Competition with non-native invasive species is considered a major threat to many rare native species. As such, invasives removals are a common management strategy. Rare native species that interact uniquely with other organisms in their community (e.g., hemiparasitic plants) may be adversely affected by removing invasives. A management plan for a regionally rare hemiparasitic plant in Massachusetts, Pedicularis lanceolata Michx., identified invasives as a threat, but more quantitative evidence is needed to determine how P. lanceolata's persistence is influenced by its co-occurrence with native or invasive hosts. This research asks how P. lanceolata is affected by growth with native versus invasive hosts. Chapter I describes the species associated with P. lanceolata throughout its range, comparing areas where it is considered common and rare. Relative abundances of natives, non-native invasives, non-native non-invasives, and species with both native and non-native genotypes growing with P. lanceolata did not differ significantly at sites where the species is considered common in the Midwest compared to sites where the species is considered rare in the east. Chapter II outlines greenhouse and field removal experiments in which the types of host plants growing with P. lanceolata were manipulated. In the greenhouse, P. lanceolata growth, survival, and flowering were lower when it was growing with invasive compared to native graminoids. However, differences in P. lanceolata growth and survival when natives versus non-native were removed in the field varied from year to year due to succession of native shrubs at the site during the study. Chapter III asks how the population growth of P. lanceolata differs in uninvaded and invaded patches using an Integral Projection Model to perform population projections, sensitivity and elasticity analyses, and a life table response experiment. The population growth rate of P. lanceolata in uninvaded patches was lower than in invaded patches due to the succession of native shrubs in uninvaded patches. Chapter IV describes a metapopulation model for the invaded population of P. lanceolata in Massachusetts. The quasi-extinction probability was significantly affected by probabilities of dispersal, positive correlations in vital rates between sites, and catastrophes. These data will be used to update the management plan for P. lanceolata.