An ecosystem is the collection of all living things and the natural area that they inhabit all taken together as one giant system. The organisms include all animals, plants, and bacteria present and the natural area includes all landforms and water systems, and the air and atmosphere. As you can imagine, there are a great many things going on in an ecosystem including the food chain among the living organisms (how the animals, plants, and bacteria all depend on each other for nutrition), and the water cycle and weather patterns among the natural landscape. When all of these systems is working in a relative stable balance, the ecosystem is said to be in equilibrium. Minor changes introduced to the system will be readily absorbed and not affect the system as a whole. Major shifts or changes in an ecosystem (such as those caused by humans) and the equilibrium could be thrown off balance.