This dissertation addresses the ecological changes that occurred in 19th century California due to Spanish Colonial and Mexican Period cattle ranching. Grasslands in particular are often depicted as having been rapidly invaded by exotic Old World grasses. In addition, cattle ranching and ecological change are thought to be a factor in the decision making of Native Californians to enter the Spanish missions. Wilder Ranch California State Park, formerly known as Rancho Refugio, is used as a case study to explore these issues. Originally established as a rancho of Mission Santa Cruz, Wilder Ranch remained an operating ranch until the 1970s, and thus provides an excellent venue to explore the long- term effects of cattle ranching. To analyze the rate and intensity of grassland change at Wilder Ranch, a combination of zooarchaeological analysis, archaeobotanical analysis, historical records, and agent based modeling is used. These lines of evidence together indicate that grasslands were altered by exotic grasses, but not at the rate and intensity that is often suggested. In addition, analysis of baptismal records from nearby Mission Santa Cruz indicate that most local Native Californians had joined the mission before cattle herds had grown much in size, and before extensive ecological change is expected to have occurred. Instead, a combination of drought and social network collapse seem more influential in the decision of local Native Californians to join the mission.