In order to live, all organisms on earth need to obtain nutrients, water, and energy. The minerals and other chemicals that make up nutrients, as well as water itself, are continually cycled from the nonliving environment to living organisms and back to the environment. The sun is the ultimate source of energy for all living organisms. Autotrophs capture solar energy and convert it to chemical energy. Then the autotrophs and the heterotrophs utilize the autotrophs' chemical energy. We have examined in depth the process of converting solar energy to chemical energy (i.e., photosynthesis) and the process of utilizing the chemical energy (i.e., cellular respiration). Now it is time to look at the bigger picture-how living organisms interact with each other and their surrounding environment while cycling chemicals and transferring energy. This is what ecosystems are all about, and it is not a static proposition. Ecosystems are dynamic and constantly changing, just as populations and communities are. Many of the changes occurring now, unfortunately, are harmful ones brought about by human interference and pollution.