproductive success. (2) Levels of herbivory will have significant relationships with growth and reproductive success of Plantago cordata. (3) The abundance of co-occurring species individually and considered in functional groups (graminoid, herb, woody) will have significant relationships with growth and reproductive success of Plantago cordata. (4) Stream depth will have a unimodal relationship with growth and reproductive success of Plantago cordata, with maximum performance at an intermediate stream depth. (5) Stream pH will have a unimodal relationship with growth and reproductive success of Plantago cordata. (6) Stream quality (as outlined by the QHEI assessment) will have significant relationships with growth and reproductive success of Plantago cordata. General trends in herbivory suggest that Plantago cordata could be at greatest risk of herbivory during warmest average temperatures of the study season. There is evidence to suggest that flowering individuals could have a greater threat of herbivory than other growth stages based on Kendal correlation results. A Wilcox test determined that there were no statistical differences in species richness for plots with versus without Plantago cordata. There were relatively low values for percent cover of co-occurring species when considered in functional groups and there was overall relatively low species richness across all communities. Despite this, non-metric multidimensional scaling (NMDS) of community data provided evidence to suggest that there were statistical differences in the species composition of co-occurring individual riparian species among P. cordata populations. A significant negative correlation for water depth and flowering individuals and a significant positive correlation with water depth and pH occurred. A significant positive correlation for stream pH with the numbers of seedlings and dehiscing individuals was also found. One-way analyses of variance (ANOVAs) showed that some populations containing Plantago cordata were found to have stream pH values statistically different from nearby IEPA control streams without Plantago cordata. One population of Plantago cordata was found to be significantly different than two other populations in regards to stream pH using a linear mixed model. A principal components analysis (PCA) of stream Qualitative Habitat Evaluation Index (QHEI) assessments demonstrated distinct clustering of Plantago cordata streams and control IEPA streams without Plantago cordata. These findings provide partial evidence to suggest that growth and reproductive success of Plantago cordata could potentially be impacted by the abiotic and biotic parameters explored in this thesis.