Effects of human actions on four ecological systems, with a focus on trophic relationships

There are, perhaps, no parts of earth remaining that haven't been impacted by human actions, either purposefully or unintentionally. Agriculture and livestock grazing occupy much of earth's arable land, oceans are overfished, greenhouse gas emissions are changing the climate, and human transportation has facilitated unintentional invasions of organisms. In order to become better stewards of Earth's biota, we must better understand how our actions - both direct and indirect - affect ecosystems and species. I examine four ecological systems and the direct and indirect effects of human change on them, with a focus on trophic (eating) relationships. First, I analyze the environmental impact of beef production, especially in the U.S., which has the most industrialized system in the world. Next, I present the results of an experiment testing the effects of plant diversity on the biological control of an agricultural pest species. An invasive species, the soybean aphid, has become a major pest of soybean in the U.S. in the past fifteen years, and efforts to control the aphid with natural enemies could reduce the need for pesticides. Third, I analyze a twenty-year seed addition experiment to investigate the long-term dynamics of plant communities. The results highlight the importance of dispersal limitation combined with local competition and movement of species over time. Finally, I describe a model that captures the dynamics of an emerging wildlife disease: bovine tuberculosis in lions in Kruger National Park, South Africa. The modeling results reveal the most likely long-term effects of the disease in this lion population and efficacy of intervention approaches.