Changing Rivers and Streams: The Effects of Anthropogenic Alteration of Riparian Systems on Mimulus guttatus

DOCTORATE DISSERTATION: The anthropogenic alteration of river and stream flow due to the building of dams throughout the Western United States has dramatically changed riparian plant and arthropod communities. To document these changes and to examine their potential impacts on a common riparian species, Mimulus guttatus, the common monkeyflower, I conducted a series of vegetation surveys along dammed and non-dammed streams as well as a set of experiments to tease apart observed differences in vegetation and arthropod community structure. Along rivers and streams in the Sierra Nevada Mountains of California, I found vegetative community structure differs with increased grass species coverage in areas below dams as compared to naturally flowing streams. Additional differences included an increase in leaf litter/grass thatch coverage below dams and an increase in shade coverage as the distance from the stream increased. These changes in community structure may be responsible for the overall observed changes in forb species communities when comparing across dammed and non-dammed streams. Similar results were also documented for a set of surveys conducted along the Carmel River in the Coastal Mountains of California. In a set of greenhouse and field experiments examining the impacts of increased leaf litter and shading on M. guttatus, I found that leaf litter accumulation decreased germination and a combination of high levels of shading and leaf litter detrimentally impacted both the germination and growth of the species. To examine how changes in flooding regimes may influence M. guttatus via changes in arthropod communities, I used a set of field exclosures in flooded and non-flooded areas. I found that changes in insect herbivores altered M. guttatus survival with decreased survival in non-flooded areas, predatory wolf spiders decreased the amount of herbivore damage during years with low herbivore populations but lowering herbivore damage did not positively affect M. guttatus growth and reproduction. This research highlights the ecological impacts of anthropogenic alteration of flooding regimes through changes in interactions between species. When considering the restoration of floodplains through controlled releases below dams, these potential changes should be considered.