Effects of the spatial heterogeneity formed by Ambrosia dumosa on individual and population growth of the invasive annual grass Schismus barbatus

Natural ecosystems are exposed to numerous factors that determine their structure and dynamics. Although change is typically inherent in to biological systems, very few factors change a natural ecosystem as drastically as the arrival and successful establishment of non-native species. The impact of the Mediterranean grass (Schismus barbatus) in the southwestern U.S. exemplifies such dynamics.;In my dissertation, I empirically and theoretically explore two components of Schismus invasion. First, native shrubs may promote Schismus invasion if Schismus benefits from the favorable growth conditions that shrubs provide. Second, Schismus may threaten ecosystem level properties if Schismus precludes establishement of shrubs. Thus Schismus may compromise the maintenance of Islands of fertility that favor growth of native plant community. To meet these objectives, I explore Schismus performance with respect to the location and size of one of the most abundant native shrubs in the Sonoran and Mojave Deserts, the Bur-sage shrub, Ambrosia dumosa .;As a first approximation to the study system, I describe the natural distribution of Schismus with respect to the location of native shrubs (Chapter 1). Then, I examine factors that control Schismus-Ambrosia interaction using a three-year field experiment (Chapter 2), and subsequent characterization of soil nutrients changes around Ambrosia shrubs in the context of Schismus invasion (Chapter 3).;I found that Schismus barbatus responds to the spatial heterogeneity formed by Ambrosia. This response is strongly controlled by water availability (Chapters 1 and 2). Ambrosia creates a dichotomous landscape formed by nutrient rich areas below their canopies and nutrient poor areas at inter-shrub spaces (Chapter 2). In this landscape, performance of Schismus is favored around Ambrosia during years with good precipitation. Nevertheless, at any given growing season, shading and water interception by shrub canopies are the most influential factors on Schismus performance.;Given that germination was consistently low below Ambrosia canopies, the consequences of Schismus-Ambrosia interactions might not be as straightforward as predicted by the individual effect of Ambrosia on Schismus. In fact, Ambrosia might actually reduce realized Schismus densities. Conversely, inter-specific competition with Schismus regulates Ambrosia seedling survival. Thus, high-density stands of Schismus might generate a negative feedback on the dynamics of favorable areas associated with shrubs; ultimately limiting Schismus abundance. I design a modeling framework to test this hypothesis and present several predictions about the long-term demographic consequences of reciprocal interactions between Schismus and Ambrosia (Chapter 4).