We would do well to close the general discussion of the study of natural ecosystems with an example that illustrates the value of studying the whole ecosystem as well as the component parts, even the system is much more complex than a small fish pond or field. A tropical biotic reef represents one of the most beautiful and well – adapted ecosystems to be found in the world. Corals, small animals with hard calcareous calcereous skeletons, and calcareous algae build up the reef substrate which is the home of numerous organisms. Embedded in the tissues of the coral, and also in and on the skeleton of many animals and the general calcareous substrate are numerous algae. If supplied with abundant zooplankton food, some coral species can be maintained in ecosystem laboratory tanks without the algal associates. However, when the metabolism of a whole reef is measured (as for example, by measuring diurnal changes in oxygen as water passes over the reef – a modification of the method just described for assaying the metabolism of a pond), the input – output budget indicates not enough animal food suspended in the water to completely support the corals. In such a situation there must be supplemental sources of food, perhaps that produced by algal associates. ecosystem Tracer experiments have shown that exchanges of organic matter between plant and animal tissues within the colony do occur. Also, it has been clearly demonstrated that mineral nutrients are recycled back and forth between animal plant components so the colony does not require a high rate of fertilization from without. These discoveries indicate that, in nature, coral animals and algae are metabolically linked and dependent on one another. The history of recent research on coral reefs bears out the point we have already emphasized: the behavior of an isolated component (coral in a tank) may not be the same as the behavior of the same component in its intact ecosystem (the reef) where a available sources and nutrient constraints may be quite different. And the corollary to this: to understand the ecosystem, the whole as will as the part must be studied.