Invasion threatens the integrity of native ecosystems by harming biodiversity and ecosystem services, costing the United States nearly $120 billion/year. Mixed grass prairies, invaluable for their utility as working rangelands, are susceptible to damage from invasive plants. While invasive plants directly compete with native plant communities, often decreasing forage value, these invasive plants can also directly and indirectly (via changes in native plant communities) affect other trophic levels. Further, invasive grasses can alter soil nutrient availability, which can have cascading consequences for other trophic levels. Understanding the effects of invasion on long-term rangeland sustainability requires taking a cross-trophic level approach. We evaluated the relationship between invasion by Bromus tectorum and B. arvensis and native community trophic dynamics in Thunder Basin National Grassland, Wyoming. We established plots along 10 invasion gradients (n = 5 gradients/invasive species). Each gradient has 5, 1m2 plots covering a range of invasion (0%, 25%, 50%, 75%, and 100% relative cover) of each respective invasive grass (n = 25 plots/species). At each plot, we collected plant, insect, and soil microbial community data, as well as several abiotic factors (soil temperature, moisture, light availability, nitrogen, phosphorus, and carbon).
Results/Conclusions
Invasion by B. arvensis and B. tectorum was not significantly correlated with any of the measured abiotic soil variables (soil temperature, moisture, nitrogen, phosphorus, or carbon); however, light at the soil surface significantly decreased as invasion percent increased. Surprisingly, the relative invasion percent (cover) and corresponding decrease in light does not drive native plant species richness or evenness. Instead, invasion decreases the abundance of Bouteloua gracilis, one of the dominant native grass species, thus drastically decreasing palatable forage availability. The decreased light availability due to bromes could contribute to the invasives’ competitive dominance over the native species in rangelands. Further, preliminary analyses suggest that invasion percent of these bromes affects the insect and soil microbial community to a greater extent than the native plant community. From this work, we gain a better understanding of how two well-established invasive grasses can impact various trophic levels of rangeland communities in an unmanipulated system. This provides valuable insight for rangeland management and sustainability, but further work is needed to establish causal relationships between invasive species and the native communities they affect.