A Non-equilibrium Approach to Rangeland Riparian Management: A Study at Tejon Ranch, California
Riparian areas in arid rangelands provide a wide range of ecosystem services and are critical areas for conservation of landscape and regional biological diversity. Creeks and their associated riparian habitats also provide unique resources for livestock management, however these systems have proven to be sensitive to disturbance from livestock. Ecological site concepts and associated state-and-transition models are useful tools for understanding rangeland plant succession and the relative influence of management actions. These models have been widely developed for upland rangeland systems; however, extension of these models to rangeland riparian systems is still in its infancy. In this study I classified two ecological sites along four streams on the Tejon Ranch in the southern San Joaquin Valley, California. Using vegetation data gathered over four years at fifteen study reaches I identified three distinct vegetation states and plausible conditions precipitating transitions between states. I monitored large mammal and herpetofauna activity at each of the study reaches and compared the predictive power of the categorical ecological site and state-and-transition models in predicting wildlife activity to models based on continuous environmental variables. Finally I investigated the role of cattle and feral pig management on riparian vegetation and wildlife. Ecological sites proved to be a good predictor of observed wildlife community assemblages and was included in top models predicting activity of three of five mammal species: bobcat (Lynx rufus) deer (Odocoileus hemionus) and feral pig (Sus scrofa); and two of three reptile species: western fence lizard (Sceloporus occidentalis) and Gilbert’s skink (Plestiodon gilberti). In contrast, vegetation states proved to be a poor predictor compared to continuous variables. Cattle and feral pigs appear to have played a role in the one observed vegetation transition, and cattle and feral pig activity was correlated with increased cover of hydrophyllic vegetation and bare soil, and decreased cover of upland vegetation and forbs. Summer cattle activity and spring pig activity were the most strongly correlated with changes in vegetation attributes. Results from this study illustrate the utility of ecological sites to model riparian vegetation change and provide a framework for understanding the effect of rangeland management on riparian vegetation and wildlife.