Ecosystems are complex interactive systems that include both living and non-living things. Ecosystems consist of several hierarchical structures, wherein groups of same organism form populations. Individual organisms or populations live, grow, or reproduce within ecosystems. How does this happen? This happens through interdependent relationships with other organisms or populations in their physical environment. These interactions restrain growth; enhance or limit the size of populations; or maintain the balance between resources and those who consume them. Ecosystems are sustained by continuous flow of energy, originating primarily from the sun, and the recycling of matter and nutrients within systems. However, ecosystems experience shifts in population composition and abundance; and changes in the physical environment over time.