Although the harmful effect of disease on agricultural crops and livestock is well-known, scientists are just beginning to recognize the importance of disease in wild plants and animals. In addition to killing infected individuals and reducing populations of plants and animals, diseases can affect the species composition of ecological communities. For example, when species compete for resources, such as food and water, and also share a disease, the disease can prevent the better competitor from dominating and allow the species to coexist. Alternatively, the disease could wipe out the less competitive species more quickly. This study will investigate whether a fungal disease that kills grass seeds is contributing to the invasion and dominance of a harmful exotic grass species, cheatgrass, in the U.S. Intermountain West. The invasive grass degrades rangeland and promotes fire, but also spreads a fungal disease that kills seeds of native grasses as well as cheatgrass. The experiment will measure the disease in field populations in Utah, and calculate its impact on cheatgrass invasion and native plant persistence using mathematical models. The study will also manipulate fall rainfall to determine whether the disease's impacts depend on climate conditions.
This research will improve scientific understanding of the effect of disease on a harmful grass invasion. The results will inform management decisions based on using disease to combat the invasive grass. Expanding scientific understanding of the impacts of wildlife disease will also aid conservation of other communities of interest such as tropical forests and coral reef fishes. In addition to its scientific impacts, this project will promote research experience for undergraduates. The research will also be incorporated into teaching materials for undergraduate courses. This project has already provided research experience for five undergraduates (four female), and it will result in the completion of a doctoral dissertation by a female mathematical biologist, an underrepresented minority in this field.
Invasive species have a range of impacts on the ecosystems they invade. For example, invasive species can compete with native species, over-exploit their prey species, and modify the cycles of fire and nutrient flow that maintain healthy ecosystems. Another important impact that ecologists are beginning to recognize is pathogen spillover, which occurs when an invasive species supports a pathogen that also infects the native species. In this project, we investigated the effect of pathogen spillover in cheatgrass-invaded grasslands in the Intermountain West, USA. Cheatgrass (Bromus tectorum), a dominant invasive annual grass from Eurasia, supports a fungal seed pathogen called Black Fingers of Death (Pyrenophora semeniperda), which spreads from cheatgrass seeds to native grass seeds. We used mathematical models combined with data on plant reproduction, competition, and survival to understand the impact of pathogen spillover on cheatgrass and a native grass, squirreltail (Elymus elymoides). Surprisingly, we found that even though cheatgrass tends to promote pathogen spillover, the pathogen is more harmful to cheatgrass than to the native grass. The native grass population was buffered against the loss of seeds because of its perennial life history strategy, contrasting with the annual strategy of the invasive grass. We also found that cheatgrass was more susceptible to the pathogen than the native grass, suggesting that the fungus may have adapted to infect the extremely common invasive species more effectively than the rare native species. Finally, we found that direct competition between cheatgrass and squirreltail was weak, allowing the species to coexist in areas where healthy adult populations of squirreltail remain. This suggests that disturbances such as fire and over-grazing may have had a larger impact on the native grasses, by killing off their long-lived adults, than cheatgrass itself. The plant demographic data this project produced will be valuable for understanding which processes limit native plant populations, and how to best suppress cheatgrass and re-establish native plants. The mathematical model we produced will be useful to scientists investigating the impact of other processes, such as grazing, on invasive and native plant populations in this and related plant ecosystems. For example, researchers are applying modified versions of the model to study interactions between understory plants in North Carolina and native and exotic grasses in California.
