Invasive plant species often enter communities as passengers, yet later become drivers of community dynamics, ultimately altering the functioning of ecosystems. Early successional old fields are ideal for addressing both the susceptibility of communities to plant invasions, but also the impact plant invasions may have on the structure and function of such ecosystems given the predominance of old fields across terrestrial landscapes in time and space and also due to the presence of a variety of invasive species. Here, we review and synthesize: (1) Patterns and underlying processes determining exotic plant richness and the abundance of invasive plant species in old fields across spatial scales, and (2) The impacts of invasive plant species on the structure of old-field communities across spatial scales. Across 17 old-fields, we established 250 1-m2 plots nested within 50-m2 transects to investigate the role of exotic and invasive plant species as drivers and passengers of old-field community structure at neighborhood (1-m2 plots), community (50-m2 transects) and landscape (> than 2,600 m2 old-fields) scales.
We found that native richness best predicted exotic richness across spatial scales, with greater exotic richness associated with greater native richness. On the other hand, nitrogen fixer abundance, not native richness, best predicted the abundance of the most common invasive plant species, Lespedeza cuneata, across spatial scales; with the abundance of Lespedeza cuneata declining with the increase in abundance of native nitrogen fixing species. In turn, both exotic richness and the invasive species Lespedeza cuneata significantly altered patterns of co-occurrence and overall composition of old-field communities. Old-field communities with greater invader richness or those dominated by Lespedeza cuneata became more randomly arranged (i.e., less aggregated) than communities with lower invader richness and lower Lespedeza cuneata dominance. Additionally, both invader richness and the presence of an invasive species altered old-field species composition. Our findings suggest that biotic factors shaping exotic richness differ from factors shaping the abundance of an invasive plant species. We also find that invader richness and the presence of invasive plant species can influence patterns of co-occurrence and overall species composition of old-field communities potentially altering the function of such systems. Future studies need to investigate the spatial-temporal aspects of invader richness and invasive plant species in old-field ecosystems to better predict factors associated with the passenger versus the driver stage of invasive plant species along with ecological consequences of plant invasions.