An environmentalist's concern includes the ECOSYSTEM, ENERGY, SMOG, FAUNA, OZONE, ecology, EROSION, acid rain, ionosphere, OZONE layer, rainforest, and climate change. The ECOSYSTEM is a complex network of living organisms and their environment. ENERGY is the capacity to do work or cause physical change. SMOG is a type of air pollution caused by emissions from vehicles and factories. FAUNA refers to the animal life of a particular region. OZONE is a form of oxygen that occurs naturally in the atmosphere. Ecology is the study of the relationships between living organisms and their environment. EROSION is the process of wearing away of the land surface by water, wind, or other natural agents. Acid rain is precipitation that has been made acidic by pollutants in the atmosphere. The ionosphere is a layer of the Earth's atmosphere that is ionized by solar radiation. The OZONE layer is a region of the Earth's stratosphere that contains a high concentration of OZONE. Rainforests are tropical forests with high levels of rainfall. Climate change is a long-term shift in global weather patterns.