This textbook chapter summarizes the changes in the ecosystems found in the world's forests. This is chapter 2 in the unit, A New World View, which introduces global systems science, emphasizing the concepts of environmental diversity, Earth as a system, and sustainability. The resource includes a textbook chapter, integrated hands-on and inquiry activities, links to current news articles, and a suite of pre- and post-unit assessments. A teacher's guide supports classroom use. The resource is part of Global Systems Science (GSS), an interdisciplinary course for high school students that emphasizes how scientists from a wide variety of fields work together to understand significant problems of global impact.