The document defines key terms related to ecosystems, including environment, ecosystem, population, community, habitat. It explains that living parts of an ecosystem depend on non-living parts like water, air, sunlight and on other living parts for food. It provides examples of coastal redwood trees and foxes/rabbits to illustrate these relationships. It describes how a habitat must provide everything a plant or animal needs, using the example of a sidewinder snake in the desert. It defines a population as a group of the same species that live together, and a community as all living things that share a place. Finally, it explains how ecosystems can change over time, such as due to increased rain or a fire altering plant and animal populations