Grasslands of the northern Rocky Mountains are being transformed. Exotic forbs such as spotted knapweed, leafy spurge, and Dalmatian toadflax have overrun vast areas, displacing native plants. These exotic invaders appear to differ from native forbs in subtle but important ways that may alter the structure and function of grassland systems.
Research Approach & Results
Our recent work in western Montana (Pearson et al. 2012) showed that dominant exotic forb species tend to put more energy into flowering structures and to flower later than dominant natives, while natives tend to put relatively more energy into vegetative growth and to synchronize vegetative growth and flowering earlier in the season. Furthermore, senescence of flowering stems also differed dramatically, with exotics producing far more persistent flowering structures. These differences depict a distinct niche separation between the dominant natives and invading exotics, which has important implications for understanding both invasibility of the system and invader impacts. With regard to the invasibility of these systems, the fact that the five species of invaders examined represent five distinct plant families yet exhibit similar plant traits suggests that the success of these species may hinge on their similar ability to exploit an "empty niche" within these communities. If so, these plant traits might be used to predict future invasions. With regard to invader impacts, the distinctive shift in the structure and function of the community indicates an invasion trajectory that predicts changes in community interactions and ecosystem services. For example, later flowering times may affect pollinators, later growing periods may influence how fire interacts with the vegetation, and shifts in the types of biomass produced and decomposition rates may affect nutrient cycling.
The above postulated effects are reasonable, but speculated based on research in other systems. However, we did quantify the extensive outcomes of one subtle shift within these communities - those resulting from changes in plant architecture (Pearson 2009). We found that the larger, more expansive, and more persistent flowering stems of the exotics allowed populations of native web-building spiders to increase by 80-fold. Additionally, native spiders built larger webs on the exotic substrates that allowed each spider to double its prey captures. The combined effect of increased spider numbers and increased lethality of each spider has allowed spiders in invaded habitats to suppress some prey populations to such an extent that their prey's prey are released from predation (Pearson 2010). These results show how even subtle changes caused by invasion can have extensive impacts that transmit through four trophic levels.
- University of Montana
- Montana Fish Wildlife and Parks
- City of Missoula
- Confederated Salish and Kootenai Tribes.