Invasive spread dynamics of Anthriscus caucalis at an ecosystem scale: propagule pressure, grazing disturbance and plant community susceptibility in canyon grasslands

Several factors may influence invasive spread dynamics once a species becomes naturalized. Propagule pressure generated from established populations is likely mediated by local scale factors such as plant community characteristics and disturbance regimes as well as ecosystem scale factors such as landscape features that shape dispersal patterns and propagule availability. Using a multi-scale observational approach, we explore how propagule pressure generated from naturalized populations of Anthriscus caucalis (M.-Bier) influences invasive spread dynamics in the Snake River canyon grassland ecosystem of the Intermountain Pacific Northwest, USA. In this study, we utilize demographic insights into A. caucalis naturalization across plant communities and a companion ecosystem scale distribution survey to elucidate invasive spread dynamics using spatially-explicit SDMs. We modeled A. caucalis occurrence as a function of plant community type, grazing disturbance, and propagule pressure generated from high-shrub communities. A. caucalis was almost uniformly present in low-and high-shrub (>90 %) communities. Within bunchgrass communities, A. caucalis occurrence was significantly lower within grazed paddocks (3 %) compared to ungrazed paddocks (52 %). The probability of A. caucalis occurrence in bunchgrass communities within ungrazed paddocks also increased at a faster rate as the relative area of high-shrub communities increased. These results suggest propagule pressure generated from high-shrub communities strongly influence invasive spread into bunchgrass communities. Our results also suggest that livestock disturbance slows invasive spread by decreasing propagule supply from high-shrub communities. Plant community composition also contributed to A. caucalis occurrence patterns within bunchgrass communities. A. caucalis occurrence declined as native bunchgrass cover increased, whereas occurrence increased with increasing cover of exotic annual species. As native bunchgrass cover declined, A. caucalis occurrence increased in locations with greater relative area of high-shrub communities. These results suggest that undisturbed bunchgrass communities are resistant to A. caucalis invasion regardless of propagule supply. In disturbed bunchgrass communities dominated by exotic annuals, A. caucalis invasion levels were positively associated with increased propagule pressure generated from high-shrub communities. In conclusion, the coupling of comparative demographic observations with spatially-explicit SDMs yielded significant insights into invasive spread dynamics. This approach should be applicable for elucidating invasive spread dynamics of other invading plant species and ecosystems.