18: Ecosystems Last updated Save as PDF Page ID5595 Contributed by CK12CK12 18.1: Biological Communities18.2: Roles in an Ecosystem18.3: Flow of Energy in Ecosystems18.4: Flow of Matter in Ecosystems18.5: Nitrogen Cycle in Ecosystems18.6: Ecosystems- Challenge 118.7: Fresh Water Ecosystems18.8: Types of Marine Organisms18.9: Marine Food Chains18.10: Ocean Ecosystems18.11: Ecosystems- Challenge 2