Abstract
Through the past 150 years over 90% of California’s vernal pool wetland habitat has been lost to agriculture and urbanization. Knowledge of the factors that drive species diversity in California’s vernal pools is required to enable proper management and restoration of these habitats in the future. While many factors have been identified as contributors to maintenance of species diversity in vernal pools, the system is far from being fully understood. One factor that has not been investigated is the effect of parasitic plants. Cuscuta howelliana is an abundant endemic parasitic plant that inhabits California vernal pools. The hypothesis tested in this research was that C. howelliana would act as a keystone predator and increase plant species diversity in vernal pools. This study took place at Beale Air Force Base in the northeastern Sacramento Valley where there are ~1,000 vernal pools. In 15 vernal pools, experimental manipulation of the presence versus absence of C. howelliana was conducted. An additional five vernal pools where C. howelliana was not naturally present were also monitored. Eryngium castrense and Navarretia leucocephala were found to be the preferred host plant species of C. howelliana. Vernal pools without C. howelliana naturally present had lower plant species richness compared to pools where C. howelliana does occur naturally. Plots in vernal pools where C. howelliana was manually removed also had lower plant species richness compared to plots where C. howelliana was present. There was no single plant species that showed a significant trend towards being absent in C. howelliana removal plots. Eryngium castrense, one of the preferred host plants, had lower percent cover in plots where C. howelliana was present, which may have explained differences in species richness. However, most plant species on average showed a higher percent cover in C. howelliana present plots, but this did not become significant until all plant species were considered on one model. Grazing regime differences among vernal pools with C. howelliana naturally present and naturally absent may have also contributed to the observed difference in plant species richness. In conclusion, C. howelliana presence was related to increases in species richness, consistent with what is expected from the effects of a keystone predator. Interactions among species, including parasitic plants, needs to be considered in restoration and management of California vernal pools.