Title of Project
Characteristics of plant communities invaded by Dittrichia graveolens (Asteraceae), as it spreads away from roadsides in Santa Clara County
Division
Physical and Biological Sciences
Course or Program
BIOE 195
Description / Abstract
Plant invasions are often facilitated by transport corridors due to high levels of bare ground and low species diversity. Some introduced plants are restricted to these disturbed environments, while others are able to spread away from corridors and become invasive. Understanding the factors that contribute to this invasion process can help direct time and money more effectively. Dittrichia graveolens (L.) Greuter is an annual plant in the Asteraceae family native to the Mediterranean region. Dittrichia was first observed in California in the 1980’s and is often found growing along roadsides and in disturbed soils. Since its introduction, Dittrichia has spread across the state and has now invaded about three-quarters of California counties. More recently, the species has been observed spreading away from roadsides and into natural habitat off-road. We hypothesized Dittrichia invasion would be highest in disturbed sites off-road, but would be limited in more diverse communities. We surveyed 15 population pairs in Santa Clara County by selecting Dittrichia populations from off-road sites, with each being paired to the nearest roadside site, which we presumed to be the most likely source of Dittrichia invasion. Using quadrats, we measured percent cover of Dittrichia, other vegetation, and bare ground, and estimated species diversity within the Dittrichia population. We predicted that away from roads, Dittrichia populations would be larger in more disturbed sites and that Dittrichia cover would be lower in more diverse plots. We found that these predictions were not supported. Roadside sites had greater percent cover of bare ground, while percent cover of other vegetation and alpha diversity were similar for roadside and off-road sites. Population size was larger in roadside sites, although percent cover Dittrichia was significantly higher for off-road sites. Interestingly, disturbance only significantly predicted Dittrichia density at the quadrat level. Lastly, species richness was a poor predictor of Dittrichia density and population size for off-road sites. We suspect that environmental factors that increase species diversity may also favor Dittrichia invasion. These results provide implications for protecting habitat with high conservation value and may offer insight for directing resources when managing Dittrichia.
