Understanding how energy moves through an ecosystem can help you understand how the system will respond to change, how organisms interact with each other, and how communities form. We use stable isotope analysis to trace energy through ecosystems from the primary producers (plants) to top carnivores (big fish). By idetifying major sources of energy we can also identify habitats that are critical for important species. For example juvenile striped bass (Morone saxatilis) get approximately half of their energy from saltmarsh habitats. This understanding allows us to classify saltmarshes as nursery habitat for striped bass.