Biodiversity is the variety of living things found in an ecosystem. It's an important way the natural world is kept in balance. When species begin dying out, it upsets this delicate balance and can cause ecological crises. For example, Earth's rainforests are some of the planet's most biodiverse places. However, the rainforests are also one of Earth's most threatened habitats. Readers discover why a loss of biodiversity in places such as rainforests is such a problem in this guide to environmental science that is timely and informative. Colorful photographs of biodiverse habitats transport readers to parts of the world in need of protection.