Individual organisms contribute to nutrient cycling in ecological systems, which is shown to be a mechanism of homeostasis at that level. The phosphorus and nitrogen cycles are used to illustrate effects of changes in populations or communities on the cycling of these nutrients. Major disturbances such as deforestation and global climate change disrupt nutrient cycles and ecological system homeostasis. Data are examined to determine effects of deforestation on nutrient cycling. Increasing atmospheric carbon dioxide and global climate change are disrupting ecological systems' homeostasis, and several studies are used to show how this is happening, including changes in primary production, temperature and precipitation patterns. This book also discusses the role of individual species in filtering contaminants and pollutants from ecological systems.