This thesis focuses on interactions between two invasive species, the western mosquitofish (Gambusia affinis) and the red swamp crayfish (Procambarus clarkii), and two desert fishes, the Amargosa pupfish (Cyprinodon nevadensis) and the endangered Pahrump poolfish (Empetrichthys latos latos). Chapter one is a literature review on the impacts of invasive species on various desert fishes. Chapter Two reports the results of multiple mesocosm experiments that show poolfish are unlikely to persist in the presence of mosquitofish. Chapter Three reports the results of a mesocosm experiment that tested the combined impacts of crayfish and mosquitofish on poolfish populations. Chapter Three mimics a historic case study in Nevada, where the Lake Harriett poolfish population crashed following the introduction of crayfish and mosquitofish. Chapter Four reports results of a mesocosm experiment that revealed density dependent effects of invasive crayfish on pupfish populations, with pupfish populations negatively affected when crayfish were at high densities.