Population density can influence all three phases of natal dispersal: departure 20 from the place of birth, searching the landscape, and selecting a new site in which to settle. The 21 direction of the effect of density on dispersal should be affected by the relative costs and benefits 22 of living in an area with high population density. Animals may benefit from high population 23 density due to mate availability and predator risk dilution, but may also face increased 24 competition in high density areas. These conflicting mechanisms should influence the pattern of 25 change in population density between pre-and post-dispersal locations: do dispersing individuals 26 choose to move to areas of higher or lower population density than that at their natal site? We 27 examined the influence of density on dispersal in brush mice (Peromyscus boylii). We 28 documented pre-and post-dispersal locations of individuals using both radio telemetry and live-29 trapping, and used a spatially explicit capture-recapture model to estimate density across the 30 landscape. We also tested for a relationship between dispersal distance and local population 31 density at the natal site. Animals tended to settle in areas with higher population densities than 32 where they were born. This pattern held when landscape-level changes in population density 33 were incorporated: the magnitude of change in local population density between the pre-and 34 post-dispersal locations of a given individual tended to be greater than would be explained by 35 increasing population density across the landscape alone. Further, dispersal distances were 36 shorter when local natal population density was higher. 37 38

Invasive species impact ecosystems through their large abundances and strong per capita effects. Enemies can regulate abundances and per capita effects, but are notably absent for many new invaders. However, invaders acquire enemies over time and as they spread; processes hypothesized to mitigate negative invader impacts by reducing abundance or per capita effects. Alternatively, properties of invaders or acquired enemies, such as an enemy's ability to attack multiple species, may hinder enemy mitigation of invader impacts. We used field experiments to evaluate disease mitigation of invader impacts using the invasive grass Microstegium vimineum, which hosts an emerging fungal disease, and a native grass competitor, Elymus virginicus. We manipulated competition through density gradients of each plant species, and we reduced ambient foliar diseases with fungicide and autoclaving. We then modeled long-term population dynamics with field-estimated parameters. In the field, disease did not reduce invader abundance or per capita effects. The invader amplified disease on itself and the competitor, and disease reduced invader and competitor fitness components (e.g., germination). The dynamical model predicted that disease impacts on the competitor are greater than on the invader, such that disease will reduce invader abundance by 18%, and competitor abundance by 88%, over time. Our study suggests that enemies acquired by invaders will not necessarily mitigate invader impacts if the invader amplifies the enemy and the enemy attacks and suppresses competitor species.

Invasive plants, which cause substantial economic and ecological impacts, acquire both pathogens and beneficial microbes in their introduced ranges. Communities of fungal endophytes are known to mediate impacts of pathogens on plant fitness, but few studies have examined the temporal dynamics of fungal communities on invasive plants. The annual grass Microstegium vimineum, an invader of forests and riparian areas throughout the eastern US, experiences annual epidemics of disease caused by Bipolaris pathogens. Our objective was to characterize the dynamics of foliar fungal communities on M. vimineum over a growing season during a foliar disease epidemic. First, we asked how the fungal community in the phyllosphere changed over two months that corresponded with increasing disease severity. Second, we experimentally suppressed disease with fungicide in half the plots and asked how the treatment affected fungal community diversity and composition. We found increasingly diverse foliar fungal communities and substantial changes in community composition between timepoints using high-throughput amplicon sequencing of the ITS2 region. Monthly fungicide application caused shifts in fungal community composition relative to control samples. Fungicide application increased diversity at the late-season timepoint, suggesting it suppressed dominant fungicide sensitive taxa and allowed other fungal taxa to flourish. These results raise new questions regarding the roles of putative endophytes found in the phyllosphere given the limited number of pathogens known to cause disease on M. vimineum in its invasive range.

Plant-pathogen interactions occur throughout the process of plant invasion: pathogens can acutely influence plant survival and reproduction, while the large densities and spatial distributions of invasive plant species can influence pathogen communities. However, interactions between invasive plants and pathogens are often overlooked during the early stages of invasion. As with introductions of invasive plants, the introduction of agricultural crops to new areas can also generate novel host-pathogen interactions. The close monitoring of agricultural plants and resulting insights can inform hypotheses for invasive plants where research on pathogen interactions is lacking. This chapter reviews the known and hypothesized effects of pathogens on the invasion process and the effects of plant invasion on pathogens and infectious disease dynamics throughout the process of invasion. Initially, pathogens may inhibit the transport of potentially invasive plants. After arrival in a new range, pathogens can facilitate or inhibit establishment success of introduced plants depending on their relative impacts on the introduced plants and resident species. As invasive plants spread, they may encounter novel pathogens and alter the abundance and geographic range of pathogens. Pathogens can mediate interactions between invasive plants and resident species and may influence the long-term impacts of invasive plants on ecosystems. As invasive plants shift the composition of pathogen communities, resident species could be subject to higher disease risk. We highlight gaps in invasion biology research by providing examples from the agricultural literature and propose topics that have received little attention from either field.

Plant-pathogen interactions occur throughout the process of plant invasion: pathogens can acutely influence plant survival and reproduction, while the large densities and spatial distributions of invasive plant species can influence pathogen communities. However, interactions between invasive plants and pathogens are often overlooked during the early stages of invasion. As with introductions of invasive plants, the introduction of agricultural crops to new areas can also generate novel host-pathogen interactions. The close monitoring of agricultural plants and resulting insights can inform hypotheses for invasive plants where research on pathogen interactions is lacking. This chapter reviews the known and hypothesized effects of pathogens on the invasion process and the effects of plant invasion on pathogens and infectious disease dynamics throughout the process of invasion. Initially, pathogens may inhibit the transport of potentially invasive plants. After arrival in a new range, pathogens can facilitate or inhibit establishment success of introduced plants depending on their relative impacts on the introduced plants and resident species. As invasive plants spread, they may encounter novel pathogens and alter the abundance and geographic range of pathogens. Pathogens can mediate interactions between invasive plants and resident species and may influence the long-term impacts of invasive plants on ecosystems. As invasive plants shift the composition of pathogen communities, resident species could be subject to higher disease risk. We highlight gaps in invasion biology research by providing examples from the agricultural literature and propose topics that have received little attention from either field.