Science Objects are two hour on-line interactive inquiry-based content modules that help teachers better understand the science content they teach. This Science Object, co-developed between NOAA and NSTA, is the second of four Science Objects in the Coral Reef Ecosystems SciPack. It investigates the abiotic characteristics that affect the coral reef ecosystem. The number and kinds of organisms found along each reef depend on the physical conditions of the environment and resources available, including food, light, water quality, temperature, and other organisms living in the reef. If conditions change significantly due to changes in climate, loss of food sources, excessive predation, or loss of habitat, the health and stability of the ecosystem will be affected. Like many complex systems, coral ecosystems tend to have cyclic fluctuations around a state of rough equilibrium. In the long run, if conditions remain reasonably constant a coral ecosystem can be stable for hundreds of years.