These systems work together to ensure the successful functioning of the organism, e.g., in getting food and protecting itself from the environment, and in how breathing and heart rate change in relation to exercise, sleeping, and normal activities. All organisms are composed of cells. Cells of plants and animals show differences and similarities. Energy is provided to ecosystems primarily in the form of sunlight. This food can be used immediately, stared for later use, or used by other organisms Organism of different types interact with one another in various ways besides providing food.
Ecosystems can change over time in response to physical conditions or interactions among organisms.
Changes may be the result of predictable succession or the result of catastrophes, e.g. volcanic eruption or ice storms. Dead plants and animals are broken down by other living organisms.