Just as the microscope extends our power of observation of the details of structure of components in the ecosystem, so tracers extend our power of observation of function. By tracers we mean small amounts of easily detected substances that can be used to follow and quantify the flow of materials or movement of organisms not otherwise visible or detectable by ordinary means. Tracers can take many forms ranging from days used to trace water movement to isotopes that can be used to measure nutrient exchange between organisms and environment because they are easily ecosystem detectable by special instruments. Justifiable concern about radioactive pollution has overshadowed the fact that radioactive tracers provide valuable tools for study. Many vital elements have radioactive isotopes with short half – lives (that is, decay to nonradioactive form in a few days), which can be detected and measured in such small quantities so that tracer amounts introduced into the ecosystem will have no  measurable effect on the process being measured or the organisms present. And an isotope does not have to be radioactive to be useful as a tracer. The nonradioactive isotope nitrogen – 15, for example, has been very useful in the study of the all important ecosystem nitrogen cycle.
ecosystem laboratory experiments in pond

       Experiments with radioactive tracers provide excellent ecosystem laboratory experiments to follow a field study of the pond. For example paired bottles of filtered pond of filtered pond water can be set up. Each ‘’spiked’’ with tracer amount of radioactive phosphorus (32p). in one of a pair of bottles a gram o two of a large, leafy submerged aquatic plant is placed, and in the other, a known weight of filamentous or phytoplankton algae. The uptake by the plants is easily monitored by withdrawing and filtering small samples of water at intervals over a period of several hours, and counting the samples in a suitable; decrease in radioactivity of the water provides a relative measure of the amount of phosphorus moving into the plant biomass. The much more rapid uptake per unit weight of small plants, as compared with the large one, provides a dramatic illustration of the differences ecosystem in nutrient turnover rates, as previously discussed.

             As was made clear in the previous chapter, the ecosystem is the basic functional unit with which we must deal since it includes both the organisms and the nonliving environment, each influencing the properties of the other necessary for maintenance of life as we have it on the earth. By considering the ecosystem first we are in a very real sense beginning our study of ecology with the gross anatomy and physiology of nature, much as a beginning medical student might begin his study with the gross anatomy and physiology of the human body. Once a clear image of overall structure and function has been obtained, component parts and functions, such as populations, the cyclic behavior of nitrogen, or plant productivity can be considered.