Abstract
Large numbers of thecamoebae are found in the sand and detritus substrate of small streams in the Amazonian terra firme forest. Their relative population densities can be determined by searching and counting thecae under a dissecting microscope for a standard time interval (constant effort sampling). The total population per sample can be calculated from a gradual linear depletion during successive such counts in a given sample (removal trapping). From a series of such regressions the total number of thecamoebae of any sample can be estimated from a single, first count. The method depends on specific conditions with regard to size of area searched, quantity of substrate per area, density of organisms per substrate and performance of the observers. These conditions are generally valid for similar methodical treatment of any population of small organisms in any type of dense substrate. The linear regression of the depletion effect implies a constant mean probability for any thecamoeba to be found in the specified substrate during the specified time interval by any of the three observers involved in the study, and this despite the uncontrollable subjectivity of visual search in a microscopic field.