Invasion of smooth brome into North American tall-grass prairies: Impact on native plant species and mechanisms responsible for successful invasion

Background/Question/Methods Within the tall-grass prairies of North America, the grass, Bromus inermis (smooth brome) was introduced through repeated introductions for soil retention and animal graze. Since the introduction of brome it has spread beyond its intended range and has become a widespread invasive species within native prairies. Currently, almost nothing is known about the mechanisms responsible for the successful invasion of brome and its impact on native communities. Here, we tested two hypotheses commonly attributed to the successful invasion of non-native plants: (1) non-native plants that become invasive are dominant competitors capable of displacing native species across a wide range of microhabitats; and (2) following introduction, non-native plants are able to become invasive due to a release from natural enemies. To assess the impact of brome on the patch dynamics (growth, colonization and extinction risk) of a dominant native plant, prairie cordgrass (Spartina pectinata), we conducted a six year survey of the spatial distribution of brome and cordgrass. We also conducted a manipulative field experiment to assess microhabitat preferences and competitive interactions of smooth brome and cordgrass. Finally we conducted a field survey to determine if brome had lower herbivore biomass, richness and diversity than other dominant native plant species.
Results/Conclusions Based on an analysis of the changes in the spatial distributions of brome and cordgrass over six years, we determined that cordgrass growth was 2 times greater in areas not invaded with brome relative to areas heavily invaded with brome. The probability of cordgrass colonization and extinction averaged 1.3 times higher and 7.8 times lower, respectively, in areas of low versus high brome coverage. Based on field experiments, we determined that germination did not differ between species (F_1,611=2.7, p>0.05) but was strongly affected by the interaction among microhabitat type, plant species and soil conditions. Furthermore, in areas where brome and cordgrass grew in competition, brome had a strong negative effect on cordgrass growth and persistence. (F_1,1474=53.3, p<0.001). Finally, the analysis of herbivore abundance and species richness between brome and native species is not yet complete. Future research plans include an assessment of how the invasion of brome affects higher trophic levels (i.e. total arthropod community) on cordgrass. Given our findings that brome is rapidly spreading and displacing native plants (and likely associated arthropods), it is imperative that a management plan be developed and acted upon in order to combat future effects on native species.